Development of an ω-Phase in Grade 2 Titanium Processed by HPT at High Hydrostatic Pressure

Abstract

High-pressure torsion was used to process grade 2 titanium to 10 turns using different pressures up to a maximum of 8 GPa. X ray diffraction showed that the as-received material and the material processed at pressures of 2 GPa and 4 GPa were single phase but the material processed at 8 GPa contained both the h.c.p. and the simple hexagonal ω-phase. Nanoindentation was used to determine the mechanical behavior and the results show a pronounced increase in hardness due both to the severe plastic deformation processing and to the phase transformation at the highest pressure. A maximum hardness of ~4.7 GPa was attained.

Phase Transformations in MgH2–TiH2 Hydrogen Storage System by High‐Pressure Torsion Process

“…Moreover, the intensity of the γ phase peaks increases with increasing the number of turns, that is, with increasing the shear strain ( γ = 2π rN / h ; γ : shear strain; r : distance from disc center, N : number of turns, h : disc thickness). The current results are consistent with an earlier publication on HPT processing of MgH 2 and numerous publications on the effect of shear strain on facilitation of phase transformations in metallic and non‐metallic materials . However, as shown in Figure b and c, no phase transformation is detected in TiH 2 after HPT processing at room and cryogenic temperatures.…”

“…The main difference between the HPT method and other mechanochemical methods is the introduction of large shear strains under high pressure . Application of shear strain during the HPT process significantly reduces the energy barrier for phase transformation and improves the kinetics . Moreover, since large fractions of defects such as vacancies, dislocations, and grain boundaries are introduced during shearing by the HPT process, the internal energy of the material and thus the thermodynamic stability of phases changes under the process.…”

“…High-pressure torsion (HPT) method, which is mainly used as a severe plastic deformation (SPD) technique for grain refinement in metals, [35][36][37] is a promising process to synthesize metastable phases in different groups of materials. [38] The method has been successfully used by different groups including the group of authors to control the phase transformation in pure metals (Ti, [39,40] Zr, [41,42] Co [43,44] ), semiconductor elements (Si, [45,46] Ge, [47,48] ), carbon polymorphs (graphite, [44,49] diamond, [49] fullerene [50,51] ), metallic alloys, [52][53][54][55][56] oxides (Al 2 O 3 , [57] ZrO 2 , [58] BaTiO 3 , [59] TiO 2 , [60,61] Y 2 O 3 , [62] ZnO, [63] SiO 2 , [64] VO , [64] ), nitrides (BN, [65] ), carbides (SiC [66] ), hydrides (MgH [13,67] ), and polymers. [68,69] Moreover, the method has been successfully employed to hydrogen storage materials to enhance the kinetics, [70][71][72] to improve the activation [73,…”

Nanostructuring Ti-Alloys by HPT: Phase Transformation, Mechanical and Corrosion Properties, and Bioactivation

“…Processing titanium and its alloys via HPT can lead to mechanically induced phase transformation, where metastable phases may precipitate. 19,20,6063) When samples are deformed by HPT, there are indications that grains larger than the submicron can induce the ¢ to ½ transformation under a relatively low pressure of 3 GPa 60) in high alloyed ¢-TiNb TaZrO alloy or transformation of ¡ phase to ½ phase by applying loads as high 6 GPa or higher no matter the number of revolutions or rotation speed in the Ti6Al7Nb alloy 69) or commercially pure Ti, 61,70) or yet, also in cp-Ti, the amount of ½ phase is dependent on the rotation speed and the number of rotations 71) applying 5 GPa, the fraction of the ½-phase decreases when the rotation speed is increased. There is a substantial augment of such a phase with the number of rotations.…”