Search

Commercially pure titanium (CP-Ti) has seen major applications in biomedical equipment. The use of selective laser melting (SLM) has become more common concerning the fabrication of CP-Ti components with a complicated shape in biomedical implants, where high strength and ductility are required. However, SLM manufactured CP-Ti often exhibits high strength and low ductility, as well as mechanical anisotropy because SLM process typically results in the formation of long columnar grains comprising of fine acicular α′ martensite. Heat treatment must be preceded to transform the acicular α′ to equiaxed α grain. This study demonstrated that annealing at 650 °C of SLM CP-Ti can create an equiaxed structure, resulting in a weakened texture. The formation of equiaxed grains occurred at two types of microstructural features, acicular α′ and irregular massive (α′m) martensite, which exhibited different sizes, morphologies, and contained different types of substructures under SLM as-fabricated condition. The formation mechanism of equiaxed grains in the α′ region is dominated by the coalescence of acicular α′ of the same variant and the dissolution of α′ into the matrix, whereas the formation mechanism in the α′m region is primarily dominated by the growth of the preexisting subgrains. A small number of equiaxed grains with new orientations were formed in the α′m region. The majority of equiaxed grains were formed in the acicular α′ region and inherited the grain orientations of the preexisting α′ or matrix. Therefore, a similar but weakened texture was inherited from the microstructure of the SLM CP-Ti after annealing.

Aluminum matrix composites (AMCs) reinforced with graphene nanoplatelets (GNPs) were fabricated by using an accumulative roll-compositing (ARC) process. Microstructure, mechanical and electrical properties of the nanostructured AMCs were characterized. The results showed that small addition (0.2 vol% and 0.5 vol%) of GNPs can lead to a simultaneous increase in the tensile strength and ductility of the GNPs/Al nanocomposites, as compared with the same processed pure Al. With increasing GNPs content, the tensile strength of the GNPs/Al nanocomposites can be enhanced to 387 MPa with retained elongation of 15%. Meanwhile, the GNPs/Al nanocomposites exhibited a good electrical conductivity of 77.8%–86.1% that of annealed pure Al. The excellent properties (high strength, high ductility and high conductivity) of the GNPs/Al are associated with the particular ARC process, which facilitates the uniform dispersion of GNPs in the matrix and formation of ultrafine-grained Al matrix. The strengthening and toughening of the GNPs/Al nanocomposites were discussed considering different mechanisms and the unique effect of GNPs.

The present work investigates the thermal stability and mechanical properties of a Co–20Cr–15W–10Ni (wt%) alloy with a bimodal grain (BG) structure. The BG structure consisting of fine grains (FGs) and coarse grains (CGs) is thermally stable under high-temperature exposure treatments of 760 °C for 100 h and 870 °C for 100–1000 h. The size of both FGs and CGs remains no significant changes after thermal exposure treatments. The microstructural stability is associated with the slow kinetics of grain growth and the pinning of carbides. The thermal stability enables to maintain the BG structures, leading to the same mechanical properties as the sample without thermal exposure treatment. In particular, the BG alloy samples after thermal exposure treatment exhibit superior mechanical properties of both high strength and high ductility compared to the unimodal grain (UG) structured ones. The BG structure of the alloy samples after thermal exposure is capable of avoiding severe loss of ductility and retaining high strength. More specifically, the ductility of the BG alloy samples after thermal exposure treatments of 870 °C for 500–1000 h is ten times higher (44.6% vs. 3.5% and 52.6% vs. 5.0%) than that of the UG ones. The finding in the present work may give new insights into high-temperature applications of the Co–20Cr–15W–10Ni alloy and other metallic materials with a BG structure.

