An axially elongated cylindrical tool has a protrusion at a lower end thereof which is inserted into a metallic work piece substrate under pressure during rotation of the tool and displacement thereof in one direction. Mixing of a matrix of metal and particles by the protrusion occurs within a stir zone underlying the substrate surface which is thereby treated by dispersion of the particles therein in response to rotation of the tool. Influx of the hard particles into the work piece substrate is effected during said mixing by the rotating tool.