Suppressing embrittlement and enhancing thermal resistance of bulk superlattice alloys by controllable grain-boundary segregation

Abstract

Severe grain-boundary embrittlement at ambient temperatures poses one of the most critical challenges for wide applications of superlattice alloys as high-performance structural materials. Indispensable active constituents like Al are recognized as the major cause of such embrittlement by releasing atomic hydrogen from moisture, which violently weakens grain boundaries (GBs) and promotes stress localization. Challenging conventional wisdom, here we surprisingly discover an anomalous ductilization effect in the L1₂-structured Co₃Ti alloys, where Al alloying conversely suppresses the intergranular brittleness and meanwhile dramatically increases tensile ductility from ∼4.1% to 30%. Further experiments and calculations revealed that the grain-boundary brittleness in bulk L1₂ Co₃Ti alloys is directly related to the preservation of L1₂ chemical order up to the boundary plane, which fortunately, can be destroyed by inducing Co-atom segregation through the alloying of a L1₂ destabilizer Al, as well as Fe. Such chemical-partitioning-induced disordered intergranular buffer significantly reduces the resistance to dislocation slip across GBs, which retards the development of slip-induced stress concentrations at GBs and hence reduces the likelihood of intergranular fracture. Moreover, the Co-atom segregation-induced grain-boundary phase together with the secondary L2₁ Co₂AlTi phase in the Al-alloyed alloy significantly improves thermal resistance to grain coarsening. The kinetic exponent and apparent activation energy for grain boundary migration in unalloyed Co₃Ti increases dramatically from 3.2 and 263.7 kJ/mol to 5.2 and 641.0 kJ/mol, which surpass documented values of Ni- and Co- based superalloys, suggesting their promising potential as heat-resistant materials. These findings pave a new way for developing high-performance, heat-resistant superlattice alloys.