The influence of pre-strain on the formation of bimodal grain structures and tensile properties of a Co–20Cr–15W–10Ni alloy was investigated. The bimodal grain structures consist of fine grains (FGs; 2–3 μm in diameter) and coarse grains (CGs; 8–16 μm in diameter), which can be manipulated by changing the pre-strain (ɛ = 0.3–0.7) and annealing temperatures (1000–1100 °C). High pre-strain applied in the samples can intensify the plasticity heterogeneity through increasing the total dislocation density and the local volumes of high-density dislocations. This can essentially result in finer FGs, a higher FG volume fraction, and overall grain refinement in the samples after annealing. High-temperature essentially increases both the size and volume fraction of CGs, leading to an increase in the average grain size. The tensile test suggests that the bimodal grain structured samples exhibited both high strength and ductility, yield strengths of 621–877 MPa and ultimate tensile strengths of 1187–1367 MPa with uniform elongations of 55.0%–71.4%. The superior strength-ductility combination of the samples arises from the specific deformation mechanisms of the bimodal grain structures. The tensile properties strongly depend on the size ratio and volume fraction of FGs/CGs in addition to the average grain size in the bimodal grain structures. The grain structures can be modified via changing the pre-strain and annealing temperature.

We used selective laser melting (SLM) to produce a three-dimensional specimen of pure Ti and found that an equiaxed grain structure and weak crystallographic texture could be achieved simultaneously via subsequent heat treatment. These traits have never been attained simultaneously in pure Ti prepared by traditional methods such as casting and rolling. This remarkable achievement was possible because recrystallization occurred during heat treatment without plastic deformation; such deformation introduces stored energy that drives recrystallization but inevitably causes a strong texture to develop. The occurrence of recrystallization was attributed to unique features of the SLM process including a very high cooling rate and repetitive layer stacking. These features generated considerable stored energy by affecting solidification and the β → α phase transformation. Steep in-grain orientation gradients and a fine lath structure also contributed to the activation of recrystallization by facilitating recrystallization nucleation. The heat-treated specimen showed tensile properties with significantly reduced anisotropy. This finding will provide new strategies for developing isotropic metallic materials and may introduce new applications of SLM.

A nearly fully dense β stabilized γ-TiAl alloy was additively manufactured by electron beam melting. The as-fabricated specimen exhibited a fine structure consisting of α2/γ colonies ( 50%) in the sample at 800 °C. Annealing and subsequent stabilization treatments resulted in α2/γ colonies (

Ti-2Al-9.2Mo-2Fe is a low-cost β titanium alloy with well-balanced strength and ductility, but hot working of this alloy is complex and unfamiliar. Understanding the nonlinear relationships among the strain, strain rate, temperature, and flow stress of this alloy is essential to optimize the hot working process. In this study, a deep neural network (DNN) model was developed to correlate flow stress with a wide range of strains (0.025–0.6), strain rates (0.01–10 s−1) and temperatures (750–1000 °C). The model, which was tested with 96 unseen datasets, showed better performance than existing models, with a correlation coefficient of 0.999. The processing map constructed using the DNN model was effective in predicting the microstructural evolution of the alloy. Moreover, it led to the optimization of hot-working conditions to avoid the formation of brittle precipitates (temperatures of 820–1000 °C and strain rates of 0.01–0.1 s−1).

Bimodal grain structures are often produced in a metal by employing severe plastic deformation followed by low-homologous-temperature annealing (≤0.4Tm, where Tm is melting temperature). Here, a bimodal grain structure was achieved in a Co–20Cr–15W–10Ni–C superalloy via moderate cold rolling (strain ≈0.3) and ultrahigh-homologous-temperature annealing (≥0.8Tm). During annealing, carbide-assisted heterogeneous static-recrystallization played a key role in forming the bimodal grain structure. A 40% increase in combined strength and ductility was observed.

