Cyclic deformation mechanisms in precipitated NiTi shape memory alloys

Results are presented on the cyclic deformation of single crystal NiTi containing Ti₃Ni₄ precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti₃Ni₄ precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti₃Ni₄ precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti₃Ni₄ precipitates (500 nm) consistently showed significant dislocation activity due to mechanical cycling in addition to stabilized martensite colonies. The first cycle stress–strain hysteresis was found to correlate to the fatigue resistance of the material. Samples demonstrating large inherent hysteresis, with different heat treatments and orientations, showed poor fatigue performance. Rational for the observed behaviors is discussed in terms of operant deformation mechanisms and ramifications on modeling the cyclic deformation of NiTi are presented.     

Martensite stabilization and thermal cycling stability of two-phase NiMnGa-based high-temperature shape memory alloys

The martensite stabilization and thermal cycling stability of four types of two-phase NiMnGa-based high-temperature shape memory alloy, including Ni_56+xMn₂₅Ga_19?x (x = 0, 1, 2, 3, 4), Ni₅₆Mn_25?yFe_yGa₁₉ (y = 4, 8, 9, 12, 16), Ni₅₆Mn_25?zCo_zGa₁₉ (z = 4, 6, 8) and Ni₅₆Mn_25?wCu_wGa₁₉ (w = 2, 4, 8) alloys, were investigated. It is found that the martensite stabilization is closely related to the strength of the alloy and the volume fraction of γ phase; and increases as the alloy strength decreases. It is also found that in Ni₅₆Mn_25?yFe_yGa₁₉ alloys, with increasing Fe content to 12 and 16 at.%, the volume fraction of γ phase increases and the martensite stabilization decreases. The thermal cycling stability differs among different alloy systems and is related to the microstructural changes during thermal cycling and to the strength of the γ phase. Poor thermal cycling stability is observed in Ni_56+xMn₂₅Ga_19?x (x > 0), Ni₅₆Mn_25?zCo_zGa₁₉ and Ni₅₆Mn_25?wCu_wGa₁₉ alloys due to the formation of the ordered γ′ phase and the high strength of the γ phase. Results further show that Fe addition to Ni₅₆Mn₂₅Ga₁₉ alloy can broaden the (bcc + γ) two-phase region and shift it to the Ni–Ga and Ni–Mn sides, hence stabilizing the two-phase region to lower temperatures. These effects can retard the formation of the ordered γ′ phase in the Ni₅₆Mn_25?yFe_yGa₁₉ system during thermal cycling, thus leading to good thermal cycling stability.     

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect(TWSME) of polycrystalline Ni_(56)Mn_(25-x)Co_xGa_(18.9)Gd_(0.1) alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in Ni_(56)Mn_(21)Co_4Ga_(18.9)Gd_(0.1) alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.     

Temperature memory effect in Cu–Al–Ni shape memory alloys studied by adiabatic calorimetry

The temperature memory effect exhibited by Cu–Al–Ni shape memory alloys was studied by means of adiabatic calorimetry and microscopic observations. The harmonic, anharmonic and electronic contributions to the lattice specific heat were estimated by using the experimental data of the metallic components. The obtained results provide an accurate baseline for the quantitative study of the martensitic phase transformations as a function of the thermal history in these alloys. The specific heat of a Cu–Al–Ni sample was measured from 140 to 350 K throughout the phase transition region, and the temperature memory effect was carefully studied. These results are in good agreement with the optical observations as a function of temperature. The global behaviour of the martensitic transformation as regards the temperature memory effect is discussed and interpreted in terms of the microscopic mechanisms of nucleation and motion of the martensite plates.     

The high-temperature deformation and tensile ductility of Al alloys

Tensile ductilities consistently in excess of 100 percent are produced at elevated temperatures in aluminum solid-solution alloys containing magnesium. The alloys that produce such enhanced ductility include commercially available 5XXX-series alloys and course-to fine-grained Al-Mg alloys. Eric M. Taleff earned his Ph.D. in mechanical engineering at Stanford University in 1995. He is currently an assistant professor of mechanical engineering at the University of Texas at Austin. Dr. Taleff is a member of TMS. Peter J. Nevland earned his B.S. in mechanical engineering at the University of Texas at Austin in 1997. He is currently an M.S. candidate in materials science and engineering at the University of Texas at Austin. Mr. Nevland is a student member of TMS.     

铜基形状记忆合金马氏体相变宏观形状应变特征

利用X射线衍射仪、透射电镜、扫描电镜、原子力显微镜等研究了Cu-18Al-9Mn-3.4Zn(摩尔分数,%)形状记忆合金的马氏体晶体结构、亚结构以及马氏体相变宏观形状应变特征.结果表明:该合金的马氏体晶体结构为18R结构,亚结构为层错;单变体马氏体表面浮凸呈"( ) "型,浮凸高度为400～500nm,浮凸宽度为2 000～2 400 nm;多变体马氏体表面浮凸呈"N"型和"山尖"型,浮凸高度为200～400 nm,浮凸宽度为1 000～1 800nm;浮凸角均为8°～12°;马氏体相变符合G-T模型的双切变特征,惯习面为(113)面.     

