Critical Stress For Slip Research Articles

Effect of Cold Rolling and Hot Rolling on Recrystallization Texture and Superelastic Properties of Ti–30Zr–10Nb–2Sn Shape Memory Alloy

A comparative study on the microstructure, texture evolution, and superelasticity of cold-rolled and hot-rolled Ti–30Zr–10Nb–2Sn (at.%) alloys was performed using electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and tensile test. The cold-rolled sheet was solution-treated at 1173 K after a 98.0% cold rolling at room temperature. Hot rolling was carried out at 1273 K. Both cold-rolled and hot-rolled sheets were solution-treated at 1173 K for 1.8 ks. Both solution-treated alloy specimens consisted of a single β-phase at room temperature. A very high (200)β peak was observed for the cold-rolled specimen, and a high (110)β peak was observed for the hot-rolled specimen after the solution treatment at 1173 K for 1.8 ks. The {001}β 〈110〉 β recrystallization texture was obtained in the 1173 K solution-treated alloy specimen owing to grain growth after recrystallization. The {001}β 〈110〉 β recrystallization texture was not observed in the hot-rolled specimen; instead, a (221)β[¯1¯14]β recrystallization texture was observed. The 1173 K solution-treated alloy specimen exhibited superelasticity with complete recovery resulting from the {001}β 〈110〉 β recrystallization texture. Superelasticity was also observed for the hot-rolled specimen, but was not complete, probably because of the low critical stress for slip.

Improvement in the Mechanical Properties of High Temperature Shape Memory Alloy (Ti50Ni25Pd25) by Copper Addition

High temperature shape memory alloys Ti50Ni25Pd25and Ti50Ni20Pd25Cu5were developed, characterized, and tensile tested in both martensite ( Mf− 50°C) and austenite ( Af+ 50°C) phases. The transformation temperatures of ternary Ti50Ni25Pd25alloy were increased by 11 to 12.5°C by substitution of Ni with 5 at% Cu. At the same time, transformation heat absorbed and released during forward and reverse martensitic transformation was also increased. In the martensite phase, the mechanical properties, that is, the stress for reorientation of martensite variants and fracture stress, were increased by 33 and 60 MPa, respectively, whereas the fracture strain was decreased by 1.5%. In the austenite phase, the critical stress for slip and fracture stress were increased by 62 and 40.9 MPa, respectively, whereas the fracture strain was decreased by 1.2%. The increase in both stresses was attributed to the solid solution strengthening by substitution of Ni atoms with relatively greater atomic radius of copper (Cu) atoms. The overall results suggest that the addition of 5 at% Cu in place of Ni in Ti50Ni25Pd25alloy is very beneficial to improving the mechanical and shape memory properties and increasing the transformation temperatures.

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti55.5Ni45.5−xCux (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti55.4Ni32.8Cu11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti2Ni, Ti2Cu and TiNi, Ti2Cu grains grew faster than Ti2Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.

Effects of Ti content on microstructure and shape memory behavior of TixNi(84.5−x)Cu15.5 (x = 44.6–55.4) thin films

Ti–Ni–Cu films with Cu and Ti contents of 15.5 and 44.6–55.4at.%, respectively, were prepared by sputtering and their structure and shape memory behavior after annealing for 1h at 773, 873 and 973K were investigated. The Cu content of the B2 phase was constant for the Ti-rich films, but decreased with decreasing Ti content for the (Ni,Cu)-rich films. The (Ni,Cu)-rich films annealed at 773K were supersaturated with Ni. The grain size of the Ti-rich films was smaller than that of the (Ni,Cu)-rich films. These differences corresponded to the difference in the shape memory behavior. The martensitic transformation start temperature was almost constant for the Ti-rich films annealed at 873 and 973K, but decreased with decreasing Ti content for the (Ni,Cu)-rich films. The critical stress for slip was larger for the Ti-rich films than for the (Ni,Cu)-rich films. The maximum recoverable strain was smaller for the Ti-rich films than for the (Ni,Cu)-rich films. The relationship between the structure and shape memory behavior is discussed.

