Fatigue behavior of Cu-Al-Be shape memory single crystals

The fatigue behavior of Cu-Al-Be shape memory single crystals is studied in cyclic loading mechanical tests. Based on literature and on tensile tests performed at various temperatures, a model is proposed to explain the mechanism of fatigue. This model is based on the idea that during cyclings, the different zones of the samples spend various lengths of time in the martensitic state. During that time, martensite evolves because of the occurrence of some reordering or other diffusional phenomena inside, and consequently, the value of the martensite start temperature for each zone changes. The kinetics of the change in mechanical behavior along with the cycles, as a function of the test temperature, are accurately described by a Johnson–Mehl relation. Relaxation tests suggest that the mechanism described not only depends on time and temperature but is also re-enforced by the movement of the martensite-matrix interface. From the kinetics of this fatigue behavior, an empirical activation energy related to the mechanism is inferred for this Cu-Al-Be alloy.     

Applying infrared thermography to analyse martensitic microstructures in a Cu-Al-Be shape-memory alloy subjected to a cyclic loading

This study aims at revealing martensitic microstructures that exist in a shape-memory alloy. Infrared thermography was used for this purpose. Experiments were performed on a Cu-Al-Be single crystal which features a pseudoelastic response at ambient temperature. The specimen was first partially transformed to martensite by mechanical loading. Then a small cyclic loading was applied while the temperature evolution on the specimen surface was captured by an infrared camera. Thermal images obtained were then processed to extract two types of quantities: the maps of heat sources produced by the material and the maps of temperature oscillation amplitudes. Two thermomechanical couplings are revealed: the thermoelastic coupling and the latent heat due to the small cyclic movement of austenite-martensite interfaces, thus highlighting the martensitic microstructure distribution in the specimen.     

Martensitic transformation and shape memory properties of Ti-Ta-Sn high temperature shape memory alloys

The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti-Ta-Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti-Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti-Ta-Sn alloys. A Ti-20Ta-3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti-Ta binary alloy showing similar martensitic transformation temperature.     

Effect of mechanical cutting and polishing on the shape memory transformation behavior of NiTi alloy

The results of the studies carried out on the effect of mechanical cutting and polishing on the transformation behavior of Ti-rich NiTi shape memory alloy (SMA) heat-treated at 560 and 660 °C, are presented in this article. Among the samples heat-treated at 560 °C, the sample subjected to mechanical cutting and polishing (HT560MC) shows M → A on reverse phase transformation and A → R → M on forward transformation whereas chemically etched sample (HT560CE) shows the intermediate R-phase both during forward and reverse transformations. Heat treatment at a higher temperature of 660 °C does not facilitate the formation of R-phase in the reverse transformation and helps the formation of R-phase to a very small extent, that too only in HT660MC, during forward transformation due to the reduced dislocation density. Both the heat-treated samples show the effect of mechanical cutting and polishing even at 150th thermal cycling.     

Martensitic Transformation of TiNi Shape Memory Alloy Fiber Reinforced Ni Matrix Composites

In this paper, a TiNi shape memory alloy fiber Ni matrix composite was fabricated by an electroplating method using TiNi alloy as the cathode and Ni as the anode. The constrained martensitic transformation behaviors of the TiNi alloy were studied by differential scanning calorimeter (DSC), and the results showed that two endothermic peaks appear on the DSC heating curves and the reverse transformation temperatures increase with increasing prestrain levels. Moreover, comparing to the free transformation, the temperature window of the constrained reverse transformation is widely expanded due to the influence of recovery stress.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.     

Recent development of TiNi-based shape memory alloys

Recent developments in China on TiNi-based shape memory alloys (SMAs), including TiNi binary SMAs, TiNiNb wide hysteresis SMAs, TiNiCu narrow hysteresis SMAs, TiNiHf high temperature SMAs and TiNiRE SMAs were concisely reviewed. The damping characteristics and corresponding mechanisms of TiNi and TiNiNb alloys were described and discussed. Some surface modifications of TiNi binary alloys were employed to improve the corrosion resistance and the biocompatibility, which are very useful for the medical applications. Shape memory effect and mechanical properties of TiNiHf alloys were enhanced by aging Ni-rich TiNiHf alloys, while the Ms still remain enough high. The authors found that the addition of RE to TiNi alloys increases the phase transformation temperatures and even changes the transformation sequence. Fundamental research on the TiNi-based alloys is still in the ascendant, leading to increasingly extending of the applications.     

