Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy
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IN recent years, considerable interest has been placed in near-equiatomic Ni-Ti alloys due to their unique shape memory effect (SME)[1,2,3] and outstanding mechanical properties.[4,5,6] In addition, Ni-Ti alloys exhibit excellent corrosion resistance[7–10] and are biocompatible in the human body environment[9,10] making them excellent candidates for the manufacture of implant devices. In these alloys, thermalmechanical processing[11–14] or additions of other alloying elements[15,16] are often used to modify the austenite-martensite (A-M) phase transformation temperatures, as well as the alloy shape memory properties. Nevertheless, small deviations from stochiometry can give rise to significant second-phase precipitation,[17,18] which in turn leads to changes in both the SME and the mechanical properties. The flow behavior exhibited by Ni-Ti alloys is strongly influenced by the type of applied stresses. In general, they can be thermoelastically deformed up to 8 pct. However, the amount of recoverable strain is strongly influenced by the Ni content, thermal-mechanical treatment, and the transformation sequence exhibited by the alloy during thermal cycling.[11–14] Although most of the work published has been focused in near stoichiometric or Ni-rich alloys, Ti-rich Ni-Ti alloys also exhibit thermoelastic behavior. In this case, the peak temperature for the transformation from martensite to austenite increases rapidly to values near 373 K, for Ti contents H.F. LOPEZ, Professor, is with the Materials Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. A. SALINAS, Professor, is with CINVESTAV-Unidad Saltillo, 25000 Saltillo, Coahuila, Mexico. H. ´ N, Professor, is with ESFM-National Polytechnic of Mexico, CALDERO 07300 Mexico, D.F., Mexico. Manuscript submitted June 30, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
in excess of 52 at. pct.[4,9] Moreover, the occurrence of a significant precipitation of inherently brittle, coarse NiTi2 particles[19] may severely impair the alloy mechanical properties. According to the Ni-Ti phase diagram, a peritectic reaction is expected to occur in alloys that exhibit a significant deviation from stoichiometry.[20] Thus, relatively large volume fractions of precipitate particles can be formed, which in turn can (a) provide preferential sites for martensite nucleation and (b) act as effective obstacles for the propagation of martensite interfaces. In a previous article,[19] it was reported that the NiTi2 phase forms as an external precipitate envelope surrounding a Ti-rich metastable solid probably as a result of a nonequilibrium peritectic reaction during solidification. These inherently brittle and hard particles can act as preferential sites for the buildup of local internal stresses when the alloy is subjected to thermal and/or mechanical processing. In the case of incoherent phases, the main contribution to the development of internal stresses arises from size and elastic moduli mismatches between matrix and precipitates. Externally imposed plastic deformati
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