Achieving superior strength-ductility synergy in a unique multi-scale heterostructured titanium laminate fabricated by temperature-controlled rolling and subsequent annealing

Abstract

Here we proposed a low-cost and high-efficiency strategy for fabricating novel multi-scale gradient-structured/heterogeneous lamella structured (GS/HLS) pure titanium laminates with superior strength-ductility synergy, by temperature-controlled rolling and subsequent annealing of as-sintered multilayered pure titanium foils with preset grain size gradient. A preliminary gradient structure with the average grain size decreasing from the center layer (alternating stacking of equiaxed fine grain bands and elongated grain bands) to the surface layer (equiaxed fine grains) is formed by optimizing the rolling process. Generalized Schmidt factor analysis based on electron backscatter diffraction indicated that there were slip-dominated and twin-dominated grain refinement mechanisms during the rolling process. In addition to the prismatic slip, two kinds of contraction twinning could be activated to achieve the grain refinement in hard domains where slip systems were hard to be activated. After subsequent annealing, equiaxed fine grains in the laminates did not significantly coarsen, while the elongated grains in the center layer had partially merged and grew up into the elongated coarse grains. Thus, the microstructure of the center layer displayed a typical heterogeneous lamella structure composed of alternately stacking of elongated coarse grains and fine grain bands, together with the grain size gradient, a unique multi-scale heterostructured titanium laminate was formed. Tensile test indicated that the as-rolled Ti laminates exhibited extremely high ultimate tensile strength (UTS) of 930 MPa with an acceptable total elongation to fracture (ETF) of 12.3 %, while GS/HLS Ti laminates after annealing at 425 °C possessed a superior combination of high UTS (823 MPa) and high ETF (22.4 %). The main reason was that the annealing treatment significantly reduced the dislocation density in the elongated coarse grains of GS/HLS Ti laminates. This meant that more geometrically necessary dislocations (GNDs) could accumulate in the plastic deformation process, resulting in the hetero-deformation induced (HDI) strengthening and an extra HDI hardening caused by the unique GS/HLS structure, therefore, an excellent strength-ductility synergy of final GS/HLS Ti laminates was achieved.