Effect of oversolidus barothermal treatment on the microstructure and properties of binary hypereutectic Al-Cu alloys

This study investigates the effect of oversolidus barothermal treatment (BTT) on microstructure and properties of binary alloys Al-22Cu and Al-27Cu (at.%), focusing on the pressure-induced shifts in solidus and liquidus temperatures. The pressure shift of the solidus and liquidus temperatures was established at a hydrostatic argon pressure of ~ 100 MPa using the differential barothermal analysis (DBA). The solidus temperature of both alloys increased by approximately 8 ℃ during heating, aligning closely with the equilibrium state diagram upon cooling. The liquidus temperature of the alloys at 100 MPa increased by 11 ℃ for Al-22Cu and by 14 ℃ for Al-27Cu compared to the phase diagram. Taking these results into account, barothermal treatment of the alloys was carried out at 100 MPa and 570 ℃ for 240 min (BTT1), as well as at 100 MPa and 562 ℃ for 10 min (BTT2). Processing of alloys at BTT1 leads to the primary Al2Cu particle size significant increase to lengths of 350–1500 μm and widths of 150–400 μm. During BTT1, the structure of the initial lamellar eutectic Al-Al2Cu is modified with the formation of discrete intermetallic particles, with the length and width being 15–240 and 1–3 µm, respectively. With a decrease in temperature and exposure time, a significantly less intense increase in the size of primary intermetallic compounds occurs, and a noticeable increase in discrete eutectic intermetallic compounds occurs. The microhardness reached 2043 MPa for the eutectic (initial Al-22Cu). For the intermetallic and the eutectic, the microhardness was 4189 and 2699 MPa, respectively (initial Al-27Cu); for intermetallic and the eutectic 4754 and 1382 MPa, respectively, for the Al-22Cu alloy processed at BTT1, and the eutectic microhardness for the Al-22Cu composition processed at BTT2 was 2409 MPa. The bending strength of alloys decreases after oversolidus BTT.