Superior strength-ductility synergy by microstructural heterogeneities in pure titanium

Abstract

Optimizing strength and ductility by uniting microstructural heterogeneity and profitable crystallographic orientation is a challenge due to processing difficulties. Here, we synthesize a new kind of highly-oriented heterostructure (HOH) in pure titanium (Ti) by controlling hot extrusion, cold swaging, and annealing processing. The processed microstructure is characterized by (i) obvious grain size heterogeneity and (ii) highly-preferred crystallographic orientation with the c-axis of most grains perpendicular to the rolling direction (RD). The HOH Ti exhibits superior strength-ductility synergy in the RD, while higher strength but lower ductility in the transverse direction (TD). When the sample is stretched along the RD, obvious geometrically necessary dislocation accumulation and strain gradients near grain boundaries in a soft zone are experimentally verified, inducing extraordinary strengthening/hardening by producing hetero-deformation-induced stress and promoting dislocation storage. In the TD, {10–12} twins are observed in a hard zone while no plastic deformation occurs in the soft zone. It is further revealed that both the hard and soft zones exhibit high Schmid factors (m) for the prismatic, pyramidal <a> slip and pyr1/pyr2 <c + a> slip in the RD, indicating a soft slip mode. On the other hand, the soft zone becomes “hard” due to very low m values for the prismatic <a> slip in the TD, resulting in fewer slip systems in the soft zone. These observations suggest that the heterostructure design with profitable crystallographic orientation is an effective approach to achieving high strength-ductility synergy.