Particle strengthening

Coarse particles of a second phase influence aU bulk properties of a material. If, for example, Young s modulus of the second phase is larger than that of the matrix, load will be transferred from the matrix to the particle if the material is stressed elastically, and the stiffness increases. In the context of fibre- and plate-shaped particles this will be discussed further in chapter 9. The amount of load transfer depends strongly on the shape of the particles and their arrangement. 

Other physical properties behave similarly to Young s modulus. If, for example, copper is used to dissipate heat from a ceramic structure, the coefficients of thermal expansion of the copper alloy and the ceramic should not differ too much to reduce thermal stresses at the interface. Adding tungsten particles to copper reduces its coefficient of thermal expansion so that it becomes closer to that of the ceramic. 

The wear resistance of a material can also be improved by adding particles of a second phase. If the particles are hard, the softer matrix material will wear off first, resulting in the hard particles sticking out of the surface and then determining the wear properties. This method is used, for example, in aluminium cylinder liners in combustion engines whose wear resistance is improved by silicon precipitates, or in cast iron (figure 6.42). 

Coarse particles also influence the plastic properties. If their Young s modulus is larger than that of the matrix, load transfer reduces the stress in the matrix, increasing its yield strength. For this to be possible, the strength of 

Although coarse second-phase particles do not strengthen a material as efficiently as fine-grained particles, they offer a multitude of ways to influence material properties. 

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Shin, KE., et al. High-Temperature Properties of Particle-Strengthened W-Re, J. of Metals (August 1990). 

Fig. 11.34. Comparison of theoretical and observed increment in the flow stress for particle strengthened AI-AI2O3 (adapted from Embury et al. (1989)).

Particle Strengthening of Metals and Alloys by E. Nembach, John Wiley and Sons, Inc., New York New York, 1997. A book which takes stock of the current understanding of the field. 

Plastic Deformation and Fracture of Materials edited by H. Mughrabi. This book is Vol. 6 in the series Materials Science and Technology edited by R. W. Cahn, P. Haasen and E. J. Kramer, VCH Publishers, Inc., New York New York, 1993. Of especial interest concerning the present discussion see chap. 6 on the subject of solid solution strengthening by H. Neuhauser and C. Schwink and chap. 8 by B. Reppich on the subject of particle strengthening. 

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. 

Large particle reinforced composite systems are utilised with all three types of materials (metals, ceramics and polymers). Concrete is a common large particle strengthened composite where both matrix and particulate phases are ceramic materials. 

Table 6.6 summarises the strength of some particle-strengthened aluminium alloys. 

Table 6.6. Effect of particle strengthening on yield strength Rpo.2 and fracture strain A of aluminium alloys after en 485. The specified values are minimum values for sheets with a thickness of more than 12.5 mm, according to the standard. The state remarks describe the heat treatment...

Precipitation-hardened alloys can be very strong, but their service temperature is limited. For example, long-time application of precipitation-hardened aluminium alloys at temperatures above 200°C is impossible due to excessive coarsening of the precipitates. To use high-strength materials at high temperatures, another method of particle strengthening can be achieved by... 

In composites, different materials are combined to exploit favourable properties of each. That such combinations may be attractive was already shown in section 6.4.4 for particle strengthening of metals and in section 7.5 for dispersion-strengthened ceramics. 

How particles can be used to change material properties was already discussed in different contexts. Precipitates in metals were covered in section 6.4.4, particle strengthening in ceramics in section 7.5, and copolymers in section 8.7. 

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. 

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