The control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering

Abstract

Grain boundaries can be effectively controlled to produce or enhance their beneficial effects and also to diminish or reduce their detrimental effects on bulk properties in polycrystalline materials. Particular attention has been paid to the control of intergranular brittleness which remains a serious problem of material processing and development. Recent studies are presented and discussed, which have been successfully performed to control intergranular brittleness of “intrinsically brittle” materials such as the refractory metal molybdenum and the ordered intermetallic alloy Ni₃Al and to produce superplasticity in an Al–Li alloy, by grain boundary engineering through controlling a new microstructural factor termed the grain boundary character distribution (GBCD). The optimization of GBCD and the grain boundary connectivity has been found to be a key to produce desirable bulk mechanical properties in both structural and functional polycrystalline materials.

Introduction

Grain boundaries are important elements associated with microstructural heterogeneity in single phase polycrystalline materials, and additionally interphase boundaries in multiphase ones. These boundaries strongly affect bulk properties, particularly mechanical properties controlled by deformation and fracture in polycrystalline materials. So far both beneficial and detrimental effects of grain boundaries (hereafter including interphase boundaries in multiphase materials) on mechanical properties have been observed. It is well known that grain refinement, i.e. an increase in the density of grain boundaries, can produce an improvement in the strength of polycrystals, as predicted by the Hall–Petch relationship [1]. This is one of the well established beneficial effects of grain boundaries on mechanical properties. On the other hand, intergranular fracture is often the primary origin of severe brittleness of polycrystalline materials, because grain boundaries can be preferential sites for crack nucleation and propagation [2].

Intergranular brittleness which takes place in various types of engineering materials often causes serious problems in their processing or service resulting from premature fracture and leading to catastrophic failure of machine components and structures; intergranular stress corrosion cracking is often involved in accidents which have occurred at nuclear power stations. Accordingly the control of material brittleness associated with intergranular fracture has been strongly required and extensively studied but unfortunately has not been fully solved yet. Until recently there was no underlying principle and approach to the control of intergranular brittleness in polycrystalline materials. Nowadays the importance of the effects of the type, structure, and character distribution of grain boundaries on bulk properties has been recognized on the basis of the relationship between grain boundary structure and properties on which many pioneering researchers were deeply involved 3, 4, 5. In 1984 one of the present authors proposed the concept of grain boundary design and control for strong and ductile polycrystals on the basis of structure-dependent intergranular properties, aiming at the control of intergranular fracture [6]. Since then rapid progress has been made in grain boundary design and control for the control of intergranular brittleness in different types of materials. It is only recently that grain boundary engineering has been successfully established, as demonstrated by development of high ductility in high silicon iron alloy [7] and B-free polycrystalline intermetallic Ni₃Al [8] and high stress-corrosion resistance in Inconel alloy for nuclear reactor [9] and lead alloys for lead-acid batteries [10].

This paper first discusses fundamentals of fracture processes in polycrystals in which intergranular fracture takes place depending strongly on the type and the frequency of grain boundaries, i.e. the grain boundary character distribution (GBCD) and geometrical configuration of grain boundaries (the grain boundary connectivity). The importance of GBCD and the grain boundary connectivity is discussed on the basis of the results of the modelling of GBCD-controlled fracture processes and on experimental observations of fracture processes in real materials. Moreover, the results from recent studies of grain boundary engineering for the control of intergranular brittleness in polycrystalline molybdenum [11] and for development of superplasticity in aluminium–lithium alloys [12] are reported which clearly demonstrate the potential of grain boundary engineering. Finally we introduce a recent successful development of high performance ferromagnetic Fe–Pd alloy which shows high magnetorestriction-induced strain and excellent shape memory characteristics for future development of actuator and micromachines [13].