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The evolution of martensite and the tensile deformation behavior of Ti-7333 alloy with different β grain size was studied. It is found that the β grain size (45–260 μm) significantly affects the triggering stress of stress-induced α″ martensite (SIMα″), and then affects the distribution and morphology of α″ martensite. At the initial stage of deformation, the larger β grain can inhibit the stress-induced secondary α″ martensitic transformation and the formation of α″ twinning. While under the same strain, the stress-induced martensite and martensite twinning can be activated simultaneously in the alloy with the smaller β grain, including {111}α″ type Ⅰ twinning, {130}α″ compound twinning, α″ type Ⅱ twinning, {021}α″ type Ⅱ twinning, which can effectively accommodate localized strains and improve the work hardening rate of Ti-7333 alloy. When the β grain size varies in the range of 45–84 μm, there is a higher work hardening plateau in the deformation stage II of Ti-7333 alloy. The yield strength and β grain size are in accordance with the Hall-Petch relation. As the grain size increases to 128 even to 260 μm, the work hardening rate continues to decrease and the yield strength inversely increases. It is demonstrated that the triggering stress for SIMα″ exhibits U-shaped fluctuations as β grain size increases, and the inflection point of Ti-7333 alloy is between 84 μm and 260 μm.

Fe-Mn-Si shape memory alloys (SMAs) have gained significant attention due to their unique characteristics. However, there remains a gap in the literature regarding the fabrication of these alloys using laser-directed energy deposition (LDED). This study fills this void, investigating the properties of Fe-Mn-Si SMAs produced by LDED. The shape memory performance of as-deposited Fe-Mn-Si SMAs was studied using a tensile method. Alloys underwent different degrees of deformation to assess their shape memory effect. Microstructural evaluations were conducted post-deformation to observe the internal structures of the alloys. The tensile tests revealed that shape recovery rates for deformation levels of 3%, 7%, 11%, and 15% were 68.1%, 44.2%, 31.7%, and 17.6%, respectively. Notably, the maximum recoverable deformation of the LDED-formed Fe-Mn-Si-based shape memory alloy reached 3.49%, surpassing the traditional deformation processing SMAs (<3%). The presence of a significant number of stacking faults was linked to the enhanced shape memory performance. The LDED technique demonstrates promising potential for the fabrication of Fe-Mn-Si SMAs, producing alloys with enhanced shape memory performance compared to traditionally processed SMAs. The study's findings offer new insights and broaden the applicability of LDED in the field of SMAs. [ABSTRACT FROM AUTHOR]

Infrared thermography (IRT) and heat source reconstruction (HSR) were used in the study to measure two thermophysical properties of superelastic nickel-titanium (NiTi) shape memory alloy (SMA) wires, namely thermal conductivity and specific heat capacity. Since the values are potentially phase dependent, the identification was carried out at ambient temperature with and without mechanical loading, i.e., in the stress-induced martensite state and in the austenite state, respectively. A uniaxial testing machine was used to apply constant deformation while allowing temperature measurements by IRT during Joule heating and natural return the thermal equilibrium. The data processing by HSR, including preliminary filtering operations, was preliminarily evaluated from temperatures provided by a model (with added noise). It gave an error of ± 0.5% for the thermal conductivity and ± 1% for the specific heat capacity. The experimental analysis showed that the thermal conductivity of stress-induced martensite is 26% higher than that of austenite at the same temperature (here the ambient temperature). Regarding the specific heat capacity, that of stress-induced martensite is 4.7% lower than that of austenite. Explanations for these differences were proposed from solid-state physics theory. The measured values were also compared to data collected in the literature (obtained at zero stress at different temperatures). [ABSTRACT FROM AUTHOR]

In this paper, high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy (SMA) under different mean strains are investigated. The uni-axial cyclic tensile tests with small amplitude of 0.25% are operated at three different states, i.e. austenite/elastic region, austenite and martensite coexisted region and martensite/elastic region with a pre-strain. Mechanical results indicate that the cyclic deformation strain is mainly in the inhomogeneous deformation band when the sample cycling in the B2 + B19′ phase coexist status. Cyclic deformation in the B2 + B19’ phase showed cyclic hardening effect. With increasing of the mean strain, the saturation stress is decreased, and saturation rate is reduced. The mean strain effect on the high-cycle fatigue life of NiTi is attributed to the inhomogeneous transformation of martensite. High resolution electron microscopy (HREM) observations and atomic simulations showed that partial dislocations with the Burgers vectors of 1/2 are induced on (001)M twin planes, the interaction of dislocations and edge dislocations in martensite results in cyclic hardening in martensite. While cyclic softening in martensite at the upper plateau of stress–strain (S–S) curve is attributed to the grain reorientation/rotation phenomenon. It suggests that newly generated stress-induced martensite at the interface of martensite and austenite could increase the high-cycle fatigue of NiTi SMA.

