4 - Plastic deformation

Summary

Why do many MMCs often behave asymmetrically in tension and compression? Why do they frequently have higher ultimate tensile strengths, yet lower proportional limits than unreinforced alloys? Since the reinforcement usually remains elastic as the composite is loaded, the answers to these and other questions concerning the mechanical behaviour of MMCs lie with the factors which govern matrix plasticity. These can be broadly divided into two areas; those which affect the stress state of the matrix, and those which alter the flow properties of the matrix via changes in microstructure induced by incorporation of the reinforcement. This chapter illustrates, with the aid of relatively simple models, how these two aspects interact and combine to determine the behaviour, from the onset of flow to the development of large plastic strains.

A considerable body of mechanical test data for discontinuously reinforced MMCs is now available, although some of these results have been obtained with rather poor quality material. However, study of data such as those for Al/SiC summarised in Table 4.1, reveals some systematic trends:

• the incorporation of reinforcement improves both yield stress (0.2% proof stress) and ultimate tensile stress (UTS)

• whiskers provide more effective reinforcement than particles

• yield stress rises with increasing volume fraction; UTS is not always similarly affected

• for whisker-reinforced composites, increases in yield strength are often much greater in compression than in tension

• for whisker-reinforced composites, increases in tensile yield strength are greater transverse to the whisker alignment than parallel to it

• there is a wide scatter in experimental results