Effect of room-temperature aging on shape memory characteristics of Ti–10Nb–10Zr–11Ta (at.%) alloy

In this study, the effect of room-temperature (RT) aging on the shape memory characteristics of the Ti–10Nb–10Zr–11Ta (at.%) alloy was investigated by tensile tests. Ingots were prepared using the arc melting method and then cold-rolled at a reduction of up to 95%. After cold-rolling, the plates were solution-treated at 1173K for 1.8ks, followed by aging at RT and temperatures up to ∼573K for various periods of times. Superior superelasticity was observed at RT in the solution-treated specimen. The critical stress for inducing martensitic transformation (σSIM), tensile strength, and critical stress for slip (σS) of specimens aged at RT increased with increasing aging time up to 60 days, showing no noticeable changes with further increases in the aging time. On the other hand, in the specimens aged at 373K, 423K, and 473K for 3.6ks, the values of σSIM and the tensile strength increased with increasing aging temperature, while the specimen aged at 573K exhibited mature fractures. There were little change in σSIM and σS of the specimen that was solution-treated followed by aging at 373K for 3.6ks during RT aging. This result indicated that aging at 373K resulted in good resistance against the effect of RT aging.

Effect of warm-working rate on shape memory properties in Ti–49.7at.%Ni alloy

Abstract This study investigated the effect of warm-working rate on the shape memory properties of Ti–49.7 at.%Ni alloy. The ingot was prepared by using an arc melting method and then warm-rolled at 673 K to reduce the thickness by 77%, 85%, and 89%. For comparison, an ingot was cold-rolled at room temperature to reduce the thickness up to 30%. The warm-worked specimens were annealed at 623–723 K for 3.6 ks, and the cold-worked specimens were annealed at 673 K for 3.6 ks. The transformation behavior was analyzed by means of differential scanning calorimeter (DSC) and tensile tests. The critical stress for slip increases with increasing warm-working rate and decreasing annealing temperature. The martensitic transformation temperature ( T R→M ) and the reverse transformation temperature ( T M→B2 ) increase with decreasing warm-working rate and increasing annealing temperature. The critical stress for slip in the warm-worked specimens is higher than that of the specimens annealed at 673 K after cold-working. Therefore, warm-working at 673 K improves the shape memory characteristics of Ti-rich Ti–Ni alloys. This suggests that Ti-rich Ti–Ni alloys are good candidate materials for actuators.

Effect of Oxygen Content on Shape Memory Characteristics of Ti-18Nb-6Zr-XO (X = 0~1.5at%) Alloys

Abstract The effect of oxygen on the shape memory characteristics in Ti-18Nb-6Zr-XO (X = 0-1.5 at%) biomedical alloyswas investigated by tensile tests. The alloys were fabricated by an arc melting method at Ar atmosphere. The ingots were cold-rolled to 0.45 mm with a reduction up to 95% in thickness. After severe cold-rolling, the plate was solution-treated at 1173K for 1.8 ks. The fracture stress of the solution-treated specimens increased from 450 Mpa to 880 MPa with an increasingoxygen content up to 1.5%. The fracture stress increased by 287MPa with 1 at% increase of oxygen content. The critical stressfor slip increased from 430 MPa to 695 MPa with an increasing oxygen content up to 1.5 at%. The maximum recovery strainof 4.1% was obtained in the Ti-18Nb-6Zr-0.5O (at%) alloy. The martensitic transformation temperature decreased by 140 Kwith a 1.0 at% increase in O content, which is lower than that of Ti-22Nb-(0-2.0)O (at%) by 20 K. This may have been causedby the effect of the addition of Zr. This study confirmed that addition of oxygen to the Ti-Nb-Zr alloy increases the criticalstress for slip due to solid solution hardening without being detrimental to the maximum recovery strain.Key wordsshape memory effect, superelasticity, biomaterials, martenite, Ti based alloy.

Effect of Temperature on Critical Stress for Slip of Ti-Ni Shape Memory Alloy

This study describes the effect of temperature on critical stress for slip of Ti-Ni shape memory alloy. Tensile loading-unloading tests has been carried out using Ti-50.3at%Ni in the temperature range from 193K to 423K in order to investigate the temperature dependence of critical stress for slip of Ti-Ni shape memory alloy. The critical stress for slip is different in the three regions which are the martensitic single phase state in the range of 193K-273K, the mixture state of the martensitic and parent phases in the range of 273K-373K and the parent single phase state in the range of 373K-423K. The critical stress for slip of the martensitic single phase is about 200MPa. In the case of the mixture state of the martensitic and parent phases, the critical stress for slip increases with increasing temperature, and reaches the maximum stress of 652MPa at 373K. The critical stress for slip of the parent phase decreases with increasing temperature.