The isothermal compression tests were carried out to study the hot deformation behavior and microstructure evolution of Ti–19Al–22Mo alloy. The samples were deformed in the temperature range from 1100 to 1250 °C with an interval of 50 °C, strain rate ranging from 0.01 to 1 s−1 and the height reduction of 50% using Gleeble-3800 thermal–mechanical simulator. By using this experimental data an artificial neural network (ANN) model was developed and evaluated with unseen data. Further, the developed ANN model was used to predict flow stress correction from adiabatic heating at finer intervals of strain rates and temperatures. The predicted isothermal flow stress values were utilized to construct processing maps for Ti–19Al–22Mo alloy at true strain of 0.4 and 0.6. The maximum efficiency was noticed at 1100 °C with the strain rate of 0.01 s−1 associated with dynamic recrystallization and dynamic recovery. The deformation conditions of the instability domains in processing map showed wedge cracking and flow localization. Using the processing maps safe working parameters for hot deformation of Ti–19Al–22Mo alloy was identified.

A bimodal grain structure consisting of fine grains (d ~ 1–2 µm) and coarse grains (d ~ 10–20 µm) was achieved in the biomedical Co–20Cr–15W–10Ni alloy via medium cold rolling (area reduction of 50%) followed by short-term annealing (15 min) at relatively low temperatures (950–1100 °C). The medium cold rolling induced a heterogeneous microstructure featuring the coexistence of severely deformed regions and weakly deformed domains. With short-term annealing at low temperatures, fine and coarse grains were preferentially recrystallized in severely deformed regions and weakly deformed domains, respectively, resulting a bimodal grain size distribution. During tension, dislocations were more rapidly generated in the fine grains, thus increasing the strain hardening, while they glided longer on the {111} planes in the coarse grains, contributing a high ductility. The combination of these two factors provided well-balanced strength–ductility behavior with an ultimate tensile strength of 1278 MPa, a yield strength of 787 MPa, and an elongation to fracture of 53%, making the alloy suitable for surgical implant and stent applications where the strength and ductility are both important to ensure mechanical reliability in a human body.

The use of selective laser melting (SLM) has become more common with regard to the fabrication of high-strength Ti-6Al-4V components. However, the as-SLMed Ti-6Al-4V alloy parts typically exhibit low ductility because of the formation of acicular and brittle α′ martensites. It is essential to use post-heat treatment to change their microstructures to achieve superior mechanical properties. In this study, a particular multi-step heat treatment (MSHT) was applied to the Ti-6Al-4V alloy samples fabricated via SLM. Conventional post-heat treatments were conducted for comparison. The microstructures and tensile properties of the as-SLMed and heat-treated samples were investigated. The as-SLMed Ti-6Al-4V sample is dominated by plate and acicular α′ martensites, which exhibit high strength (1280 MPa) and low ductility (9.0%). The conventional solution plus aging or stress-relieving treatment cannot result in a good combination of strength and ductility. A single step annealing process at 700 °C for 2 h changes the martensitic structure to a fine lamellar α+β structure, which provides a strong and ductile Ti-6Al-4V with an ultimate tensile strength of 1108 MPa and a total elongation of 17.6%. In contrast, the MSHT process facilitates the globularization of α and, thereby generates a nearly equiaxed structure, resulting in superior ductility (21.8% of total elongation), while maintaining a moderate ultimate tensile strength of 953 MPa.

The present work investigates the microstructural evolution of a biomedical Co-20Cr-15W-10Ni alloy during multi-pass thermomechanical processing (TMP) and clarifies its effect on the tensile properties. The results indicate that multi-pass TMP consisting of repetitive cold groove-rolling followed by annealing is attractive for fabricating the alloy rod for fine wire and mini-tube making. The multi-pass TMP provided significant grain refinement from 100 to 4 μm via five repetition and, can create bimodal grain structures consisting of fine (≈3 μm) and coarse grains (4−16 μm). The bimodal grain structure provided a superior strength-ductility combination, a yield strength of 593−738 MPa and ultimate strength of 1197−1304 MPa with a uniform elongation of 54.7−61.1%. Changing the annealing condition in the final TMP step enables to further refine the grains to 2 μm and also modify the grain size distribution and, the dispersion and fraction of nano-carbides. Thus, the mechanical properties of the Co-20Cr-15W-10Ni alloy can be further optimized, leading to a yield strength of 795−890 MPa and ultimate strength of 1328−1363 MPa with a uniform elongation of 49.4−64.0%.

