Critical Stress For Slip Deformation Research Articles

The effects of Ru, Cu, Zr and Hf on mechanical properties in Ti-Pt high temperature shape memory alloys

Shape memory alloys (SMAs) have been widely used in the fields of actuators and aerospace industry due to their pseudo-elasticity and shape memory effect which are displayed in phase transformations. The martensitic transformations (MT) of TiPt is much higher, at approximately 1273 K and this is considered to be of potential technological interest for elevated temperature SMA applications. TiPt based alloys exhibit very low shape memory effect due to low critical stress for slip deformation compared to the stress required for martensitic transformation, hence it is necessary to enhance the mechanical properties of the equiatomic alloy. The first principles approach was employed to study the effect of the third element (M = Ru, Cu, Zr, and Hf) on the TiPt shape memory alloy. The supercell approach in VASP was used to substitute Pt with Ru and Cu, Ti with Zr and Hf on the TiPt structure to evaluate their mechanical stability from elastic properties for actuators and higher temperature applications. The Ti50Pt50-xRux and Ti50Pt50-xCux decreases in density with increase in Ru and Cu concentration, whilst the Ti50-xZrxPt50 and Ti50-xHfxPt50 substitution increases with an increase in their concentration, which result in larger lattice parameters. The heats of formation suggest that Ti50Pt50-xRux substitution is more thermodynamically stable than Ti50Pt50-xCux substitution, and Ti50-xHfxPt50 substitution is more stable than Ti50-xZrxPt50. The elastic properties suggest that the ternary structures become mechanically stable with an increase of the third element. The Ti50Pt50-xRux and Ti50Pt50-xCux substitution became more ductile with the increase in concentration. Zr and Hf substitution became more ductile at higher compositions (31.75 – 43.75 at.%). The Ru and Hf substitutions have potential to be used for high-temperature applications.

Enhancement of superelastic property in Ti–Zr–Ni–Cu alloy by using glass alloy precursor with high glass forming ability

The glass forming ability (GFA), crystallization behavior and subsequent superelastic property after crystallization in Ti50-xZrxNi35Cu15 (x = 5, 10, 15, 20 at%) alloys have been investigated with an aim to develop glassy alloy precursor for shape memory alloy having both high glass forming ability and superior superelastic property. The addition of Zr is effective in improving the GFA exhibiting a critical thickness larger than 100 μm by increasing both liquid stability and resistance to crystallization. The glassy Ti50-xZrxNi35Cu15 alloys are polymorphically crystallized into B2 structure. With increasing Zr content, the grain size after crystallization decreases significantly by accelerated nucleation process. Thus, the stability of austenite increases, resulting in the decrease of Ms. The critical stress for slip deformation gradually increases with alloying Zr up to 15 at%, improving the superelastic recovery. The Ti35Zr15Ni35Cu15 alloy exhibits the highest remnant depth ratio value of 7.2%. By contrast, the superelasticity is significantly deteriorated in the Ti30Zr20Ni35Cu15 nanocrystalline alloy due to the difficulty in stress-induced martensitic transformation which is caused by extremely small grain size and irregular morphology of grain boundary. Considering both glass forming ability and superelastic property, the Ti35Zr15Ni35Cu15 alloy is confirmed to be the most suitable alloy satisfying wide supercooled liquid region (∼40 K), large maximum thickness for amorphous alloy formation (∼100 μm) and excellent superelasticity after crystallization.

Effect of Annealing Temperature on Annealing Twins and Shape Memory Effect in Hot-Forged Co-23.9Ni-5.6Si Alloy

Effects of annealing temperature on shape memory effect (SME), annealing twins, and deformation behavior in a hot-forged Co-23.9Ni-5.6Si alloy were studied. The results showed that a low density of the annealing twin boundaries tended to cause a good SME in the hot-forged Co-23.9Ni-5.6Si alloy. The best SME was obtained in the hot-forged Co-23.9Ni-5.6Si alloy annealed at 1073 K. A high recovery strain of 4 pct was realized when deformed at 77 K. The low density of the annealing twin boundaries and lots of introduced stacking faults were responsible for this large recovery strain. The pre-existence of too many tangled dislocations was the main reason why the SME was poor in the as-forged alloy and the alloy annealed below 1073 K although their densities of annealing twin boundaries were also low. The significant improvement of SME after deformation at 77 K resulted from the remarkable increase of the critical stress for slip deformation due to the drop in the thermal activation. When the critical temperature of thermal activation (T0) was above the forward martensitic transformation temperature (Ms), a negative temperature dependence of the critical stress for stress-induced martensitic transformation occurred between the Ms and T0 temperatures.

