Research on Dispersion Hardening

Résumé du livre

This report is an attempt to better understand the mechanism by which second-phase particles strengthen alloys. Hardening in age-hardened Ni-Al and Cu-Co alloys was found to be due to the precipitation of coherent ordered Ni3Al, or Co particles, respectively. Both showed coherency strain fields, but they were particularly clear in the latter. The main contrast effects expected from coherent spherical particles were consistent with those observed at Co particles. The shape and size of the Co particles agreed with earlier conclusions based on bulk magnetic measurements. Overaging in Cu-Co was partly due to loss of full coherency which is helped by deformation. Particles slightly decreased the spacing of fine slip lines in stretched samples. Dislocations apparently passed through the particles in Ni-Al. Dislocations commonly bowed out between particles indicating local pinning. Particles substantially increased the dislocation density produced by stretching. After 5% strain when the particle size was over about 300 A, dislocations formed a network with particles at the nodes and in Ni-Al, dislocation tangles formed round each particle. In Ni-Al, particles substantially reduced or even eliminated the temperature dependence of the flow stress. This was first attributed to the elimination of thermally activated cross slip, but electron microscopic observations did not show cross slip to be common in the solid solution. Preliminary observations indicated that the effect of particles on recovery is complex. The results emphasize the need for a more statistical theoretical approach to the problem of particle hardening, taking into account the actual distribution of particle sizes and spacings and the complex arrangements of dislocations actually present.