Martensitic Transformation in Magnetically Controlled Shape Memory Alloys Co50Ni20Ga30

The martensitic transformation for Co50Ni20Ga30 ribbon synthesized by the melt-spinning technique was studied by means of X-ray diffraction and ac magnetic susceptibility. The Co50Ni20Ga30 ribbon, having bcc phase with calculated lattice parameters of a=0.57431 nm at 313 K. It exhibits a structure transition from parent phase to martensite during cooling. The martensitic phase in Co50Ni20Ga30 ribbon is tetragonal structure with lattice parameters of a=b=0.5422 nm and c=0.6401 nm. (c/a>1). According to the changing of diffraction intensity for martensite and the change of ac magnetic susceptibility, the process of the martensitic transformation can be divided into three parts during cooling from 283 K to 213 K. When the temperature decreasing sequentially from 193 K to 110 K, the structure of the martensite has a change in which the a-axis decreases and c-axis increases. The morphologies of selfaccommodation were observeds. The parallelogram morphology, the diamond morphology and the fork morphology were     

Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni_50.8Ti_49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2_2 0 0 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈1 0 0〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M_s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state.     

High-throughput characterization of mechanical properties of Ti-Ni-Cu shape memory thin films at elevated temperature

Hardness and Young's moduli values for Ti_xNi_90−xCu₁₀ (37 at.% < x < 67 at.%) thin films from a continuous composition spread type materials library, annealed at 500 °C for 1 h, were determined at room temperature (martensitic state) and 80 °C (austenitic state) using high-throughput nanoindentation experiments. These values are found to increase as the compositions deviate from Ti contents close to 50 at.%. The increases in hardness is correlated to the presence of Ti-rich and (Ni,Cu)-rich precipitates resulting in precipitate hardening and grain size refinement (Hall-Petch effect). The increase of the Young's moduli is rationalized by considering the significantly higher Young's moduli of the different precipitate phases and applying the rule of mixtures. The contributions of the precipitate phases and the matrix to the combined Young's modulus were estimated by evaluating the load-displacement curves in detail. The obtained results are in good agreement with the Young's moduli determined from thin film curvature measurements [R. Zarnetta et al., Smart Mater. Struct. 19 (2010) 65032]. Thus, the experimental restrictions for nanoindentation experiments at elevated temperatures are concluded to not adversely affect the validity of the results.     

Degeneration and rejuvenation of shape memory effect associated with the precipitation of coherent nano-particles in a Co-Ni-Si shape memory alloy

In a solution treated Co-20Ni-6Si shape memory alloy,coherent nano-particles were precipitated after annealing at 873 K for 1 min,but the shape memory effect almost vanished.It is attributed to that the coherent nano-particles not only suppressed the stress-induced face-centered cubic to close-packed hexagonal martensite transformation but also damaged the crystallographic reversibility of reverse martensite transformation.After further annealing at 1073 K for 1 min,the shape memory effect was reju-venated owing to the dissolution of nano-particles.Besides,the recovery strain significantly increased to 5.1％ from the solution treatment of 3.1％ after annealing at 1073 K for 1 min.     

Anisotropy in microdevices produced by micromachining of cold-rolled NiTi sheets

The process of fabrication and characterisation of three types of NiTi microdevice are described: a double-beam cantilever of constant beam width and two designs of cantilever of stress-optimised width profile, with thicknesses ranging from 17 to 95 μm produced by cold rolling. The technology of electrolytic photoetching was used as a versatile tool for micromachining. The transformation behavior of the specimens was investigated by differential scanning calorimetry and electrical resistance measurements. Anisotropies caused by rolling-induced textures have been investigated by beam-bending experiments. Strong anisotropies were found in results on thickness-dependent Young’s modulus in the parent phase as well as in the beam-bending characteristics in the rhombohedral and martensitic phase. For beam directions transverse to the rolling direction, beam-bending experiments revealed a thickness dependence of hysteresis associated with the martensitic transformation, which was not observed in the rolling direction. Cantilevers with stress-optimised width profile display favorable beam-bending characteristics compared with cantilevers of constant beam width due to an optimum use of the shape memory effect.     

