Temperature strongly affects stress-strain curves. Generally, strength decreases and ductility increases. However, this trend does change according to the micro­structural evolution such as precipitation, strain aging, or recrystallization that may take place during testing. Thermally activated processes help in deformation pro­cess and result in reducing the strength. Figure 5.9 shows the stress-strain curves of a mild steel at three different temperatures.

Understanding of thermally activated deformation is important for structural materials serving at elevated homologous temperatures. The flow stress (shown as shear component) of a pure metal is composed of two parts:

t = t* + Tg,

where t* and tG are the thermally activated stress and athermal (temperature — independent) stress components, respectively. There are two types of obstacles in a material: long-range obstacles and short-range obstacles. The influence of long-range obstacles occurs over several atom distances and is difficult to surmount through pure thermal fluctuations. Hence, the athermal stress component comes from the long — range obstacles. The long-range stress field is not generally affected by temperature or strain rate except for the change in modulus due to temperature change (which is purely due to reduction in interatomic bonding forces). On the other hand, the short- range obstacles (less than 10 atom diameters) for which dislocations can surmount

these barriers with thermal fluctuations result in temperature-sensitive strengthening. These short-range obstacles are also known as thermal barriers and their influence on flow stress strongly depends on temperature and strain rate. Figure 5.10 shows the variation of normalized flow stress (tested at a strain rate of 3 x 10~4 s-1) as a function of temperature in a high-purity titanium.