Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy

Abstract

The microstructural evolution and grain refinement mechanisms of a Ti-10V-2Fe-3Al alloy, b-solution quenched and cold forged (CF) to strains of 0.1, 0.35, and 1.2 have been investigated using optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the stress-induced martensitic transformation became a predominant deformation mode in the metastable Ti-10V-2Fe-3Al alloy during cold forging. These martensites a¢¢ repeatedly divided the original b parent phase into a large number of micronsized blocks when the forging strain was 0.1. Shear bands were observed to traverse a¢¢/b lamellae and resulted in a significant grain refinement of the b phase, as the forging strain increased to 0.35. The degree of grain refinement inside shear bands was higher than the outside. Nanocrystalline and amorphous structures were produced in local areas of the original b phase, when the forging strain rose to 1.2. This dramatic grain refinement in the metastable Ti-10V-2Fe-3Al alloy could be attributed to the stress-induced martensitic transformation promoting the initiation and growth of shear bands across a¢¢/b lamellae. More dislocations were produced and accumulated inside grains to accommodate plastic deformation. The crystal structure was collapsed and an amorphous structure was formed as soon as the dislocation density was accumulated to a critical value of 10 14 /cm 2 . Moreover, some of the reverse martensitic phase transformation, a¢¢fib, was observed to contribute to grain refinement of Ti-10V-2Fe-3Al alloy as well.

Formation of nanocrystalline β structure in metastable beta Ti alloy during high pressure torsion: The role played by stress induced martensitic transformation

“…It is observed that the amount of the stress induced a 00 decreases with increasing strain (completely disappears at the highest equivalent strain applied of 242), consistent with the observations earlier [14][15][16][17]. Although the reverse a 00 to b transformation has been proposed to be responsible [14][15][16][17], the speculated cause -local temperature rise above A s due to SPD -is not justified.…”

“…Similar mechanisms of grain refinement were proposed in a Ti-10V-2F e-3Al alloy processed by cold forging to large strains, with the final nano b grains produced by rotational dynamic recrystallisation in the spreading shear bands [17]. The lack of a 00 in the final microstructure, however, prompted the suggestion that local temperature increases to above the so-called austenitic starting temperature (A s ) [13] [14][15][16][17]. Xu et al [14][15][16] believed that dislocation pileups at the a 00 /b interfaces may have resulted in the dissipation of some mechanical energy as heat, leading to a local increase in temperature, whereas Chen et al [17] attributed the temperature rise to the formation of adiabatic shear bands.…”

“…The a 00 phase would then transform back to b in the reverse transformation, causing further dislocation movement and forming the final nanocrystalline b structure. Similar mechanisms of grain refinement were proposed in a Ti-10V-2F e-3Al alloy processed by cold forging to large strains, with the final nano b grains produced by rotational dynamic recrystallisation in the spreading shear bands [17]. The lack of a 00 in the final microstructure, however, prompted the suggestion that local temperature increases to above the so-called austenitic starting temperature (A s ) [13] [14][15][16][17].…”

“…In contrast, the stabilised b-Ti alloys feature more of the ductile BCC phase and are expected to be more amenable to SPD. Although no martensitic phase transformations take place upon quenching, it has been shown that a stress induced orthorhombic martensitic phase known as a 00 may form during plastic deformation of some less stabilised b alloys [11][12][13], and the stress induced phase transformation would facilitate the formation of nano-sized b grains by introducing additional grain refinement mechanisms [14][15][16][17][18][19], leading to the production of nanocrystalline b Ti alloys by SPD.…”

“…The lack of a 00 in the final microstructure, however, prompted the suggestion that local temperature increases to above the so-called austenitic starting temperature (A s ) [13] [14][15][16][17]. Xu et al [14][15][16] believed that dislocation pileups at the a 00 /b interfaces may have resulted in the dissipation of some mechanical energy as heat, leading to a local increase in temperature, whereas Chen et al [17] attributed the temperature rise to the formation of adiabatic shear bands. Further, it has been implied that the nano-sized b grains were too small for any formation of new a 00 [16].…”