Effects of soaking time on the microstructure and mechanical properties of Nb-NbC/Fe core–shell rod-reinforced cast-iron-matrix composite fabricated through two-step in situ solid-phase diffusion

Abstract

To overcome the disadvantages of low toughness of conventional particle reinforced steel/iron-based composites, a novel niobium-niobium carbide/iron (Nb-NbC/Fe) core–shell rod-reinforced cast-iron-matrix composites were fabricated through two-step in situ solid-phase diffusion at 1172 °C for 15 min, followed by heat treatment at 1120 °C for 1, 4, 7, and 10 h (1172 °C × 15 min + 1120 °C × (1, 4, 7, 10 h)). The effects of soaking time on the microstructure and mechanical properties of the composites were investigated. Results indicated that the phase composition of the Nb-NbC/Fe core–shell rod-reinforced cast-iron-matrix composites were NbC, Nb, α-Fe and, graphite (G) phases, where Nb existed as core at the center surrounded by NbC/Fe shell. With the extension of the soaking time, the phase composition of the composites did not change, but the thickness of the NbC/Fe shell increased from 90 ± 3 μm to 233 ± 5 μm. The average hardness of the NbC/Fe shell was 12.86 GPa, which was approximately 2.6 times than that of the cast-iron-matrix due to high volume fraction of NbC particles. The yield strength and strain of the composite prepared at 7 h reached the maximum value of 1794.4 MPa and 11.6%, respectively. The excellent yield strength was attributed mostly to NbC particles, which induced the load-transfer strengthening and the mismatch of thermal-expansion-coefficient strengthening. Meanwhile, the Nb and α-Fe effectively hindered microcrack propagation, which contributed to the reasonable plasticity of the composite.

Keywords

• Nb-NbC/Fe core–shell rod
• Cast-iron-matrix composites
• Two-step in situ solid-phase diffusion
• Microstructure
• Mechanical properties