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

Ti 5.5Al 2Fe alloy has been theoretically designed by modification of Ti 6Al 4V alloy, using ubiquitously alloying elements in order to establish the strategic method for suppressing utilization of rare metals. The utilization of the cold crucible levitation melting (CCLM) is very useful, because titanium is very chemically reactive at high temperature. The experimental alloys with high purity and without contaminations from a crucible were prepared, and the homogeneous melt was also achieved by the diffusion mixing effect of CCLM. The microstructure, phase stability, strength, corrosion resistance and workable properties of the designed alloy, were comparable to those of Ti 6Al 4V. The alloy design can be successfully carried out even using ubiquitously alloying elements by the d electrons concept, which leads to the establishment of one method for the strategic utilization of rare metals.

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

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