Effect of Carbon Addition of Shape Memory Properties of TiNb Alloys

In order to increase critical stress for slip in Ti-Nb base shape memory alloys, strengthening by carbon additions (0.2 and 0.5mass%C) to Ti-27mol%Nb was investigated. It was found that all the alloys were  (bcc) phase at room temperature, and TiC existed in C-added alloys. The grain size was decreased with carbon content due to grain boundary pinning. Texture measurement revealed that strong {112}<110> recrystallization texture was formed in C-free alloy and that weak {001}<110> texture in C-added alloys. Tensile tests revealed that clear superelasticity appeared in C-free alloy but that stress-induced martensitic transformation seems to be suppressed by TiC in C-added alloys. The critical stress for slip was linearly increased by carbon content. Then, carbon addition affects the shape memory properties of TiNb alloys, and is effective to enhance the critical stress for slip.

904 Ti-Ni形状記憶合金のすべり臨界応力に及ぼす温度の影響(OS9-1 形状記憶材料の特性と応用)

904 Ti-Ni形状記憶合金のすべり臨界応力に及ぼす温度の影響(OS9-1 形状記憶材料の特性と応用)

EFFECT OF ANNEALING ON SHAPE MEMORY CHARACTERISTICS OF Ti-50.85at.%Ni ALLOY

In order to clarify the effect of annealing on the shape memory behavior of Ti -50.85at.% Ni alloy, the deformation and transformation behavior were investigated using tensile tests and differential scanning calorimeter (DSC). The martensitic transformation temperature increases with increasing annealing temperature until it reach as a maximum, and then decreases with further increasing annealing temperature. This can be rationalized by interaction between the distribution of Ti 3 Ni 4 precipitates and recovery of cold-worked structure. The R-phase transformation temperature increases with increasing annealing temperature until reaching a maximum, and then decreases with a further increase of annealing temperature. This is attributed to the change of Ni content in the matrix caused by precipitation of Ti 3 Ni 4. The critical stress for slip decreases rapidly with increasing annealing temperature, influenced by interaction between the distribution of Ti 3 Ni 4 precipitates and recovery of cold-worked structure.

Effect of Boron Concentration on Martensitic Transformation Temperatures, Stress for Inducing Martensite and Slip Stress of Ti-24 mol%Nb-3 mol%Al Superelastic Alloy

Effects of boron (B) addition on martensitic transformation temperature, stress for inducing martensite and slip stress of a Ti-24 mol%Nb-3mol%Al (TiNbAl) superelastic alloy were investigated in a composition range from 0 to 0.10mass%B. It was found that a second phase is formed by the B addition being higher than 0.05 mass%B. This second phase was estimated to be TiB. The averaged grain size of TiNbAl was decreased by the B addition being higher than 0.05 mass%B. This decrease must be explained by the suppression of grain growth by the second phase. The second phase plays a role of pining sites of grain boundary movement during the solution treatment. The martensitic transformation temperatures (M s ) measured by differential scanning calorimetry were decreased by the B addition. Superelastic behavior was evaluated by a cyclic loading-unloading tensile test at room temperature with a constant strain increment of 1%, and it was found that superelasticity appeared regardless of the amount of B addition. It was also found that stress for inducing martensitic transformation (σ SIM ) increased with increasing B concentration up to 0.05 mass%B. This increase of σ SIΜ by the B addition can be explained by the lowering of M s by the B addition. The critical stress for slip (σ SLIP ) increased with increasing the B concentration being up to 0.05 mass%B. The increase of σ SLIP by B addition was evaluated to be 3GPa/mass%B, and it was significantly higher than that of σ SIM (500MPa/mass%B) when the B concentration was less than 0.05 mass%B. These results indicate that B addition is effective to reduce the permanent unrecoverable strain introduced during deformation.

Effect of boron addition on transformation behavior and tensile properties of Ti–Nb–Al alloy

Effects of 0.05 mass% boron addition on the transformation behavior and the tensile properties at room temperature were investigated for Ti–24 at.% Nb–3 at.% Al (TiNbAl). The martensitic transformation temperatures ( M s) measured by differential scanning calorimetry decreased upon the addition of boron. The tensile properties were evaluated by a cyclic loading–unloading tensile test with a constant strain increment of 1% carried out at room temperature. It was found that superelasticity appeared regardless of boron addition. By considering the residual strain and the maximum applied stress per cyclic deformation, relationships between maximum stress and residual strain were evaluated. It was found that flow-stress of the B-added TiNbAl was higher than that of TiNbAl, and that the critical stress for slip ( σ SLIP) became higher upon boron addition: boron addition is effective for strengthening. The stress for inducing the martensitic transformation ( σ SIMT) became higher by the boron addition, mainly due to the lowering of martensitic transformation temperature. When increasing the number of deformation cycles, σ SIMT becomes lower regardless of boron addition. However, the reduction of σ SIMT per cycle is smaller in B-added TiNbAl than TiNbAl, due to strengthening by boron addition.