Fe-Mn-Si shape memory alloys (SMAs) have gained significant attention due to their unique characteristics. However, there remains a gap in the literature regarding the fabrication of these alloys using laser-directed energy deposition (LDED). This study fills this void, investigating the properties of Fe-Mn-Si SMAs produced by LDED. The shape memory performance of as-deposited Fe-Mn-Si SMAs was studied using a tensile method. Alloys underwent different degrees of deformation to assess their shape memory effect. Microstructural evaluations were conducted post-deformation to observe the internal structures of the alloys. The tensile tests revealed that shape recovery rates for deformation levels of 3%, 7%, 11%, and 15% were 68.1%, 44.2%, 31.7%, and 17.6%, respectively. Notably, the maximum recoverable deformation of the LDED-formed Fe-Mn-Si-based shape memory alloy reached 3.49%, surpassing the traditional deformation processing SMAs (

The present study focuses on modifying the microstructures and mechanical properties of a recently developed metastable β Ti–5Al–5V–5Mo–3Cr (Ti-5553) alloy through cold rolling technique. Three different degrees of deformations (5 %, 10 %, and 20 % thickness reductions) are employed on the as-cast Ti-5553 alloy. Microstructural analysis reveals presence of single β-phase in the as-cast alloy, which is further confirmed through X-ray diffraction technique. Cold rolling however activates different deformation mechanisms with increasing degree of rolling and hence evolve the microstructure as well. While the alloy subjected to the lowest extent of thickness reduction, deforms mainly via slip, that with the highest degree of rolling deforms through formation of slip lines, zigzag slip lines, and deformation bands. Overall, activation of specific twins such as {332} <113> β along the closed packed direction of the β matrix is noted for the optimally cold rolled specimen. Most interestingly, occurrence of stress-induced martensitic phase is also confirmed for all the cold rolled specimens with its volume percentage being directly proportional to the degree of deformation. This cold rolling imparted microstructural modifications also influence the local and global mechanical performance of Ti-5553 alloy. Highest values of hardness and compressive yield strength are noted for the Ti-5553 alloy cold rolled up to the highest extent. This systematic study is the first of its kind to report the influence of cold rolling on the stress induced martensite formation and hence establishes the processing-structure-property correlation for Ti-5553 alloy. [Display omitted] • Presence of stress-induced martensite is noted in all the cold rolled Ti-5553 alloy. • {332} <113> β twin activates along closed packed direction of the β matrix. • Hardness and compressive yield strength increase with increase in degree of cold rolling. • A novel hardness-strength relationship is established for Ti-5553 alloy. • Localized strain concentration at 45° to compressive loading axis is observed for as-cast and cold rolled Ti-5553 alloys. [ABSTRACT FROM AUTHOR]

Based on the modified Hopkinson torsion bar, a high-temperature dynamic shear test method was proposed for the Ti-1023 alloy, and the microstructural evolution of the tested material at different temperatures was studied. By using the modified high-temperature Hopkinson torsion bar, high-temperature testing within 1000 °C can be achieved. As the specimen-heating rate was fast, and the temperature gradient of the experimental environment was small, valid experimental data can be ensured during the experiment. The experimental results show that stress-induced martensite can significantly enhance the strength of the Ti-1023 alloy. Dynamically recrystallized grains similar to those in adiabatic shear bands appear in the microstructure of the Ti-1023 alloy after severe plastic deformation. Therefore, it is possible to regulate the content of stress-induced martensite in the microstructure to improve the mechanical properties of other alloys that are similar to β titanium alloys. [ABSTRACT FROM AUTHOR]

This study explores cardiovascular stents fabricated using laser powder bed fusion (LPBF) which is an emerging method to offer patient-specific customisable parts. Here, the shape memory alloy NiTi, in a near equiatomic composition, was investigated to deconvolve the material response from macroscopic component effects. Specifically, stress-geometry interactions were revealed, in-situ, for a minaturised cardiovascular stent subjected to an externally applied cylindrical stress whilst acquiring synchrotron X-ray imaging and diffraction data. The approach enabled the collection of spatially resolved micromechanical deformation data; the formation of stress-induced martensite and R-phase was evident, occurring in locations near junctions between stent ligaments where stress concentrations exist. In the as-fabricated condition, hardness maps were obtained through nanoindentation, demonstrating that the localised deformation and deformation patterning is further controlled by porosity and microstructural heterogeneity. Electron backscatter diffraction (EBSD) supported these observations, showing a finer grain structure near stent junctions with higher associated lattice curvature. These features, combined with stress concentrations when loaded will initiate localised phase transformations. If the stent was subjected to repeated loading, representing in-vivo conditions, these regions would be susceptible to cyclic damage through transformation memory loss, leading to premature component failure. This study highlights the challenges that must be addressed for the post-processing treatment of LPBF-processed stents for healthcare-related applications. [Display omitted] • X-ray imaging & diffraction of a NiTi stent reveal material-geometry interactions. • AM/LPBF miniaturised cardiovascular stents specially designed for in-situ testing. • When loaded, preferential locations have R-phase and martensite phases forming. • Stress concentrations and local microstructures promote phase transformations. • Knowledge of geometry-material effects critical to guide post-AM heat treatments. [ABSTRACT FROM AUTHOR]

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ​K or 773 ​K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ​K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications. [Display omitted] • This study systematically investigated the effect of annealing temperature on microstructure and mechanical properties. • This study analyzed the kinetics of β grain growth in Ti–38Nb-0.2O alloy. • The dependence of critical stresses of SIM and dislocation slip on grain size was comparatively analyzed. [ABSTRACT FROM AUTHOR]

Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is simultaneously achieving an ultra-low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi-component body-centered cubic (BCC) biomedical high-entropy alloy (Bio-HEA), which is composed of non-toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio-HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress-induced martensite transformation with extremely low trigger stress. The transformation-induced plasticity and work hardening capacity are achieved via the stress-induced martensite transformation. Additionally, the hierarchical stress-induced martensite twin structure and crystalline-to-amorphous phase transformation provide robust toughening mechanisms in the Bio-HEA. The cytotoxicity test confirms that this Bio-HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra-low Young's modulus, excellent biocompatibility, and decent ductility., (© 2024 Wiley‐VCH GmbH.)

