Microstructure, deformation, and fracture characteristics of an Al 67 Pd 8 Ti 25 intermetallic alloy
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I.
INTRODUCTION
RAMANarid Schubert[1] first reported that the
tetragonat D02.2 crystal structure of A13Ti can be changed to the related cubic Lt2 crystal structure by substituting small amounts of Ni, Cu, or Zn for AI. Markiv and Burnashova[2~and Siebold[3] have shown that substitution of Fe for Al in AI3Ti produces the same change. The amounts of Ni, Cu, Zn, or Fe required to change the crystal structure from D022 to LI2 vary but are typically in the range of 5 to 15 at. pct. Following the convention established by Nash et al., [4] who assessed the ternary A1-Ni-Ti phase diagram, the Ll2 phase in (A1, X)3Ti alloys wilI be denoted "~ phase" throughout this paper. Kumar and Pickens t5'6] investigated the compression stress-strain behavior of the 7r-phase alloys AI67Fe9Ti24 and AI67FegTi2zV2 over the temperature range of 298 to 1050 K. A mild positive temperature dependence of yield strength was found for both of the 7r-phase alloys, a frequent observation for intermetallic alloys with the LI~ crystal structure (cf: References 7 and 8). Kumar and Pickens [5,6] also reported the resistance to microcracking during hardness indentation at room temperature for A13Ti and a variety of 7r-phase alloys. Cracking resistance was observed to increase in the order: A13Ti, AI62CuI3Ti25, A167Fe9Ti24,, AI67Fe9Ti22V2. Tensile deformation characteristics of these materials were not reported. The microstructure, room-temperature deformation, and fracture characteristics of the 7r-phase alloy A167NisTi25 have been reported by Wert et al. [9'1~ In compression, the alloy has a 0.2 pct offset yield strength of 355 MPa and exhibits substantial plasticity. The mechanism of plastic deformation in compression was found to be glide of undissociated ao(110) {11 1} dislocations. Upon application of a tensile stress, AI67NisTi25 fails in a brittle manner by transgranular cleavage fracture with no evidence of associated plasticity. Analysis of the fracture characteristics led to the conclusion that brittle fracture of AI67NisTi25 is a consequence of an activation barrier
W.O. POWERS, Research Scientist, is with the Metals Research Laboratories, OLIN Corporation, New Haven, CT 06511. J.A. WERT, Associate Professor, is with the Department of Materials Science, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted February 9, 1989. METALLURGICAL TRANSACTIONS A
for emission of dislocations from crack tips, which prevents plastic blunting of cracks. [12-19]This conclusion indicates that low toughness is an intrinsic characteristic of the (A1, Ni)3Ti 7r-phase alloy. The present paper describes results of an investigation of crystal structure, microstructure characteristics, indentation hardness, and fracture of A167Pd8Ti25. The goals of this study were to determine if substituting Pd for AI in AI3Ti produces a 7r-phase intermeta!lic alloy and, if so, to ev.aluate the deformation and fracture behavior of this new 7r-phase alloy. The element Pd was chosen because it has an electronic c
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