Ti-content and annealing temperature dependence of deformation characteristics of Ti XNi (92− X) Cu 8 shape memory alloys

The Ti-content and annealing temperature dependence of the deformation characteristics of Ti X --Ni (92− X) Cu 8 (at.%) ( X=49.0–51.0) alloys was investigated by constant load thermal cycling tests by varying the annealing temperature between 673 and 1273 K. It was found that the stress-induced martensite transformation strain of a 50.0 at.% Ti alloy is larger than those of Ti-poor and Ti-rich alloys due to the precipitates of Ti(Ni 1− X Cu X ) 2 and Ti 2(Ni 1− Y Cu Y ) existing in the Ti-poor and Ti-rich alloys, respectively, acting as obstacles to the growth of preferential martensite variants. By annealing at 1273 K to cause the precipitates of Ti(Ni 1− X Cu X ) 2 being dissolved into the matrix of the Ti-poor alloys, the stress-induced martensite transformation strain exhibits little dependence on Ti-content in the composition range of 49.0–50.0 at.% Ti. The plastic strain, critical stress for slip and recovery strain also show a unique Ti-content and annealing temperature dependence.

Strengthening of Ti-Ni shape-memory films by coherent subnanometric plate precipitates

It has been found that subnanometric thin plate precipitates are formed in sputter-deposited Ti-rich Ti-Ni shape-memory films. This has been achieved by heating the as-sputtered amorphous sample at relatively low temperatures. The precipitates are produced along the {100} planes of the bcc (B2) parent phase and are completely coherent with the matrix. They have a disc-like shape with a thickness of only two to three lattice planes and a diameter of 5-10 nm. The distance between neighbouring precipitates is dependent on the heat treatment temperature, varying in the range 5-30 nm. Owing to the existence of these precipitates in the parent phase, the critical stress for slip in the parent phase is greatly increased and, as a result, excellent shape-memory properties, such as 6% recoverable shape strain and 670 MPa recovery strength, are obtained for Ti-Ni shape-memory thin films. It is concluded that the precipitates have a Ti-rich composition and are produced only in the off-stoichiometric amorphous Ti-Ni ...

Characteristics of Ti50Pd30Ni20 high-temperature shape memory alloy

The decrease in critical stress for slip with increasing temperature leads to the introduction of permanent strain and thus deteriorates shape memory (SM) characteristics of high-temperature SM alloys. Thermomechanical treatment consisting of cold-rolling and subsequent annealing at various temperatures, was applied for the first time to improve the high-temperature SM characteristics of the Ti50Pd30Ni20 alloy. Then, a systematic study of transformation temperatures, microstructure and associated mechanical behaviour at 443–548K was carried out. The remarkable improvement in SM characteristics was found for the alloy cold-rolled to 24–25% reduction in thickness and annealed at 673K for 3.6 ks. In addition, superelasticity was observed for the first time in this alloy. Finally, the key factor for improvement of high-temperature SM characteristics in the alloy is pointed out.

Wall slip of molten high density polyethylenes. II. Capillary rheometer studies

Above the critical stress for slip, the procedures normally used to analyze the results of capillary flow data give anomalous results. In particular, the Bagley plots are curved, even when a variation of viscosity with pressure is not anticipated, and the Mooney technique used to calculate the slip velocity gives results that indicate that the slip velocity depends on the L/D ratio. It is proposed that these phenomena arise from the dependence of the slip velocity on the wall normal stress, which implies a dependence on pressure. Based on this hypothesis, an approximate method is developed for interpreting the results of capillary flow experiments to determine the slip velocity as a function of both the wall shear stress and the pressure. The large available data set was used to incorporate into the model the effects of molecular weight parameters and temperature on the slip velocity. Finally, a detailed model for slip flow in a capillary was formulated that takes into account that the slip velocity and wall shear stress vary along the flow direction due to the pressure gradient. This model was used to evaluate the validity of the approximations used in the approximate data analysis technique for determining the slip velocity.