Ultrahigh thermal stability and hardness of nano-mixed fcc-Al and amorphous phases for multicomponent Al-based alloys

Abstract

Abstract Fcc-Al (α-Al) + amorphous phase mixtures with ultrahigh hardness are formed by a heating-induced reverse transition from the primary precipitates of α-Al and AlxMy in residual amorphous phase for melt-spun Al84Y9Ni4Co1.5Fe0.5TM1 (TM = V, Nb, Cr, Mo, Mn, Fe, Co, Ni or Cu) amorphous alloys. The resulting particle diameter and volume fraction of the α-Al phase are 5–15 nm and 60–70%, and no internal defects are observed. The solute content in the α-Al phase is 8–11% for the TM = V or Cu alloys. In contrast, the reverse transition is not found for the TM = Zr, Ag or Au alloys, presumably because these elements have atomic radii larger than Al and positive heats of mixing with other solute elements hindering the atomic rearrangements to decompose the AlxMy compound. The [α-Al + amorphous] phase mixture is maintained up to about 700 K and exhibits a Vickers hardness of 550–580, much higher than for the corresponding crystalline alloys. The high hardness at elevated temperature is due to the coexistence of perfect crystal α-Al and high solute-content residual amorphous phase. For a cast conical rod of the TM = Cu alloy, the microstructure is fully amorphous up to a diameter of 0.82 mm, and is an [α-Al + amorphous] phase mixture at larger diameters up to 1.2 mm. The formation of the highly stable [α-Al + amorphous] phase mixtures by the heating-induced reverse transition, as well as the bulk formation (in cast rods) of similar phase mixtures for the TM = V, Nb, Cr, Mo, Mn, Fe, Co, Ni or Cu alloys is promising for future structural and coating materials owing to their high hardness and high elevated-temperature strength.