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A novel two-step constrained aging (CA) treatment, in the form of CA firstly at 500 °C for 1 h and then CA at 300 °C for 39 h, is proposed to obtain enhanced two-way shape memory effect (TWSME) and elevated R-phase transformation temperatures of the Ni51Ti49 alloy strip fabricated by rapid solidification technique. The two-step constrained aged Ni51Ti49 alloy strip possesses a much increased recovery ratio of 96.4% in the fully martensitic state, and elevated R-phase transformation start and finish temperatures of 60.2 °C and 40.9 °C respectively, compared with these of the alloy subjected to the one-step CA treatment. Higher recovery ratio and elevated R-phase transformation temperatures are attributed to preferentially oriented lenticular coherent Ni4Ti3 precipitates, with an average length of 149.1 ± 14.3 nm, dispersed homogeneously in the NiTi matrix. The first-step CA with a relatively high aging temperature triggers the fast growth of Ni4Ti3 precipitates, while the second-step CA with a low aging temperature leads to decrease of the solid solubility of Ni atoms and promotes further precipitation of Ni atoms and thus induces formation and growth of Ni4Ti3 precipitates.

The influence of the chemical composition of a NiTi alloy on the martensite stabilization effect was studied. The Ni- 50.0 at. %Ti, Ni – 49.5 at. % Ti and Ni – 49.0 at. %Ti alloys were quenched from 900 °C (10 min) into water and after this heat treatment, the alloys underwent the B2 ↔ B19′ transformation on cooling and heating without the R phase formation. The martensite stabilization effect was observed in NiTi alloys regardless of the chemical composition and value of the preliminary strain. The value of the martensite stabilization effect was measured as the difference between the temperatures that were measured during the first and the second heating. When the residual strain was less than 2.5%, the martensite stabilization effect values were close to each other in all studied alloys. Otherwise, if the residual strain exceeded 2.5%, the martensite stabilization effect values in the Ni- 50.0 at. %Ti and Ni – 49.5 at. % Ti alloys were larger than in the Ni – 49.0 at. %Ti alloy. It was shown that there was no correspondence between the values of the martensite stabilization effect and the irreversible plastic strain that appeared in the samples during the preliminary deformation. The martensite stabilization effect was found in the Ni – 49.0 at. %Ti alloy after preliminary deformation up to 10% or less that was not accompanied by a plastic strain. Thus, it was shown that the plastic strain was not the main reason for the martensite stabilization effect. A new hypothesis was assumed that a loss in the coherency of the interface that was caused by the martensite reorientation and detwinning during the preliminary deformation might be responsible for an increase in the temperatures of the reverse transformation that occurred on the first heating.

Iron-doped Ni-Ti shape memory alloys are known for their R-phase transformation. The stress-induced R-phase transformation and its reverse transformation can be utilized to create a temperature change based on elastocaloric heating and cooling. We studied the elastocaloric properties of hot rolled Ni48Ti50Fe2 and Ni46Ti50Fe4 alloys with three different approaches: 1) the calorimetry and specific heat measurements, 2) mechanical stress-strain behaviour and Maxwell relation based calculations and 3) direct measurement of adiabatic temperature change by stress application and removal. We compared the results related to different approaches and present the essential properties related to elastocaloric effect in these alloys. We show that the temperature change due to energy dissipation of the R-phase transformation is minor in comparison with the elastocaloric temperature change. The maximum adiabatic temperature change expected from the latent heat of R-B2 transformation is 12.4 K for the Ni48Ti50Fe2 alloy and 8.7 K for the Ni46Ti50Fe4 alloy. We also report an increasing tendency of the peak temperature for the elastocaloric effect when the applied stress increases.