Low frequency damping properties of a NiMnTi shape memory alloy

The present study investigated the low frequency damping properties of a NiMnTi shape memory alloy (SMA) for the first time. The NiMnTi SMA had a high β?θ′ internal friction peak at approximately 125 °C and a low relaxation peak at approximately − 45 °C in the dynamic mechanical analysis cooling tan δ curve. The relaxation peak possessed an activation energy of 0.64 ± 0.03 eV and its damping capacity gradually decreased with the increase of thermal cycling. The NiMnTi SMA also had a good inherent internal friction with tan δ = 0.009 at approximately 140 °C and is a promising high damping alloy for high temperature applications.     

Study by resonant ultrasound spectroscopy of the elastic constants of the β phase in Cu---Al---Ni shape memory alloys

The evolution with temperature of the elastic constants of the metastable β phase in a Cu-27.96 at.%Al-3.62 at.% Ni shape memory alloy (SMA) has been studied by resonant ultrasound spectroscopy (RUS). The corresponding elastic constants of this cubic phase have been measured near the martensitic transformation temperature in a single crystal. Above the martensitic transformation temperature, an anomalous behavior has been found in the C′elastic constant which shows a softening as the temperature decreases. The internal friction value Q⁻¹ has been obtained in this temperature range from the RUS spectra. The mechanisms associated with the Q⁻¹ increase must be related to {1 1 0} shear.     

Microstructure characterization of the non-modulated martensite in Ni–Mn–Ga alloy

The microstructure of the non-modulated martensite in a Ni–Mn–Ga alloy has been characterized in detail by conventional transmission electron microscopy. Bright field images show that the martensite exhibits an internal substructure consisting of a high density of narrow twins. Using electron diffraction, it is found that the martensite has a tetragonal crystal structure. The lattice correspondence between the parent phase and the non-modulated martensite is investigated. Furthermore, the four twinning elements describing the microtwinning have been graphically and quantitatively determined. The results indicate that the microtwinning within the non-modulated martensite belongs to the compound type.     

Ti-content and annealing temperature dependence of transformation behavior of TiXNi(92-XCu8 shape memory alloys

Ti-content and annealing temperature dependence of the transformation behavior of Ti_XNi_(92-X)Cu_8.0 (at,%) (X = 49.0–5l.0) alloys was investigated by varying the annealing temperature from 573 to 1273 K. It was found that the peak temperature of B2–B19 transformation (O^*) increases with increasing annealing temperature from 673 to 873 K for all of the alloys. With annealing at temperatures above 873 K, the influence of annealing on O^* depends on Ti-content. In the range of 50.4–51.0 at.% Ti, O^* shows little dependence on annealing temperature. In the range of 49.3–50.2 at.% Ti, O^* firstly decreases and then keeps constant with increasing annealing temperature. For the alloy of 49.0 at.% Ti, O^* continuously decreases with increasing annealing temperature from 873 to 1273 K. On the basis of the above data, a partial phase diagram of Ti-Ni-8.0Cu (at.%) was proposed. The transformation hysteresis also showed unique Ti-content and annealing temperature dependence.     

Comparative study of martensitic transformation behavior of NiAlMn and NiAlMnFe high temperature shape memory alloys

A comparative study of microstructure and martensitic transformation (MT) behavior of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ high temperature shape memory alloys (SMAs) has been performed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), optical microscopy, and micro-hardness testing. The MT temperature (MTT) of Ni₅₉Al₁₁Mn₃₀ alloy is higher than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy, and both alloys’ MTT increases with increasing annealing temperature. The temperature hysteresis and hardness of Ni₅₉Al₁₁Mn₃₀ alloy are smaller than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy. The MT behavior of Ni₆₀Al₁₉Mn₁₆Fe₅ is sensitive to aging temperature and its MTT and hysteresis decrease with increasing aging temperature. However, the MT behavior of Ni₅₉Al₁₁Mn₃₀ alloy is not sensitive to aging temperature. The MT stabilization effects appear in both alloys during thermal cycles. This stabilization effect vanishes from the second thermal cycle. The quenched microstructure of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ alloys is M plus gamma phase, in which the volume fraction of gamma phase is about 40 and 20%, respectively, and the microhardness of M is higher than that of gamma phase. No aging effects were found in both alloys after aging at 400 °C.     