Ni_(54)Mn_(25)Ga_(15)Al_6高温形状记忆合金的微观组织和相变行为(英文)

研究Ni54Mn25Ga15Al6高温形状记忆合金的微观组织、马氏体相变特性、力学性能和形状记忆效应。通过与Ni54Mn25Ga21合金对比,分析添加第四组元Al对Ni-Mn-Ga合金性能的影响。结果表明:Ni54Mn25Ga15Al6合金为单一的四方结构非调制马氏体相并呈片状的马氏体孪晶板条形貌。该合金的马氏体相变开始温度超过190°C,具有发展成为高温形状记忆合金的潜力。在Ni-Mn-Ga合金中添加Al会降低马氏体相变温度,这主要归因于Al添加引入的晶格尺寸因素的改变。添加Al元素能有效提高合金的强度和塑性,但降低合金的形状记忆性能。     

Martensitic transformation and mechanical properties of NiMnGaV high-temperature shape memory alloys

The effect of V substitution on microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni₅₆Mn₂₅Ga_19-xV_x (x = 0, 1, 2, 4, 6 at.%) alloys was investigated. Single phase of non-modulated martensite with tetragonal structure is observed for x = 0 and x = 1, and dual phases with tetragonal martensite and face-centered cubic γ phase are present for x ≥ 2. The volume fraction of the γ phase increase with the increase of V content up to 41 vol%. The martensitic transformation temperatures decrease with V content increasing from 1 to 6 at.%, which is mainly attributed to the reduction of electron concentration of martensite. The compressive fracture strength and strain increase from 346 MPa and 10.0% for x = 0 to 1429 MPa and 31.0% for x = 6. Therefore, γ phase can markedly enhance the mechanical properties. As γ phase particles on martensite are barriers to its shape recovery, the shape memory strains decrease a little with increasing V content when x ≤ 2, and then drop remarkably from x = 2 to x = 6.     

Investigation of microcrystalline NiAl-based alloys with high-temperature thermoelastic martensitic transformation: II. Construction of isothermal diagrams of decomposition of a supersaturated β solid solution of Ni65Al35 and Ni56Al34Co10 alloys

Diagrams of the onset of decomposition of two functional microcrystalline alloys (Ni₆₅Al₃₅ and Ni₅₆Al₃₄Co₁₀) with a thermoelastic reversible martensitic transformation prepared by ultrarapid quenching from the melt have been constructed based on the results of isothermal measurements of electrical resistance during various annealings. A multi-stage nature of the diffusive decomposition of a β solid solution supersaturated with Ni has been revealed, and the temperature range of its maximum thermal stability has been found. The retardation effect of cobalt on the decomposition of high-nickel martensitic Ni-Al alloys has been determined.     

Martensitic and magnetic transformation of Co41Ni32Al24Sb3 and Co41Ni32Al27 alloys

The martensitic transformation and magnetic property of Co41Ni32Al27 and Co41Ni32Al24Sb3 alloys were investigated by optical microscopy（OM）, scanning electric microscopy（SEM）, energy dispersive X-ray spectroscopy（EDS）, X-ray diffractometry （XRD）, differential scanning calorimeter analysis（DSC） and vibration sample magnetometer（VSM） methods. The results show that martensitic crystal structure of Co41Ni32Al24Sb3 alloy is still Llo type. Both martensitic transformation temperature Tm and Curie point Tc are in linear relation to quenching temperature. Tm increases by 9 K and Tc increases by 7.5 K for every 10 K increasing in quenched temperature. Quenched from same temperature, Tm of Co41Ni32Al24Sb3 alloy is higher than that of Co41Ni32Al27 alloy by 76 K, meanwhile Tc is higher by 18 K. The melting point of Co-Ni-Al alloy is decreased by the Sb addition, eutectic structure appears in Co41Ni32Al24Sb3 alloy annealed at l 573 K, which indicates that the alloy is partially melted, whereas Co41Ni32Al27 alloy can be annealed at 1 623 K without melted. The martensitic transformation temperature range of Co41Ni32Al24Sb3 alloy is 22-29 K, only half that of Co41Ni32Al27 alloy. This is a very important result to benefit the achievement of large magnetic field induced strain on Co-Ni-Al based alloy. The results of Tm and Tc were explained by total average s＋d electron concentration and magnetic valence number Zm respectively.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.     

Ti-50.1Ni形状记忆合金相变与形变行为

用热重分析仪、X射线衍射仪和拉伸试验研究了退火温度、变形温度对Ti-50.1Ni形状记忆(SME)合金丝的相变、形变的影响.Ti-50.1Ni合金加热氧化过程中温度超过600℃后氧化加剧,故退火温度不宜超过600℃.该合金奥氏体相变开始温度(As)高于室温,室温相为马氏体,呈SME特性.350～600℃退火态Ti-50.1Ni合金在室温下均呈SME.     

Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti₅₀Ni₃₀Pt₂₀ (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.     

Microstructure and phase equilibria in Ni–Al–Cr–Co alloys

Several alloys of Ni–Al–Cr–Co system with Ni content close to 60 at.% were prepared and annealed at 1273 and 1373 K with the aim to reach the state of thermodynamic equilibrium. The microstructure of quenched samples was studied by means of scanning and transmission electron microscopy. Phases equilibrated at high temperature were identified using selected area diffraction in transmission electron microscope. Energy dispersive X-ray analysis was used for evaluation of nominal alloy compositions as well as the chemical composition of individual phases. The results obtained experimentally are compared with the results of thermodynamic calculation using software ThermoCalc and a commercial database of thermodynamic data for Ni-based alloys. In most cases the results of calculation show good agreement with experiment. A few differences of the results obtained by the two approaches are pointed out and briefly discussed.