It is known to produce dispersion hardened aluminum-base alloys by powder metallurgical methods and, more particularly, to use the process known as mechanical alloying in the production of such alloys. Generally, a mechanically alloyed (or otherwise formed) aluminum powder containing a dispersoid is hot compressed in a vacuum and consolidated and formed by extrusion. A problem exists in producing useful shapes from the dispersion hardened aluminum bar stock provided by extrusion when the bar stock contains significant amounts of dispersed, transition metal, intermetallic phase insoluble in the solid aluminum matrix.
Ordinarily a cheap, generally applicable metallurgical solution to providing useful shapes from extruded or otherwise formed bar stock is hot working by forging, rolling or the like. In such processes, unlike extrusion, metal is free to expand in more than one direction. Generally speaking, such forging, rolling and the like is done hot because at high temperatures metal is weaker and has good ductility. At high temperatures precipitated strengthening phases dissolve; matrices change from one phase to another, e.g. ferrite to austenite; and generally workability as indicated by tensile elongation is enhanced. An exception exists in the case of mechanically alloyed dispersion-hardened aluminum containing insoluble intermetallic dispersoid. It has been observed in mechanically alloyed aluminum-base alloys containing Al.sub.3 Ti dispersant that, as the test temperature rises, while the strength of dispersion-hardened aluminum alloys decreases, the ductility as measured by elongation in tensile testing, also decreases.
The ductility of two- (or multi-) phase alloys is most commonly discussed in the art in terms of the volume fraction of the hard phases. Previous theoretical as well as experimental studies have demonstrated that at a given temperature, particularly at room temperature, alloy ductility (as evidenced by the elongation to fracture during a tensile test) decreases sharply as the volume fraction of the hard phase increases. From previous empirical work, a simple relationship has been developed relating ductility and hard-phase volume fraction: ##EQU1##
In this equation k is an empirical constant (whose value depends upon the characteristics of the matrix alloy), and f is the volume fraction of the hard phase. The above relationship has been shown to hold approximately true at room temperature for a variety of dual or multi-phase alloys, including Al-SiC composites.