We have investigated the propagation of {332} twins in a multilayered Ti-10Mo-xFe (x = 1–3) alloy fabricated by multi-pass hot rolling. The material contains a macroscopic Fe-graded structure (about 130 μm width) between 1 and 3 wt% Fe in the direction perpendicular to rolling. We observe strong influence of the Fe-graded structure in the twin propagation behavior. The propagation of {332} twins that are nucleated in Fe-lean regions (~1 wt% Fe) is interrupted in the grain interiors at a specific Fe content, namely, about 2 wt% Fe. We ascribe this effect to the role of Fe content in solid solution on the stress for twin propagation. The interruption of twins in the grain interiors results in the development of characteristic dislocation configurations such as highly dense dislocation walls (HDDWs) associated to strain localization phenomena. The nucleation and propagation of these dislocation configurations is ascribed to the underlying plastic accommodation mechanisms of the stress field...

A study on microstructural evolution and age hardening behavior of a new metastable beta titanium alloy Ti–2Al–9.2Mo–2Fe was undertaken by microscopic observation, Vickers hardness and tensile tests in this research. The result showed that the athermal ω phase was formed in the beta matrix after solution treatment and followed by water quench, but it seemed that the athermal ω phase did not result in a considerable hardening (300 HV in hardness and 670 MPa in YS in ST condition). However, the isothermal ω phases with 10–40 nm and nano-scaled α platelets with 30–100 nm were observed in the alloy aged at lower temperatures (300–450 °C). The nano-scaled ω and α phase led to an attractive hardening effect (400–500 HV in hardness and above 1500 MPa in YS). However, micro-scaled α phase with 0.5–3 μm obtained in the samples aged at temperatures (500–600 °C) showed a moderate hardening (350–450 HV in hardness and 1100–1500 MPa in YS). The hardening went through an under-aging, peak-aging and over-aging due to the continuation of nucleation and growth of the α phases and subsequent coarsening. The coarsen α phases (3–5 μm) and grain boundary α layers (0.1–0.5 μm in thickness) obtained at high temperatures aging (650–750 °C) showed a poor hardening or even a softening (around 300 HV in hardness and below 1000 MPa in YS). The Ti–2Al–9.2Mo–2Fe alloy had a fast aging response, and can be age hardened to high hardness above 400 HV within 30 min at a broad aging temperature region. The age hardening curves can also provide a basic criterion for selecting aging treatment for a given stress level.

Ti-6Al-4V plates in thickness of 8 mm were directly hot rolled and then heat treated in various routes by a single electron beam cold hearth melted (EBCHM) ingot, in which the weight percentage of recycled titanium scraps was at least about 50 wt%. The dynamic stress-strain behaviors of the plates with typical microstructures were investigated by Split Hopkinson Pressure Bar system at the strain rates of 2000 s-1~ 3000 s-1. The results show that the dynamic compression properties of EBCHM-ingot Ti-6Al-4V plates are relative to that of commercial VAR-ingot plates. The dynamic properties in normal direction of the plates are not sensitive to variations in the fraction of primary alpha phase.

The present study describes the influence of β stabilizers (namely Fe and Cr) on the microstructural evolution and mechanical behavior of Ti–6Al–4V alloy fabricated by direct energy deposition. As the β stabilizer content increases from 1 to 4 wt%, the coarse columnar-grained morphology typical of additively manufactured Ti–6Al–4V is significantly refined, with a columnar-to-equiaxed transition. Furthermore, reheating of the deposited layers by the deposition of overlaying layers during fabrication induces an element partitioning effect, resulting in the formation of fine-grained α and isothermal ω. Hence, the heat profile during direct energy deposition resembles conventional solution and aging treatments. In addition, increasing the amount of β stabilizer steadily increases the room temperature strength of the Ti–6Al–4V alloy. The alloy with 4 wt% Fe showed the maximum strength, of ∼1.5 GPa.

