Concentration-Dependent High-Temperature Deformation Mechanism of Hexagonal Close-Packed Ti-Al Alloys
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, KUN XU, QINGGANG LI, JUNYAN WU, and ZHI WANG
The dependence of Al alloying on the high-temperature deformation physics of Ti-Al alloys was investigated in the present study. The mechanisms of planar dislocation slip, enhanced hc + ai activity and stacking faults were operative in low-alloyed Ti-Al model materials upon plastic deformation 600 C. Increasing Al content up to 10.2 at. pct triggered significant pyramidal at slip and 10 12 twins, highlighting the role of Al alloying. Atomic-scale characterization revealed the nature of the topologically discontinuous ‘‘stepped’’ twin boundary, consisting of alternately stacked basal/prismatic interface couples without a traditional rigorous twin invariant plane. This is the first time significant twinning has been observed in severely alloyed polycrystalline hcp metallic alloys, especially plastically deforming at 600 C with a low strain rate of 1 9 104 s1. A lattice-reorientation-based twinning mechanism, as well as its influence on mechanical properties, was proposed and discussed. https://doi.org/10.1007/s11661-020-05731-2 The Minerals, Metals & Materials Society and ASM International 2020
I.
INTRODUCTION
TITANIUM and its alloys constitute a very important class of structural materials, both technically and industrially, with engineering applications in aerospace, automobile, and aircraft manufacturing.[1,2] As one of the typical hexagonal close packed (hcp) metals, pure Ti has a c/a ratio lower than the ideal value (1.633), making prismatic slip the easiest to trigger.[3] Plastic deformation along the crystallographic hci direction is only dependent on the gliding of hc + ai dislocations, twinning, or both.[4] Therefore, easy prismatic slip in combination with excessive tensile stress (normal to the basal plane) produces severe deformation anisotropy with highly localized damage accumulation and consequently causes rapid failure.[5] This is also the reason for the poor tensile ductility of strongly textured hcp metallic sheets when processed by rolling or extrusion.[6] Of the substitutional elements that alloy to Ti, Al is the most commonly used. Replacing Ti by Al elevated the threshold stress required for basal and prismatic slip known as solute solution strengthening; however, the effect is much more pronounced for easier prismatic
H. WU, Q. LI, J. WU, and Z. WANG are with the School of Materials Science and Engineering, University of Jinan, Jinan, 250022, P.R. China. Contact e-mail: [email protected], [email protected] and K. XU is with Anhui Dexinjia Biopharm Co., Ltd, Taihe, 236600, P.R. China. Manuscript submitted September 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
slip.[3] At 6.6 wt pct Al, these effects contributed equally with significant deformation isotropy.[7] This is because of preferred atomic substitution on prismatic planes as revealed by atomic-scale geometric phase analysis coupled with ab initio calculations.[8] This anisotropic substitution behavior may facilitate local short- or long-range Al ordering. Another role of Al
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