Interface response to solidification in sapphire-reinforced Ni-base composites

Abstract

Controlled solidification of sapphire-NiAl and sapphire-Hastelloy composites was carried out to evaluate the influence of processing and alloying on the microstructure, chemistry, and interface and fiber strengths. Pressure casting, gravity casting, and zone directional solidification (DS) techniques were used to synthesize the composites. Both gravity casting and DS yielded higher interfacial shear strength in the composites compared to the solid-state powder cloth (PC) techniques. Large columnar β-NiAl grains surrounding the fibers in the DS material decreased the propensity for interfacial decohesion, resulting in a higher interface strength than in fine, equiaxed grains of gravity cast and PC composites. Alloying of NiAl matrix with Cr, W and Yb increased the interface strength relative to unalloyed NiAl but led to fiber degradation, with Yb causing the most extensive fiber damage. Pressure casting is viable to make high fiber volume fraction sapphire-reinforced Ni-base composites; however, fibers suffer strength loss (about 65 pct. relative to the virgin fiber) due to chemical attack. The residual fiber strengths are consistent with a Weibull distribution function. Controlling the strength-limiting reactions by matrix modification, use of barrier coatings, and process control (e.g. reduced temperatures) would permit exploitation of the unique potential of solidification techniques to design the composites for toughening and strengthening.

Keywords:

• Zone directional solidification
• interfacial shear strength
• pressure casting
• nickel-base high-temperature composites
• sapphire-reinforced NiAl
• fiber push-out test
• interfacial reactions
• fiber residual strength