A Theoretical Interpretation of the Experimental Results of an Investigation of the Thermal Expansion Mismatch Strengthening in an Al-Based Metal Matrix Composite Reinforced with Nano-Sized TiCN Particulates.

The coefficient of thermal expansion mismatch (CTEM) strengthening is presumed to be among the main strengthening factors in metal matrix composites reinforced with micro- and nano-sized particulates. It is assumed that the CTEM-generated dislocations obstruct the movement of those generated, when the composite is under external load, thus strengthening the composite. An experimental study, which was carried out on as-produced aluminum-based composite, reinforced with 5 and 10 wt.% nano-sized TiCN particulates (volume fraction of TiCN, f_v = 0.0306 and 0.0625), failed to detect the predicted by the theory CTEM-generated dislocations. Moreover, it was found that the reinforcement-matrix adhesion forces become weaker with the increase in the reinforcement size. An analysis of the experimental results, intended to investigate the contradiction between the experiment and the postulates of the CTEM strengthening theory, was carried out. The analysis, that implements the dislocation nucleation (Cottrell) and vacancy "drift-diffusion" (Einstein–Smoluchowski) theories, revealed that the classical CTEM strengthening mechanism is active only above a certain, critical size, of the reinforcement. It was found that below that critical size, the composite strength is controlled by the adhesion forces between the reinforcement and the matrix. The theories above predicted that the composite strength (i) would improve as the size of the reinforcement tends to the nano-scale sizes, and (ii) is at its lowest when the size of the reinforcement is commensurate with the aforementioned critical size. [ABSTRACT FROM AUTHOR]