It is well known that a small amount addition of Mn to aluminum alloys can play an interesting role in fracture toughness, recrystallization and grain refinement. According to the recent investigation of Nam et al., the Mn-dispersoids in a new Al-Zn-Mg-Mn alloy act as a major role in increasing the strength without sacrificing ductility, improving fatigue life of low cycle and high cycle fatigue, enhancing the fracture toughness and increasing corrosion resistance. However, no results have been reported for the controlling growth mechanism and the effects on the deformation mechanism of Mn-dispersoid in Al-Zn-Mg alloy.
The present research was undertaken to investigate the effect of the Mn-dispersoid in an Al-Zn-Mg alloys on deformation mechanism by using the in-situ tensile test and in-situ fatigue test and to see the orientation relationship between the dispersoid and the matrix, through image analysis and computer simulation programs.
The chemical composition of the investigated Al-Zn-Mg alloy is 4.6wt.%Zn, 2.6wt%Mg and 0~1.0wt.%Mn and the ingots were hot-extruded and peak-aged.
Sphere- or rod-shaped Mn-dispersoids in the size range of 0.05~0.5㎛ are formed by the addition of Mn in Al-Zn-Mg alloy. The activation energy for growth of Mn-dispersoid, regardless of the difference in shape, was determined to be 200kJ/mole. It is concluded that the growth of the Mn-dispersoid is controlled by the diffusion of manganese in the matrix. The Mn-dispersoid was identified as $Al_6Mn$ and the crystallographic relation between the $Al_6Mn$ and the matrix, even though the dispersoid had different shape, was found to be $<213>_{matrix} \parallel(010)_{dispersoid}$ ; $[031]_m \parallel [100]_d$. The enhancement of strength by the presence of Mn-dispersoid regardless of their shape and size, without decreasing the ductility of the Mn-containing alloy is explained as a result of the dispersoids being incoherent with the matrix thus hindering the motion of dislocations.
From the results of in-situ tensile test of Mn00(no Mn alloy), in the early stage of tensile deformation, it was found that inhomogeneous slip trace is easily generated. As increasing the strain, the fracture occurred partially intergranular by the intense inhomogeneous slip trace. Mn78 (containing 0.78wt% Mn alloy) has higher resistance to inhomogeneous deformation and transgranular ductile fracture surface with micro-dimples was observed. These phenomena are due to the improved load bearing capacity and the effective accommodation of the applied stress. One can conclude the homogeneous deformation caused by the Mn-dispersoid disperses the slip and prevents the stress and/or strain concentration.
From the in-situ fatigue test, it was found that the fatigue life of Mn78 alloy was longer than that of Mn00, and it is explicitly observed in in-situ SEM and TEM work. It is believed that the dispersive distribution of Mn-dispersoids in Mn78 alloy promote homogeneous deformation by converting the deformation mechanism from planar slip to wavy slip. This results in retarding the fatigue crack initiation, as well as crack propagation, generated by the inhomogeneous deformation on the grain boundary.
