Sintering and dispersion hardening of iron-based alloys.

Powder metallurgy techniques have been used to study the dispersion strengthening in iron-based alloys containing alumina, zirconia, or titania. The degree of densification obtained from sintering experiments decreased with increasing oxide content in the carbonyl-iron: the presence of zirconia virtually prevent densification even at 1500 C. These particles, including zirconia, produce less inhibition in densification in stainless steel matrix. The final particle morphology, studied by optical and electron microscopy, was found to be closely related to the sinterability of the oxide phase, post-sintered particle size, and the fabrication technique used. In the case of alloys which had been sintered at 1350 C, and then forged and cold-rolled, the room-temperature strengthening effect of alumina was found to be significantly higher than that of zirconia; whereas, for alloys sintered at the same temperature but tested in the extruded condition there was no such difference in strengthening effect. Furthermore, sintering at 1200 C followed by forging and cold-rolling produced a similar degree of strengthening for each of the two dispersoids. These effects are interpreted in terms of the variation of particle morphology with production history. The analysis of yielding, yield-point phenomena, and work hardening characteristics of alloys containing various amounts of alumina or zirconia indicate a basic difference in the group and dynamic behaviour of dislocationsbetween iron-alumina and iron- zirconia alloys. The strain-rate-change experiments have been used to determine the activation volume and dislocation velocity exponent, and these have been correlated with the dispersion parameters. The activation volume decreased and the dislocation velocity exponent increased with decreasing free-spacing between the particles. Tensile properties were also determined at temperatures in the range 20-600 C for the extruded alloys. The dispersion strengthening effect decreased with increasing temperature in a manner which is consistent with the experimentally determined recrystallization temperature of these alloys