Microstructures and Thermal Stability of the Metastable-Phase Precipitates Formed in an Al-Cu Alloy

Abstract

The precipitation behavior in an Al-Cu alloy isothermally annealed at 373K～493K was studied, using the Vickers microhardness tests, Differential scanning calorimetry(DSC) and Transmission electron microscope(TEM) observations. An additional endothermic peak was found in DSC measurements which may be attributed to the θ″-phase independent from the formations of the G.P.(Ⅱ) and the θ′-phase at the aging temperatures. High resolution transmission electron microscopy (HRTEM) revealed that the G.P.(Ⅰ) is formed at a very early stage of isothermal aging at 463K. Comparing the Vickers microhardness and TEM images, it was concluded that G.P.(Ⅱ) is mainly responsible for the peak hardness.

A Quantitative Study of Precipitation of Metastable Phases in an Al-1.94 at%Cu Alloy during Isothermal Aging at 373 K

“…After passing over the plateau, the curve again increased until it attained a peak hardness in the third stage after aging for 3:0 Â 10 7 s. In the fourth stage, the HV hardness decreased with aging time. Figure 2 shows the DSC curves with endothermic reactions that are attributed to the dissolution of metastable phases in an Al-1.94 at%Cu specimen aged at 373 K. Systematic DSC measurements at various times up to 3:0 Â 10 7 s revealed the existence of three endothermic peaks in the DSC thermograms under 550 K. From the interpretation in previous papers, [5][6][7] we concluded that the first endothermic peak corresponds to the dissolution of solute clusters, the second to G.P. (I) dissolution, and the third to G.P.…”

“…Since it was clarified in previous reports by the present authors [5][6][7] that four types of metastable phase precipitates are formed before the intermediate 0 phase formation, we have adopted a new interpretation of the precipitation sequence in an Al-Cu alloy: supersaturated solid solution (ssss) ! solute clusters !…”

“…(II) and the 00 phase have slightly different thermal stabilities, and thus the two particles should be considered as two different metastable phases. [4][5][6][7] Apart from the dispute over the stability of metastable phases, it is also recognized that the size distribution and density of precipitates, as well as the characteristics of the precipitate phases, significantly influence the properties of precipitationhardening alloys. Therefore, the quantitative study of the precipitation process is regarded as another important direction to conduct from both the experimental and theoretical points of view.…”