For age hardenable aluminum and titanium alloys, changes in hardness are indicated by changes in:

In the context of age-hardenable aluminum and titanium alloys, changes in hardness are primarily indicated by changes in conductivity. As these alloys undergo heat treatment processes such as solution treatment and aging, their microstructure evolves, affecting the distribution and density of solute atoms within the matrix. These changes contribute to the mechanical properties, particularly hardness.

The relationship between electrical conductivity and hardness is significant because as aging occurs, the precipitation of hardening phases within the alloy can reduce the number of free electrons, leading to a decrease in conductivity. This is due to increased scattering of electrons by precipitates, which can be used as a qualitative indicator of aging and hardening progress.

In contrast, retentivity, permeability, and magnetostriction pertain to magnetic properties or the behavior of materials under magnetic fields, which are not directly related to the hardness changes in age-hardenable aluminum and titanium alloys. Hence, conductivity serves as the most relevant indicator for these specific alloys in assessing changes in hardness over the aging process.