Enhancement of Superelastic Property in Ti-Zr-Ni-Cu Alloy by Using Glass Alloy Precursor with High Glass Forming Ability

The glass forming ability (GFA), crystallization behavior and subsequent superelastic property after crystallization in Ti50-xZrxNi35Cu15 (x= 5, 10, 15, 20 at. %) alloys have been investigated with an aim to develop glassy alloy precursor for shape memory alloy having both high glass forming ability and superior superelastic property. The addition of Zr is effective in improving the GFA exhibiting a critical thickness larger than 100 by increasing both liquid stability and resistance to crystallization. The glassy Ti50-xZrxNi35Cu15 alloys are polymorphically crystallized into B2 structure With increasing Zr content, the grain size after crystallization decreases significantly by accelerated nucleation process. Thus, the stability of austenite increases, resulting in the decrease of Ms. The critical stress for slip deformation gradually increases with alloying Zr up to 15 at.%, improving the superelastic recovery. The Ti35Zr15Ni35Cu15 alloy exhibits the highest remnant depth ratio value of 7.2 %. By contrast, the superelasticity is greatly deteriorated in the Ti35Zr20Ni35Cu15 nanocrystalline alloy due to the change of deformation mechanism following inverse Hall-Petch relation. Considering both glass forming ability and sueprelastic property, the Ti35Zr15Ni35Cu15 alloy is confirmed to be the most suitable alloy satisfying wide supercooled liquid region (~40 K), large maximum thickness for amorphous alloy formation (~100 m) and excellent superelasticity after crystallization.

Effect of B addition on the microstructure and superelastic properties of a Ti-26Nb alloy

The mechanical properties and shape memory behavior of Ti–26Nb–(0–1.0)B and Ti–27Nb alloys were investigated. The stress for inducing martensite and the critical stress for slip deformation were greatly affected by increasing B content up to 0.1at%, and less affected by further increase in B content up to 0.7at%. The martensitic transformation start temperature decreased suddenly with a slope of 350K/at% B when B content was less than 0.1at% and decreased with a smaller slope of 15K/at% B with further increase in B content up to 1.0at%. In all samples tested, only superelasticity was observed at room temperature. TEM investigations revealed that the solubility of B in Ti–26Nb alloy was around 0.1at%. In spite of the low solubility limit, B was effective to stabilize the superelastic behavior by effectively increasing the critical stress for slip deformation. On the other hand, while the stress for inducing martensite increased effectively by the addition of 1.0at% Nb to the Ti–26Nb alloy, the critical stress for slip deformation remained almost constant and hence superelasticity was deteriorated.

Martensitic transformation behavior of Ti–Ni–Ag alloys

Microstructures and martensitic transformation behavior of Ti–Ni–Ag alloys prepared by arc melting were investigated by means of scanning electron microscopy (SEM), electron probe micro analysis (EPMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermal cycling tests under constant load. Ti–Ni–Ag alloys consisted of Ti–Ni–Ag matrices, Ti2Ni and TiAg phases. Ti–Ni–Ag matrices contained 0.27–0.52 at.% of solute Ag atoms depending on alloy compositions. The B2–B19′ transformation occurred in Ti–50.1Ni–0.7Ag, Ti–49.2Ni–0.9Ag, Ti–49.2Ni–0.6Ag and Ti–49.0Ni–0.7Ag alloys, while the B2-R-B19′ transformation did in Ti–47.5Ni–1.3Ag and Ti–44.4Ni–1.1Ag alloys. Thermo-mechanical treatment separated the B2-R from the R–B19′ transformation clearly and improved shape recovery by increasing the critical stress for slip deformation in a Ti–50.0Ni–0.7Ag alloy.

