High strength copper alloys by thermomechanical treatments

A thermomechanical processing technique for in creasing the strength of copper alloys is described. Alloys studied include phosphor bronze (5 pct Sn), nickel silver (12 pct Ni-28 pct Zn), tin-modified cupronickel (9 pct Ni-2 pctSn), and Cu?Be (2 pct Be). In this technique, the material is cold-rolled to about 95 pct reduction in thickness followed by heat treatment below the recrystallization temperature. The severe cold work results in increased strenght through strain hardening and texture strengthening, but at the expense of decreased ductility. The terminal heat treatment recovers the ductility while maintaining or increasing the strength imparted by cold work alone. Preliminary results indicate that cold work-accelerated precipitation is chiefly responsible for the strength increase during heat treatment. As a result of the present processing, the copper alloys exhibit higher yield strengths for given amounts of ductility than have heretofore been attained.     

Electrospark powders of shape memory alloys

Powders of Ti-Ni and Ni-Mn-Ga alloys are obtained by the electrospark method. It is established that the largest particles are of irregular shape and the smaller ones are spherical. Some spherical particles are hollow. No carbon contamination is found within the Ti-Ni and Ni-Mn-Ga powders obtained in cryogen liquids. The Ti-Ni powders of all compositions are mixtures of austenite and martensite phases. Preliminary heat treatment of spherical-particle Ni-Mn-Ga alloys is needed to activate their martensitic transformation. The martensitic transformation proceeds over quite a wide temperature range, which is indicative of high dispersion, and the martensitic temperatures increase, which is indicative of a minor difference between the chemical compositions of powders and massive samples. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 3–15, 2007.     

Wear characteristics of TiNi shape memory alloys

The wear characteristics of TiNi shape memory alloys against Cr-steel have been studied. Experimental results indicate that the Ti₄₉Ni₅₁ alloy can exhibit a better wear resistance than Ti₅₀Ni₅₀ alloy due to their higher hardness and pseudoelastic behaviors. Four main mechanisms, adhesion, abrasion, surface fatigue, and brinelling, are found to have important contributions to the wear characteristics of TiNi alloys. The weight loss increases with increasing wear load and sliding distance but decreases with increasing sliding speed. The contact area during sliding wear will be increased due to the variant accommodation and/or pseudoelasticity and, hence, will reduce the average compressive stress and wear damage. Variant accommodation and/or pseudoelasticity can also stabilize the crack tips and hinder crack propagation, hence improving the wear characteristics of TiNi alloys.     

The structure of NiTiCu shape memory alloys

A combined electron microscopy and X-ray diffraction study has been made of NiTiCu shape memory alloys where Cu is substituted for Ni in binaryβ NiTi. It is shown that the high temperature phase with the CsCl structure is retained for copper contents of up to 30 wt pct, and that this phase transforms on cooling to a monoclinic martensite with similar morphology and lattice parameters to those previously reported for the binary NiTi alloy. Initial substitution of Cu for Ni produced slight changes in the lattice parameters of both the high and low temperature phases, then these remain remarkably constant on further addition of Cu. The chief effect of the Cu substitutions is to reduce the distortion needed to form martensite from the parent austenite. An attempt to explain the effect of the Cu is made in terms of a few simple alloying theories, but it is concluded that an effective treatment must consider the interplay of several material parameters. Formerly Visiting Scientist at Brown Boveri Research Center on leave of absence from Cambridge University, England     

Damping characteristics of TiNi shape memory alloys

The damping characteristics of TiNi SMAs have been systematically studied by using techniques of resonant-bar and low-frequency inverted torsion pendulum. Experimental results show that both the martensite phase (M) and R phase (R) have high damping due to the movement of twin boundaries. Because the B2 parent phase (B2) has smaller damping, it is suggested that this may come from the dynamic ordering process of lattice defects. In the transformation re-gions of B2 ↔ M, B2 ↔ R, and R ↔ M, there are maxima of the damping capacity which are attributed to two contributions. One arises from the plastic strain and twin-interface move-ment during the thermal transformation, which obeys a linear variation of peak heightsQ ⁻¹ _max vst att ≥ 1 °C/min. The other originates from the stress-induced transformation formed by the applied external stress which dominates atT < 1 °C/min. The elastic modulusE of martensite and the R phase is lower than that of the B2 phase, and a modulus minimum appears in the transformation region.     

