Compression-Induced Martensitic Transformations in Cu-Zn-Al Alloys with e/a>1.50
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A.Cuniberti (1) and R.Romero " (2)
(1) Instituto de Fisica de Materiales Tandil, Facultad de Ciencias Exactas, Universidad Nacional del Centro de la Provincia de Buenos Aires. Pinto 399, 7000 Tandil, ARGENTINA. Tel/fax +54-293-42821 (2) Comisi6n de Investigaciones Cientificas de la Provincia de Buenos Aires. ABSTRACT Results are reported about the pseudoelastic cycles 13-martensite obtained by compression of P3 Cu-Zn-Al single crystals with electronic concentration 1.52. Stress-strain curves at different temperatures and in situ observations were carried out. The results are compared with those obtained from tension test. INTRODUCTION In Cu-Zn-A1 alloys the 13phase is metastable at room temperature and can be retained by quenching, suffering two successive orderings A 2->B2->L21 . The martensitic phases can be either thermally-induced, spontaneous transformation, or stress-induced, cooling or stressing the 03phase. Because of the non-diffusive character of the transformation, the martensite inherits the atomic order from the 13phase. The martensitic structures are based on the stacking sequence of the close-packed planes, they are 9R, 18R and 2H where the number indicates the stacking sequence period and the letter the structure simetry: R rhomboedric and H hexagonal. Each martensite can be obtained from the 13 phase depending on the electronic concentration, temperature and stress conditions [1]. 13Cu-Zn-Al single crystals with e/a higher than 1.50 transform spontaneously to 2H martensite, by tensile load it is possible to obtain 18R and 2H martensite depending on temperature conditions [2] [3]. At temperatures above about Ms+20K the 18R phase is induced and the pseudoelastic cycles are similar to those for alloys with e/a 1.48, with a small stress hysteresis equivalent to 3-4K. Below Ms+20K the martensite induced is 2H, the stress-strain curves show a marked yield point followed by a plateau at a lower stress and a stress increase on further loading, an important stress hysteresis is observed and commonly no reverse transformation takes place. Although the martensitic transformations induced by tensile load had already been studied in detail, up to now, there was not any information about the phase transformations that take place when the applied load is compressive. In this work, the martensitic transformations occurring in 13Cu-Zn-AI single crystals with e/a> t.50 are studied by compressive pseudoelastic cycles at different temperatures and in situ observations. EXPERIMENTAL The composition of Cu-Zn-Al single crystals used in the present study is Cu-8.54atZn21.75at%Al, e/a=1.52 and Ms=264K, determined by electrical resistivity [2]. They were grown starting from alloys prepared with high purity materials (5N). 439 Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society
The load axes orientation of each single crystal was determined in 13phase by Laue technique. Cylindrical samples of 3mm diameter and 10mm length for compression tests were spark machined. After machining, the samples were kept for
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