Stress relaxation testing of aluminum - 30-45% silicon powder alloy in and around macroscopic yield point determined the effect of stress increase under the conditions of constant deformation. The increase is determined by mass transfer diffusion processes under the implied stress in a long period of time. The processes are oriented in a way to structural balance of the material. According to SEM data the structure after the testing becomes more equilibrium. A large number of rounded grains can be observed. The average size of small grains inside dendrite cells increases. Number of nano-size grains (150 nm or less) decreased by 20%. The quantity of pores decreases. Pores become more rounded in shape. Structural changing described above cause the increase of material strength properties. We recommend to apply the term strain age-hardening for the observed effect unlike generally accepted strain ageing or strain hardening.
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