Architecture design and strengthening-toughening mechanisms in heterogeneous-structured medium/high entropy alloys
Metallic materials are indispensable structural materials in modern industry and play an important role in energy,environmental protection,advanced manufacturing,aerospace and aviation,owing to their high strength,high thermal conductivity,high electrical conductivity and processability.To meet the increasingly diversified engineering applications,metallic materials are demanded to exhibit excellent mechanical properties and performances,such as high strength,high toughness,corrosion resistance,creep resistance and good thermal stability.As two most important mechanical properties of metallic materials,strength and ductility are often mutually exclusive,which is attributed to a complex coupling between mechanical properties and microstructures of metallic materials.Traditional strengthening methods have been utilized to enhance strength by impeding the dislocation motion or improve the interactions among dislocations.However,traditional strengthening methods can limit dislocation multiplication and storage capacity,leading to insufficient work hardening capacity,premature plastic destabilization and consequent reduction in ductility.As a novel class of alloys,medium/high entropy alloys are a mixture of three or more metallic elements with equiatomic or near equiatomic ratio,with each element being regarded as a principal component.Different from the conventional alloys,medium/high-entropy alloys demonstrate unique structural features and exhibit the medium/high-entropy effect,lattice distortion effect,sluggish diffusion effect,and"cocktail"effect,which is attributed to the presence of the multi-principal elements.Medium-high entropy alloys possess superior mechanical properties,such as ultrahigh strength,excellent ductility,remarkable wear resistance and corrosion resistance.However,like traditional metal and alloys,medium/high entropy alloys encounter a persistent dilemma of balancing strength and ductility,i.e.a strength-ductility trade-off.Biomaterials in nature exhibit excellent mechanical properties,which is mainly attributed to their heterogeneous structures across multiple scales.Drawing inspiration from this natural heterogeneity,the concept of heterogeneous design has been proposed and applied to metallic materials to overcome the strength-ductility trade-off.Consequently,a new class of structural metallic materials(known as heterogeneous structured metals/alloys)is emerging.By manipulating the element composition and refining the processing techniques,stable heterogeneous structures across various scales can be introduced into the medium/high entropy alloys.These heterogeneous structures include the local chemical ordering at atomic scale,precipitates at nanoscale,second phases or intra-grain defects(such as nanotwins and dislocations)at sub-microscale,and multi-modal grain sizes at microscale.These cross-scale heterogeneous structures,either introduced through elaborately designed processing techniques or formed during plastic deformation,can significantly affect the plastic deformation mechanisms of medium/high-entropy alloys.These structures can impede dislocation motion and enhance work hardening rate,consequently improving both strength and ductility of medium/high entropy alloys.Therefore,the heterogeneous-structured design contributes to achieving a strength-ductility synergy in medium/high-entropy alloys.This paper provides a summary and review of recent studies about the heterogeneous-structured design in the medium/high-entropy alloys,and reveals the associated strengthening-toughening mechanisms.The typical strengthening-toughening mechanisms include hetero-genous structures blocking dislocation motion and promoting dislocation interaction,twinning induced plasticity,transformation induced plasticity,and nucleation and accumulation of geometrically necessary dislocations and resultant back stress.Finally,this paper summarizes the correlation among heterogeneous structures,deformation mechanisms,and mechanical properties in medium/high-entropy alloys,and outlines future research focuses and directions.Three prospective research directions for future include customizing the elemental composition to create the specific heterogeneous structures in order to achieve the strength-ductility synergy,investigating the relationships between microstructures and additive manufacturing processes,and studying the mechanical properties and deformation mechanisms under extreme conditions.
万方数据
维普
