Phase transformations in Ti 2 Ni 3 precipitates
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I.
INTRODUCTION
THE TiNi alloys of near equiatomic composition are one of the most technologically important shape memory alloys. It is known that the precipitation sequence 1'2 in Ni-rich (Ni -> 50.6 at. pet) TiNi alloys is as follows: fl0 ~
fll + T i u N i i 4
> •2
+ TizNi3
) f13 + TiENi3
where flo is the original supersaturated Ni-rich alloy, fll is the composition of the matrix in equilibrium with Til 1Ni ~4, and so on. Among these decomposition phases, the Ti2Ni3 showed interesting microstructures as described below. Surface relief was observed in the optical micrographs. 1 In the transmission electron microscopy studies, needle and antiphase-like microdomains were observed, and the former corresponded to the surface relief. 2 The purpose of the present paper is to clarify details of the microstructural aspects of the TizNi3 phase in an aged Ti52 at. pct Ni alloy.
II.
EXPERIMENTAL PROCEDURE
The Ti-52 at. pct Ni alloy used in the present study was the same as that described in previous papers, t,2 The fabrication and solution treatment of the alloy and specimen preparation for optical microscopy and TEM observations were also as described. In order to form only the Ti2Ni3, an aging treatment was performed under conditions which were M. NISHIDA, formerly with the University of Illinois, is with the Department of Metallurgy, Kumamoto University, Kumamoto, Japan. C. M. WAYMAN is Professor, Department of Materials Science and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted September 25, 1986. METALLURGICALTRANSACTIONSA
prescribed from TTT diagrams describing precipitation behavior for the alloy. Optical microscope observations were made using a Leitz MM5 metallograph with polarized light. TEM observations were carried out in a Hitachi H-800 microscope operated at 200 kV and equipped with -+60 deg tilt and 360 deg rotation and single tilt heating stage holders. III.
RESULTS AND DISCUSSION
A. Optical Microscopy Observations Striated structures were seen in the coarse Ti2Ni3 particles in previous studies 1 of precipitation processes in Ti52 at. pct Ni. The striations were first thought to be due to an etching effect. Therefore, a specimen aged at 700 ~ for 100 hours was observed immediately after electropolishing and without etching under polarized light. The results obtained are shown in Figure 1. Parallel, band-shaped structures are clearly seen in the Ti2Ni3 particles. These are similar to the surface relief for the R-phase transformation in TisoNi47.sFe2.5,3 or the martensitic transformation in Mn-NiC,4 Mn_Cu,5 and so on. It is well known that a surface relief effect is usually associated with a shear transformation. This suggests that a shear transformation takes place in the Ti2Ni3 precipitate phase.
B. TEM Observations Typical microstructures of Ti2Ni3 phase Figure 2 shows typical microstructures in Ti2Ni3 phases which form following several aging conditions. Band or needle-like and antiphase-like microdomains are seen in all cases.
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