Particle strengthening mechanism

Fig. 8.2. Representative data associated with four of the key strengthening mechanisms within a material, (a) Increase in flow stress as a function of concentration of substitutional impurities, (b) Dependence of flow stress on mean particle size for material in which there are second-phase particles, (c) Dependence of yield stress on mean grain size of material, (d) Relation between yield stress and mean dislocation density. (Adapted from (a) Neuhauser and Schwink (1993), (b) Reppich (1993), (c) Hansen (1985), (d) Basinski and Basinski (1979).)...

As noted above, our working hypothesis concerning the various hardening mechanisms is that chemical impurities, second-phase particles and even other dislocations serve as obstacles to the motion of a given dislocation. As a result of the presence of these obstacles, the intrinsic lattice resistance tp is supplemented by additional terms related to the various strengthening mechanisms. We further assume that the flow stress can be written as... 

Particle reinforced composite systems can be either large particle or dispersion strengthened. If a composite is reinforced by large particles (larger than 0.1 [xm and equiaxed, which are harder and stiffer than the matrix), mechanical properties are dependent on volume fractions of both components and are enhanced by increase of particulate content. Concrete is a common large particle strengthened composite where both matrix and particulate phases are ceramic materials. 

Small particles of a second phase, evenly distributed in the grains of the first phase, form a strong barrier to dislocation motion. This was previously discussed in section 6.3, and we saw there that there are two possible ways to overcome such obstacles, the Orowan mechanism and cutting of the particles. The mechanism actually occurring depends on the strength of the obstacles and on their distance. This strengthening mechanism is frequently called precipitation hardening, because the particles are usually created by a precipitation process, described below. 

Nano-size dispersed particles drastically change the fracture mode from intergranular fracture to transgranular fracture, and also improve the fracture strength of ceramics markedly, especially after annealing, mentioned previously. To explain these phenomena, we consider a strengthening mechanism in nanocomposites... 

Nanoparticle-reinforced MMCs usually exhibit higher microhardness than that of the unreinforced matrix alloy. The increase in hardness can be described as follows (1) strengthening mechanisms arising from restricting or impeding the motion of dislocation, (2) the presence and near-uniform distribution of second-phase particles achieved by optimal processing, and (3) a refinement in grain size. 

R.L. Klueh, N. Hashimoto, P.J. Maziasz, New nano-particle-strengthened ferritic/ martensitic steels by conventional thermo-mechanical treatment, J. Nucl. Mater. 367-370 (2007) 48-53. 

Cite the difference in strengthening mechanism for large-particle and dispersion-strengthened particle-reinforced composites. 

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Mechanical alloying is another method of producing dispersion-strengthened metals. In this process, the powdered constituents of the ahoy are treated in an attrition mih. A finely distributed layer of the dispersed phase is distributed on particles of the base metal. Subsequent pressing and sintering strengthens the dispersion (25). 

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