Improvement of shape memory effect in Fe-Mn-Si-Cr-Ni alloys

Three kinds of Fe-Mn-Si-Cr-Ni alloys, which contain cerium, titanium, and nitrogen, respectively, were employed to study the shape memory effect (SME). Experimental results have shown that the SME can be improved significantly by alloying with Ce, Ti, and N, deforming at a low temperature (77 K), and using a series of heat treatments. By alloying with Ce, the SME can be enhanced greatly, so adding the rare-earth elements is an effective way to improve the SME. When prestrained at 77 K, the alloy containing N shows an excellent SME, with a fully reversible strain of more than 3.2 pct, and 6 pct of the net reversible strain can be obtained under the prestrain of 8 pct. Each alloy has a best solution-treated temperature, at which a better SME can be obtained. All these three alloys exhibit an excellent SME after a series of heat treatments which include aging, resolution treatment after tensile deformation, and thermomechanical training. A small two-way SME has been observed in the alloy containing N.     

Ti-V-Al轻质记忆合金的研究进展

Ti-V-Al合金基于热弹性马氏体相变而呈现出形状记忆效应。同时,Ti-V-Al合金不仅呈现出良好的冷热加工性能,还具有较低的密度,这可满足当今航空航天领域对轻量化制造的需求。文中主要综述国内外研究学者在Ti-V-Al轻质记忆合金研究方面的重要工作和进展,其中重点阐述了Ti-V-Al轻质记忆合金热循环稳定性、力学性能与功能特性方面的研究。最后,简单阐述了Ti-V-Al轻质记忆合金功能特性的演化规律与机制,并对后续Ti-V-Al轻质记忆合金的发展方向进行了展望。      

Precipitation processes in near-equiatomic TiNi shape memory alloys

Metallographic studies have been made of precipitation processes in Ti-50 pct Ni and Ti-52 pct Ni (at. pct) shape memory alloys. The eutectoid and peritectoid reactions previously reported for near-equiatomic and Ni-rich TiNi alloys were not observed for either composition. In the Ti-52Ni alloy, diffusional transformations take place, similar to those in supersaturated alloys. The precipitation sequence can be written asβ ₀ → Ti₁₁Ni₁₄ → Ti₂Ni₃ → TiNi₃. The solidus line of the TiNi phase in the Ti-52Ni alloy lies at 812 ± 22 °C. Morphological characteristics of the various precipitate phases are described in detail. M. NISHIDA, formerly with the University of Illinois     

Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic transformations: Part I. shape memory behavior

As part of a study on newly developed Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed (fcc-hcp) martensitic transformations, the shape memory behavior is reported and various physical factors influencing it discussed, based on experimental results from dilatometry and bending tests. It is shown that the alloys studied exhibit a good shape memory effect (SME) without special training. The SME is significantly improved by prestraining at low temperatures. A higher net reversible strain is made available by over-prestraining, although the fully reversible prestrain usually does not exceed 2 pct. Super-elasticity, the two-way shape memory, and a softening effect of the matrix at low temperatures are briefly discussed.     

Progress in the understanding of NiTi shape memory alloys

Shape memory alloys (SMAs) are a special class of metallic materials which respond with a considerable change in their properties to small changes in temperature or stress. The SMAs offer two interesting characteristics, viz., shape memory effect (SME) and superelasticity (SE), also called pseudoelasticity which make them attractive for applications in engineering and biomedical fields. Among the various SMAs, NiTi base alloys have been the most commercially exploited ones because of their superior SME and SE, mechanical properties, corrosion resistance and biocompatibility. Since the pioneering discovery of NiTi SMA in early 1960s, significant progress has been made in the processing and understanding of the behaviour of these alloys. In spite of these efforts, the NiTi SMAs continue to offer challenges to the scientists and engineers, and new findings are being made continuously. This paper provides an overview of the developments in NiTi SMAs.