Effect of electroplastic rolling on the ductility and superelasticity of TiNi shape memory alloy

The brittle TiNi shape memory alloy strips were processed by electroplastic rolling (EPR) without intermediate annealing. The maximum thickness reduction of the alloy strip in the individual EPR pass can be 21.6% and the total deformation in terms of thickness reduction can reach 74% in the seven passes of EPR processing. The ductility of the TiNi alloy is enhanced by electropulse due to the dynamic recrystallization in the process. The rolling separating force in EPR drops compared to the cold rolling. Furthermore, the EPR processed TiNi after annealing exhibits an excellent superelasticity and the recoverable strain of 6% is obtained.     

承受各种循环加载的TiNi形状记忆合金的超弹性变形行为

TiNi形状记忆合金由于其优良的机械性能、抗腐蚀能力和生物适应性得到广泛的使用。超弹性是TiNi形状记忆合金重要的力学性能之一。本文通过实验研究了不同加载速率和不同实验温度下承受完全循环加载以及部分加载卸载的TiNi形状记忆合金超弹性变形行为。分析了循环变形期间马氏体相变应力和弹性模量变化的特性。研究表明在完全循环加载过程中，由于残余应变的存在，马氏体相变应力随循环增加而减小。马氏体相变应力的变化量（即残余应力）与残余应变成线形关系。对于受过循环变形的机械训练的TiNi形状记忆合金，研究了部分加载和卸载情况下其超弹性变形，分析了相变开始与结束的应力特性。     

形状记忆合金的力学及界面参数和体积分数对大块金属玻璃基复合材料增韧的影响

采用考虑塑性的超弹性材料模型和基于损伤塑性的准脆性材料模型,建立了三维单胞有限元模型,模拟了形状记忆合金颗粒增韧大块金属玻璃基复合材料的单调拉伸行为。讨论了形状记忆合金的力学参数、体积分数、界面厚度和界面材料参数对金属玻璃增韧效果的影响。结果表明:提高形状记忆合金的相变应变和马氏体塑性屈服应力将显著提高形状记忆合金颗粒增韧大块金属玻璃基复合材料的拉伸失效应变;形状记忆合金弹性模量超过50.0GPa、马氏体塑性屈服应力超过1.8GPa后,复合材料的拉伸失效应变变化不大。能同时兼顾失效应变和失效应力的形状记忆合金体积分数为15%左右。复合材料界面弹性模量和界面屈服应力的增加将提高复合材料的失效应力,但对失效应变影响不大;复合材料界面厚度的增加在提高失效应变的同时,也降低了复合材料的失效应力。     

Mechanical properties and damping capacity of SiCp/TiNif/Al composite with different volume fraction of SiC particle

SiC_p/TiNi_f/Al composite with 20 Vol.% TiNi fibers were fabricated by pressure infiltration method. The effect of volume fraction of SiC particle on the mechanical properties and damping capacity of the composite were studied. Four different volume fractions of SiC particle in the composite were 0%, 5%, 20% and 35% respectively. The microstructure and damping capacity of the composites was studied by SEM and DMA respectively. As the gliding of dislocation in the Al matrix was hindered by SiC particle, the yield strength and elastic modulus of the composites increased, while the elongation decreased with the increase in volume fraction of SiC particle. Furthermore, the damping capacity of the composites at room temperature was decreased when the mount of strain was more than 1 × 10⁻⁴. In the heating process, the damping peak at the temperature of 135 °C was attributed to the reverse martensitic transformation from B19′ to B2 in the TiNi fibers.     

电轧AZ31镁合金在模拟海水中腐蚀行为研究

为对比研究高能电脉冲轧制工艺和冷轧工艺对AZ31镁合金腐蚀性能的影响,采用腐蚀形貌观察、动电位极化测试、电化学阻抗谱与腐蚀速度测试等方法系统地研究了高能电脉冲轧制和冷轧AZ31镁合金带材在模拟海水(3.5%NaCl)中的腐蚀行为.结果表明:在同样变形量下,与冷轧AZ31镁合金相比,电轧AZ31镁合金的耐腐蚀性略有提高.这与电轧AZ31镁合金再结晶比例大,位错密度小,具有较低能态的位错组态及能形成较稳定的腐蚀产物膜有关.     

Experimental investigation on formability and microstructure of AZ31B alloy in electropulse-assisted incremental forming

Based on a CNC machine tool and a high-energy electropulse generator, a self-built experiment platform was employed to conduct electropulse-assisted incremental forming (EAIF) of AZ31B alloy. The experimental results show that the electroplastic effect increases with the increase of the root mean square (RMS) current density of electropulse, and the forming limit angle in EAIF have been up to 72° from the previous 39.6° without electropulse. The tests prove that the peak current density plays a leading role in determining the materials formability when the RMS current density remains to be approximate. In addition, the microstructure transformations of the formed specimens were studied by means of optical microscopy and scanning electron microscopy. It is found that the electropulse can reduce the AZ31B dynamic recrystallization (DRX) temperature and accelerate the DRX progress, and it can also restrain the crack growth of the tested materials which in turn improves its formability.     

