Work hardening in ultrafine-grained titanium: Multilayering and grading

Ultrafine-grained (UFG) metals exhibit excellent mechanical strength and high hardness at room temperature; however, most have negligible work-hardening ability. In this study we promote the work-hardening ability of UFG Ti through multilayering and grading. Cryorolling in conjunction with surface mechanical attrition treatment have been applied to commercial-purity Ti as a strategy to engineer a multilayered hierarchical structure with a graded microstructural transition between successive layers. When deformed in compression, UFG Ti with a multilayered hierarchical structure shows significant work hardening without sacrificing strength. This behavior has been attributed to the reinforcing effect caused by the juxtaposition of multiple layers and the gradient transformation of the fracture propagation in the junctions between layers. The micromechanical gradation across the layers and in the junctions between layers achieves a more gradual stress redistribution, providing increased resistance to catastrophic failure. A fracture mechanics model is presented to explain the crack-stopping effect of the reinforcing layer. The strategy reported here provides a valuable guide for the development of the nanograined/ultrafine-grained materials with high strength and pronounced work hardening.