Deformation of NiTiCu shape memory single crystals in compression
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SEVERAL research results have been published on the shape memory behavior of NiTi10Cu polycrystalline alloys.[1–10] These materials exhibit lower transformation stress and a two-step transformation strain compared to the NiTi alloys. The transformation from B2 (austenitic) phase to an intermediate orthorhombic phase (B19) occurs first followed by transformation to the monoclinic martensite phase (B19⬘). The NiTi10Cu exhibits a narrower hysteresis and a narrower transformation temperature range compared to NiTi alloys. We note, however, that the tails of the hysteresis can extend further than in case of the binary NiTi.[10] These features can be exploited in specific applications. One of the advantages of these alloys is that aging is not required to achieve pseudoleastic stress-strain response at room temperature (for R.T. ⱖ Af where Af is the austenite finish temperature). Upon slow cooling, Cu clusters emerge from solution and raise the martensite start (Ms) temperature of B2 → B19 transformation to values near 270 K. Profit et al.[10] showed the benefits of NiTiCu alloys compared to the NiTi alloys as having a stable Ms temperature (as opposed to a decrease in Ms with cycling) and nearly constant recoverable strains under cyclic loading. Retaining shape memory HUSEYIN SEHITOGLU, Grayce Wicall Gauthier Professor, IBRAHIM KARAMAN, Graduate Research Assistant, X. ZHANG, Postdoctoral Research Associate, and HONG KIM, Undergraduate Student, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. YURIY CHUMLYAKOV, Professor, and I. KIREEVA, Research Associate, are with the Siberian Physical-Technical Institute, Tomsk 634050, Russia. HANS J. MAIER, Professor, is with the Lehrstuhl f. Werkstoffkunde, (Materials Science) University Paderborn, 33098 Paderborn, Germany. Manuscript submitted February 18, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
properties under fatigue conditions increases the reliability when designing with NiTiCu alloys and makes them more attractive in specific applications compared to other shape memory alloys. However, the lower strength of NiTiCu can produce loss of shape memory properties with number of fatigue cycles in certain cases. Despite the potential interest in this class of alloys, there has been significantly less work reported compared to the extensive literature on binary NiTi alloys. Saburi and co-workers[1–4] have determined the Clausius– Clapeyron relation for the cubic to orthorhombic transformation and reported the transformation stress-temperature slope in these alloys as 8.4 MPa/K with a corresponding maximum recoverable strain of 3.2 pct. Their transformation stress levels in the pseudoelastic regime varied from nearly zero at Ms temperature to 200 MPa at Ms ⫹ 15 deg. As noted by Saburi, the presence of grain boundaries influenced the reorientation of the martensite variants upon heating. Saburi et al.[3] determined experimentally the macroscopic habit plane for B2 → B19 as {334} type. No information on the transforma
