G.M. Swallowe

Stress-induced martensitic transformation in Cu–Al–Zn–Mn polycrystal investigated by two in-situ neutron diffraction techniques

Dedicated to Prof. P. Lukac, Charles University, Prague on the occasion of his 65. Birthday Abstract In-situ neutron diffraction studies of internal strains, stresses and phase fractions in pseudoelastic CuAlZnMn shape memory alloy evolving during two tensile load cycles at constant temperature are reported. The results are discussed with the aim of demonstrating the applicability of this experimental technique for the shape memory alloy research. Particularly, an experimental information on the mechanisms by which individual grains of the transforming polycrystal share the macroscopic stress - load partition - as well as quantitative information on the residual stress frozen in the austenite phase after the first cycle are discussed.

Time-Temperature Equivalence

Both the modulus and the compliance of viscoelastic materials are time dependent quantities and a complete characterisation of the modulus over a wide temperature and time range can be a very time consuming process (see Viscoelasticity). It is however possible, using the concept of time-temperature equivalence, to relate the compliance at one temperature to that at another temperature by a shift of the compliance-time curve along the time axis. This is illustrated in Figure 1.

Intergranular strains in transforming NiTi alloys

Shape memory alloys (SMAs) exhibit unique thermomechanical properties due to a reversible martensitic phase transformation. Their current high cost and the insufficient predictability of the thermomechanical responses of these materials hinders further growth in their application. Neutron diffraction offers a unique tool to probe the phase transformation of bulk materials in situ during thermal and mechanical loading, allowing the simultaneous monitoring of phase fraction, texture evolution, interphase and intergranular strains. This paper describes the initial results of thermomechanical loading tests of the shape memory material NiTi as it martensitically transforms in complex thermomechanical load cycles. When fully analyzed this experimental data will provide crucial information on the interaction between the transformed and untransformed material, and on the dependence of transformation on grain level orientation.

Stress and Strain

The measurement of mechanical properties is basically the determination of the relationship between the two parameters stress and strain. Stress is simply defined as the force per unit area that the material is being subjected to and has units of kg m−2 also known as Pascals. Strain can be defined as the ratio of the change in dimension of the material measured along a particular direction to its original dimension, it is therefore a dimensionless unit. The conventional symbols for stress are the Greek letters σ and τ and the symbols used for strain are e and γ.