Unprecedented phenomena were discovered by tempering the Ti 4Fe 7Al alloy quenched from the b (bcc) field. The alloy became very hard when it was tempered at 450°C for several minutes, and severely rugged surface was generated. The inverse shape recovery phenomenon was also discovered when a quenched specimen that had been bent at room temperature was heated. The tempered microstructure showed almost b grains and some of the usual martensitic acicular structure areas. However, electron back scattering pattern (EBSP) measurements showed that the b like grain was not the bcc structure but was the hcp or orthorhombic structure. X ray diffraction (XRD) measurements clarified that an orthorhombic a″structure (a＝0.299 5 nm, b＝ 0.491 3 nm, c＝0.465 9 nm) was formed from the b phase by tempering. Moreover, this a″structure was confirmed to be a type of martensitic transformation because no concentration distribution was detected in scanning transmission electron microscopy energy dispersive spectroscopy (STEM EDS) analysis of the microstructure. It was suggested that the essential M s point of the alloy should be higher than room temperature; however, the martensite transformation could not operate by fast quenching.The newly discovered a″martensite is formed without atomic diffusion by heating. When the b grain transforms into the single a″  variant, a very huge lattice strain is generated, resulting in the severely rugged surface or the inverse shape recovery phenomenon. Keywords: martensite, M s point, heat treatment, variant, surface roughness, shape recovery

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The purpose of this study was to investigate and discuss the relationship between the crystal structure of martensites and the material properties, such as the Young's modulus and internal friction, in quenched Ti Nb alloys with plasto elastic deformation. In the alloys, minimal Young's modulus was indicated by the a′ martensite of Ti15Nb, and maximal internal friction was indicated by the a″martensite of Ti18Nb. After a weak plastic deformation of 5 rolling reduction, the Young's modulus of Ti (15,18, 20)Nb decreased, and the internal friction of Ti20Nb increased though that of Ti (15, 18)Nb hardly changed. From the XRD profiles, the extreme values of the material properties were predicted to be associated with the unit volume and phases (hexagonal close packed or orthorhombic) of the materials. It was also predicted to be associated with the change of phase from a′ to a″and from a″to a′ ＋a″or a″ ＋a″with different lattice parameters. Cyclic tensile test results after weak plastic deformation revealed characteristic changes in elastic incline, that is, an increase from ～40 GPa in Ti15Nb, and a decrease until ～40 GPa from the initial value followed by an increase in Ti18Nb and Ti20Nb. The consequent changing mechanism of material properties was suggested from the viewpoint of the changing behavior of the phase transformation and transition on martensites with plastic deformation or loaded elastic deformation.

The influence of Al content on the evolution of microstructures and mechanical properties of Ti xFe yAl alloys quenched from a b field was investigated via hardness measurements, tensile tests, scanning electron microscopy (SEM) equipped with electron back scattering pattern (EBSP) analyzer, and conventional transmission electron microscopy (TEM). The hardness of Ti (3 6)Fe yAl decreased remarkably on addition of 1 Al. In comparison to binary alloys, the maximum hardness decrease was achieved in Ti 4Fe 1Al. The hardness of Ti 4Fe yAl decreased with increasing Al content up to around 5, reaching a minimum, then gradually increased. Though all of the Ti 3Fe yAl alloys showed the martensite structure, Ti 4Fe yAl showed a structural change of b＋v → b＋v＋martensite → b＋v with increasing Al content. Consequently, Ti 4Fe 7Al had the lowest density in the b type alloys equivalent to that of Ti 6Al 4V. The ratio of reciprocal distance d 0002v /d 222b on selected area diffraction (SAD) pattern, which is often used as an indicator of {332} 〈113〉twin formation, was nearly constant up to 5 Al of Ti 4Fe yAl, but decreased over 6 Al. The tensile tests of both Ti 4Fe 5Al and Ti 4Fe 7Al showed ductility. The deformation behavior of Ti 4Fe 5Al producing strain induced martensite resembled that of Ti 8Mo. On the other hand, Ti 4Fe 7Al was deformed by a coarse slip of {112} 〈111〉in the same manner as Ti 20Mo, and the SAD pattern analysis revealed that the diffuse reflection of the v phase in the non deformed area turned into the spots at the deformation band. It was suggested that Al behaves as a b stabilizer element when added to the lowest b alloy and is available for weight saving, while maintaining the b structure. (Received January 26, 2012; Accepted February 13, 2012; Published May 1, 2012) Keywords: b stabilizer, a stabilizer, ubiquitous element, lightweight alloy, omega phase, martensite

In this study, we investigated the microstructure, hardness, Young's modulus, and tensile behavior of binary Ti 4～9 mass Mo alloys, quenched from a temperature of 1223 K. Among the solution treated binary Ti Mo alloys, the lowest Young's modulus was observed in the case of the Ti 6Mo alloy, which was the border composition of a′ /a″ . When the rolling deformation of 2 reduction was carried out, the Young's modulus of Ti Mo alloys having less than 6 mass Mo increased, whereas that of higher than 6 mass Mo reduced. The Young's modulus of the 6Mo alloy hardly changed with the rolling deformation. Ti 6Mo X (Y) alloys, where X and Y are 1 mass of Al, Sn, Cr, and Fe, were prepared in order to investigate the influence of additional elements on the microstructure and mechanical properties of the 6Mo alloy. The phase constitution of quenched Ti 6Mo X Y alloys mostly corresponded with the value of the Mo equivalency (Moeq); however, Ti 6Mo 1Al 1Fe (MAF) and Ti 6Mo 1Fe (MF) exhibited slightly more b rich structures for their respective Moeq values. The lattice parameters``a'' and``b'' of the a″  structure in the Ti 6Mo X Y alloys changed with Moeq in a manner similar to that in the Ti Mo alloy; however,``c'' exhibited a different behavior. The mechanical properties of Ti 6Mo X Y alloys except for the MF alloy were similar to those of Ti Mo alloys corresponding Moeq. However, the Young's modulus of the Ti 6Mo X Y alloys was greater than that of the 6Mo alloy. The tensile properties of the MF alloy having b＋v structure were extremely unstable with respect to the fracture elongation. This peculiar behavior is attributed to the occurrence of deformation induced b＋v→a″transformation. The formation of the a″structure caused significant softening and local intense deformation in the a″phase resulted in a brittle fracture. On the other hand, a successive a″formation induced by work hardening would result in good ductility. It was suggested that the unstable elongation in the MF alloy resulted from competition between the reverse effects.