Effect of Precipitates on Fatigue Strength

Abstract

Many high-strength alloys, such as Cu-Be alloy, Al-Cu-Mg-Zn alloy (duralumin), 17-7 PH stainless steel and maraging steel are strengthened by dispersing fine precipitates into a matrix. However, the increase of fatigue strength by precipitation hardening is usually smaller than the values expected from other mechanical properties. For example, carbon steel has a value σ_w⁄σ_B=0.4∼0.5 (σ_w: fatigue limit, σ_B: tensile strength), but as for duralumin, σ_w⁄σ_B=0.2∼0.3. These properties paticular to fatigue strength of precipitation-hardening alloys are assumed to be mainly due to the generation of lattice vacancies or interstitials during fatigue process. So, the effect of excess vacancies on ageing process during fatigue is very interesting.
Then the effects of point defects generated by fatigue during the ageing process and the influence of fine precipitates on fatigue strength were investigated for Cu-Be alloy, Fe-Ni-Ti alloy, and Fe-Cr-Mo alloy which have relatively high recrystallization temperatures.
As a result of experiments, the fatigue strength of these alloys was found to increase by precipitating fine particles in the matrix. The precipitating process is accelerated by the generation of lattice vacancies and interstitials during fatigue. When precipitation-hardening alloys are aged isothermally, the increase rate of fatigue strength is larger than that of other mechanical properties. However, the absolute value for the increase of fatigue strength is small as compared with other mechanical properties. The influence of the size and distribution of fine precipitate on fatigue strength was investigated using a transmission electron microscope.