Abstract
Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results will be presented and compared to post-mortem characterization of soft-recovered shocked specimens. Comparisons are made with previous tests where material was shock-loaded and soft recovered for microstructural analysis. Together these techniques create a consistent picture of material behavior during the shock-induced ff–! phase transformation in titanium.
Highlights
The solid-solid phase transition between hexagonal closepacked alpha and hexagonal omega phases in titanium has important implications on mechanical properties
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]
Both tests were carried out using the same shock conditions with different timing to capture early (16-1-003) and late-stage (16-1-002) material behavior
Summary
The solid-solid phase transition between hexagonal closepacked (hcp) alpha and hexagonal omega phases in titanium has important implications on mechanical properties. 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]. It was observed that when shock-loaded to ∼15 GPa, titanium forms a twophase microstructure, with characteristic omega lathes inside alpha grains (suggesting a predominately forward, α → ω pathway where the grains never reach 100% ωphase). It is expected that the α– ω phase transformation is a process characterized by both nucleation and growth Such post-mortem analyses are not well-suited to measure order of operations, relative activity of mechanisms during testing, or kinetics.
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