Influence of ordering on properties

5.6.3 Influence of ordering on properties

5.6.3.1 Specific heat

The order–disorder transformation has a marked effect on the specific heat, since energy is necessary to change atoms from one configuration to another. However, because the change in lattice arrangement takes place over a range of temperature, the specific heat–temperature curve will be of the form shown in Figure 5.3b. In practice the excess specific heat, above that given by Dulong and Petit’s law, does

not fall sharply to zero at T c owing to the existence of short-range order, which also requires extra

energy to destroy it as the temperature is increased above T c .

5.6.3.2 Electrical resistivity

As discussed in Chapter 3, any form of disorder in a metallic structure (e.g. impurities, dislocations or point defects) will make a large contribution to the electrical resistance. Accordingly, superlattices

below T c have a low electrical resistance, but on raising the temperature the resistivity increases, as shown in Figure 5.17a for ordered Cu 3 Au. The influence of order on resistivity is further demonstrated by the measurement of resistivity as a function of composition in the copper–gold alloy system. As shown in Figure 5.17b, at composition near Cu 3 Au and CuAu, where ordering is most complete, the resistivity is extremely low, while away from these stoichiometric compositions the resistivity increases; the quenched (disordered) alloys given by the dotted curve also have high resistivity values.

Disordered

Quenched Cu 3 Au alloy 15 Disordered

cm) − alloy (ohm 10 6 Annealed Cu 3 Au alloy

× Ordered Cu 3 Au

5 Ordered

alloy

5 T c Resistivity

Resistivity (arbitrary units) 60

Reduction in c.s.a. (%) (a)

Figure 5.17 Effect of temperature (a), composition (b), and deformation (c) on the resistivity of copper–gold alloys (after Barrett, 1952; courtesy of McGraw-Hill).

260 Physical Metallurgy and Advanced Materials

5.6.3.3 Mechanical properties

The mechanical properties are altered when ordering occurs. The change in yield stress is not directly related to the degree of ordering, however, and in fact Cu 3 Au crystals have a lower yield stress when well ordered than when only partially ordered. Experiments show that such effects can be accounted for if the maximum strength as a result of ordering is associated with critical domain size. In the alloy

Cu 3 Au, the maximum yield strength is exhibited by quenched samples after an annealing treatment of 5 min at 350 ◦

C, which gives a domain size of 6 nm (see Figure 5.14). However, if the alloy is well ordered and the domain size larger, the hardening is insignificant. In some alloys such as CuAu or CuPt, ordering produces a change of crystal structure and the resultant lattice strains can also lead to hardening. Thermal agitation is the most common means of destroying long-range order, but other methods (e.g. deformation) are equally effective. Figure 5.17c shows that cold work has a negligible effect upon the resistivity of the quenched (disordered) alloy but considerable influence on the well- annealed (ordered) alloy. Irradiation by neutrons or electrons also markedly affects the ordering (see Chapter 3).

5.6.3.4 Magnetic properties

The order–disorder phenomenon is of considerable importance in the application of magnetic mate- rials. The kind and degree of order affects the magnetic hardness, since small ordered regions in an otherwise disordered lattice induce strains which affect the mobility of magnetic domain boundaries (see Section 5.8.4).