Abstract
By thermal analysis and electrical resistivity measurement, the formation of a metastable state (Borelius’s ε′ and ε states) during quenching Al-rich Al–Ag alloys has been observed, separately from the cold hardening stage (η state) and the warm hardening stage (precipitation of γ′ and γ phases from the matrix δ phase). Based on the specific heat versus temperature curves obtained during the heating of specimens of various Ag concentrations immediately after quenching, the temperature limit of existence of the ε state has been determined and is shown in the phase diagram, but it does not correspond with the solubility curve of metastable fcc Ag clustering determined based on the thermodynamic data by Hillert, Averbach and Cohen. The heat absorbed during the transitional change from the ε state to the homogeneous δ state is 12 cal/mol for 0.25 atomic percent Ag alloy and increases linearly with Ag concentration to 190 cal/mol for 5.9 atomic percent alloy. It has been confirmed that the ε state is not directly responsible for the cold hardening and its reversion.
