Effect of microstructure variations on the formation of deformation-induced martensite and associated tensile properties
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I. INTRODUCTION
IT is well known that metals can deform by different mechanisms: dislocation slip, twinning, formation of deformation-induced martensite, or a combination of these. The occurrence of the different deformation mechanisms strongly depends on the matrix: on the one hand, on the critical stresses that trigger slip and twinning and, on the other hand, on the ability of a metastable matrix to transform into martensite. The interplay between the different deformation mechanisms significantly affects the mechanical behavior of the materials. The ability of the matrix to transform into martensite is, for example, known to control the behavior of shape-memory alloys.[1–4] The effect of stress and plastic strain on the martensitic transformation has been extensively studied in the case of ferrous alloys, both concerning the phase-transformation kinetics and the mechanical behavior resulting in transformation-induced plasticity.[3,5–8] In the metastable b titanium alloys, the MS temperature is below room temperature and the b phase can be retained in a metastable state. The deformation of this b phase can involve slip, {332} ^113& twinning, and the formation of stress-induced martensites or strain-induced martensites (SIMs).[9–20] In many of the less-stable b titanium alloys, stress- or strain-induced formation of hcp a8 or orthorhombic a9 martensites has been observed.[14–16,18–20] Combination T. GROSDIDIER, Senior Lecturer, formerly with the Laboratoire d’Etude ´ ´ ´ des Textures Appliquees aux Materiaux, CNRS, UMR7078, Universite de ´ Metz, is with LERMPS, Universite de Technologie de Belfort, 90010 Belfort, France. Y. COMBRES, Senior Engineer, is with the Centre de Recherche de CEZUS, 73400 Ugine, France. E. GAUTIER, CNRS Research ´ ´ ´ Director, is with the Laboratoire de Science et Genie des Materiaux Metalliques, CNRS UMR7584, 54042 Nancy, France. M.-J. PHILIPPE, Profes´ ´ sor, is with the Laboratoire d’Etude des Textures Appliquees aux Materiaux, ´ Universite de Metz, 57045 Metz, France. Manuscript submitted August 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
of slip with the formation of martensite has been reported to often lead to a good compromise in terms of enhanced ductility. Since the relative contribution of transformation and slip depends strongly on the “stability” of the matrix, it is essential to fully understand what the different factors are that control the stability of the b phase in the metastable b titanium alloys and to see to what extent they influence the occurrence of the various mechanisms. The first factor that influences the stability of the matrix is, obviously, its chemical composition. Consequently, the deformation mechanisms have been mainly studied by varying the chemical composition of alloys quenched in their b state.[14,16–18] The metastable b alloys show a good response to heat treatment, and their microstructures can be easily modified. It is, therefore, surprising to note that, whereas the influence of the chemical composition of the alloy on the competition
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