The Effect of Cooling Rate During Rapid Solidification on the Structure and Texture of NiTi
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THE EFFECT OF COOLING RATE DURING RAPID SOLIDIFICATION ON THE STRUCTURE AND TEXTURE OF NiTi A.J. Pedraza*, M.J. Godbole*, E.A. Kenik**, D.F. Pedraza** and D.H. Lowndes*** * Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2000 ** Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376 *** Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 ABSTRACT A study has been conducted on the effects of increasing cooling rate during rapid solidification of NiTi upon the phases that are produced. The hammer and anvil rapid solidification technique and laser melting with a nanosecond excimer laser were used, which allow the cooling rate to be varied by three to four orders of magnitude. Although 1/3 (1101 superlattice reflections are seen in the selected area diffraction (SAD) patterns of the splat quenched (SQ) specimens, x-ray diffraction analyses show the presence of only B2 phase and martensite. On the other hand, laser treatment (LT) of the specimens produces a layer that has a L10 structure with a slight monoclinic distortion. This phase can be envisaged as a small distortion of a B2 unit cell with a volume per atom -3.3% lower than the equilibrium B2 phase. Also martensite is present in the layer. SQ alloys exhibit a marked (2001 texture due to columnar growth opposite to the direction of heat extraction, while LT produces epitaxial regrowth of the melted layer. No substantial disordering is obtained in NiTi rapidly solidified alloys. INTRODUCTION Intermetallic Ni-Ti alloys of near equiatomic composition have a high temperature B2 (CsCl) ordered structure up to the melting point. These compounds exhibit a martensitic transformation into an orthorhombic phase, B19', with a small monoclinic distortion [1] and intermediate transitions between the austenitic and the martensitic phases. Those transitions together with accompanying changes in various physical properties are widely known as premartensitic phenomena. Great effort has been devoted to the study of these phenomena [1-6]. However, there is still considerable controversy regarding the nature of the transitions and the crystalline structure of the Dhases that NiTi alloys of near equiatomic composition exhibit [6,7]. Among the factors that may play a role in the transition phenomena, the cooling rate appears to be an important one, as indicated for instance by the experiments of Shabalovskaya [8]. Buschow studied the thermal stability of amorphous alloys of the NixTiIx system in the range 0.24 4 x'4 0.64, produced by melt spinning [9]. For x 4 0.40 and x > 0.58, the alloys were produced in the fully amorphous state. Glass forming tendency, however, decreased in the range 40-58 at. pct. Ni. *Research sponsored in Department of Energy, under Energy Systems, Inc. and in the University of Tennessee
Mat. Res. Soc. Symp. Proc. Vol. 74.
part by the Division of Materials Sciences, U.S. contract DE-AC05-840R21400 with Martin Marietta part by the Center for Materials Proce
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