The objective of this study was to develop a thermo-mechanical strategy to create a radial elasticity gradient in a β metastable Ti-Nb-Zr alloy, and to characterize it in terms of microstructural and mechanical properties. A first investigation was conducted on thin samples of Ti-20Nb-6Zr (at.%) submitted to various thermo-mechanical treatments. Microstructure-properties relationships and elastic variability of this alloy were determined performing uniaxial tensile tests, X-ray diffraction and scanning and transmission electron microscopies. Based on these preliminary results, mechanical deformation was identified as a potential way to lower the elastic modulus of the alloy. In order to create elastically graded pieces, shot-peening was therefore carried out on thicker samples to engender surface deformation. In this second part of the work, local mechanical properties were evaluated by instrumented micro-indentation. Experimental observations demonstrated that shot-peening enabled to locally induce martensitic transformation on surface, and a decrease in indentation elastic modulus from 85 to 65 GPa over 400 μm was highlighted. Surface deformation proved to be an efficient way of creating an elasticity gradient in β metastable titanium alloys. This combination of material and process could be suitable to produce dental implants with mechanically enhanced biocompatibility.

Based on the modified Hopkinson torsion bar, a high-temperature dynamic shear test method was proposed for the Ti-1023 alloy, and the microstructural evolution of the tested material at different temperatures was studied. By using the modified high-temperature Hopkinson torsion bar, high-temperature testing within 1000 °C can be achieved. As the specimen-heating rate was fast, and the temperature gradient of the experimental environment was small, valid experimental data can be ensured during the experiment. The experimental results show that stress-induced martensite can significantly enhance the strength of the Ti-1023 alloy. Dynamically recrystallized grains similar to those in adiabatic shear bands appear in the microstructure of the Ti-1023 alloy after severe plastic deformation. Therefore, it is possible to regulate the content of stress-induced martensite in the microstructure to improve the mechanical properties of other alloys that are similar to β titanium alloys.

Metastable β-type Ti-Nb based alloys are extensively applied in the fields of surgical implantation and wound repair owing to their low elastic modulus, excellent biocompatibility as well as superelastic properties. Based on the d-electron theory, a novel metastable β-type Ti-25Nb-3Zr-2Sn-3Mo (wt %) (TLM) alloy was developed using the Bo (d orbital electron bonding strength) -Md (d electron bonding energy) diagram. The microstructures and phase constituents of the prepared alloys under different solution conditions were investigated. The unidirectional tensile tests and cyclic loading-unloading experiments were conducted to analyze the mechanical behavior and superelastic behavior. Moreover, the microstructural characteristics near the fracture for the samples under different solution conditions were also investigated. Such results show that α” quenched martensite phase is generated within the equiaxed β grains of the TLM alloys which are solution-treated in the β or (α + β) phase regions. The typical double yielding of the TLM alloys is presented during the deformation at room temperature. The alloy solution-treated at 750°C has the strain hardening rate of ~1,100 MPa and the uniform elongation of ~32.5%. The stress-induced martensite phase and deformation twins are simultaneously generated in the microstructures of the TLM alloys during the deformation. Such alloys exhibit a certain superelastic effect due to the reverse transformation between the stress-induced martensite α” phase and β parent phase during the cyclic loading-unloading at room temperature. Furthermore, the underlying relationship between the properties of the TLM alloys and their microstructures are discussed.

Deformation behavior of Ti–12Mo alloy which is primarily deformed by {332}<113> deformation twinning was studied by in situ neutron diffraction measurements and deformation structure analyses, focusing on the formation of stress-induced α" martensite phase. We directly observed for the first time that the α" phase was formed at the initial stage of the plastic deformation and the volume fraction of the phase increased with increasing plastic deformation. This stress-induced α" martensite phase acts as a nucleation site for the {332}<113> twins. Further increase in stress results in a replacement of the α" phase by the {332}<113> twins. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The interaction between the stress-induced martensitic transformation and resistivity behavior of superelastic NiTi shape memory alloy (SMA) was studied. Strain-controlled low-cycle fatigue up to 6% was monitored by in situ electrical resistivity measurements. The experimental results show that a great motion of martensite fronts results in a significant accumulation of defects, as evidenced by transmission electron microscopy (TEM), before and after the tensile cycles. This gives rise to an overall increase of the resistivity values up to the maximum deformation. Therefore, the research suggests that shape memory alloy wire has great potential as a stress sensor inside bulk materials.

Fe-Mn-Si shape memory alloys (SMAs) have gained significant attention due to their unique characteristics. However, there remains a gap in the literature regarding the fabrication of these alloys using laser-directed energy deposition (LDED). This study fills this void, investigating the properties of Fe-Mn-Si SMAs produced by LDED. The shape memory performance of as-deposited Fe-Mn-Si SMAs was studied using a tensile method. Alloys underwent different degrees of deformation to assess their shape memory effect. Microstructural evaluations were conducted post-deformation to observe the internal structures of the alloys. The tensile tests revealed that shape recovery rates for deformation levels of 3%, 7%, 11%, and 15% were 68.1%, 44.2%, 31.7%, and 17.6%, respectively. Notably, the maximum recoverable deformation of the LDED-formed Fe-Mn-Si-based shape memory alloy reached 3.49%, surpassing the traditional deformation processing SMAs (<3%). The presence of a significant number of stacking faults was linked to the enhanced shape memory performance. The LDED technique demonstrates promising potential for the fabrication of Fe-Mn-Si SMAs, producing alloys with enhanced shape memory performance compared to traditionally processed SMAs. The study's findings offer new insights and broaden the applicability of LDED in the field of SMAs.

