The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys
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I.
INTRODUCTION
T R A N S F O R M A T I O N - i n d u c e d plasticity (TRIP) refers to enhanced toughness which results from the transformation of one phase to another during deformation. In so-called TRIP steels, retained austenite transforms to martensite during straining; the resulting volume change relieves some of the applied stress and thereby increases the steel's resistance to fracture. This method of improving toughness is, in fact, employed in various other materials. [11Improved ductility resulting from deformationinduced transformation was achieved, for example, in cast irons t21 and partially stabilized zirconia, f3~ It is well known that metastable/3 phase in (a + /3)and/3-type titanium alloys either twins or transforms to a ' (hexagonal martensite), to a" (orthorhombic martensite), or to to phase during straining, t4-7] The possibility thus exists that these reactions can improve the toughness and the ductility of titanium alloys in the same manner as the retained austenite ----> martensite reaction in steels. Although the effect of deformation-induced transformation on the fatigue strength of titanium alloys has been reported, lm there is little information concerning the influence of deformation-induced transformation on fracture toughness. (Transformation which is induced by either stress or strain is defined here as "deformationinduced transformation. ")
M. NI1NOMI, formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA, is Associate Professor with the Department of Production Systems Engineering at Toyohashi University of Technology. T. KOBAYASHI, Professor, is with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 440, Japan. I. INAGAKI, formerly with the Department of Production Systems Engineering, Toyohashi University of Technology, is with Sumitomo Metal Industries, Osaka Steel Works, Konohana-ku, Osaka 554, Japan. A.W. THOMPSON, Professor and Department Head, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted July 18, 1989. METALLURGICAL TRANSACTIONS A
This study was undertaken to determine the effect of deformation-induced transformation on mechanical properties and particularly on the toughness of two classes of titanium alloys: (or + /3) type and/3 type. Tensile tests, Charpy impact tests, and static and dynamic fracture toughness tests were performed on two typical (a + /3)type alloys (Ti-6A1-2Sn-4Zr-6Mo and Ti-6A1-4V) and on two typical /3-type alloys (Ti-15V-3A1-3Sn-3Cr and Ti-3AI-8V-6Cr-4Mo-4Zr). Both groups of alloys were heat-treated so as to produce a large amount of retained /3 phase. II.
EXPERIMENTAL PROCEDURES
Materials used in this study were hot-forged bars of Ti-6A1-2Sn-4Zr-6Mo (Ti-6.2.4.6) and Ti-3A1-8V-6Cr4Mo-4Zr (Ti-3.8.6.4.4) and hot-rolled plates of extralow interstitial (ELI) grade Ti-6A1-4V (Ti-6.4) and Ti-15V-3A1-3Sn-3Cr (Ti-15
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