The Strength and Ductility of Precipitation-Hardened Fe–Si–Ti Alloys

The tensile and microstructural properties of a Fe–2·5% Si–1.4% Ti alloy have been examined after various ageing and mechanical treatments. In specimens aged to peak hardness the alloy had high strength (1100–1200 MPa) but extremely poor ductility. Overageing improved ductility but only at the expense of decreased strength. Cold work before ageing, despite producing a further increase in the strength of the aged material, also improved ductility, but a simultaneous combination of high strength and toughness was not achieved. In all cases, the alloy was more brittle than commercial quenched and tempered steels of the same strength.

Although the cleavage fracture strength of the aged material was higher than that of the alloy in the solution-treated condition, this was not thought to be directly attributable to the precipitate. Indeed, the precipitate promoted planar slip during deformation and this was regarded as the most likely mechanism of cleavage-crack nucleation.

Changing the distribution of the precipitate (by overageing) had no measurable effect on fracture strength and the increased ductility was attributed to the decrease in yield strength. The cold-worked and aged alloys retained a dislocation substructure (even after prolonged heat-treatment) that raised the fracture stress and improved ductility.