A polymer-like ultrahigh-strength metal alloy

Futuristic technologies like morphing aircrafts and superstrong artificial muscles are hinged on metal alloys being as strong as an ultrahigh-strength steel (with a high yield strength σ_y >1 GPa) yet as flexible as a polymer (with an ultralow elastic modulus E ~10 GPa)^1-3. However, achieving such “strong yet flexible” alloys has proven challenging^4-9. The difficulty lies in an inevitable trade-off between strength and flexibility^5,8,10, which precludes a high-strength alloy from being of polymer-like ultralow modulus. Here we report a Ti-50.8 at.% Ni strain glass alloy showing an unprecedented combination of an ultrahigh yield strength σ_y ~1.8 GPa with a polymer-like ultralow elastic modulus E ~10.5 GPa, together with a superlarge rubber-like J-shaped elastic strain of ~8%. As a result, it possesses the highest flexibility figure of merit σ_y/E ~0.17 which far exceeds that of existing structural materials. This alloy was fabricated by a simple 3-step thermomechanical treatment, which leads to not only ultrahigh strength but also ultralow modulus through forming a unique “dual-seed strain glass” (DS-STG) microstructure, being a strain glass matrix embedded with a small amount of R and B19' martensites. In-situ x-ray diffractometry reveals that the DS-STG enables a nucleation-free reversible transition between strain glass and R and B19’ martensites during stress loading/unloading, thereby leading to ultralow modulus and large recoverable strain with narrow hysteresis. Our finding may open a new horizon for designing and mass-producing strong and flexible alloys and such alloys may lead to important applications.