Unprecedented Phenomena with Tempering of Ti-4Fe-7Al Alloy

Abstract

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

Computational thermodynamics and kinetics of displacive transformations in titanium-based alloys

“…One particular interesting phenomenon is the -catalysis‖ effect: For some as-quenched alloy with β phase partially transformed to martensite, a short heat treatment can make the transformation complete. This has been reported in various metastable-β Ti alloys [75,91,99,100]. For the thermodynamic and kinetic modeling of martensitic transformation in such systems, one has to keep in mind that ω phase or its embryo may conceal the real M S temperature, and that the kinetics of ω phase may have a timedependent effect on martensitic kinetics.…”

“…Mo (Figure 7). However, there is evidence that such a β-stabilizing effect of Al, Sn and Zr also exists in other systems like Ti-Fe/Cr/Mo-based ternary alloys [91][92][93][94]. Simply using fixed chemical interactions will need system-by-system assessments.…”

Peculiar Transformation Behavior and Microstructure Evolution with Tempering of Ti-4Fe-7Al Alloy

“…these results are shown along with the results of a previous study 8) in which the used alloy batch was different from that used in this study. …”

Recent Topics of Titanium Research and Development in Japan

“…The alloy shows a significant hardening by aging at 723 K after solution treatment as shown in Fig. 21 [11]. The constituent phases after solution treatment are the and phases; by aging, these phases transform to " martensite.…”