In the work, (1- x ) (K 0.48 Na 0.52 ) (Nb 0.96 Sb 0.04 )O 3 - x (Bi 0.5 K 0.5 ) 0.7 Ca 0.3 ZrO 3 [(1- x )KNNS– x BKCZ] lead-free piezoceramics are prepared to investigate location and associated impacts of non-equivalent ionic dopants on phase transition and property enhancement. Results of XRD refinement and analysis of transmission electron microscopes (TEM) strongly prove that Rhombohedral-Tetragonal (R-O-T) phase coexistence for 0 ≤ x ≤ 0.04. According to lattice parameters, R phase should be ascribed to lattice deformation subjected to shear stress owing to the cooperation of multiple ionic dopants in A sites and B sites. Based on the variation of temperature-dependent capacitances with the frequencies and atomic-resolution HR-O-TEM image, much more oxygen vacancies are investigated because of major substitution of Ca 2+ in B site, which leads to an obvious decrease of T C in (1- x )KNNS– x BKCZ besides c / a of O phase. Additionally, domain wall motion is demonstrated to become easier through calculating intrinsic piezoelectric response using Rayleigh law and consequently, piezoelectric properties of (1- x )KNNS– x BKCZ are greatly enhanced ( d 33 ∼440 pC/N and d* 33 –500 pm/V) in R-O-T phase coexistence region.

The effect of compositional segregation on the properties of relaxor ferroelectric single crystal are attracting increasing attention for its serious limitation on the application field. In this paper, the composition, phase structure, optical properties and electric properties of Mn-doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystal were investigated. The real composition of the single crystal fluctuated around the original ratio, except for MnO2. The single crystal contained less Mn element after removing inclusions compared with the starting ratio. The single crystal samples exhibited rhombohedral phase (R phase), orthorhombic phase (O phase) and tetragonal phase (T phase) structures with different composition due to the segregation. The rhombohedral to tetragonal phase transition temperature (TR-T) shifted to a lower value along the growth direction, while the Curie Temperature (TC) shifted to a higher value instead. The relaxation level of the single crystal went down to a lower level when the crystal structure changed from R phase to T and O phase. The coercive field (EC) with [001] poling in R phase remained stable before a rise in O phase and T phase. However, the composition segregation didn't obviously affected the internal bias field and kept around 0.2 kV/cm. The piezoelectric coefficient d33 and unipolar strain with poling direction along [001] had the same trend along growth direction. The curves of them both took a rapid fall at the top of the single crystal boule, which related to the T phase.

In some experiments, an intermediate phase, i.e., the rhombohedral phase (R-phase), can be observed in the phase transformation process between austenite and martensite for NiTi shape memory alloys. To predict phase transformations among these three phases, a one-dimensional constitutive model for shape memory alloys considering the inelastic strains caused by the R-phase transformation is proposed. Firstly, a temperature-stress phase diagram reflecting martensitic transformation as well as R-phase transformation is provided. On the basis of this phase diagram, the novel model for shape memory alloys is developed in the framework of thermo-mechanics taking into account various inelastic deformation mechanisms, involving the independent phase transformation (e.g., the phase transformation between pure austenite and pure R-phase or the one between pure R-phase and pure martensite) and the mixed phase transformation (e.g., the phase transformation between mixed phases of ‘austenite + R-phase’ and pure martensite). Through numerical examples, it is demonstrated that the proposed model is capable to predict the deformation properties of shape memory alloys associated with the R-phase transformation, including shape memory effect and pseudo-elasticity, as well as internal sub-loops due to the incomplete phase transformation. Furthermore, the numerical results are compared with experimental results and show good agreement with the experimental ones, which confirms the feasibility of the proposed model.

The (1-x)Bi1+y(Fe0.96Mg0.02Ti0.02)O3-xBaTiO3 (abbreviated as B1+yFMT-xBT) system was designed to explore the morphotropic phase boundary (MPB) separating rhombohedral (R) and tetragonal (T) phases, and the mechanism of performance enhancement. The phase structure of samples was discreetly investigated by X-ray diffraction, and Rietveld refinement results of the XRD data revealed that the B1+yFMT-xBT ceramics consist of an R phase at x = 0.24, two R and T phases at 0.26 ≤ x ≤ 0.32, and a single T phase at x = 0.34. Because of the MPB and the improvement of resistivity with adding excess Bi compensation, excellent piezoelectric properties of d33 = 198 pC/N and kp = 30.9%, as well as high Curie temperature TC = 497 °C were achieved in the B1+yFMT-xBT system at x = 0.30 and y = 0.02. Here, the large d33 = 198 pC/N which is the maximum value for the BF-BT-Bi(Mg,Ti)O3 system so far, and high TC = 497 °C suggest that Bi(Fe,Mg,Ti)O3-BaTiO3 ceramics have good prospects in high-temperature piezoelectric devices.

