The α–ω phase transition in shock-loaded titanium

Abstract

We present a series of experiments probing the martensitic α–ω (hexagonal close-packed to simple hexagonal) transition in titanium under shock-loading to peak stresses around 15 GPa. Gas-gun plate impact techniques were used to locate the α–ω transition stress with a laser-based velocimetry diagnostic. A change in the shock-wave profile at 10.1 GPa suggests the transition begins at this stress. A second experiment shock-loaded and then soft-recovered a similar titanium sample. This recovered material was then analyzed with electron-backscatter diffraction methods, revealing on average approximately 65% retained ω phase. Furthermore, based on careful analysis of the microstructure, we propose that the titanium never reached a full ω state, and that there was no observed phase-reversion from ω to α. Texture analysis suggests that any α titanium found in the recovered sample is the original α. At these stresses, the data show that both the α and ω phases are stable and can coexist even though the shock-wave presents as steady-state.

In-situ Experiments to Capture the Evolution of Microstructure During Phase Transformation of Titanium Under Dynamic Loading

“…High-purity (99.995%) polycrystalline titanium was used for this study. The material chemistry and processing was the same as a previous study [7]. Briefly, the material was supplied by American Elements (USA) as 20 mm thick plate.…”

“…The specimen was fixed over a hole on a brass holder and bolted to a larger target assembly to interface with the EPJ Web of Conferences 183, (2018) https://doi.org/10.1051/epjconf/201818303020 DYMAT 2018 Table 1. Chemical composition of as-received titanium plate, in ppm [7].…”

“…Omega phase can form in hcp metals under shock loading conditions, and, as a metastable phase at ambient pressure, can be retained in the microstructure after unloading, where it can contribute to subsequent deformation behavior [1][2][3][4][5]. Previous studies have examined the shock response of Ti single and multicrystals [6], and polycrystals [7][8][9]. Post-mortem microstructural investigations are an essential piece to determination of phase transformation pathways, but can only infer likely operations based on beginning and end states.…”

STRAIN INDUCED PHASE TRANSFORMATION IN Ti-15Mo β ALLOY

“…It is preserved by the addi on of alloying elements called β-phase stabilizers [7]. The metastable nature of the β-Ti phase has caused a lot of confusion about the phase transforma on of β-Ti alloys in processing, par cularly in cold working/annealing, and therefore about the strengthening mechanisms under different condi ons [19][20][21][22][23][24][25]. The phase transforma on of β-Ti to α-Ti through a series of β + β‫׳‬ and ω, α‫׳‬ or α‫״‬ etc.…”

“…The structure of the two β products are BCC, as same as the parent β phase, resul ng in no extra diffrac on spots other than those of β in a selected area electron diffrac on (SAED) pa�ern [35]. The phase transforma on is further complicated by the fact that α to ω and β to ω transforma on are enhanced by stress, resul ng in combined phase transforma on induced plas city (TRIP) and twinning induced plas city (TWIP) [21,22,[36][37][38].…”