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

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