The Study Of The Liquid Structure And Solidification Process Of The Al-rich Al-Mn Alloys

Many researches have been conducted on the alloys which can form quasicrystal.In the Al-rich Al-Mn alloys the icosahedral quasicrystal was firstly discovered.The study on the liquid structure and solidification process of Al-rich Al-Mn alloys should make insight into the mechanism of quasicrystal forming ability.In this dissertation,the liquid structure and the solidification of three Al-Mn alloys have been studied byθ-θX-ray diffraction,DSC and the Reverse Monte Carlo (RMC)simulation.The DSC curves show that the peritectic reactions of the Al-Mn₁₀,Al-Mn₁₄ and Al-Mn₂₀alloys are prevented in the cooling process of Al-Mn alloys.All the temperatures of the peritectic reactions of the Al-rich Al-Mn alloys from DSC are lower than the equilibrium temperature at 20℃/min cooling rate and the undercooling degree is approximately 50℃for the first peritectic reaction,but only 30℃for the second one.The DSC curves also show that the eutectic reaction temperatures are identical with the equilibrium temperature.The liquid structures of the the Al-rich Al-Mn alloys have been studied byθ-θX-ray diffraction.The results show that all of the mean nearest neighbor distances decrease with increasing temperature,which indicates that the micro-thermal contraction phenomenon of atom clusters exists in the the Al-rich Al-Mn alloys.In the Al-Mn₁₀,Al-Mn₁₄and Al-Mn₂₀alloys,the second peaks of the structure factors split into two peaks gradually with decreasing temperature.The splitting of the second peaks at positions equal to 1.7q1 and 2q1(q1 is the position of the first peak of the structure factor),suggesting the predominance of local icosahedral order in the Al-rich Al-Mn alloys.The x-ray diffraction experimental results show that the icosahedral phase forms at 20℃/min cooling rate in the Al-rich Al-Mn alloys.On the basis of the structure analysis the three alloys during the solidification process,it is indicated that the local structure in the liquid may persist into the solid state in the cooling process,and there are correlations between the structure of liquid state and that of the solid state in the Al-rich Al-Mn alloys.The RMC computer simulation shows the predominance of the five-fold symmetry around Mn atoms in the liquid Al-Mn₂₀alloy.