镍-钛合金弹性模量的测定

采用静态拉伸法和纳米压入法对具有伪弹性行为的镍-钛合金的弹性模量进行了研究.结果表明,镍-钛合金的弹性模量随其相组成的变化而变化,母相状态时的弹性模量为24.5 GPa,而马氏体相状态时的弹性模量为11.6 GPa.多次重复拉伸使其弹性模量降低并呈现非线性的变化趋势.纳米压入法测量镍-钛合金的弹性模量同拉伸法测量结果有较大的偏差,其原因同纳米压入法未考虑伪弹性变形行为有关.     

Two-stage recovery strain of prestrained TiNi shape memory alloy after phase transformations under constraint

A two-stage strain recovery was observed in a TiNi alloy fiber, which had been prestrained and experienced a constraint heating and cooling cycle with a fixed strain constraint. The results of differential scanning calorimeter (DSC) measurements showed that after the thermomechanical process, the self-accommodation martensite in the TiNi alloy was divided into two sorts of martensite, the redeformed martensite and the inheriting deformation martensite. The first stage of the recovery strain vs. temperature curve corresponds to the transformation from the inheriting deformation martensite to parent, and the second one corresponds to the transformation from the redeformed martensite to parent.     

Lowering of elastic modulus in the near-beta Ti–13Nb–13Zr alloy through heat treatment

ABSTRACT

The bio-implant devices require adequate strength and low elastic modulus, for compatibility with human bone. In this study, the near-beta Ti–13Nb–13Zr alloy was subjected to two different solutionising temperatures and quenched at different temperatures. Microstructure modifications and its influence on microhardness, elastic modulus and tensile properties were investigated. Elastic modulus was decreased with increase in cooling rate from solutioning temperature. The samples solutionised at 900°C and quenched at sub-zero temperature contained α^″ martensite along with α′ and β phases and these lowered the elastic modulus. Among all the heat-treated samples, the one solutionised at 900°C and quenched at sub-zero exhibited lowest elastic modulus of 59?GPa and adequate tensile properties for the application as bioimplants.     

The strain rate and temperature dependence of microstructural evolution of Ti–15Mo–5Zr–3Al alloy

A compressive split-Hopkinson pressure bar apparatus and transmission electron microscopy (TEM) are used to investigate the deformation behaviour and microstructural evolution of Ti–15Mo–5Zr–3Al alloy deformed at strain rates ranging from 8 × 10² s⁻¹ to 8 × 10³ s⁻¹ and temperatures between 25 °C and 900 °C. In general, it is observed that the flow stress increases with increasing strain rate, but decreases with increasing temperature. The microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result, primarily, of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate, but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. The amount of α phase increases with increasing temperature below the β transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain under the current loading conditions.     

Microstructure of TiNi shape-memory alloy synthesized by explosive shock-wave compression of Ti–Ni powder mixture

Cylinders of TiNi shape-memory alloy were synthesized from mixtures of equiatomic fine irregular titanium and nickel powders by explosive-wave compression with a detonation velocity of about 6500 m s^-1. B2 type parent phase, R phase, B19′ type martensite, Ti₂Ni, Ti₃Ni₄ and Ti₂Ni₃ phases were observed in this as-synthesized material. In the B2 matrix high density dislocations existed. The Burgers vectors of many dislocations were determined to be parallel to directions. The R phase variants formed (0 0 1) B2 twinning structure. The substructure of the B19′ martensite was (0 0 1) B19′ type I twin and stacking faults on the (0 0 1) B19′ plane. When increasing the temperature of the as-synthesized material in a differential scanning calorimeter, no B19 ′ → R → B2 transitions were observed on the temperature range −50 to 100 °C. However, B2 → B19′(R) transitions occurred during the cooling cycle. After heat treating the specimen at 800 °C for 1 h and then ageing at 400 °C for 10 min, both B2 → R → B19′ and B19′(R) → B2 phase transitions were observed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

TiNi合金在恒应力约束下的不完全相变

利用DSC对预应变TiNi形状记忆合金丝在恒应力约束下的马氏体不完全逆相变进行了研究,发现不完全相变热循环样品在第二次自由态加热过程中出现两步马氏体逆转变和两段应变回复现象.分析认为:经过恒应力约束下的不完全逆相变后,TiNi样品中存在不同的马氏体,在随后的加热过程中先后逆转变,产生两段回复应变.     

Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium–scandium (Al–Sc) alloy

Abstract

The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium–scandium (Al–Sc) alloy at temperatures ranging from ? 100 to 300°C and strain rates of 1·2 × 10³, 3·2×10³ and 5·8 × 10³ s^?1. The relationship between the dynamic mechanical behaviour of the Al–Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress–strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli–Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al–Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al–Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 10³ s^?1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al₃Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size.     