High-temperature mechanical and shape memory properties of TiPt–Zr and TiPt–Ru alloys

Ti–50Pt (all compositions in atomic %) alloy exhibits phase transformation from B2 phase having bcc crystal structure to orthorhombic martensitic (B19) phase above 1200K. However, it exhibits a partial shape memory effect due to its low critical stress for slip deformation compared with the stress required for martensitic transformation. In order to improve the shape memory properties and strength of Ti–50Pt alloys at high temperature, this study examined the effects of partial substitution (e.g. 5at%) of Ti with group IV (e.g. Zr) transition metals and the effects of partial substitution of Pt with platinum group (e.g. Ru) transition metals. The compressive strength and shape memory properties at high temperature were improved by partial substitution of Ti with Zr as well as by partial substitution of Pt with Ru in Ti–50Pt alloys: e.g., 1468MPa for TiPt–Zr, 712MPa for TiPt–Ru and 485MPa for TiPt alloys. Furthermore, Ti–50Pt–5Zr alloy exhibited much higher strength and better shape memory effect than Ti–45Pt–5Ru alloy.

Strengthening of β Ti–6Cr–3Sn alloy through β grain refinement, α phase precipitation and resulting effects on shape memory properties

Our recently developed Ti–6Cr–3Sn alloy exhibited shape memory effect and large fracture strain. However Ti–6Cr–3Sn alloy exhibited partial shape memory effect and low ultimate tensile strength (UTS). Then in this study, effects of decrease in β grain size on the mechanical and shape memory properties of β Ti–6Cr–3Sn alloy were investigated. Results of decrease in β grain size were also compared with our recently reported uniform α phase precipitated Ti–6Cr–3Sn alloy. It was investigated that decrease in β grain size was found effective in improving the strength, fracture strain and shape memory properties of β Ti–6Cr–3Sn alloy due to the increase in critical stress for slip deformation. On the other hand, uniform precipitation of α phase in β matrix of Ti–6Cr–3Sn alloy was found much more effective than decrease in β grain size in improving the strength and shape memory properties of β Ti–6Cr–3Sn alloy. However fracture strain of uniform α phase precipitated Ti–6Cr–3Sn alloy was lower than coarse and fine β grain Ti–6Cr–3Sn alloy. It is concluded that decreasing β grain size was found as effective way to improve the mechanical and shape memory properties altogether of β Ti–6Cr–3Sn shape memory alloy. However decrease in β grain size was found less effective than our recently reported effect of uniform precipitation of α phase in β matrix of β Ti–6Cr–3Sn alloy.

Effect of uniform distribution of α phase on mechanical, shape memory and pseudoelastic properties of Ti–6Cr–3Sn alloy

Recently, among our developed Ti–Cr–3Sn alloys, the Ti–6Cr–3Sn alloy exhibited good mechanical, shape memory and pseudoelastic properties. However, the shape memory effect was incomplete, the pseudoelastic recovered strain was small and the ultimate tensile strength (UTS) was low. Therefore, in this study, the effect of a step heat treatment at 973K (SHT973K) in the (α+β) region with the purpose of creating a uniform distribution of the α phase in the β phase matrix using ω phase precursors was investigated. It was determined that the step heat treatment in the 973K (α+β) region was effective for improving the strength, shape memory and pseudoelastic properties of the Ti–6Cr–3Sn alloy due to the uniform distribution of the α phase in the β phase matrix, which resulted in a more effective increase in the critical stress for slip deformation. It is concluded that the uniform distribution of the α phase in the β matrix is an excellent method for improving the mechanical, shape memory and pseudoelastic properties of ω-sensitive β Ti alloys.