Destruction of microstructural stability caused by topologically close-packed (TCP) phase precipitation restricted the industrial application of advanced single crystal superalloys. However, systematic investigations on microstructural nature and evolution of Co-rich TCP phases in Ru-containing single crystal superalloys with high level of Co addition have been rarely reported. In this study, experimental results indicated that the main TCP phase was μ phase after thermal exposure at 950 °C for 1000 h, while R phase was the main phase after thermal exposure at 1100 °C for 1000 h in the alloys containing high level of Co and different levels of Cr and Mo. The experimental and thermodynamic calculation results showed that R phase was more stable at 1100 °C than that of μ phase in the experimental alloys. Meanwhile, μ phase started to transform to R phase at 1100 °C for 50 h, and the stress accelerated the transformation of μ phase and R phase during creep test at 1100 °C/140 MPa in an alloy containing high levels of Co, Cr and Mo additions. Both R phase and μ phase were enriched in Re, W, Mo, Cr and Co, while R phase contained more Cr and μ phase was more enriched in Mo. This study contributes to better understand the microstructural evolution of TCP phases at different temperatures and to optimize the alloy design and thermodynamic database of Ru-containing single crystal superalloys with high level of Co addition.

In this study, a high-performance Mg–8Gd–2Y–1Zn–6Li (wt%) alloy having a TYS of 202 ± 2.6 MPa, UTS of 243 ± 2.1 MPa and EL of 10.7 ± 2.3% was successfully fabricated by casting, heat treatment and hot extrusion. The effects of intermetallic compounds on microstructure and mechanical properties were deeply investigated. The results show that the studied alloy consists of coarse deformed α-Mg grains with (0001) basal texture, fine recrystallized β-Li grains with (002) texture and three Mg3RE phase variants, which are bulk Mg3RE phase with a size range of 10–30 µm, spot Mg3RE phase with a size range of 1–2 µm and spherical Mg3RE phase with a size range of 0.3–1 µm, respectively. The spherical Mg3RE phase has a strong dispersion strengthening effect, while the bulk Mg3RE phase acts as crack resources and deteriorates the ductility. A novel island-like β 1 R phase (FCC, a = 0.78 nm) is found for the first time in the Mg-Gd-Li based alloys. It has a coherent ( 1 12 ) β 1 R // ( 10 1 0 ) α interface at the broad facet and a semi-coherent ( 1 11 ) β 1 R // ( 11 2 0 ) α interface at the end facet. The β 1 R phase transforms from β H phase at the semi-coherent interface by atomic shear of RE and Mg atoms. The β 1 R phase provides a significant precipitation strengthening effect to the studied alloy, while the dissolution of β 1 R phase results in the age-softening phenomenon. The high-performance of the studied alloy is mainly attributed to the α/β duplex structure, three Mg3RE phase variants, and β 1 R phase.

A polycrystalline copper alloy with 13.7 wt% aluminum and 4.0 wt% nickel, produced by plasma fusion followed by injection molding, was investigated for the reversible martensitic transformations (RMT) occurring in association with load-free thermal cycling treatments (TCT's). The as-received quenched alloy was subjected to distinct TCT's up to 500 cycles within the temperature interval from 258 K (below M f ) to 373 K (above A f ). Each distinct TCT was finished at room temperature (RT) either by half cooling cycle (258 K to RT) or by half heating cycle (373 K to RT). Transformations were characterized by X-ray diffraction, differential scanning calorimetry, optical microscopy and microhardness. The results revealed a complex sequence of transformations involving the high temperature β 1 phase, the metastable γ 1 ′ and β 1 ′ martensitic phases, and the intermediate Al 7 Cu 4 Ni phase, known as the R phase. Depending on the number of cycles and the finishing way, the sequence of transformations was partial or complete. The RMT with TCT promotes different changes in the microstructure, including an apparent pull out of grains. Microhardness was also sensibly affected owing to the complex participation of different phases.

This in situ TEM study investigated the effect of sample thickness on the martensitic transformation of NiTi. The transformation temperature gradually decreased with decreasing sample thickness. No transformation was observed when the thickness was below a critical value. The critical thickness values for the R phase and martensitic transformations were determined to be ∼22 nm and ∼50 nm, respectively, for the Ti-50.8 at.%Ni alloy studied. This apparent “size effect” was attributed to the depletion of Ti in the matrix caused by surface oxidation, as confirmed by oxygen content measurements. These findings demonstrate that natural oxidation and its influence on the transformation temperature must be taken into consideration in the fabrication of micro-electro-mechanical devices using NiTi.