A constitutive model for cyclic actuation of high-temperature shape memory alloys

In this work, a three dimensional constitutive model for High Temperature Shape Memory Alloys (HTSMAs) is presented. To describe the evolution of the cyclic actuation behavior of such alloys, viscoplastic mechanisms and transformation induced plasticity are introduced in addition to the classical transformation behavior of shape memory alloys. Based on continuum thermodynamics, the evolution of phase transformation, plasticity induced transformation, retained martensite and viscoplasticity are described. Deformation mechanisms that occur over the operational range of such HTSMAs have been identified from the thermomechanical behavior of a NiTiPd alloy. The proposed model has therefore been calibrated and validated based on the thermomechanical response of the studied NiTiPd HTSMA alloy during thermal cycles under compression. Careful attention is devoted to the calibration procedure to identify the contribution of the different mechanisms independently. Finite Element Analysis (FEA) is performed to demonstrate the capabilities of the model to describe the cyclic behavior of HTSMA devices.     

Effects of molybdenum content on the structure and mechanical properties of as-cast Ti-10Zr-based alloys for biomedical applications

The effects of molybdenum on the structure and mechanical properties of a Ti-10Zr-based system were studied with an emphasis on improving the strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti-10Zr and a series of Ti-10Zr-xMo (x = 1, 3, 5, 7.5, 10, 12.5, 15, 17.5 and 20 wt.%) alloys prepared using a commercial arc-melting vacuum pressure casting system were investigated. X-ray diffraction (XRD) for phase analysis was conducted with a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens. The experimental results indicated that these alloys had different structures and mechanical properties when various amounts of Mo were added. The as-cast Ti-10Zr has a hexagonal α′ phase, and when 1 wt.% Mo was introduced into the Ti-10Zr alloy, the structure remained essentially unchanged. However, with 3 or 5 wt.%, the martensitic α″ structure was found. When increased to 7.5 wt.% or greater, retention of the metastable β phase began. The ω phase was observed only in the Ti-10Zr-7.5Mo alloy. Among all Ti-10Zr-xMo alloys, the α″-phase Ti-10Zr-5Mo alloy had the lowest elastic modulus. It is noteworthy that all the Ti-10Zr and Ti-10Zr-xMo alloys had good ductility. In addition, the Ti-10Zr-5Mo and Ti-10Zr-12.5Mo alloys exhibited higher bending strength/modulus ratios at 20.1 and 20.4, respectively. Furthermore, the elastically recoverable angles of these two alloys (26.4° and 24.6°, respectively) were much greater than those of c.p. Ti (2.7°). Given the importance of these properties for implant materials, the low modulus, excellent elastic recovery capability and high strength/modulus ratio of α″ phase Ti-10Zr-5Mo and β phase Ti-10Zr-12.5Mo alloys appear to make them promising candidates.     

Factors affecting recovery stress in Fe-Mn-Si-Cr-Ni-C shape memory alloys

The effects of the heating temperature, the carbon content, the amount of pre-strain, the annealing temperature, and the conventional training on the recovery stresses at room temperature of cold-drawn Fe-Mn-Si-Cr-Ni-C shape memory alloys were studied. The results showed that the addition of carbon or the refining of grains could more significantly enhance the recovery stress than the conventional training. The recovery stress of a cold-drawn Fe-Mn-Si-Cr-Ni alloy with 0.18%C could reach 565 MPa only after annealed at 1023 K. Its recovery stress was only improved to 452 MPa after it was subjected to annealing at conventional temperature 1373 K and then four cycles of the conventional training. The dominating factors affecting the recovery stress were the amount of the plastic deformation and that of the martensitic transformation induced by the recovery stress in cooling, not the recovery strain under no constraints.