大ひずみ多軸鍛造による結晶粒微細化プロセスと機構

Abstract

The evolution processes of new high-angle boundaries as well as ultrafine grains processed by severe multidirectional forging (MDF) are studied in ferritic, austenitic steels and copper at low temperatures below 0.5Tm (Tm is the melting point), and aluminum and magnesium alloys at high temperatures above 0.5Tm, where dynamic recovery operates mainly as a restoration process. The structural changes can be characterized by the evolution of deformation bands such as microshear or kink bands at moderate strains. MDF promotes the multiple shearing, which results in the formation of spatial net of mutually crossed bands subdividing the original grains. This is a mechanical induced event and so an athermal process. On the other hand, in large strain the fast operation of recovery processes accelerates the kinetics of ultrafine grain evolution with increasing temperature at low temperatures below 0.5Tm. In contrast, the volume fraction of new grains decreases in large strain through hard development of deformation bands with increasing temperature at elevated temperatures above 0.5Tm. The misorientations between (sub)grains evolved gradually increase with increase in strain, finally leading to the development of a new fine-grained structure. Temperature effect and the mechanism of strain-induced grain formation are discussed in detail.