Study On The Microstructure And Properties Of Aluminium Alloys And Cu-Based Bulk Metallic Glasses Solidified By Phase-Transition Cooling Medium

A new rapid-cooling technology using a copper mould cooled by a phase-transition medium was used to prepare cast aluminium-A356 alloy and Cu-based bulk metallic glass by solidification of the melt in this paper.The temperature fields of the casting and the mould with phase-transition cooling were simulated using the Ansys software package. The simulated results showed that the cooling rates of the melt could be controlled by using different phase-transition materials and their volumes. Sodium, tin and zinc were selected as the phase-transition materials in this paper. The cooling rate of melt obtained by using Sodium as phase-transition medium is the highest one of these three media, followed by tin, zinc. This was caused by their different thermal physical properties. The simulated curves of melt and media were in good agreement with the experiment curves.The microstructures and mechanical properties of aluminium-A356 alloy cast with this technology were studied in this paper. The results showed that the solidification conditions affected the microstructures of this alloy. The effect of refining on the microstructures was weakened in the order: sodium, tin, and then zinc. The refining ability of copper was between sodium and tin. The different ratios of volumes between the casting and the media also affected the microstructures of this alloy. The microstructure was refined as the ratios increasing. The microstructure was the finest when the ratio was 5.25, and when higher than this ratio the microstructure became coarser. The microstructure was modified by adding Al-10%Sr and refined by adding Al-5%Ti-B to the melt. The microstructure was refined obviously and the microhardness was improved when the amount of Al-Sr10% was 0.04% or the amount of Al-5%Ti-B was 0.03%. However, the microstructure became coarser when both Al-10% Sr and Al-5%Ti-B were added in the melt, and the microhardness decreased. Compounds formed by the combination of modifier and refiner weakened the effect of refining on the microstructure.The microstructures of the casting solidifying with a phase-transition material and copper were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-rays diffraction (XRD). The results showed that the microstructure obtained by the phase-transition cooling method was finer than that obtained by copper mould. The solubility of silicon inα-Al was 2.7 at. % in specimens solidifying in phase transition cooling medium and 2.4 at. % in specimens solidifying in copper mould. The ultimate tensile strengths (UTS) and elongation of A356 alloy cast by phase-transition cooling method were improved 22.9 % and 33.3%, respectively. The optimal heat treatment of T6 is 175℃×8h and the UTS is 320MPa. The optimal heat treatment of T5 is 140℃×4h and the UTS is 305MPa.Bulk metallic glasses of Cu50Zr40Ti10 with a diameter of 4 mm and Cu47.5Zr47.5Al5 with a diameter of 5 mm were prepared by using phase-transition cooling methods and their crystallization kinetics and mechanical properties have also been investigated in this paper. The crystallization kinetics of these two kinds of bulk metallic glasses was investigated by means of DSC under nonisothermal conditions. The results showed that the crystallization process of these two kinds of bulk metallic glasses could be regarded as a kinetically modified thermodynamic phase transformation process.Real-time detection of the crystallization behavior of Cu50Zr40Ti10 bulk metallic glass from the glassy state was studied using X-ray diffraction during in-situ heat treatment. The results showed that monoclinic CuZr metastable phase is firstly precipitated in the glassy matrix during the continuous heating process, and the equilibrium phases were orthorhombic Cu10Zr7, orthorhombic Cu8Zr3 and triclinic Cu3Ti2 at the final stage of crystallization.The compressive deformation behaviors of these two kinds of bulk metallic glasses at room temperature were investigated at strain rate of 10-4 s-1. The results showed that both the strength and the strain of Cu50Zr40Ti10 bulk metallic glass are much high than those of Cu47.5Zr47.5Al5 bulk metallic glasses.