Electrochemical response of Type 304 stainless steel, with different levels of plastic deformation, was investigated in solutions containing different amounts of H2SO4 and KSCN at room temperature. Potentiodynamic polarization and double-loop electrochemical potentiokinetic reactivation (DL-EPR) techniques were used to characterize different levels of plastic deformation. The results have indicated that DL-EPR values and passivation current density increased with increase in plastic deformation. The DL-EPR values were correlated well with hardness and fraction of strain-induced martensite. Attack during DL-EPR tests occurred on strain regions within the matrix such as dislocations in austenite and martensite and/or interface between austenite and martensite. No well-defined correlation was observed between the levels of plastic deformation and potentiodynamic polarization behavior. [ABSTRACT FROM AUTHOR]

Abstract A semi-empirical approach based on the compositional average electron-to-atom ratio (e / a ¯) and atomic radius difference (∆ r ¯) was proposed to refine the “d-electron method”. The e / a ¯ − ∆ r ¯ diagram shows that Twinning/Stress-Induced Martensite (SIM) activates when ∆ r ¯ > −2.5, and a low e / a ¯ or absolute value of ∆ r ¯ favors Twinning/SIM by reducing the resistance of lattice shear. In addition to the phase stability, it suggests that the valence electron number and atomic radius of alloying element also determine the deformation mechanism in body-centered cubic Ti alloys. This alloy design method was verified by the tensile results of Ti-4Mo-4Co and Ti-6Mo-4Zr (at.%) alloys. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The influence of cold rolling and subsequent annealing on the microstructure and mechanical properties of a beta titanium alloy with the chemical composition of Ti-4Al-7Mo-3V-3Cr (Ti-4733) was investigated. The cold rolling was applied at room temperature up to 70% thickness reduction without cracking. The stress-induced martensitic transformation and dislocation slip were found to be the deformation mechanisms at the initial stage of deformation. Microstructural examinations revealed that several deformation bands form within the grains and also reverse transformation of α to β phase takes place with increasing deformation. The formation of band structures explains the high workability for Ti-4733 at room temperature. A short time annealing of the cold-rolled specimen resulted in a fine-grained structure with the average grain size of 22 µm. The reverse α to β phase transformation and recrystallization of deformed β phase are suggested to be the grain refinement mechanisms. The cold-rolled and annealed specimens exhibited enhanced tensile properties, so that the strength and elongation increased from 790 MPa and 13% in the solution treated condition to 900 MPa and 35%, respectively, after 70% cold rolling and annealing at 880 °C for 10 min. [ABSTRACT FROM AUTHOR]

Three beta titanium alloys in the Ti-Al-Mo-Cr-V system have been designed using the d-electron method with the aim of activating different combinations of deformation mechanisms. In this regard Ti-4Al-7Mo-3V-3Cr (Ti-4733), Ti-3Al-5Mo-7V-3Cr (Ti-3573) and Ti-3Al-8Mo-7V-3Cr (Ti-3873) alloys have been designed and compared with a commercial Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy. To evaluate the accuracy of the d-electron theoretical predictions, uniaxial compression tests were performed at room temperature. Different characterization methods including X-ray diffraction, electron backscatter diffraction, optical and transmission electron microscopy were used to investigate the microstructural evolution and deformation mechanisms. As a result of the modified deformation mechanisms, all the designed alloys showed enhanced compressive properties in comparison to the Ti-5553 alloy in beta single phase state. It was found that with increasing stability of the beta phase, the deformation mechanism of the new Ti-alloys gradually changes from slip + stress-induced martensitic transformation to slip + stress-induced martensitic transformation + mechanical twinning and finally to slip + mechanical twinning. The results showed that in the case of twinning the prediction by the d-electron method is consistent with experimental observations whereas regarding the stress-induced martensitic transformation this method should be used with considering an expanded martensite region on the lower portion of the d-electron phase stability map. [ABSTRACT FROM AUTHOR]

The sequence of activation of deformation mechanisms in a near-β Ti-5Al-5Mo-5V-3Cr alloy quenched from the β-field is investigated as a function of strain during uniaxial compression at 0.001 s −1 up to ε = 0.041. Bulk in situ high energy synchrotron X-ray diffraction is applied to follow continuously the evolution of the microstructure of the alloy during deformation and is complemented with metallographic analysis. Stress induced β → α″ transformation takes place together with grain rotation of β upon reaching the yield stress of the alloy at ε = 0.021. During further loading, formation of β sub-grain cells is activated at ε ≥ 0.03. The stress-induced martensite forms with a strong texture of {200} α″ planes perpendicular to the compression direction. [ABSTRACT FROM AUTHOR]

A transmission electron microscopy and in situ X-ray diffractometry study of Ti-50.61 at.% Ni shape memory alloy was carried out with an objective to clarify the stress-induced transformation mechanism in nanostructured B2-austenite. Extremely small austenite structure elements (grains/subgrains), about 15 nm in size, were obtained via a thermomechanical treatment comprising moderate cold rolling with a true strain of e = 0.55, followed by post-deformation annealing at 300 and 350 °C. It was unambiguously evidenced that the martensitic transformation preserves its “discrete” (discontinuous) nature even when the size of the parent structure elements (grains/subgrains) decreases down to a lower bound of the nanometer range. [ABSTRACT FROM AUTHOR]

Phase transformation in shape memory alloys is the most important factor in their unique behavior. In this paper, the formation of stress induced martensite phase transformation in a superelastic NiTi (50.8% Ni) shape memory alloy was investigated by using the photo-stress method. First, the material's fabrication procedure has been described and then the material was studied using the metallurgical tests such as differential scanning calorimetry and X-ray diffraction to characterize the material features and the mechanical tensile test to investigate the superelastic behavior. As a new method in observation of the phase transformation, photo-stress pictures showed the formation of stress induced martensite in a superelastic dog-bone specimen during loading and subsequently it's disappearing during unloading. Finally, finite element analysis was implemented using the constitutive equations derived based on the Boyd-Lagoudas phenomenological model. [ABSTRACT FROM AUTHOR]

