Diffusion-controlled mechanisms.

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 1, 2). For aluminium 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 that quenched-in vacancies result in clustered vacancy defects and these may harden the material. Similarly, irradiation by high energy particles may produce irradiation-hardening (see Figure 2).

Figure 1. Effect of quenching on the stress–strain curves from (a) aluminium (after Maddin and Cottrell, 1955), and (b) gold (after Adams and Smallman, unpublished).

Figure 2. (a) Stress–strain curves for unirradiated and irradiated fine-grained polycrystalline copper, tested at 20 °C; (b) variation of yield stress with grain size and neutron dose (after Adams and Higgins, 1959).

This dependence of the yield stress 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 key 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 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 characterized by a low initial rate of work hardening (see Figure 1). 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 Modern Physical Metallurgy and Materials Engineering 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 contact. 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.

2. Look through the text and find cases of:

1) Present Perfect (Passive Voice);

2) Modal Verbs;

3) nouns with the suffix – tion.

Give summary of the text A (in 8-9 sentences).

Read the text and translate it into Russian.

Text B.

1. Read the text B and choose the best title for it: