Thermal stability analysis of Cu-11.8 wt%Al milled samples by TEM and HT-XRD

- The thermal stability of samples of Cu-11.8 wt% obtained by reactive milling was study.
- After a heat treatment, the (5 ± 3) mass% of the samples are martensite phases.
- Quenched planetary sample shows a calorimetric event related to γ′ → β1 transformation.
- Quenched horizontal sample doesn′t show calorimetric event due to γ2 precipitates.
- Quenched planetary mill sample is better precursor to produce a shape memory alloy.

In this work, the thermal stability of two samples of Cu-11.8 wt%Al obtained by different milling processes is analyzed. Several TEM techniques were used and HT-XRD experiments performed to determine the crystal structure and the morphological microstructure of the samples obtained during different heat treatments. The heat treatments were: quenching from 850 °C to room temperature and two consecutive calorimetric runs at 5 °C/min. After the quenching,α2 is the major phase observed, reaching 95 mass%. The remaining 5 mass% consisted of martensitic phases: one sample had γ′, a hexagonal structure, and the other β1′, a rhombohedral structure. During the first calorimetric run, the sample containing the γ′ phase exhibited a calorimetric event and the sample containing the β1′ phase did not. The calorimetric event is attributed to the austenitic transformation γ′ → β1. The lack of calorimetric event in the sample containing the β1′ is associated with the inhibition of the transformation β1′ → β1 because of the precipitation of the γ2 phase. Finally, the absence of a calorimetric event in the second run with the first sample is associated with the retransformation to β1′ instead to γ′ phase during cooling of the first calorimetric run. These studies determined that the first sample is a better candidate than the second sample to produce a shape memory alloy after thermo-mechanical treatments of the milled powders.

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