Crystallography of Martensite in TiAu Shape Memory Alloy
- PDF / 793,251 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 28 Downloads / 231 Views
TRODUCTION
THE shape memory effect is not operative in TiNi above the martensitic transformation temperature (Ms) of TiNi binary alloy, which does not exceed 373 K (100 C).[1] Thus, the development of shape memory alloys (SMAs) with Ms higher than 373 K (100 C) is critical for expanding the range of SMA applications.[2] Meanwhile, TiNi SMA has been used recently as an implant material in medical applications. However, this material might increase the risk of Ni sensitization and hypersensitivity reactions in patients.[3] Therefore, an alternative SMA containing hypoallergenic materials is also desired strongly in addition to a high-temperature SMA.[4] TiPd, TiPt, and TiAu alloys are known to undergo a martensitic transformation from a B2 (cubic) to B19 (orthorhombic) structure at Ms values much greater than 373 K (100 C). These alloys also exhibit the shape memory effect,[5–7] and Ms of TiPd and TiAu can be controlled by ternary addition of various elements.[8–11] Moreover, Ti, Au, and Pt are known to be hypoallergenic in the human body, and alloys of these elements are, thus, attractive as the base alloy of hightemperature SMAs and new biomedical SMAs. Shape memory properties such as the recovery strain, hysteresis, critical stress to induce martensite, and orientation dependence can be determined by a crystallographic analysis of the martensitic transformation. The crystallography of the B19 martensite in TiPd and TiPt has been examined in detail[12,13]; however, only a limited information has been reported for the martensite in TiAu, although Wu and Wayman[8] have reported the morphology and internal twins of the martensite. The objective of the current study is to clarify in detail the crystallographic features of martensite in an T. INAMURA, Associate Professor, and H. HOSODA, Professor, are with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan. Contact e-mail: inamura. [email protected] Manuscript submitted May 23, 2010. Article published online October 9, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
equiatomic TiAu alloy. We focus on the complete set of crystallographic parameters that are determined by the invariant plane strain condition, as well as a microstructure in which energy is minimized.
II.
EXPERIMENTAL PROCEDURE
Nominal composition of alloy ingot was 50 mol pct Ti-50 mol pct Au (TiAu). The purity of the starting materials was 99.99 pct. Ingot was fabricated by Ar arc melting in an Ar/H2 (1 pct) reducing atmosphere using a nonconsumable W electrode and a water-cooled Cu hearth. The ingot was melted six times and was flipped after each melting. The chemical composition of the fabricated ingot was determined to be 50.6 mol pct Ti-49.4 mol pct Au by electron probe microanalysis (JEOL JXA-8200, Japan Electron Optics Laboratory, Akishima, Tokyo, Japan). The ingot was homogenized at 1173 K (900 C) for 3.6 ks and then hot forged at 1373 K (1100 C) for 10.8 ks in vacuum (~10–1 Pa) to form a 1-mm-thick plate. The surface scale was removed mechanic
Data Loading...
