Matrix hardening in dispersion strengthened powder products

Dispersion hardening is one of the methods of strengthening a soft matrix. Other methods such as grain boundary hardening and solid solution hardening are known. Experimental data for various dispersion hardened systems have been examined in order to investigate whether it is possible to add the stress contributions from these three different strengthening processes.

The effect of dispersed particles, has been accounted for by the Orowan-expression σp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$= 2\frac{{G.b}}{A}$$\end{document} The effect of grain boundaries, has been described by the Petch-expression σgb = kD-1/2.

The contribution from solid solution is entered as the empirically found value. Considering the flow stress values (0. 2% offset) it has been determined to what extent the various contributions can be added independently, i. e. whether \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma = {\sigma _o} + {\sigma _p} + {\sigma _{gb}} + {\sigma _{sol}}$$\end{document} is valid when inserting the stress values expected if the individual strengthening agents had occurred alone in the matrix.

Data have been obtained from the following systems: A1-A12O3, Ni-ThO2, Zr-Y2O3, and Fe-carbides. The upper limit for the amount of dispersed phase has been about 10 vol. pct. The grain sizes have been in the range from 0. 3 μ to the order of 1 mm (i. e. extruded, cold worked, and recrystallized materials).

Within some limitations the strength data at room temperature indicate that the contributions from the various strengthening agents can be added, i.e. that each mechanism acts independently of the others. At higher temperatures in the range 0.5 to 0.8 Tm, the three strengthening processes may all contribute to the tensile strength and to the creep strength, but the use of the simple additive rule was inconclusive.

At temperatures above 0.8Tm dispersion hardening is predominant.