The first study of the effect of the initial microstructure on its evolution under hydrostatic compression before, during, and after the irreversible α → ω phase transformation and during pressure release in Zr using in situ x-ray diffraction is presented. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α → ω initiates at lower pressure for pre-deformed sample but above volume fraction of ω Zr c = 0.7, larger volume fraction is observed for the annealed sample. This implies that the general theory based on the proportionality between the athermal resistance to the transformation and the yield strength must be essentially advanced. The crystal domain size significantly reduces, and microstrain and dislocation density increase during loading for both α and ω phases in their single-phase regions. For the α phase, domain sizes are much smaller for prestrained Zr, while microstrain and dislocation densities are much higher. For the cold-rolled sample at 5.9 GPa (just before initiation of transformation), domain size in α Zr decreased to ∼ 45 nm and dislocation density increased to 1.1 × 1015 lines/m2 , values similar to those after severe plastic deformation under high pressure. Despite the generally accepted concept that hydrostatic pressure does not cause plastic straining, it does and is estimated. During transformation, the first rule was found: The average domain size, microstrain, and dislocation density in ω Zr for c < 0.8 are functions of the volume fraction of ω Zr only, which are independent of the plastic strain tensor prior to transformation and pressure. The microstructure is not inherited during phase transformation. Surprisingly, for the annealed sample, the final dislocation density and average microstrain after pressure release in the ω phase are larger than for the severely pre-deformed sample. The significant evolution of the microstructure and its effect on phase transformation demonstrates that their postmortem evaluation does not represent the actual conditions during loading. A simple model for the initiation of the phase transformation involving microstrain is suggested. The results suggest that an extended experimental basis is required for the predictive models for the combined pressure-induced phase transformations and microstructure evolutions.
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Lattice Parameter Evolution during the β-to-α and β-to-ω Transformations of Iron- and Aluminum-Modified Ti-11Cr(at.%)
β-titanium (β-Ti) alloys are useful in diverse industries because their mechanical properties can be tuned by transforming the metastable β phase into other metastable and stable phases. Relationships between lattice parameter and β-Ti alloy concentrations have been explored, but the lattice parameter evolution during β-phase transformations is not well understood. In this work, the β-Ti alloys, Ti-11Cr, Ti-11Cr-0.85Fe, Ti-11Cr-5.3Al, and Ti-11Cr-0.85Fe-5.3Al (all in at.%), underwent a 400 °C aging treatment for up to 12 h to induce the β-to-ω and β-to-α phase transformations. Phase identification and lattice parameters were measured in situ using high-temperature X-ray diffraction. Phase compositions were measured ex situ using atom probe tomography. During the phase transformations, Cr and Fe diffused from the ω and α phases into the β matrix, and the β-phase lattice parameter exhibited a corresponding decrease. The decrease in β-phase lattice parameter affected the α- and ω-phase lattice parameters. The α phase in the Fe-free alloys exhibited α-phase c/a ratios close to those of pure Ti. A larger β-phase composition change in Ti-11Cr resulted in larger ω-phase lattice parameter changes than in Ti-11Cr-0.85Fe. This work illuminates the complex relationship between diffusion, composition, and structure for these diffusive/displacive transformations.
more » « less- Award ID(s):
- 1607942
- NSF-PAR ID:
- 10491947
- Publisher / Repository:
- mdpi
- Date Published:
- Journal Name:
- Crystals
- Volume:
- 14
- Issue:
- 2
- ISSN:
- 2073-4352
- Page Range / eLocation ID:
- 145
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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