The deformation behavior of a Ni-rich Ni 55 Ti 45 (at.%) alloy, commonly known as 60NiTi (as designated in wt.%), was analyzed using neutron and synchrotron x-ray diffraction during in situ isothermal tension and compression loading, and pre and post-test electron microscopy. The alloy was shown to exhibit remarkable strength and high hardness resulting from a high density of fine Ni 4 Ti 3 precipitates (size ∼67 nm), which were uniformly dispersed throughout the matrix after a solution treatment and oil quench. The precipitate volume fraction was 55 ± 3%, determined from both the neutron Rietveld refinement and conventional x-ray measurements. Non-linear stress-strain behavior was observed in tension (but not in compression) and was attributed to reversible stress-induced martensite (SIM) that forms to accommodate the stress as revealed by neutron diffraction measurements. The tensile and compressive neutron data also showed peak broadening and residual lattice strains. Transmission and scanning electron microscopy revealed stress-induced coarsening of Ni 4 Ti 3 precipitates in both tension and compression tested samples, but precipitation and growth of the stable Ni 3 Ti phase was observed only after tensile testing. Finally, the potential ramifications of these microstructural changes are discussed. [ABSTRACT FROM AUTHOR]

In this study, Cu–Al–Mn–V alloys with bcc phase separation were designed through changing V and Mn contents, resulting in complex microstructure consisted of L2 1 parent, small amounts of δ(V, Mn) and 2H(γ ' 1 ) martensite. The amounts of δ(V, Mn) phase and 2H(γ ' 1 ) martensite increase with the increase of V content. The results further show that the stabilization of stress-induced 2H(γ ' 1 ) martensite from L2 1 parent occurs among all Cu–Al–Mn–V alloys during deformation. Thus the same alloy not only exhibits superelasticity, but also shape memory effect after unloading when heated. The superelasticity and shape memory effect are summarized to three situations. Situation I includes Cu 63.7 Al 25.3 V 1.9 Mn 9.1 and Cu 63.7 Al 25.8 V 2.2 Mn 8.3 alloys (at.%), which have excellent superelasticity strains being up to 5.4% and 3.9% respectively. Cu 63.8 Al 25.4 V 4.2 Mn 6.6 and Cu 63.0 Al 26.2 V 4.7 Mn 6.1 alloys are situation II, and exhibit good shape memory effects being up to 3.5% and 3.9% respectively. The superelasticity and shape memory effect of Cu 62.8 Al 26.2 V 2.6 Mn 8.4 alloy that is situation III are between the above two situations. The obtained results may open an avenue to design copper-based shape memory alloys simultaneously have superelasticity and shape memory effect under the same composition and deformation temperature through applying stress. [ABSTRACT FROM AUTHOR]

Currently, β-type Ti-Nb-Ta-Zr alloys are gaining attention owing to their feasibility for use as biomedical materials. However, the deformation behaviors of these alloys have not yet been clarified. In this study, the Nb content of the Ti-30Nb-10Ta-5Zr (TNTZ) alloy was altered between 1.0 and 3.0 mass% from 30 mass%, and the corresponding changes in the superelasticity and shape-memory characteristics were investigated by tensile loading-unloading tests, X-ray diffraction (XRD) analysis, and microstructural analysis using a transmission electron microscopy (TEM). When the Nb content is less than approximately 29 mass%, the loading-unloading stress-strain curves show two-step gradients and are similar to those of common shape-memory alloys such as Ti-Ni alloys; these gradients result from stress-induced martensitic transformation and its reversion. When the Nb content is approximately 30 mass%, the tensile loading-unloading curves show a nonlinear gradient corresponding to superelastic behavior, which can not be explained by XRD and TEM microstructural analyses on stress-induced martensitic transformation and its reversion in the present state. When the Nb content is approximately 31 mass%, the tensile loading-unloading curves show a single gradient. The elastic deformation is mainly caused by the elastic strain in the lattice. Changes in the chemical content of Nb in TNTZ within a very narrow range are found to alter the superelastic behavior of this alloy.

In situ synchrotron X-ray diffraction was used to study the deformation behavior of a Ti-17Nb-1Cr alloy at room temperature. Stress-induced martensite (SIM) transformation increased the total recoverable strain. A maximum of ∼3.5% strain recovery was obtained after proper heat treatment. Unlike other alloys with a bcc crystal structure, the β-phase has a strong (210)β texture, supporting the previous suggestion that all meta-stable β-Ti alloys may have a similar texture instead of the well-known (110)β texture after axisymmetric deformation. The ω-phase has a ()ω texture. The (041)α″ fiber texture in α″-phase was formed from the (210)β texture based on their crystal orientation relationship. Impact Statement: This manuscript reports the stress-induced phase transformation in a new β-Ti alloy with large recoverable strain and unique textures that are not widely known. [ABSTRACT FROM AUTHOR]

Fracture properties of polycrystalline NiTi shape memory alloy have been investigated with the tension experiments on smooth and notch samples at different temperatures. The dog-bone tension tests demonstrate that above room temperature, NiTi with superelasticity effect presents enhancive ductility with the increase of temperature. Above the ceasing temperature of stress-induced martensite M d , EBSD investigation on crack indicates that the crack tends to propagate along the easy cleavage plane of {110} or {100}. If the apparent fracture toughness is larger than the critical stress intensity factor for expansion of martensite, the crack tip can be trapped by continual expansion of martensite, and the samples fracture in a ductile way. Otherwise, the crack tip could not be trapped by martensite expansion, the martensite transformation reduces the fracture toughness of NiTi, and the samples fracture in a quasi-brittle way. [ABSTRACT FROM AUTHOR]

