Anomalous strain rate dependence of ultra-low temperature strength and ductility of an electron beam additively manufactured near alpha titanium alloy.

• EBSM-built Ti alloy exhibits an irregular drop of UTS at the maximal strain rate at 20 K. • YS rises and ductility reduces monotonically as strain rate rises at 77 and 20 K. • A higher strain rate promotes pyramidal slipping and <10 1 ¯ 0 >34° twinning at 20 K. • Enhanced slip CRSS and more pyramidal slips result in the monotone increase of YS. • Serration flow vanishes at the maximal strain rate due to drastic strain localization. The strain rate (ε ˙) dependence of ultra-low temperature strength and ductility was investigated systematically on a cryogenic near alpha titanium alloy Ti-3Al-3Mo-3Zr-0.2Y additively manufactured by electron beam selective melting (EBSM). As ε ˙ increases under quasi-static tension at 77 and 20 K, ductility monotonically decreases when yield strength (YS) keeps ascending. As ε ˙ increases from 5.6 × 10^–4 s⁻¹ once by one order of magnitude, elongation-to-fracture declines from 20.0 % to 16.5 % and 15.7 % at 20 K. However, unlike the regular monotone increase at 298 and 77 K, ultimate tensile strength (UTS) at 20 K rises to 1460 MPa first then drops to 1320 MPa. This study further examined how strain rate affects slipping, twinning and strain hardening behavior. The monotone increase in YS is primarily attributed to the increased CRSS for slips and the enhanced tendency of pyramidal 〈 a 〉 and 〈 c + a 〉 slipping and <10 1 ¯ 0 > 34° twinning at 20 K. On the other hand, the monotone decrease of ductility is essentially ascribed to the intensified deformation localization characterized by micro shear bands, multiple necking and single necking. More importantly, the mechanisms of abnormal UTS variation and disappearance of serration flow at 20 K are discussed in terms of strain hardening levels, degree of shear deformation localization and the interaction of slips and twins. This study provides deep insights into strain rate effect on cryogenic mechanical behavior of EBSM-built titanium alloy. [Display omitted] [ABSTRACT FROM AUTHOR]