Powder-metallurgy fabrication of ZrB2-hardened Zr3Al2intermetallic composites by high-energy ball-milling and reactive spark-plasma sintering

Abstract

A powder metallurgy route combining high-energy ball-milling (HEBM) of elemental powders and reactive spark-plasma sintering (SPS) is proposed for the controlled fabrication of novel composites based on a Zr-Al intermetallic matrix hardened with a ceramic second-phase. As proof-of-concept, its suitability is demonstrated on ZrB2-hardened Zr3Al2. Specifically, commercially available powders of ZrH2, Al, and B were first combined in molar ratios of 2:1:1 to give an intermetallic-ceramic composite nominally formed by ∼76.8 vol.% Zr3Al2 plus 23.2 vol.% ZrB2, and were intimately mixed and mechanically activated by HEBM in the form of dry shaker milling for 30 min, next identifying by a dilatometric SPS test at 50 MPa pressure that the densification window of these composites is ∼975-1275 °C. Subsequent densification SPS tests at 50 MPa pressure in that temperature interval, and also at 1350 °C, plus the microstructural and mechanical characterisations of the resulting materials, established 1175 °C as the optimal SPS temperature. It was also identified that densification takes place by transient liquid-phase sintering with molten Al, and that it occurs gradually, not abruptly, because most molten Al disappears in a flash by reacting with Zr to form in situ the intermetallic. It is also shown that the combination of HEBM plus reactive SPS yields Zr3Al2+ZrB2 composites with fine-grained microstructures formed essentially by multitudinous ZrB2 nanograins dispersed within a matrix of submicrometre, or nearly submicrometre, Zr3Al2 grains. Importantly, these intermetallic-ceramic composites were found to be very hard (i.e., ∼11.5 GPa), attributable to the hardening provided by the ZrB2 nanograins, and fairly tough (i.e., ∼4.5 MPa·m1/2), and therefore potential candidate materials for a multitude of structural-tribological applications. Finally, implications for future study are discussed.