The antiphase boundary (APB)-like structure of B19′ martensite via R-phase transformation in a Ti–Ni–Fe alloy was investigated by means of transmission electron microscopy. The APB-like structure exhibited shifts along the (0 1 0) B19 ′ , (0 0 1) B19 ′ , and (1 0 0) B19 ′ planes; that is, it exhibited facets composed of those planes at the atomic level. This atomic displacement reflects the atomic movement stemming from the R-phase transformation.

A high Ti-rich Ti52.6Ni46.8Si0.6 ribbon was investigated to reveal the precipitation hardening effect caused by the nano-scale Ti2Ni precipitates. TEM observations revealed only nano-scale Ti2Ni second phase in the ribbon after aging treatment, which induced the R phase transformation detected in DSC tests. Nanoindentation measurements demonstrated the increment of hardness from 2.43 GPa to 2.98 GPa, an increment of about 22%, in the first 3 h of aging treatment at 500 °C. The pseudoelasticity (PE) of the ribbon, evaluated by depth and energy recovery ratios of nanoindentation tests, also revealed that 3 h-aged ribbon had the best recovery performance. The shape memory effect (SME) of the 3 h-aged ribbon exhibited the best performance with a recoverable strain of 3.5% and an irrecoverable strain smaller than 0.1%. Both PE and SME tests demonstrated that the precipitation hardening effect of nano-scale Ti2Ni precipitates can effectively improve the shape memory properties of high Ti-rich TiNi shape memory ribbons.

Microstructural features and martensitic transformation behavior of Ti–Ni–Sn alloys were investigated by means of electron probe microanalysis (EPMA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). In a Ti–49.5Ni–0.5Sn alloy, Ti 2 Ni phase embedded in Ti–Ni matrix containing 0.7 at%Sn was observed. In a Ti–49.0Ni–1.0Sn alloy, Ti 2 Ni, unknown phase and Ti–Ni matrix containing 1.1 at%Sn were observed, while Ti 2 Ni and two different Ti–Ni matrices with 2.1 at%Sn and without Sn were observed in a Ti–50.0Ni–1.0Sn alloy. In a Ti–48.5Ni–1.5Sn alloy, Ti 2 Ni, Ti 3 Sn and Ti–Ni matrix containing Sn about 1.2 at% were observed. The B2–R–B19′ transformation occurred in Ti–49.5Ni–0.5Sn, Ti–49.0Ni–1.0Sn and Ti–48.5Ni–1.5Sn alloys, while the B2–B19′ transformation occurred in a Ti–50.0Ni–1.0Sn alloy. Substitution of Sn for Ni of an equiatomic TiNi alloy was effective to induce the R phase transformation.

Ni-rich TiNi alloys were subjected to the effect of multiple equal channel angular extrusion (ECAE) treatments by B C path at 500 °C. The characteristics of R phase transformation in aging treatment were dissimilar in the appearance and the temperature range to those counterparts induced by ECAE treatments. The fine lens-like shape Ti 3 Ni 4 particles precipitated mainly in the regions of near grain boundaries and on the tangled grain boundaries after ECAE treatments. The effects and mechanisms of aging treatments and ECAE treatments on R phase transformation behaviors and Ti 3 Ni 4 precipitates were investigated and discussed.

Ni-rich TiNi shape memory alloys were subjected to the effect of equal channel angular extrusion (ECAE) processes at 773 K by Bc path. The effects of ECAE processes on microstructures evolution and phase transformation behaviors were investigated. The initial 60–80 μm equiaxed coarse grains of samples were elongated along the shearing force direction of ECAE and refined to 300–400 nm after eight passes ECAE. The R phase transformation of Ni-rich TiNi shape memory alloys was stimulated by ECAE processes within a larger temperature range. The martensite transformation peak temperature (Mp) dropped in previous 1–3 ECAE treatments, but the dropped Mp increased gradually with the increase of ECAE processes. Ti 3 Ni 4 phase was observed in the regions with high density of dislocations after ECAE treatment. Reasons for microstructures evolution and phase transformation changes were also discussed.

