The application of alloys consisting of two and more phases made possible to obtain composite materials in which high temperature strength combines with low temperature ductility and other useful properties. Strength characteristics of composite materials can be estimated from the mixing low, taking  into account the dependence of properties for every phases from the size of structural elements (for example, by Hall-Petch law) and contiguity of phases.

For example hardness of well known composite material WC-Co can be calculated by equation proposed by Lee and Gurland [1]

 H=H_WCV_WCC+H_m(1-V_WCC)   (1)

In this equation H – hardness of WC-Co, H_WC – hardness of binderless polycrystalline WC, H_m – hardness of the binder phase in WC-Co, C – contiguity of the WC grains, V_WC – volume fraction of the WC phase (0.9 in WC-6wt%Co).

It was shown that H_WC and H_m follow the Hall-Petch relationships of the type:

H_WC=H_0WC+K_0WCd^-1/2   (2)

Hm=H_0m+K_0ml^-1/2   (3)

where d is the mean WC grain size, l the mean free path in the binder phase, and H_0WC, K_0WC, H_0m and K_0m are parameters that were determined experimentally.

The following relationship exists between d and l:

l=V_md/((1-V_m)(1-C))   (4)

where V_m is the volume fraction of the binder (0.1 in WC-6wt%Co).

By substituting (2), (3) and (4) into (1) obtains [2]

H=H₀+K_yd^-1/2   (5)

where:

H₀=H_0WCV_WCC+H_0m(1-V_WCC)   (6)

K_y=K_0WCV_WCC+K_0m(1-V_WCC)B^-1/2   (7)

Equation (7) suggests that when the cobalt content is constant, a Hall-Petch-type relationship exists between the hardness of  composite WC-Co and the WC grain size.

It was shown that equation (5) is true for WC-Co alloys in the wide temperature range.

The estimation of composite plasticity is the more difficult problem.

So, if composite consists of two phases: hard and metallic soft ones, elongation to fracture in tension tests can be calculated using supposition that the soft phase is only deformed.

For characterization of plasticity of brittle at standard mechanical tests composites the indentation method can be used [3]. Plasticity characteristic was introduced as a dimensionless parameter which is determined by the fraction of the plastic strain e_p in the total elastoplastic strain ε_t   δ_H=ε_p/ε_t.

For Vickers method

δ_H = 1-14.3(1-ν-2ν²)HV/E,

where n is Poisson’s ratio, HV is the Vickers hardness and E is Young’s modulus. The dependence of d_H on the temperature for different composites is discussed.

Dispersion hardened metallic alloys is the very important class of composite materials. Mechanisms of deformation and fracture of these materials are investigated the most fully and discussed in the presentation. Some new possibilities of dispersion hardening are considered: effectiveness of dispersion hardening metallic alloys by quasicrystalline particles and hardening by two ensembles of disperse particles, possibility of dispersion hardening metallic glasses by nano size crystalline particles etc.

For example in aluminum alloys of Al-Zn-Mg-Sc system two ensembles of disperse particle are used for hardening: h’-phase and intermetallic Al₃Sc. Variation of size and concentration of these particles made possible to find optimal values of strength and plasticity.

It was shown that in metallic glass Al₉₁Ce_9-xSc_x nano size particles of crystalline aluminum which are formed at x = 3 in amorphous matrix increase hardness from 2 to 3.5 GPa.

Eutectic alloys are discussed as the natural composites, which have the most advantageous mutual orientation of phases, high thermal stability and increased high temperature strength. Effectiveness of creating eutectic alloys in quasibinary sections of three components systems (e.g. Al-Ti-Cr) is considered.

In last years it was shown that one component metals of technical purity (in which phase transitions are absent) can behave as many phases composites. At that texture components play a part of phases. Texture components, which are formed in the process of thermomechanical treatment, have different dislocation density, different mechanical properties and recrystallization temperature.

The different properties of the texture components can lead to the so called 45⁰-britleness of Mo and W alloys.

References

1.               Lee, H.C. & Gurland, J. Mater. Sci. Engng 33 (1978) 125.

2.               Milman, Yu.V, Chugunova, S., Goncharuk, V., Luyckx, S., Northrop, I.T. Int. J. of Refractory Metals & Hard Materials 15 (1997) 97.

3.               Milman, Yu.V. J. of Physics D: Applied Physics 41 (2008) 074013.

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