TiNi形状记忆合金在定应变约束下的马氏体相变

通过DSC及回复应变的测量,研究了定应变约束态加热-冷却过程对预应变TiNi形状记忆合金相变行为的影响。结果表明,在约束态相变中,逆相变温度区间拓宽;取向马氏体除向母相转变外,应变还要进一步增大;在正相变过程中,从母相生成的马氏体也具有变形结构。约束态不完全相变后,样品中存在两种马氏体;再变形马氏体和继承变形马氏体,在随后的无约束逆相变过程中,前者的相变温度高于后者,并且输出两段回复应变。     

Effect of compressive stress aging on transformation strain and microstructure of Ni-rich TiNi alloy

The Ti–50.7%Ni (atom fraction) alloy rods were compressive stress aged at 400 °C, 450 °C and 500 °C for different time, their strain behaviors accompanied by temperature elevation were investigated, and their microstructures were observed. It is found that the compressive stress aged TiNi alloy rod displays an obvious contractive strain behavior in the stress direction as the temperature is elevated from approximately 55–75 °C. Compressive stress causes the parallel alignment of the aging precipitate Ti₃Ni₄ in the TiNi alloy, which controls the martensitic transformation (B19′ transformation) and its reverse transformation, leading to its contractive strain behavior accompanied by temperature elevation. The contractive strain of the TiNi alloy compressive stress aged at 400 °C for 100 h is increased with increasing compressive stress up to 140 MPa. Higher aging temperature and longer aging time lead to the coarsening of the precipitates and the enlarging of the inter-precipitate spacing, and therefore result in a decrease in the contractive strain.     

Effects of aging on martensitic transformation of Ti50Ni25Cu25 shape memory alloy

Martensitic transformation has been studied in Ti₅₀Ni₂₅Cu₂₅ shape memory alloy by internal friction (IF) measurement and X-ray diffraction. It shows that the martensitic transformation proceeds from B2 to B19 for the solution-treated Ti₅₀Ni₂₅Cu₂₅ alloy. B2 phase is stabilized, and aging the alloy at 723–923 K decreases internal friction values. Part of the remaining B2 parent phase transform to B19′ monoclinic martensite at much lower temperatures.     

Structure and properties of ductile CuAlMn shape memory alloy synthesized by mechanical alloying and powder metallurgy

A ductile Cu–Al–Mn–Ti–B shape memory alloy with high fatigue strength has been prepared via mechanical alloying and powder metallurgy. With increasing milling time, the size of the crystallite grains decreases. Cu diffraction pattern appeared only after milling at a speed of 300 rpm for 25 h. The single phase CuAlMnTiB solid solution powder after 35 h milling was hot-pressed and extruded to form the final alloy. The quenched alloy had a single β phase at room temperature and its yield strength, maximum strength and strain were measured to be 390 MPa, 1015 MPa and 14.4%, respectively. The aged alloy showed a martensite structure at room temperature and had a shape memory recovery of 92% after 120 cycles.     

Electro-plastic effect on tensile deformation behaviour and microstructural mechanism of AZ31B alloy

The influences of electropulse on mechanical properties of AZ31B alloy were investigated by the electro-plastic (EP) tensile tests. The results show that the flow stress decreases with the increase of the root mean square (RMS) current density, while the elongation to fracture almost remains unchanged after it reaches a certain value. The higher peak current density can lead to a more potent EP effect when the RMS current density remains approximate. The results of microstructure analysis indicate that, the electropulse can reduce the dynamic recrystallisation temperature and promote the grain boundary sliding. The inverse eutectic reaction (α?+?β?=?L) will take place at the necking zone under high electropulse, which can optimise the deformation mechanism before the liquid phase is too much.     

Evaluation of changes in X-ray elastic constants and residual stress as a function of cold rolling of austenitic steels

Abstract

Austenitic steels rapidly attain high mechanical strength when subjected to cold working. The heterogeneous plastic deformation produced in cross section of the specimen, development of preferred orientation and martensitic transformation contribute to the occurrence of residual stress in cold worked steels. AISI 304 and 316 steels were cold rolled at room temperature from 10% up to 70% deformations in steps of 10%. The formation and sigmoidal growth of martensite caused by cold rolling (CR) 304 steel was studied by X-ray diffraction. The residual stresses generated were evaluated in both the austenite and martensite phases using sin² ψ technique. The accurate determination of residual stress by X-ray diffraction requires experimental determination of X-ray elastic constants for both the austenite and martensite phases. The changes in X-ray elastic constants as a function of CR of 304 and 316 steels were measured and their effect on residual stress values was established. The results show that tensile stress was generated initially on cold working in the austenite phase in both steels and in the dominant martensite phase in 304 steel, which decreases, passes through zero and becomes compressive at higher deformations. X-ray elastic constants were found to decrease in all cases and a maximum reduction of 15% was found.