A new metastable β titanium alloy, Ti-4Al-7Mo-3V-3Cr (wt.%), was designed using d-electron method, aiming to tailor the deformation mechanism. Microstructural and X-ray diffraction analysis of deformed specimens conforming with the theoretical prediction of d-electron method, confirmed the formation of stress-induced martensite (SIM) and mechanical twinning as the deformation mechanisms. The effect of initial grain size, strain level and strain rate on the formation of SIM were investigated. The results showed that in a given grain size, the volume fraction of SIM initially increases intensively and then follows with lower rate reaching a saturation at 35% reduction. It was observed that after a decrease in the martensite laths interspace down to less than 2 μm as a result of increasing the strain, some secondary martensite laths forms within the primary ones. It was found that with an increase in grain size from 150 μm to 250 μm, the volume fraction of SIM increases while a further increase in grain size up to 500 μm leads to a decrease in SIM volume fraction. The compression test results at different strain rates ranging from 0.7 × 10 −4 to 0.7 × 10 −1 s −1 showed SIM transformation occurs at all strain rates and although the triggering stress for SIM transformation increases continuously with an increase in strain rate, the volume fraction of SIM is independent of the strain rate. [ABSTRACT FROM AUTHOR]

The phenomenological theory of martensitic transformation is well understood that the displacive phase transformations are mainly influenced by the externally applied stress. Martensitic transformation occurs with 24 possible Kurdjomov-Sachs (K-S)variants, where eachvariantshows a distinct lattice orientation. The eleganttransformation texturemodel of Kundu and Bhadeshia for crystallographicvariant selectionof martensite in metastable austenite at various stress/strain levels has been assessed in this present research. The corresponding interaction energies have also been evaluated. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed at different stress/strain levels. It has been investigated that the mechanical driving force alone is able to explain the observed martensite microtextures at all stress/strain levels under uniaxial tensile deformation of metastable austenite under low temperature at a slow strain rate. The present investigation also proves that the Patel and Cohen’s classical theory can be utilized to predict the crystallographicvariant selection, if it is correctly used along with the phenomenological theory of martensite crystallography. [ABSTRACT FROM AUTHOR]

Metastable austenitic stainless steels are prone to form deformation-induced martensite under the influence of externally applied stress. Crystallographicvariant selectionduring martensitic transformation of metastable austenite has been investigated thoroughly with respect to the interaction between the applied uniaxial cyclic stress and the resulting accumulated plastic strain during cyclic plastic deformation. The orientation of all the Kurdjomov–Sachs (K-S) variants has been evaluated extensively and compared with the measured orientation of martensite with their corresponding interaction energies by applying the eleganttransformation texturemodel recently developed by Kundu and Bhadeshia. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed. It has been found that both the applied uniaxial cyclic stress and the accumulated plastic strain are having strong influence on crystallographicvariant selectionduring cyclic plastic deformation. Patel and Cohen’s classical theory can be utilized to predict the crystallographicvariant selection, if it is correctly used along with the phenomenological theory of martensite crystallography. [ABSTRACT FROM AUTHOR]

A metastable β-Ti alloy, Ti–10V–3Fe–3Al (wt.%), was subjected to thermomechanical processing (TMP), where the temperature of isothermal holding in the α + β phase field was varied in order to change the volume fraction of the α phase and, correspondingly, the β phase stability. Following TMP, compression tests were performed at room temperature to evaluate the deformation mode. Microstructural features induced by compression were identified using transmission electron microscopy. It was found that {3 3 2}〈1 1 3〉β deformation twinning along with stress-induced products (α″ martensite and ω lamellae) and slip were operational in the least stable β. The co-existence of {3 3 2}〈1 1 3〉β and {1 1 2}〈1 1 1〉β twinning was found at intermediate β stability along with other deformation products. With further increasing of β phase stability, no {3 3 2}〈1 1 3〉β twinning was detected whereas other deformation modes remained unchanged. In stable β phase, dislocation glide was the only deformation mode to be found. It was revealed that triggering stress required inducing the deformation products increases with the β phase stability. Based on the findings, a modification of the lower portion of the Bo ‾ – Md ‾ phase stability diagram is proposed. [ABSTRACT FROM AUTHOR]

A powder metallurgy processed metastable Ti-10V-3Fe-3Al alloy is thermo-mechanically processed for obtaining two phase (α and β) microstructure. Subsequently, the alloy is compressed up to 0.4 (true) strain. Deformation mechanisms of the compressed sample are then thoroughly investigated. Stress-strain curve uniquely displays double yielding with serrated flow that are characteristics of formation of deformation twins, martensite in the constituent phases. In α phase, { 1 ̅ 011 } 〈 10 12 ̅ 〉α deformation twin is found to be operative. In β phase, {112}〈111〉β twin with stress-induced α″ martensite are formed. For the first time, a simultaneous operation of twinning in both β (bcc) and α (hcp) is noticed in a titanium alloy. α″ martensite layer formation at β/α interface is also observed. These operational mechanisms have been discussed based on the β phase stability and atomic movement in (1 1 ̅ 01) planes of α phase. These results will greatly benefit in designing in new titanium alloy with desired mechanical properties. • Deformation mechanism of metastable Ti-10V-3Fe-3Al alloy is revealed. • Stress-strain curve displays double yielding with stress plateau and serrated flow. • First time, twinning in both ⍺ and β phases is found to be operative simultaneously in titanium alloy. • Twinning in ⍺ and β phases is explained in terms of atomic movement in (1 1 ̅ 01) planes ⍺ phase and β stability, respectively. [ABSTRACT FROM AUTHOR]

