Pseudo-Elastic Behaviour in Rapidly-Quenched Dilute Tin Alloys
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PSEUDO-ELASTIC BEHAVIOUR IN RAPIDLY-QUENCHED DILUTE TIN ALLOYS
W.M. CALLERNFAULT AND R.W. SMITH Department of Metallurgical Engineering, Queen's University, Kingston, Canada
ABSTRACT Ribbons of pure tin (99.99q9%) and dilute Sn-Ge alloys, prepared by melt spinning using a 4" diameter copper cylinder, were strained in tension. Reversible strains of a few percent were recorded in the dilute alloys but not in the pure tin. X-ray diffraction examination suggests that this pseudoelasticity is due to twinning rather than a martensitic transformation.
INTRODUCTION Normally metals and alloys deform plastically when the applied stress exceeds the elastic limit. However, there is an increasingly large body materials in which large mechanical strains may be produced far in excess the elastic limit, and which, when the applied stress is removed, revert Such materials to their original shape with little or no permanent set. said to be pseudo-elastic. Two types of pseudo-elasticity have been identified: Mi) Transformation Pseudo-elasticity e.g. (1] (ii) Twinning Pseudo-elasticity e.g. [2] Case Mi) is associated with a stress-induced martensitic transformation above the Ms temperature and proceeds with increased strains according to the Clausuis - Clapeyon equation:
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(da/dT) = (-AHT /T.Ac) a = critical stress T = temperature AFT enthalpy change associated with transformation Ac = strain associated with stressinduced transformation As the stress is removed then the specimen reverts to the parent phase. is caused by the revisible movement of twin boundaries without Case (ii) It has been proposed by Lieberman et al [3] that involving a phase change. the twinning mechanism is a combination of a quick twinning shear and a slower subsequent, shuffling process. (The shuffling effect changes the atomic arrangement into the correct twin positions, vis-a-vis the original twin boudary sites.) Unfortunately, this model is unable to account for the cycle of stress application in actual residual strain observed on the first systems. In this paper, we note a pseudo-elastic effect may be observed in rapidly quenched samples of dilute tin-germanium alloys and attempt to characterise it.
430 EXPERIMENT Samples of pure tin and dilute tin-germanium alloys % containing Sn, 1.0 wt/Ge and 2 wt/Ge alloys respectively, were prepared from 99.9999 Sn Ge (manufacturer's specification) The samples were melted in an and qq9q9. argon atmosphere, heated to approximately 1200 K and cast into a copper mold. These specimens then were re-melted in a silica glass tube and rapidly quenched by melt spinning using a 4" copper drum to yield continuous ribbons From these ribbons 3 mm wide and with a thickness varying from 20-80 m. tensile specimens were cut to a guage length of 6 cm and strained at a rate Other samples were cm/cm/sec to yield load versus % strain curves. of 10 analyzed by means of reflected X-rays; in this test a crude tensile stage was fabricated so that the X-ray beam impinged transver
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