Microstructural and mechanical characterization of Nb-based in situ composites from Nb-Si-Ti ternary system

This study deals with the Nb-niobium silicide-based composites developed by the hot-pressing of Nb-Si-Ti ternary powder mixtures with a fixed Ti addition (6 at.%) and Si content ranging from hypereutectic (I I at.%) to near-eutectic compositions (18 at.%). The effects of Si content, Ti addition and strain rates on the sample microstructural characterization, flexural strength, fracture toughness, quasistatic compressive deformation and failure processes were investigated. It was revealed that the volume fraction of silicides increased with increasing Si content, and most of the Ti atoms dissolved into the niobium silicides to form (Nb,Ti)(5)Si-3 solid solutions instead of binary titanium silicides. The experimental evidence showed that a moderate improvement in the flexural strength, fracture toughness and compressive yield stress of the composites was achieved by the addition of Ti. Higher Si additions produced a much more remarkable enhancement in the compressive yield stress and bulk hardness, whereas both the flexural strength and fracture toughness decreased with increasing Si content owing to the existence of residual porosities in the samples. The composites showed remarkable superiority to the are-melted Nb-Si alloys and monolithic niobium silicides in fracture toughness (8.3-13.0 MPa root m vs. 4.5 MPa root m), where the toughening effect was attributed mainly to crack bridging and crack deflection by the remaining ductile Nb phase. Moreover, quasi-static uniaxial compression tests at strain rates between 10 (5) and 10 (3) S-1 indicated that the deformation behavior and failure processes were significantly affected by Si content and strain rates. The strain-rate-hardening behavior for all the strain rates was observed in the composite materials and the strain-rate sensitivity decreased with increasing Si content. At a lower strain rate, the composite materials with a hypoeutectic Si composition failed with a pseudoplastic response and at an angle of similar to 45 degrees off the compressive loading direction. However, in the case of higher strain rate and Si content, the brittle failure in the samples occurred through vertical splitting and spalling of fragments. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.