The influence of training cycles consisting of deformation by 5% at room temperature and subsequent annealing at different temperatures on the shape memory effect was studied in a Co–31Ni–3Si alloy to improve its shape memory effect. The results showed that the optimal annealing temperature in training was 973 K. A large recovery strain of 2.3% was obtained after one training cycle at the optimal annealing temperature and then bend by 3.7% at 77 K. The training promoted the stress-induced epsilon martensitic transformation and suppressed the occurrence of slip deformation due to the increase in yield strength. These results revealed that the shape memory effect in Co–Ni-based alloys stems from the stress-induced epsilon martensitic transformation. Their low yield strengths account for their poor shape memory effect. [ABSTRACT FROM AUTHOR]

A novel approach to achieve an extraordinary high stress recovery shape memory effect based on thermal cycling of stress-induced martensite is proposed. An alternative thermodynamic path is considered in order to achieve outstanding functional properties of Ni-rich NiTi alloys, which are commonly used at room or body temperature as superelastic materials. Fatigue tests revealed excellent stability of the material subjected to the novel thermomechanical path, confirming its suitability for employment in high-performance shape memory actuators. [ABSTRACT FROM AUTHOR]

Abstract: The effect of swaging on the Young's modulus of β Ti–33.6Nb–4Sn rods was investigated by X-ray diffraction, thermography, microstructural observations, deformation simulator analysis and cyclic tensile deformation. Stress-induced α″ martensite was stabilized by swaging, dependent on the diameter reduction rate during swaging. Thermography and deformation simulator analysis revealed that swaged rods were adiabatically heated, and consequently, stress-induced α″ underwent reverse transformation. Young's modulus, which was measured by the slope of the initial portion of the stress–strain curve, decreased from 56GPa in the hot-forged/quenched rod to 44GPa in the rapidly swaged rod with a high reduction rate and to 45GPa in the gradually swaged rod with a low reduction rate. The tangent modulus, which was measured by the slope of the tangent to any point on the stress–strain curve, decreased with strain even in the linear range of the stress–strain curve of the hot-forged/quenched rod and the rapidly swaged rod, while the tangent modulus remained unchanged for the gradually swaged rod. It was found that Young's moduli in swaged β Ti–33.6Nb–4Sn rods were affected by stabilized α″ martensite. Low Young's modulus of 45GPa and high strength over 800MPa were obtained when the reverse transformation by adiabatic heating was suppressed and the stress-induced α″ was sufficiently stabilized by gradual swaging to a 75% reduction in cross section area. [Copyright &y& Elsevier]

Abstract: The temperature dependence of the stress-induced martensite (SIM) formation in a Ti–10V–2Fe–3Al (Ti-1023) alloy under compressive loading has been studied. At low temperatures, the stress level at which martensite starts to form increases linearly with the deformation temperature, while the stress at which the deformation switches to regular plastic deformation is roughly temperature independent. A thermostatistical model for dislocation evolution is employed to describe deformation twinning in martensite. Combined effects of twinning induced plasticity and solid solution strengthening are considered in terms of temperature variations. The SIM effect disappears on deformation at temperatures beyond , which is close to the predicted M s temperature of 240°C. The thermostatistical model predicts a transition from twinned martensite to pure slip at 250°C. By providing a model to predict the martensite formation, and by describing deformation twinning, the present work provides a number of tools that may be employed to conceive new titanium alloys combining improved strength and ductility. [Copyright &y& Elsevier]

Abstract: The effect of stress-induced α″ martensite on Young's modulus was investigated by cyclic tensile tests, optical microscopy and X-ray diffraction analysis to determine the low Young's modulus of severely rolled or swaged, unstable β Ti alloys. An unstable β Ti–33.6%Nb–4%Sn alloy and a stable, slightly Nb-rich Ti–36%Nb–4%Sn alloy used as a reference were groove rolled and swaged after hot-forging. The groove rolling and swaging of Ti–33.6Nb–4Sn rods formed a fiber structure composed of 〈010〉 α″ and 〈110〉 β textures along the rolling direction, and stress-induced α″ martensite was stabilized by groove rolling, swaging and cyclic tensile deformation. Almost completely stabilized α″, which was attained by a 91% reduction in rod diameter via rolling followed by swaging, exhibited linear behavior in the initial stage of a stress–strain curve. Nb-rich Ti–36Nb–4Sn rods that were 91% rolled and swaged showed 〈110〉 β textures and linear behavior. From the slope of the linear portion of the stress–strain curves, Young's moduli along the rolling/swaging direction were determined to be 40GPa for Ti–33.6Nb–4Sn and 48GPa for Ti–36Nb–4Sn. Assuming that the rule of mixture is applicable to 91% rolled/swaged Ti–33.6Nb–4Sn with volume fractions of 0.81 for α″ and 0.19 for β, a low Young's modulus of 38GPa was calculated for the 〈010〉 orientation of α″ martensite. It is concluded that the fiber texture development of 〈010〉 α″ and 〈110〉 β with a low modulus is responsible for the low Young's modulus in rolled or swaged, unstable Ti–33.6Nb–4Sn. [Copyright &y& Elsevier]

Abstract: Crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined during static loading. The material composition of 50.7at.% Ni (austenitic, pseudoelastic) was investigated using high-energy synchrotron X-ray diffraction. A miniature CT specimen was developed, which is small enough to allow in situ testing in a synchrotron beam line to identify phases, textures and lattice strains in front of a crack tip. Stress-induced martensite in pseudoelastic NiTi SMAs was mapped in front of the crack of a CT specimen during static loading using synchrotron radiation. The phase volume fraction and lattice microstrain results are discussed and compared with results from thermographic measurements. The Poisson effect is observed by comparing the lattice strains in the loading direction and transverse to the loading direction. [Copyright &y& Elsevier]

