Study On Strain Rate Sensitivity And Wear Resistance Of As-cast Mg-Zn-Gd Alloy

In this paper,nanoindenter,atomic force microscopy(AFM)and electron microscopy techniques have been applied to investigate the strain rate sensitivity and to analyze wear resistance properties for a Mg-based icosahedral quasicrystalline phase.Nanoindentation test and nanoscratch test were carried out at room temperature for the quasicrystal phase and the magnesium matrix in the Mg79.4-Zn18.5-Gd2.1(at%)alloy..The indentation curves and surface morphological features of indents corresponding to different strain rates were compared,through which the deformation and wear behaviors for both the quasicrystalline phase and the magnesium matrix have been characterized.The main results are as follows:In the as-cast Mg79.4-Zn18.5-Gd2.1(at%)alloy,a large number of icosahedral quasicrystal phases were found to be formed in a network structure,and their quasiperiodic ordering degree is lower in comparison with that of typical high-quality quasicrystal.The Mg-based quasicrystalline phase exhibits definite strain rate sensitivity in contrast to magnesium matrix with quite weak effect in this aspect.It is revealed that,with increasing of indentation strain rate,elastic modulus of the magnesium matrix and quasicrystals show no change,while the hardness of quasicrystalline phase increases obveriously.For the quasicrystalline phase subjected to indentation testing,its hardness value increases rapidly followed by a rapid dropping,and then decreases slowly to reach a certain value.The higher is the strain rate applied,the higher is peak hardness of quasicrystal phase,and furthermore,the higher are the degree of softening and the final stable hardness value.Clearly,theses variations reflect the complexity of the deformation mechanism in the quasicrystal structure.Unlike the case in conventional crystal materials,increasing of the strain rate gives rise to larger indentation pile-up in the quasicrystal phase,which can be attributed to a work softening process taking place in the quasicrystal phase.In the nano-scratch test,the impacts of scratching speed and vertical loading rate on the scratching behaviors for magnesium matrix and quasicrystals have been explored.The results show that,with the scratching speed increasing,the hardness values for both the magnesium matrix and the quasicrystal phase tend to increase to some extent,and as a result their wear rates turn to decrease.Decreasing rate for wearing in the quasicrystal phase is larger than that in the magnesium matrix.It has been found that,with increasing of vertical loading rate,both scratch depth and wear rate for the quasicrystal phase and magnesium matrix increase,while the increasing rate for wearing in quasicrystal phase is smaller.Moreover,it is found through measuring the curve of scratch depth vs.scratch distance that the variation rate of scratching depth does not exhibit a simplified linear increase with increasing of vertical loading;it actually increases rapidly first to undergoes an inflection point,and then turns to increase linearly.This variation behavior do not agree with that described by Aachard wear equation,and may be attributable to the different effects from different wear mechanisms.The friction and wear behaviors of the Mg-based quasicrystal phase in the as-cast Mg79.4-Zn18.5-Gd2.1(at%)alloy is much better than that of the magnesium matrix.This means that Mg-based alloys of this type have potential to be designed and used as friction-and-wear resistant materials.