Characteristics of the martensitic transformation and the induced two-way shape memory effect after training by compress
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I.
INTRODUCTION
THE pseudoelastic effect constitutes one of the interesting effects associated with martensitic transformations in Cu-Zn-A1 alloys. Although the basic features of this effect do not depend on the signs of the applied stress, it has been shown that differences arise if tensile or compressive stresses are applied on Cu-Zn-A1 beta (bcc) single crystals, tl,2J In both training modes, at low applied stresses, an 18R single crystal is first induced. The formed variant can be predicted using the Schmid factor criteria. However, if further stresses are applied, a martensitemartensite transition or plastic deformation of the martensite occurs for tensile loading, depending on the tensile axis orientation, while plastic deformation of the martensite is found for compressive stresses in the whole range of orientations, r3,4j If pseudoelastic cycling is performed, depending on the stress mode, differences have been found related to the dislocation arrangements left in the material and on the amount of retained martensite present in the samples, t~'5-7~This has encouraged further investigations to study the effect of pseudoelastic cycling on the martensitic transformation, particularly under the effect of compressive stresses. Additionally, pseudoelastic cycling is a suitable method to induce two-way shape memory effect (TWSME).II-7I In this work, results concerning the TWSME induced under compressive stresses will be presented, and compared with previous work performed with tensile stresses/81
C. PICORNELL and E. CESARI are with the Physics Department, University of the Balearic Islands, Palma de Mallorca, Spain. M. SADE is with the National Commission of Atomic Energy, Atomic Center Bariloche, Balseiro Institute, 8400 S.C. de Bariloche, Argentina, and the National Commission of Atomic Energy, National University of Cuyo, Argentina. Manuscript received August 4, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL METHODS
Single crystals were grown using the Bridgman technique. Samples were spark machined for compression (10-mm length, 5-ram diameter). Only one sample was prepared for tensile cycling (13-mm gage length, 3-mm diameter). All the samples were kept for 30 minutes at 1100 K and air-cooled [thermal treatment (TT) 1]. After thermal treatment, the samples were maintained at room temperature at least 24 hours before the mechanical or calorimetric tests. Samples were mechanically and electrolytically polished. Specimens were pseudoelastically cycled in an Instron testing machine model 1123 at a constant temperatureTexp ~ Af. Cycling was performed between a stress at which the sample was in the beta phase and a strain which led to a complete transformation. The martensitic structure obtained in all the specimens was MI8R and will be called M hereafter. The experimental details of each sample are shown in Table I. Sample names with the same letter indicate that they were obtained from the same single crystal. Single crystal f has a nominal Ms of 233 K, and the rest have nominal Ms = 253
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