Room temperature aging behavior of Ti–Nb–Mo-based superelastic alloys

The effect of room temperature (RT) aging on the superelasticity of Ti–Nb–Mo-based superelastic alloys is investigated. The results show that annealing at relatively low temperatures such as 973K after severe cold rolling results in poor resistance to the effect of RT aging. The transformation stress increases considerably due to the formation of an isothermal ω phase at RT. Addition of Sn is partially effective in suppressing the RT aging effect in the specimens annealed at 973K. The RT aging effect is suppressed by increasing the annealing temperature, due to the annihilation of lattice defects or non-equilibrium vacancies introduced during cold rolling, which are responsible for accelerating the diffusion process, however, superelasticity is reduced by annealing at higher temperatures, due to a decrease in the critical stress for slip deformation (σCSS). The specimen annealed at 1173K followed by aging at 773K exhibits stable superelasticity with a high resistance to the effect of RT aging. Annealing at 1173K causes the annihilation of lattice defects or non-equilibrium vacancies, while aging at 773K induces precipitation of the α phase, which in turn causes an increase in σCSS, and further enhances the resistance to the RT aging effect by enriching the matrix with β-stabilizing elements.

Crystallization and grain refinement of Ti 30Ni 20Cu (at%) alloy ribbons prepared by melt spinning

Abstract Crystallization behavior of amorphous Ti 30Ni 20Cu (at%) alloy ribbons and martensitic transformation behavior, shape memory effect and superelasticity of crystallized ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, thermal cycling tests under constant load and tensile tests. Crystallization occurred in the sequence of amorphous–B2–Ti(Ni,Cu)2 and the activation energy for crystallization of the B2 phase was 198.8 kJ/mol. Average grain size of the sample annealed in two-step, i.e., annealing at 773 K for 40 s followed by annealing at 748 K for 3.6 ks was 0.25 μm, which was very small comparing with the sample annealed in one-step, i.e., annealing at 823 K for 3.6 ks (1.20 μm). Grain refinement reduced the heat of transformation (ΔH) associated with the B2–B19 transformation and increased the critical stress for slip deformation causing the perfect shape memory recovery under the applied stress of 400 MPa.

Transformation Behavior of Ti-(45-x)Ni-5Cu-xCr (at%) (x = 0.5-2.0) Shape Memory Alloys

Transformation behavior and shape memory characteristics of Ti-(45-x)Ni-5Cu-xCr (x=0.5-2.0) alloys have been investigated by means of electrical resistivity measurements, differential scanning calorimetry, X-ray diffraction and thermal cycling tests under constant load. Two-stage B2-B19-B19' transformation occurred in Ti-(45-x)Ni-5Cu-xCr alloys. The B2-B19 transformation was separated clearly from the B19-B19' transformation in Ti-44.0Ni-5Cu-1.0Cr and Ti-43.5Ni-5Cu-1.5Cr alloys. A temperature range where the B19 martensite exists was expanded with increasing Cr content because decreasing rate of Ms (85 K / % Cr) was larger than that of Ms' (17 K / % Cr). Ti-(45-x)Ni-5Cu-xCr alloys were deformed in plastic manner with a fracture strain of 68% ~ 43% depending on Cr content. Substitution of Cr for Ni improves the critical stress for slip deformation in a Ti-45Ni-5Cu alloy due to solid solution hardening.

パルス通電焼結によるTi基形状記憶合金の創製に関する研究

Recently, the fabrication of Ti-based shape memory alloys (SMAs) has been tried using the powder metallurgy (PM) process. The fabrication conditions of Ti-based SMAs by PM process and their important shape memory characteristics for applying to the elements, however, have not been discussed enough. The target of this study is to develop Ti-based SMAs having superior shape memory characteristics by PM process. At first, the fabrication conditions and shape memory characteristics of Ti-Ni alloys were examined. Next, in order to improve the critical stress for slip deformation and the shape memory characteristics of Ti-Ni alloys, the effects of aging treatment, densification processing and addition of Cu and Zr as third elements on the shape memory characteristics were investigated. In this paper, the effects of Cu addition on phase transformation behavior and thermo-mechanical properties of Ti-Ni-Cu alloy were introduced. As a result, Ti-based SMAs having superior shape memory characteristics could be developed by establishment of fabrication conditions of the alloy by the PM process.

