Strain-induced Precipitation Research Articles

Strain Aging of Mild Steel under Tensile Stress

Although there have been few subjects in the field of physical metallurgy which have attracted more interest and attention than that of aging phenomena of mild steel, but little study has been made so far on the effect of stress upon strain aging. Using a 0.05 wt% C steel, strain aging was carried out with and without tensile stress at temperatures ranging from 55°to 250°C. The specimen was plastically strained about 9% in elongation at room temperature prior to the aging.The rate of aging was much enhanced by the stress. However, the inherent mechanism of strain aging does not seem to have been changed by the stress, since the apparent activation energy for strain aging is not changed by the stress imposed on the specimens during the aging process.The activation energy of 16000cal/mole was obtained at temperatures below 100°C which is about the same as that for diffusion of nitrogen in alpha iron, while a rather low activation energy of 9600cal/mole was obtained at higher temperatures. This difference implies that strain aging is controled by two different kinds of thermally activated processes. The early stages of strain aging at low temperatures are considered to be associated with migration of solute atoms to dislocations. But in the later stages and at higher temperatures, strain aging is caused by the strain-induced precipitation of carbides or nitrides at dislocations.It requires considerably more studies to define the effect of stress on strain aging. However, it is considered that the stress effects the vibrational frequency of the atoms around the dislocations and increases the diffusion constant of the solute atoms so that the rate of strain aging is enhanced by the stress.

Relationships between impact and low-strain-rate hydrogen embrittlement of titanium alloys

Hydrogen embrittlement of titanium alloys is generally separated into two types: impact embrittlement, which occurs in alpha-titanium alloys, and low-strain-rate embrittlement, which occurs in alpha-beta alloys. Data are presented which show that either low-strain-rate embrittlement or impact embrittlement can occur in alpha-compound, alpha-beta, or beta alloys depending on alloy composition and hydrogen content. These results suggest that both types of embrittlement are different manifestations of the same basic embrittling process, the rejection of hydride from solution in titanium, and that the type of embrittlement observed is related to the kinetics of hydride rejection. If hydride precipitates rapidly, impact embrittlement is observed. If hydrogen is held in supersaturation, strain-induced precipitation leads to low-strain-rate embrittlement.

Precipitation of Carbon and Nitrogen in Cold-Worked Alpha-Iron

The strain-induced precipitation of carbon and nitrogen from supersaturated solution in alpha-iron is shown to be in agreement with a dislocation mechanism and estimates of the dislocation density required to produce the observed precipitation rates are in agreement with dislocation theory. The activation energies involved in the process are found to be 20,000 cal/mole for carbon and 17,200 for nitrogen, in agreement with published data for the activation energies of diffusion of the two solutes.