Microstructure Evolution and Phase Transformation of Ti‐1.0 wt%Fe Alloy with an Equiaxed α + β Initial Microstructure during High‐Pressure Torsion and Subsequent Annealing

Abstract

Herein, the microstructure evolution and phase transformation during high‐pressure torsion (HPT) and subsequent annealing of Ti‐1.0 wt%Fe alloy with an equiaxed α + β initial microstructure are investigated. Both α and β grains in the initial microstructure fragment into smaller and elongated areas during HPT. X‐ray diffraction (XRD) analysis reveals that the deformation‐induced α to ω and β to ω phase transformations occur from the onset of HPT deformation. The β phase totally disappears after 1.5 rotations, whereas some α phase still remains even after 4 rotations. The nano‐hardness of β grains is much larger than that of α grains in the initial microstructure, due to the nano‐sized athermal ω precipitates inside the β grains. With an increase in the HPT rotation, the nano‐hardness of prior α areas continuously increases, whereas that of the prior β areas gradually decreases. During the subsequent annealing of HPT‐deformed sample, both ω to α and ω to β reverse‐phase transformations occur, and the deformation‐induced ω phase totally disappears at annealing temperatures above 400 °C. The recrystallization of α phase completes at an annealing temperature of 500 °C. At higher annealing temperatures, secondary α precipitates with ultrafine grain sizes start to appear inside the prior β areas.

Formation and Thermal Stability of ω-Ti(Fe) in α-Phase-Based Ti(Fe) Alloys

“…The retained amount of the high-pressure ω-Ti(Fe) phase after HPT and the transformation pathway varied, depending on the initial phase fractions and the chemical compositions of the quenched phases [26]. Heat-treated alloys with Fe contents below 4 wt.% contained α -Ti martensite and/or metastable β-(Ti,Fe) after quenching [24][25][26][27][28]. During HPT, α -Ti and β-(Ti,Fe) transformed partially to ω-Ti(Fe).The Fe content in β-(Ti,Fe) depends on the overall Fe concentration in the Ti-Fe alloys and it can vary in a relatively broad range.…”

“…During HPT, α -Ti and β-(Ti,Fe) transformed partially to ω-Ti(Fe).The Fe content in β-(Ti,Fe) depends on the overall Fe concentration in the Ti-Fe alloys and it can vary in a relatively broad range. However, if β-(Ti,Fe) contains~4 wt.% Fe, athermal ω-Ti(Fe) can be formed within the β-(Ti,Fe) grains after quenching [28,29]. The phase transformation β-(Ti,Fe) → ω-Ti(Fe) is promoted for an iron content about 4 wt.% Fe, because both crystal structures possess a strong orientation relationship (OR) {111} β ||(0001) ω and 110 β || 1120 ω [18,22], and because the atomic distances within the habitus planes match perfectly together at this composition.…”

“…Thus, this phase transformation proceeds diffusionless by shearing the crystal structure of β-(Ti,Fe) in the HPT process [26]. For lower Fe contents (≤2 wt.% Fe), the HPT process induces an incomplete ω-Ti(Fe) phase transition [25,28], because the transition of α -Ti to ω-Ti(Fe) dominates the phase transformation process, which involves the redistribution of iron atoms between α -Ti and ω-Ti(Fe). The mass transfer impedes finally the phase transformation [26,28].So far, the thermal stability of HPT-induced ω-Ti(Fe) was predominantly investigated in the samples, which were annealed above the eutectoid temperature (~595 • C) prior to the HPT process and that contained α-Ti, β-(Ti,Fe) and ω-Ti(Fe) in different ratios after the HPT treatment [27,28].…”

“…Thus, this phase transformation proceeds diffusionless by shearing the crystal structure of β-(Ti,Fe) in the HPT process [26]. For lower Fe contents (≤2 wt.% Fe), the HPT process induces an incomplete ω-Ti(Fe) phase transition [25,28], because the transition of α -Ti to ω-Ti(Fe) dominates the phase transformation process, which involves the redistribution of iron atoms between α -Ti and ω-Ti(Fe). The mass transfer impedes finally the phase transformation [26,28].…”

“…So far, the thermal stability of HPT-induced ω-Ti(Fe) was predominantly investigated in the samples, which were annealed above the eutectoid temperature (~595 • C) prior to the HPT process and that contained α-Ti, β-(Ti,Fe) and ω-Ti(Fe) in different ratios after the HPT treatment [27,28]. In these samples, the HPT-induced ω-Ti(Fe) phase transformed upon heating at 380 • C into a supersaturated hexagonal α-Ti(Fe) phase.…”