First Principles Study On Deformation Mechanism Of Magnesium By Alloying Elements And Strain

Magnesium alloy is the lightest structural metal material with good mechanical properties,high specific strength,good shock absorption and noise reduction,electrical conductivity and thermal conductivity,excellent biocompatibility,and degradation and environmental protection,which are widely used in aerospace,automotive and biomedical applications.Magnesium is a densely arranged hexagonal structure,with great anisotropy,difficult deformation and great brittleness in processing,and leads to the great limitation in the large-scale application of magnesium and its alloy,which cannot meet the industrial requirements for the large-scale application of structural materials.Therefore,it is very important to study the mechanical properties of magnesium and its alloys.In this thesis,we focused on the effects of alloying elements and strain on the initiation mechanism of different slip systems of magnesium and its alloys,by using the first-principles calculations based on density functional theory.It mainly includes three aspects:(1)We calculated and compared the unstable stacking fault energies and activation probabilities of five main slip systems,and found that basal <a> slip system is the easiest to open,prism <a> slip system is the second,Py-I <a> slip system is the third.We also found that the Py-I <c + a> and Py-II <c + a> slip systems are difficult to be activated,which is consistent with the experimental observations.(2)We systematically studied the contributions of alloy elements to the unstable stacking fault energies of alloys,and found that the addition of most of the larger atomic radius elements can significantly reduce the fault of the non-based slip system and increase the activation probability.Rare earths and Ca are found to promote the activation probability of prism <a>slip system by two orders of magnitude,which is consistent with experiments that alloying Y can activate the prism <a> slip system of Mg.Moreover,Ca is found to promote the activation probability of the Py-II <c + a> slip system by one order of magnitude and dramatically improve the plasticity of Mg.(3)The effects of three different strain conditions on the five slip systems of magnesium and its alloys were calculated by combining the unstable fault energy and activation probability.The results show that the stacking fault energies of all five slip systems are proportional to strain.When the tensile strain is applied,the increase of the activation ratio of the Py-I and PyII <c + a> slip system is one to two orders of magnitude higher than that of the basal <a> slip system,proving that the <c + a> dislocation is more easily activated.Finally,the synergistic effect of alloying elements and strain was studied by activation ratio and charge differential density diagram.