Differential scanning calorimetry (DSC) results indicate that a two-step B2 → R → B19′ martensitic transformation and a one-step B19′ → B2 transformation exhibit in as-spun and in 200–600 °C annealed Ti 51 Ni 49 ribbons. Guinier-Preston (GP) zones and Ti 2 Ni precipitates are formed in ribbons annealed at ≦300 °C and ≧400 °C, respectively, and a conspicuous increase of DSC transformation peak temperature occurs in between 300 °C and 400 °C. The sizes of GP zones and Ti 2 Ni precipitates increase with increased annealing temperature. Transmission electron microscope (TEM) observations show that GP zones can induce the R-phase and both of them are formed along 〈1 0 0〉 B2 directions. DSC and TEM tests show that Ti 2 Ni precipitates can induce the R-phase more than GP zones and the induced R-phase plates are also found along 〈1 0 0〉 B2 directions. Experimental results show that the growing direction of R-phase plates is strongly confined by that of GP zones and Ti 2 Ni precipitates. The length of R-phase plates can reach about 2 μm in 300 °C annealed ribbon.

This paper studies how the R-phase developed by thermal cycling at zero stress influences two different training procedures for developing the two-way shape memory effect (TWSME) in NiTi shape memory alloys (SMAs). The two training procedures are thermal cycling under constant load and isothermal tensile deformation under martensite state. This paper studies how R-phase developed by thermal cycling at zero stress influences these training procedures and their training parameters, the two-way memory strain obtained and the transformation temperatures. The results show first, that the presence of the R-phase did not prevent substantial two-way memory strain from being obtained in any instance of training and thermal treatment; and second, that thermal cycling at zero stress prior to training has a considerable impact depending on the training method used. Finally, to obtain the best results (that is, a substantial two-way memory strain together with a minimum of final plastic strain and easy TWSME activation temperatures), the NiTi wire should be thermally cycled at zero stress prior to training until the R-phase is totally developed and trained by thermal cycling under constant load.

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni 4 Ti 3 participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.

Aging effect on a Ti 47.25 Ni 48.75 V 4 shape memory alloy is investigated. Experimental results reveal that the 400 °C aged Ti 47.25 Ni 48.75 V 4 alloy exhibits a B2 → R → B19′ two-stage martensitic transformation on cooling, but exhibits a B19′ → B2 one-stage transformation on heating. Two kinds of second phases, (Ti,V) 2 Ni and V 9 (Ti,Ni), are observed in the solution-treated Ti 47.25 Ni 48.75 V 4 alloy. (Ti,V) 2 Ni exhibits an FCC order structure with a lattice parameter of 11.1 A while V 9 (Ti,Ni) exhibits a BCC structure with a lattice parameter of 3.01 A. There appear lots of fine spherical V 9 (Ti,Ni) and coherent Ti 3 Ni 4 precipitates after 400 °C aging for the Ti 47.25 Ni 48.75 V 4 alloy. These precipitates will strengthen the alloy's matrix and improve the shape recovery ability.

The intrinsic internal friction IF I of R-phase and B19′ martensite are composed of static internal friction IF S and dynamic internal friction IF D . The tan δ values of I F S R and I F S B 1 9 ′ are both proportional to σ 0 / υ 1/2 and are related to the stress-assisted motions of twin boundaries. The tan δ values of I F S R are higher than those of I F S B 1 9 ′ is owing to the softer storage modulus E 0 in R-phase. The tan δ values of I F D B 1 9 ′ are linearly proportional to T ˙ / υ 1 / 2 . The occurrence of relaxation peak at ≈−60 °C is found to come from the I F S B 1 9 ′ , instead of the I F D B 1 9 ′ .

The R-phase was induced in a binary polycrystalline 51.2 at.% Ni–48.8 at.% Ti alloy by deformation of e =5% followed by annealing at 350 °C for 70 h. This procedure allowed us to clearly separate the B2→R transformation from the R→B19′ one. In addition, a ternary Ni 47 Ti 50 Co 3 alloy showed a gap of almost 40 °C between these two transformations during cooling. The X-ray diffraction pattern of the R-phase was refined with the Rietveld method for three structure models with the following space groups: P 3, P -3, and P 31 m , where the atomic positions were initially calculated from Patterson maps determined by the LeBail method. The best result was obtained for the model in P -3.

The two-staged thermoelastic martensitic transformation (TMT) B2-->R-->B19' in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young's modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19' martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19' phase. (C) 2000 Elsevier Science S.A. All rights reserved.