Commercially pure titanium (CP–Ti) has seen major applications in medical and chemical equipment. However, improving its strength in its coarse-grained state is expected to extend its usage in structural applications. This study demonstrated that tailoring bimodal grain structures (in terms of grain size or shape) in CP-Ti offers a new way to enhance its mechanical properties. For this, an extensive effort was made to clarify the relationship between the initial microstructures and the deformation heterogeneities of a CP-Ti (Gr. 2). Samples with initially equiaxed (EQ), bimodal (BM), and lamellar (LM) microstructures were cold-rolled and subjected to annealing at different temperatures. The results showed that variations in initial grain orientation, size, and morphology result in different deformation heterogeneities during the plastic deformation, which in turn play an important role in determining the recrystallization kinetics during annealing as well as the resulting mechanical properties. The initial BM and EQ microstructures yielded better mechanical performances in both their initial and cold-rolled states than the initial LM microstructure. Furthermore, the initial BM microstructure exhibited the best strength–ductility combination in both their partially and fully recrystallized states.

Unfortunately, the acknowledgements were incomplete in the original version of this article.

An investigation is given on the influence of heat treatment by the microstructural characteristics and tensile properties of a new high strength alloy Ti–2Al–9.2Mo–2Fe. Both of the α/β and β solution treatment (α/β-ST and β-ST), then aged at temperatures ranging from 400 °C to 600 °C, were prepared. The primary α phases having 2–5 μm are formed during the α/β-ST, and restrain the size of β grains below 10 μm. As a result of the fine β grains, the α/β-ST contributes a higher strength than the β-ST. The co-existence of α″ and athermal ω phase is found in the β-ST and water quenched samples. However, these have little influence on the alloy hardening. After aging, the alloys in the α/β-ST and β-ST condition reveal the phase transformation of β to isothermal ω, and β to α depending on the aging temperature. Although the primary α phase formed during the α/β-ST increases the stability in the β matrix, and the isothermal ω phase also appears to occur during aging at 400 °C and 450 °C for 2 h. These phenomena are less common in beta titanium alloys, when treated in the α/β-ST and aged at lower temperatures. The isothermal ω phase formed in both conditions results in high strength levels (1600 MPa of ultimate strength) with much ductility loss (2.5–4.5% of elongation) as a result of the superior hardening effect and brittleness of ω phase. However, the secondary α phase with the size of 1–3 μm leads to attractive combinations of strength and ductility (1200–1400 MPa of ultimate strength with 7.5–12.5% of elongation). The reason for that is too fine α phase below 1 μm tends to result in ultra-high strength with much ductility loss. As a whole, the alloy can be heat treated to obtain excellent balances of strength and ductility, and provided abundant stress levels with optional ductility as a usable material.

The effect of solution treatment on microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy was studied. The microstructure–mechanical property relationship is also discussed. The results show that, the Ti–6Cr–5Mo–5V–4Al alloy in solution treated condition has moderate strength levels (800–1000 MPa) with an excellent ductility (−22% of the elongation and 50–60% of the reduction of area). The alloy in the α/β-solution and aged (α/β-STA) condition shows a more attractive combination of strength and ductility than the alloy in the β-solution and aged condition (β-STA). The α/β-STA results in the occurrence of mixed α precipitates at nano-scale (−100 nm) and at micron-scale (1 μm), which contributes to the high ultimate strength levels (1400–1600 MPa). However, the fine β grains below 10 μm and the primary α phase contribute to good ductility (6–13.5% of the elongation and 22–52% of the reduction of area). The β-ST leads to equiaxed and coarsen β grains (40–100 μm), and subsequent aging contributes to the α phase with the size of 1–5 μm. The β-STA microstructure contributes to 1200–1400 MPa of the ultimate strength with 7–10% of the elongation and 22–28% of the reduction of area, which shows less attractive behavior on the properties than that of the α/β-STA.

