Compatibility of deformation in two-phase Ti-Al alloys: Dependence on microstructure and orientation relationships
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INTRODUCTION
DURING the past years, significant advances have been made in improving the ductility of two-phase alloys consisting of 3/-TiAI and cr Llj Previous studies have shown that the primary deformation mechanisms in twophase TiA1 alloys are slip of 1/2(110] ordinary dislocations and 1/6(112] {1 11} deformation twinning. I2,31 Fewer (101] and 1/2(112] superdislocations are observed in two-phase alloys in comparison with the singlephase counterparts, t4'51 This change in deformation mechanisms has been attributed to an oxygen-gettering effect of the a2 phase that has been proposed as responsible for the improved ductility of the alloys.t2] One prevalent feature of these two-phase alloys is the existence of specific orientation relationships between the phases present. In the lamellar microstructure, the alternating 3/and c~2 lamellae are oriented such that the following relationships have been confirmed to exist: tS,6j [2110]o2//(011)~ and (0001)o2//{111}v As a consequence of these relationships and due to the phase transformation sequences that occur during the thermomechanical treatments, adjacent lamellae of the 3/ phase also exhibit a well-defined orientation relationship that can be described by a multiple of 60 des rotation about a (111) pole. [7] From the six possible orientations, three different types of orientation relationships between adjacent 3/ grains can be described as a 180 des truetwin relationship, a 60 des pseudotwin relationship, and a 120 des rotation relationship that retains the ABCABC stacking sequence. This 120 des rotation can be equally well described as a 90 deg rotation about a (010) pole. While a few studies have been carried out to show that the specific orientation relationship between grains makes possible the transmission of slip between adjacent grains, t3,8-1~ no systematic studies have been performed in this area. Since the transmission of slip across grain J. LUSTER, Research Scientist, is with CASTOLIN, St. Sulpice, Switzerland. M.A. MORRIS, Professor, is with the Institute of Structural Metallurgy, University of Neuch~tel, Neuchgttel, Switzerland. Manuscript submitted June 30, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
boundaries is expected to play a dominant role in the deformation behavior of the alloys, it appears relevant to relate the latter to the rotation relationships between adjacent grains for different types of microstructure. In an effort to provide more fundamental understanding regarding the influence of microstructure on the deformation mechanisms of two-phase alloys, a detailed analysis of the active slip systems produced by roomtemperature compression has been carried out from both lamellar and equiaxed microstructures using transmission electron microscopy (TEM). The influence that the load orientation with respect to each given grain and that of the orientation relationships between adjacent grains have on the slip systems activated have been systematically compared using a geometric compatibility factor as defined in this section. The co
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