Materials Science and Engineering: A ( IF 4.652 ) Pub Date : 2020-01-02 , DOI: 10.1016/j.msea.2019.138906 Guoliang Shi; Jiawei Yuan; Ting Li; Kui Zhang; Xinggang Li; Yongjun Li; Minglong Ma
Extension twinning is not only a key plastic deformation mode of Mg and its alloys at room temperature (RT), but also introduces high density of dislocations and changes crystal orientation (texture) which exert considerable influence on precipitation strengthening and mechanical properties. Using tensile test, quasi-in-situ hardness test, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the effects of extension twinning on age-hardening, precipitation behavior and mechanical properties of an extruded AZ80 bar were investigated. Extension twins were introduced into the bar by cold compressed (H1) along extrusion direction (ED), and two kinds of aging treatments, i.e. ‘on-line quenching + aging’ (T5) and ‘on-line quenching + H1 + aging’ (T10), were conducted at 150 °C. We clarified that extension twinning can significantly accelerate precipitation kinetics. For T5 aging, there is an incubation stage in the first 15 h, and then the hardness increases continually and slowly in the following 35 h. By contrast, for T10 aging, softening is caused in the first 2.5 h, and then the hardness increases linearly until 20 h and remains stable in the next 30 h. Discontinuous precipitation is predominant in T5 aging; however, discontinuous precipitation is mostly inhibited in T10 aging and at the same time nano-sized continuous precipitates with high number density are formed in both extension twins and matrixes. Extension twins contains higher dislocation density and thus denser continuous precipitates than matrixes. Both <a> and <a+c> dislocations can be seen in extension twins and matrixes. Mechanical properties have been largely improved by extension twins. Particularly, after T10 the ED tensile yield strength (Rp0.2) remarkably increases from 144 MPa (H1) to 206 MPa, indicating that dense nano-sized continuous precipitates can effectively hinder twin-boundary migration during the detwinning. Moreover, T10 can obviously diminish the machinal property anisotropy as compared with T5.