Shape memory properties of Ti–Nb–Mo biomedical alloys

Mo is added to Ti–Nb alloys in order to enhance their superelasticity. The shape memory properties of Ti–(12–28)Nb–(0–4)Mo alloys are investigated in this paper. The Ti–27Nb, Ti–24Nb–1Mo, Ti–21Nb–2Mo and Ti–18Nb–3Mo alloys exhibit the most stable superelasticity with a narrow stress hysteresis among Ti–Nb–Mo alloys with Mo contents of 0, 1, 2 and 3 at.%, respectively. The ternary alloys reveal better superelasticity due to a higher critical stress for slip deformation and a larger transformation strain. A Ti–15Nb–4Mo alloy heat-treated at 973 K undergoes (2 1 1)〈1 1 1〉-type twinning during tensile testing. Twinning is suppressed in the alloy heat-treated at 923 K due to the precipitation of the α phase, allowing the alloy to deform via a martensitic transformation process. The Ti–15Nb–4Mo alloy exhibits stable superelasticity with a critical stress for slip deformation of 582 MPa and a total recovery strain of 3.5%.

Microstructure and superelasticity of in situ synthesized (TiB + La2O3)/Ti-alloy composites with different mass fraction of LaB6

In situ techniques have been developed to synthesize titanium matrix composites. The microstructures and the superelastic characterization of (TiB+La2O3)/(Ti–35Nb–2Ta–3Zr) TMC were examined and studied. The effects of TiB whiskers and La2O3 spherical particles on superelasticity were discussed. In the LaB6-additioned specimens, minimum size of grain around 12μm was obtained and critical stress for slip deformation was increased. Excellent superelastic strain and pure elastic strain were obtained in 0.1% and 0.2% LaB6-additioned specimens. The largest superelastic strain was observed in 0.5% LaB6-additioned specimen.

Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys

Recently, the Ti-Zr-Nb alloys have been developed as Ni-free shape memory and superelastic alloys. In this study, the effects of Nb and nitrogen (N) contents on martensitic transformation behavior, shape memory effect and superelasticity in Ti-18Zr-(12∼16)Nb-(0∼1.0)N (at%) alloys are investigated using loading and unloading tensile tests, optical microscopy and X-ray diffractometry. The shape memory effect is observed in Ti-18Zr-(12∼13)Nb and Ti-18Zr-12Nb-0.5N alloys at room temperature. The superelastic behavior appears by the increase of Nb or N content. The Ti-18Zr-(14∼15)Nb, Ti-18Zr-(13∼14)Nb-0.5N and Ti-18Zr-(12∼14)Nb-1.0N alloys exhibit the superelasticity at room temperature. The martensitic transformation start temperature decreases by 75 K with 1 at% increase of N content for the Ti-18Zr-13Nb alloy. The critical stress for slip deformation and the stress for inducing the martensitic transformation increase with increasing N content. The superelastic recovery strain is also increased by adding N. The maximum recovery strain of 5.0% is obtained in the Ti-18Zr-14Nb-0.5N alloy.

Shape Memory Characteristics and Superelasticity of Ti-Ni-Cu Alloy Ribbons with Nano Ti2Ni Particles

Microstructures and deformation behaviour of Ti-45Ni-5Cu and Ti-46Ni-5Cu alloy ribbons prepared by melt spinning were investigated by transmission electron microscopy, thermal cycling tests under constant load and tensile tests. Spherical Ti2Ni particles coherent with the B2 parent phase were observed in the alloy ribbons when the melt spinning temperature was higher than 1773 K. Average size of Ti2Ni particles in the ribbons obtained at 1873 K was 8 nm, which was smaller than that (10 nm) in the ribbons obtained at 1773 K. Volume fraction of Ti2Ni phase in the ribbons obtained at 1873 K was 40%, which was larger than that (20%) in the ribbons obtained at 1773 K. The stress required at temperatures of Af + 10 K for the stress-induced martensitic transformation increased from 93 MPa to 229 MPa and apparent elastic modulus of the B2 parent phase increased from 56 GPa to 250 GPa with increasing the melt spinning temperature from 1673 K to 1873 K in Ti-45Ni-5Cu alloy ribbons. The critical stress for slip deformation of the ribbons increased by coherent Ti2Ni particles, and thus residual elongation did not occur even at 160 MPa, while considerable plastic deformation occurred at 60 MPa in the ribbons without Ti2Ni particles. Almost perfect superelastic recovery was found in the ribbons with coherent Ti2Ni particles, while only partial superelastic recovery was observed in the ribbons without coherent Ti2Ni particles.

