Liquid sintered materials

Many metals are naturally brittle at room temperature, so must be machined when hot. However, particles of these metals, such as tungsten, chromium, molybdenum, etc., can be suspended in a ductile matrix. The resulting composite material is ductile, yet has the elevated-temperature properties of the brittle constituents. The actual process used to suspend the brittle particles is called liquid sintering and involves infiltration of the matrix material around the brittle particles. Fortunately, In the liquid sintering process, the brittle particles become rounded and therefore naturally more ductile. 

Although some of the concepts established for metal/ oxide systems are also valid for non-oxide ceramics, there are other concepts which are specific to these kinds of ceramics, owing to their predominantly covalent (SiC, BN, AIN) or metallic (TiC, TiN, WC) character. These materials seldom can be obtained as the high-purity monocrystalline specimens desirable for fundamental wetting studies. Usually, they are sintered materials with impurity contents higher than 0.1% and they often contain open porosity. Further difficulties arise from the high oxidization tendency of many of them, the presence of an oxide layer dramatically changing their wettability by liquid metals. 

J. E. Marion, A. G. Evans, M. D. Drory, and D. R. Clarke, High Temperature Failure Initiation in Liquid Phase Sintered Materials, Acta Metall., 31, 1445-1457 (1983). 

Tikare, V. and Cawley, J.D. (1998) Numerical simulation of grain growth in liquid phase sintered materials-I. 

Consider a liquid phase sintering material containing inert second-phase spheres. Sketch and explain the growth shapes of a grain around a sphere for a dihedral angle between them of 0, 90 and 160°, respectively. 

German, R. M. and Olevsky, E. A., Modeling grain growth dependence on the liquid content in liquid phase sintered materials, MetalL Mater. Tram. A, 29A, 3057-66, 1998. 

The best mechanical strength is exhibited by pressureless sintered SiC, hot-pressed [460,461] and liquid-phase-sintered materials (see Sections 4.4.1.4,4.4.1.5). 

Upon cooling, these liquid phases remain as glassy phases or as secondary crystalline phases in the sintered materials consequently, these hquid-phase-sintered ceramics are actually composite materials consisting of a matrix of grains and dispersed secondary phases. The thermal conductivity of these composite materials will depend on the amount, distribution state and thermal conductivity of each constituent phase in the structure. The effects of secondary phases on the thermal conductivity ofliquid-phase-sintered nonoxide ceramics are discussed in the following subsection. 

In liquid-phase sintered materials, a continuous glassy phase is often present along the grain boundaries. However, liquid-phase sintering requires a low dihed-... 

The CO, temperature control can be used in conventional steel molds as well as with inserts made from porous sintered material, which mainly consists of steel (TOOLVAC ). When using sintered steel, the liquid CO, flows through the capillary tubes into the expansion chamber. In the gaseous state, the CO, penetrates into the porous materiai and flows, due to the gas pressure, and is uniformly distributed over the core up to the cavity surface. There, heat can be directly withdrawn from the moided part. 

Figure 3.2. Top, vitrification the liquid phase is abundant enougfi to fill the interstices between the particles in the middle, liquid phase sintering the liquid is not suffieient to fill the interstiees bottom, solid phase sintering organization and shape of the partieles are extremely modified. This diagram does not show the grain coarsening in fact, the grains of the sintered material are appreeiably coarser than the starting particles [BRO 91 If...

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