On the correlation between indentation hardness and tensile strength in friction stir processed materials

Friction stir processing (FSP) is a well-established surface modification technique for the production of ultra-fine grained alloys and metal-matrix nanocomposites. In this context, the main goal of implementing FSP is typically for improving surface properties such as indentation hardness and wear resistance. However, the tensile behavior of modified surfaces is also essential for some specific applications, although rarely reported. Therefore, the main object of this article is to perform a detailed experimental investigation of selected material systems together with a review of the reported data in the literature to establish an empirical correlation between indentation hardness and tensile strength in friction stir processed alloys and nanocomposites. Results indicate a consistent relationship between the properties of the modified alloys and the grain refinement following FSP. Accordingly, two general correlations can be made between yield stress (YS) and ultimate tensile strength (TITS) versus the indentation hardness (HV) with proportionality constants of similar to 1.69 and 3.36, respectively. Meanwhile, it is not possible to offer a general linear correlation between hardness and tensile strength for metal-matrix nanocomposites, mainly due to the structural heterogeneity related to the type and content of reinforcing agents. The statistical analysis of data revealed only an approximate correlation of similar to 1.23 between YS and HV.