Point defect hardening

6.6.1 Point defect hardening

The introduction of point defects into materials to produce an excess concentration of either vacancies or interstitials often gives rise to a significant change in mechanical properties (Figures 6.35 and 6.36). For aluminum the shape of the stress–strain curve is very dependent on the rate of cooling and a large increase in the yield stress may occur after quenching. We have already seen in Chapter

3 that quenched-in vacancies result in clustered vacancy defects and these may harden the material.

Mechanical properties I 331

1 Slow cooled 40 2 Slow cooled and aged

⫺ 2 ) 60 Quenched

17 hours at room temperature 3 As quenched 4 Quenched and aged

30 17 hours at room

5 Immediate retest

Shear stress 1 2 3 4 5

0 0.1 (%) Strain

Strain (a)

(b) Figure 6.35 Effect of quenching on the stress–strain curves from: (a) aluminum (after Maddin

and Cottrell, 1955) and (b) gold (after Adams and Smallman, unpublished).

冣 20°C

load ig.

3 weeks in

Irradiated 3 weeks Unirradiated

or 50 flux 3 ⫻ 10 11 n.cm ⫺ 2 50

Nominal tensile stress

Irradiated 1 week

er yield tensile stress (MN m 0 w 0 5 Lo

(b) Figure 6.36 (a) Stress–strain curves for unirradiated and irradiated fine-grained polycrystalline

(a)

copper, tested at 20 ◦

C. (b) Variation of yield stress with grain size and neutron dose (after Adams and Higgins, 1959).

Similarly, irradiation by high-energy particles may produce irradiation hardening (see Figure 6.36). Information on the mechanisms of hardening can be obtained from observation of the dependence of the lower yield stress on grain size. The results, reproduced in Figure 6.36b, show that the relation

σ y =σ i +k y d −1/2 , which is a general relation describing the propagation of yielding in materials, is obeyed.

332 Physical Metallurgy and Advanced Materials This dependence of the yield stress, σ y , on grain size indicates that the hardening produced by

point defects introduced by quenching or irradiation is of two types: (1) an initial dislocation source hardening and (2) a general lattice hardening which persists after the initial yielding. The k y term would seem to indicate that the pinning of dislocations may be attributed to point defects in the form of coarsely spaced jogs, and the electron-microscope observations of jogged dislocations would seem to confirm this.

The lattice friction term σ i is clearly responsible for the general level of the stress–strain curve after yielding and arises from the large density of dislocation defects. However, the exact mechanisms whereby loops and tetrahedra give rise to an increased flow stress is still controversial. Vacancy clusters are believed to be formed in situ by the disturbance introduced by the primary collision, and hence it is not surprising that neutron irradiation at 4 K hardens the material, and that thermal activation is not essential.

Unlike dispersion-hardened alloys, the deformation of irradiated or quenched metals is charac- terized by a low initial rate of work hardening (see Figure 6.35). This has been shown to be due to the sweeping out of loops and defect clusters by the glide dislocations, leading to the formation of cleared channels. Diffusion-controlled mechanisms are not thought to be important, since defect-free channels are produced by deformation at 4 K. The removal of prismatic loops, both unfaulted and faulted, and tetrahedra can occur as a result of the strong coalescence interactions with screws to form helical configurations and jogged dislocations when the gliding dislocations and defects make con- tact. Clearly, the sweeping-up process occurs only if the helical and jogged configurations can glide easily. Resistance to glide will arise from jogs not lying in slip planes and also from the formation of sessile jogs (e.g. Lomer–Cottrell dislocations in fcc crystals).