Abstract: “Gum Metal” refers to β-Ti alloys that achieve exceptional elastic elongation and, with a specific alloy composition, appear to deform via a dislocation-free mechanism involving elastic instability at the limit of strength. This paper describes the current status of research on its strength, deformation mechanism and the possible role of stress-induced martensite. The theoretical basis for deformation at ideal strength is presented. The relevant experimental data is then discussed, including ex situ nanoindentation behavior and in situ pillar compression observed by transmission electron microscopy. [Copyright &y& Elsevier]

The temperature fields and the energy dissipations of shape memory alloys during the stress-induced martensitic transformations are studied theoretically and experimentally. The effect of the loading rate is analyzed. It was found that the temperature field inside a shape memory alloy sample varies strongly in space and time. The increase rate of the temperature is given by the difference between the rate of the latent heat release and the rate of the heat convection and conduction. The notion and the rate dependence of the energy dissipation are discussed in connection with the stress-strain hysteresis, the entropy production, and the Clausius-Duhem inequality. [ABSTRACT FROM AUTHOR]

In this work, preliminary results are presented on attempts to develop a new family of titanium alloys with high ductility induced by combined transformation-induced plasticity and twinning-induced plasticity effects. Mechanical tests carried out on a binary β-metastable Ti–12wt.% Mo alloy designed by a formulation strategy based on the “d-electron alloy design” show a very high work hardening rate. X-ray diffraction, electron backscattered diffraction and transmission electron microscopy reveal the mechanical activation of twinning and of α′, α″ and ω phases. [Copyright &y& Elsevier]

Abstract: Specimens from Fe–Mn–Si–Cr–Ni SMA, obtained by powder metallurgy and compacted through hot rolling, were subjected to tensile loading-unloading cycles. The pseudoelastic parameters were determined based on recorded stress-strain curves, and their variation tendency with increasing the number of mechanical cycles was discussed. The gauges of tensile specimens were cut after mechanical cycling and were subjected to structural and dilatometric analysis. The structure was analyzed by XRD and SEM, aiming to reveal mechanical cycling effects. The thermomechanical response on heating, of mechanically cycled specimens, was recorded by dilatometry and revealed a tendency to enhance thermal expansion as an effect of increasing the number of cycles. The microstructural changes, induced by mechanical cycling, consisted in the stress induced formation of ’ martensite. [ABSTRACT FROM AUTHOR]

Abstract: Titanium alloys dedicated to biomedical applications may display both clinical and mechanical biocompatibility. Based on nontoxic elements such as Ti, Zr, Nb, Ta, they should combine high mechanical resistance with a low elastic modulus close to the bone elasticity (E=20 GPa) to significantly improve bone remodelling and osseointegration processes. These elastic properties can be reached using both lowering of the intrinsic modulus by specific chemical alloying and superelasticity effects associated with a stress-induced phase transformation from the BCC metastable beta phase to the orthorhombic martensite. It is shown that the stability of the beta phase can be triggered using a chemical formulation strategy based on the electronic design method initially developed by Morinaga. This method is based on the calculation of two electronic parameters respectively called the bond order () and the d orbital level () for each alloy. By this method, two titanium alloys with various tantalum contents, Ti–29Nb–11Ta–5Zr and Ti–29Nb–6Ta–5Zr (wt%) were prepared. In this paper, the effect of the tantalum content on the elastic modulus/yield strength balance has been investigated and discussed regarding the deformation modes. The martensitic transformation has been observed on Ti–29Nb–6Ta–5Zr in contrast to Ti–29Nb–11Ta–5Zr highlighting the chemical influence of the Ta element on the initial beta phase stability. A formulation strategy is discussed regarding the as-mentioned electronic parameters. Respective influence of cold rolling and flash thermal treatments (in the isothermal omega phase precipitation domain) on the tensile properties has been investigated. [Copyright &y& Elsevier]

Abstract: An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1GPa) and unloading. Both macroscopic stress–strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys. [Copyright &y& Elsevier]

Abstract: This study reports on the role of repeated stress-induced martensite (SIM) transformations on the pseudoelastic (PE) behavior of solutionized Co35Ni35Al30 [001]-oriented shape memory single crystals under both isothermal and non-isothermal conditions (referred to as “training”). It is demonstrated that training results in austenite stabilization and strengthening, consequently increasing the critical transformation stress levels for the SIM (), and promoting excellent cyclic stability and reproducibility of the PE response. This is attributed to the formation of dense dislocation arrangements and fine coherent sub-nanometer hexagonal close-packed Co and γ′ (Ni3Al:L12) precipitates during training. The training that involved cyclic loading conditions was more effective than the monotonic stress–strain tests at different temperatures in modifying SIM characteristics bringing about (i) a large pseudoelastic temperature range of 350°C with levels reaching 1GPa with complete recoverable strains of 2%, and (ii) excellent stable cyclic PE response at temperatures as high as 250°C. [Copyright &y& Elsevier]

It is well known that one way shape memory effect (SME) in Fe-Mn-Si-based shape memory alloys (SMAs) is related to the thermally induced reversion of ε (hexagonal close packed, hcp) stress-induced martensite (SIM) to γ (face centered cubic, fcc) austenite. In the case of a Fe-Mn-Si-Cr-Ni SMA, this reverse martensitic transformation was analyzed in regard to the critical temperature for the beginning of austenite formation ( As) in different states characterized by quenching temperature and permanent tensile strain. For this purpose, dynamic mechanical analysis (DMA), dilatometry (DIL), differential thermal analysis (DSC), and optical microscopy (OM) were employed to determine the influence of quenching temperature and permanent tensile straining on SIM reversion to austenite during heating. [ABSTRACT FROM AUTHOR]