J. Beyer

Texture development and transformation strain of a cold-rolled Ti50-Ni45-Cu5 alloy

Shape memory alloys (SMAs) are finding increased use as functional materials in the aerospace, energy and medical industries 1 J. van Humbeeck, Shape Memory Materials and Phenomena—Fundamental Aspects and Applications, p. 3771, vol. 246, MRS, Pittsburgh, PA (1992).(1), (2) and (3). Shape memory behaviour is based on the recovery of large amounts of induced strain upon heating and/or unloading. This transformation strain is a result of the reversible growth of certain favoured martensite variants during martensite transformation and/or stressing [4] and [5]. For single crystal SMAs, the favoured variants are those which result in the maximum transformation strain for a specific orientation. This has been well established for several common single crystal SMAs such as TiNi, CuZnAl and CuAlNi [4] and [6]. For polycrystalline SMAs, it is not clear which variants are favoured. Anisotropic behaviour in SMAs has been interpreted based on the anisotropy data of single crystal materials using the concept of the selection of favoured martensite variants. This has met with only limited success in work on NiTi alloys due to the lack of information about which variants are formed [7] and [8]. An investigation of the anisotropic behaviour of textured SMAs was thus conducted in order to determine which martensite variants develop during thermal cycling of a commercial TiNiCu SMA. The relationship between the observed variant development, changes in texture and anisotropic shape memory behaviour are discussed in light of models using the concept of favoured martensite variants.

Some questions on the structure of martensite and precursor in Ni (<63at%)-Al alloys

M. CHANDRASEKARAN*, J. BEYER** and L. DELAEY*** * Universitat de les llles Balears, Departament de Fisica, Facultat de ciencies, E-07071 Palma de Mallorca, Spain (now at **). ** University of Twente, Faculty of Mechanical Engineering, Laboratory for Materials Science, P.O. Box 217, 7500 AE Enschede, The Netherlands *** Catholic University of Leuven, Department of Metallurgy and Materials Engineering, de Croylaan 2, B-3030 Heverlee, Belgium

Influence of manganese on the transformation temperatures of Ni50Ti50 shape memory alloys

The transformation temperatures of NiTi-based alloys strongly depend on the number of (valence) electrons. It has been argued that the Ms of Ni0.50−xTi0.50−xMn2x is independent of x, because e/a = 7.00, unchanged relative to Ni0.50Ti0.50, for any x. In this work, a model is presented, in which it is argued that not only changes in the number of electrons, but also band-structure changes should be considered. The calculations, made with this model, are confirmed by experimental results.

Powder metallurgically produced Ni-35 at.% al compacts — A first assessment

The intermetallic phase [beta] NiAl to be an alloy with high heat resistance and good oxidation resistance and is being developed as a potential material for high temperature applications. In addition, off-stoichiometric nickel rich [beta] Ni-Al compositions are known to undergo a thermoelastic martensitic transformation and exhibit shape memory effect at relatively high temperatures. There is no major difficulty in processing these alloys at elevated temperatures. However, thy suffer a major problem of brittle fracture and poor ductility at ambient temperature. In this regard it has been suggested that finer grained NiAl can exhibit enhanced ductility. Moreover, in quenched alloys containing [>=]35at.% Al there is a problem with reversibility of the martensitic transformation. This is because, martensite decomposes on heating by temperate and this suppresses subsequent transformation to martensite on cooling. The aim of this work was to investigate the possibility of producing nickel rich Ni-Al alloys by P/M route. It will also be shown that the tendency for martensite to temper is suppressed with such a method of preparation.

Anisotropy of tensite recovery stress and two way memory effect of cold rolled TiNi sheet

The texture of cold rolled and heat treated TiNi sheet has been measured and designated as {110} p . The.anisotropy of the tensile recovery stress, the stress dependence of the transformation temperatures, and the development of the two-way memory effect during thermal fatigue have been measured. The resulting changes of texture after thermal fatigue are presented and explained. The observed effects are explained on the basis of the parent texture and the specific martensite variants which are formed according to the lattice correspondence between austenite and martensite.