Different combinations of critical and subcritical heat treatments variously modify the initial as cast microstructure of high chromium white cast irons leading to secondary carbide precipitation of different extent and nature. Destabilisation (critical heat treatment) of austenite at 970uC for 2?5 h followed by annealing (subcritical heat treatment) at 600uC for 13 h results in massive precipitation of M 23 C 6 carbide particles along with spheroidised M 7 C 3 . The reversed order of heat treatments leads to extensive precipitation of M 7 C 3 secondary carbide particles. Mo has a favouring effect on the hardness of the microstructures containing pearlite by limiting pearlite formation. The gradual increase in the alloying additions, C and Cr, increases the hardness of the materials at the different treatment states by inducing carbide precipitation. The increase in the Si content leads to the opposite effect by favouring pearlite formation.
Al7Mg5Si-based TiC reinforced composites have been produced by casting methods. The microstructures of the monolithic alloy and the composite materials are explained in terms of solidification phenomena. The TiC presence significantly alters the matrix microstructure both by causing extensive grain refinement and by altering the extent and morphology of the Mg 2 Si phase. The TiC particles serve as potential nucleation sites for the Mg 2 Si phase. The effect of the TiC reinforcing particles on the sliding wear performance at the composites has also been investigated. TiC additions improve the overall wear rate of the monolithic alloy. A likely wear mechanism is proposed, based on wear rate and friction coefficient data along with wear track and debris examination.
A 3-5 vol.% TiC particulate Al-Si-Cu composite was prepared by diluting Al/20 vol.% TiC composite in an Al-7Si-4Cu alloy matrix. TiC particle distribution consists of isolated and clustered particles which are both located at the primary-a grain boundaries and at the areas of the last solidified liquid. Particle pushing by the solidification front is responsible for the final particle location. The solidified microstructure consists of primary and intermetallic phases formed by a sequence of possible eutectic reactions. No evidence of TiC particle degradation was observed.
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