Brittle particle

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. 

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Morphology (compaction) Compressibility Tensile strength Bonding Brittleness Particle size (flow ability)... 

Most solids are subjected to permanent deformation or breakup once the applied stresses exceed a certain limit. Hence, most solid particles may be classified into two categories elastoplastic particles and elastic-brittle particles. Typical elastoplastic materials include metals and polymers, while typical elastic-brittle materials include coal, activated carbon, and ceramics. Materials that are elastoplastic at room temperature may become brittle at low temperatures and those that are brittle at room temperature may become plastic at high temperatures. 

Suwonsichon, T., Normand, M.D., and Peleg, M. (1997). Estimation of the mechanical properties of individual brittle particles from their bulk s compressibility. J. Texture Stud,... 

Expansion of the strip after pressure release is influenced by the physical characteristics of the material to be compacted (plasticity, brittleness, particle size and distribution, particle shape, etc.), the roll diameter, the speed of rotation, and the surface configuration of the rollers. With increasing roll diameter and/or decreasing speed the expansion of compacted material is reduced due to better deaeration during densification and a more complete conversion of elastic into permanent, plastic deformation. 

Some agglomerates of different materials have been observed to fail because of internal flaws driven by a number of stresses (e.g., internal tensile stress cracks in the surface plastic flow at the surface between the agglomerate and platen and shear stress within the sphere). For brittle particle agglomerates with significant internal flaws, the tensile strength is small compared to the compressive and shear strength, and failure is likely initiated by the internal tensile stress. In any case, a careful microscopic examination of failed pieces can provide much information on the dominant failure mode (Bika et al., 2001). 

The mechanism of densification of particulate solids (Fig. 6.6) includes, as a first step, a forced rearrangement of particles requiring little pressure followed by a steep pressure rise causing brittle particles to break and malleable ones to deform plastically. During the entire process, porosity decreases so that fluids which originally occupied the pore space of the bulk feed must be able to escape and the initial elastic deformation must have sufficient time to either cause breakage or convert into plastic deformation (see also Section 8.1). These requirements limit the speed of densification and, therefore, the production capacity. 

Referring to Fig. 8.1, as a first step, pressure agglomeration achieves a rearrangement of particles which requires little force and does not change particle shape and size. This is followed by a steep rise of pressing force during which brittle particles break and malleable particles deform. Sketches 3 (brittle) and 4 (plastic) occur either/ or and often simultaneously if both brittle and malleable particles are present in the mix. 

Particle strength Zones of high shear in equipment (e.g. centrifuges) will fracture brittle particles, leading to a broadening of the particle size distribution and (generally) a reduction in cake porosity. 

The mechanical alloying of mixtures of ductile and brittle components results in the brittle particles being trapped at the layered welded interfaces of the ductile component (Benjamin 1970, Maurice and Courtney 1994). Continued milling results in fi-acture of the brittle particles and the development of a uniform distribution of particles of the brittle phase within the matrix of the ductile phase. This is in contrast to the distribution in conventional powder systems where the dispersoid particles are confined to the prior particle boundaries. Mechanically alloyed CDS alloys are an important practical example of a ductile/brittle system. If the volume fraction of the brittle phase is of the order of 0.5, the characteristic layered structure does not form. Rather, the microstructure consists of a uniformly distributed nanocrystalline mixture of the two phases (Schaffer and McCormick 1990a). 

When a mixture of brittle and ductile materials is miUed, both of the above-mentioned processes take place (Figure 2.2d). Plastic deformation of the ductile material absorbs the impact energy, resulting in much less fracture of the brittle material. Under the impact, the brittle particles can either penetrate into or sandwiched between two flattened ductile particles. Hence, repeated impacts lead to the dispersion of the brittle material within the ductile matrix. Metal matrix composites (MMC) could be synthesized in this way [8]. 

Considering a ctrrve integral of the thermal-stress-induced elastic energy density, the critical particle radii related to crack formatiorrs in ideal-brittle particle and matrix. 

The steel ball projectiles were flattened or severely deformed or fragmented, depending on impact velocity, upon impacting the SiC/SiC composite. By contrast, the projectiles that impacted the oxide/oxide composite were neither flattened nor noticeably deformed even at the highest impact velocity of 400 m/s, as shown in Fig. 2. This was due to the oxide/oxide composite s soft and open structure, compared to the hard-and-dense SiC/SiC counterpart. Often at higher impact velocities > 300 m/s, steel ball projectiles were embedded into the oxide/oxide composite, similar to the case that sharp brittle particles impacted ductile metal targets. 

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