1009 粉末冶金プロセスによるTi-Ni合金のCu添加による形状記憶特性の改善(S03-2 形状記憶合金の特性と応用技術(2),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Ti-Ni-Cu shape memory alloys were fabricated by powder metallurgy (P/M) process. The blended powders of three compositions of Ti-50.2at%Ni, Ti-40.2at%Ni-10at%Cu and Ti-30.2at%Ni-20at%Cu were milled using planetary ball mill. These powders were sintered using pulse-current sintering equipment. The effect of Cu addition on the shape recovery strain and critical stress for slip deformation of the alloys were investigated. Increase in Cu addition increases the shape recovery strain of the alloy, but it decreases the critical stress for slip deformation because of a number of pores formed by Cu addition. In the thermal cycling test, the plastic strain of the alloy of Ti-30at%Ni-20at%Cu was lower than that of the other alloys in an applied stress more than 75 MPa. The plastic strain of the alloy was 0.06% under an applied stress of 175 MPa.

Microstructures and mechanical properties of Ti–45 at.%Ni–5 at.%Cu alloy ribbons containing Ti 2Ni particles

Microstructures, shape memory characteristics and the superelasticity of Ti–45Ni–5Cu (at.%) alloy ribbons were investigated by transmission electron microscopy, thermal cycling tests under constant load and tensile tests. Coherent Ti 2Ni particles less than 13 nm in diameter are formed when the melt spinning temperature is higher than 1773 K. The ribbons with coherent Ti 2Ni particles do not show a residual elongation, which may be ascribed to the increase due to the coherent Ti 2Ni particles of the critical stress for slip deformation by the coherent Ti 2Ni particles. Almost perfect superelastic recovery is found in the ribbons containing coherent Ti 2Ni particles, while only partial superelastic recovery is observed in Ti 2Ni particle-free ribbons.

Effects of Si addition on superelastic properties of Ti–Nb–Al biomedical shape memory alloys

Effects of Si addition on microstructure, transformation temperature and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–24 at.%Nb–3 at.%Al (TiNbAl), were examined in the compositional range from 0 to 0.9 at.%Si. After the solution treatment at 1273 K, the constituent phases of the alloys with Si concentration lower than 0.7 at.% are parent β-phase (bcc) and athermal ω-phase. The alloys with Si concentration higher than 0.7 at.% contains incoherent (Ti, Nb) 3Si particles. Then, the solid solubility limit of Si in TiNbAl at 1273 K is estimated to be 0.7 at.%Si. Tensile tests revealed that the stress for inducing martensite is raised by Si addition. Thermomechanical analysis revealed that the reverse martensitic transformation finish temperature ( A f) becomes lower by Si addition by −250 K/at.%Si. The critical stress for slip deformation evaluated using residual strain increases with increasing Si concentration up to 0.7 at.%Si and the hardening rate is estimated to be 300 MPa/at.%Si. This hardening caused by Si addition is due to the solid-solution strengthening. When Si concentration exceeds 0.7 at.%, incoherent Ti 3Si-type precipitates are formed during the homogenization at 1273 K, and no further hardening occurs. Consequently, apparent transformation strain drastically decreases when Si concentration is higher than 0.7 at.%. The Ti 3Si-type particles are suggested to act as an obstacle for the stress-induced martensitic transformation. It was concluded that Si is effective to reduce M s and to raise critical stress for slip of TiNbAl, and that the formation of Ti 3Si-type precipitates exhibits negative effect for superelasticity.