For age hardenable aluminum and titanium alloys, what indicates changes in hardness?

The changes in hardness of age-hardenable aluminum and titanium alloys can be indicated by conductivity. As these alloys undergo aging treatments, their microstructure evolves, leading to variations in the arrangement and density of alloying elements, which in turn affects their electrical conductivity.

In age-hardenable materials, the formation of precipitates during the aging process impacts both the hardness and electrical properties. As the precipitates grow and the material strengthens, the conductivity generally decreases. This relationship can be utilized in non-destructive testing methods, where measuring the electrical conductivity gives insight into the material's hardness and overall mechanical properties.

Other options relate to different physical properties not directly associated with the indication of hardness changes in these particular alloys. Retentivity pertains to a material's ability to retain magnetization and does not provide information on hardness. Permeability refers to a material’s ability to support the formation of a magnetic field, which is unrelated to mechanical hardness. Magnetostriction relates to the change in dimensions of a material when exposed to a magnetic field and similarly does not correlate with hardness assessments.