Ti-Al-Mo-Fe alloys were developed as low cost beta Ti alloys for automotive springs, and designed based on Molybdenum equivalency and Bo-Md molecular orbital method. Low priced Mo-Fe master alloys were introduced as alloying elements for the cost and elastic modulus reduction. Eight Ti-Al-Mo-Fe alloy candidates were pre-designed according to the Bo-Md method. Primary laboratory scale ingots were thus melted by ISM (Induction Skull Melting). After primary property evaluation, the Ti-2Al-9.2Mo-2Fe alloy was optimized finally and large scale ingot was made by VAR for further property evaluation. Resultantly, it shows that the alloy has lower elastic modulus (60-70 GPa) and good tensile properties in the solution condition, and compares well with the other developed commercial beta alloys.

A study on the microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy were investigated. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 460 °C to 560 °C were introduced to investigate the microstructural characteristics and microstructure–property relationships. The results show that the primary α phase formed during the α/β solution treatment can increase the stability of the β matrix and then restrain the size of secondary α phase during subsequent aging. In the α/β solution plus aging condition, the secondary α phase with the size of about 1 μm results in a high strength above 1500 MPa with −6% of elongation. The β solution treatment leads to the least stable β matrix and the greatest driving force for secondary α phase, so coarser α phases tend to form during aging. The β solution plus aging does not lead to attractive high strength because of the coarser and non-uniform α phase. And it also leads to a poorer combination of strength and ductility than the α/β solution plus aging. Direct aging exhibits the best strengthening. The ultimate strength of the alloy is about 1600 MPa with acceptable elongations of 6.5–7.5% when direct aged at 500 °C for 2–8 h. It benefits from the mixed microstructure with nano-scale and micron-scale α phases. However, more detailed investigations are needed to improve the ductility. As a whole, the alloy can be heat treatable to obtain an excellent balance of strength and ductility, and provide abundant stress levels with optional ductility (900–1600 MPa in ultimate strength and 6–20% in elongation).

Ageing behaviors and microstructural characterization by aging condition of Ti-Al-Mo-Fe alloy was investigated. Due to the formation of ω phase, it occurs a drastic change in mechanical properties of β alloys. There is large increasing in hardness and yield strength, accompanied with serious ductility reduction. However, it has been proved that proper control of ω phase volume fraction can bring to improve strength with a reasonable ductility. In this work new beta titanium alloy was designed and developed in Ti-Al-Mo-Fe alloy to investigate the hardening behavior of ω and α precipitation during aging. The results showed that a small amount of athermal ω was observed in the β matrix during water quenching from above the β transus temperature. Isothermal ω formation was also found during aged at temperatures ranging from 573 K to 773 K although it has a limited time of stability at 773 K. The hardening due to isothermal ω precipitation exhibits no over-aging effect as long as ω phase exists inside the matrix. The hardness of this alloy is very sensitive to size and volume fraction of ω phase and its existence, which depends on aging temperature, time and alloy compositions.

We have investigated {332} twinning and detwinning mechanisms in a multilayered Ti-10Mo-xFe (x =1−3) alloy fabricated by multi-pass hot rolling. A strong influence of the Fe-graded structure on both phenomena is observed. The propagation of twins that are nucleated in Fe-lean regions (~1 wt.% Fe) is interrupted in the grain interiors at about 2 wt% Fe. We ascribe this effect to the role of the Fe content on the stress for twin propagation. The unusual twin structure determines the subsequent detwinning process upon thermal annealing. This process consists of two independent detwinning events that occur at two different microstructural regions, namely at twin tips located at grain interiors and at grain boundaries. Both detwinning modes can be explained from a thermodynamic standpoint whereby the boundary dissociation processes minimize the boundary free energy.