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Abstract
Titanium has a significant potential for the cryogenic industrial fields such as aerospace and liquefied gas storage and transportation due to its excellent low temperature properties. To develop and advance the technologies in cryogenic industries, it is required to fully understand the underlying deformation mechanisms of Ti under the extreme cryogenic environment. Here, we report a study of the lattice behaviour in grain families of Grade 2 CP-Ti during in-situ neutron diffraction test in tension at temperatures of 15–298 K. Combined with the neutron diffraction intensity analysis, EBSD measurements revealed that the twinning activity was more active at lower temperature, and the behaviour was complicated with decreasing temperature. The deviation of linearity in the lattice strains was caused by the load-redistribution between plastically soft and hard grain families, resulting in the three-stage hardening behaviour. The lattice strain behaviour further deviated from linearity with decreasing temperature, leading to the transition of plastically soft-to-hard or hard-to-soft characteristic of particular grain families at cryogenic temperature. The improvement of ductility can be attributed to the increased twinning activity and a significant change of lattice deformation behaviour at cryogenic temperature.
Highlights
Titanium has a significant potential for the cryogenic industrial fields such as aerospace and liquefied gas storage and transportation due to its excellent low temperature properties
We found a significant change of lattice strain evolution with the transition of plastically soft-to-hard and hard-to-soft characteristic in certain grain families at cryogenic temperature, probably suggesting that a particular slip system can be promoted with decreasing temperature
It is noteworthy that three-stage hardening behaviour—(i) stage A: a decrease of hardening rate after yielding, (ii) stage B: an increase of hardening rate and (ii) stage C: a decrease of hardening rate in plastic flow regime—is observed in both rolling direction (RD) and transverse direction (TD) samples, and the strain-hardening plateau between the stage A and B is further manifested as temperature decreases
Summary
Titanium has a significant potential for the cryogenic industrial fields such as aerospace and liquefied gas storage and transportation due to its excellent low temperature properties. Most of metallic materials exhibit a ductile-to-brittle transition at low temperature, whilst several Ti alloys with a low impurity show better structural material properties (e.g., fatigue and toughness)[3–5], and surprisingly higher ductility at cryogenic temperature[4,6,7] This signifies a potential of improving the formability of Ti through cryogenic processing techniques (e.g., cryo-rolling and -forging)[8–11]. Since the plasticity of well-oriented grains in polycrystalline α-Ti initiates with the activation of the preferred slip systems, which satisfy their CRSS in the given orientation and applied stress conditions, the local deformation response is highly sensitive to the lattice orientation[30–33]. The refined crystallographic information can be extracted by the analysis of the neutron spectra, regarding to the reflection-dependent lattice spacings, grain orientation and phase volume fraction This in-situ technique enables to acquire the diffraction data during the testing process, whereby the deformation responses of constituent microstructures can be monitored in both elastic and plastic regime, especially at the elastic-toplastic transition Section. This in-situ technique enables to acquire the diffraction data during the testing process, whereby the deformation responses of constituent microstructures can be monitored in both elastic and plastic regime, especially at the elastic-toplastic transition Section. 30–33
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