Papers by Author: Jian Feng Nie

Abstract: Sheets of a Mg60Cu29Gd11 alloy were produced by twin roll casting with all operational variables, except roll speed, being kept constant. As a function of the roll speed, the structure of the as-cast sheet changed from being crystalline to fully amorphous and then back to crystalline. Through careful selection of the casting speed that is suitable for the selected alloy system and with which the exit temperature of the sheet remains within the supercooled liquid region, a malleable sheet with no surface defects is produced. This work shows that twin roll sheet casting is a viable process for the production of magnesium-based bulk amorphous sheet in a continuous manner and on an industrial scale.

Abstract: In high strength AlMgSi alloys additions of Mn and Cr lead to the formation of dispersoid phases whose primary functions are to improve fracture toughness and control grain structure. Whether or not dispersoid phases form during heating to the homogenisation temperature and which dispersoid forms is strongly dependent on the alloy composition. By correlating dispersoid features after different homogenisation heat treatments to TEM investigations into the crystal structure, it is proposed that the crystal structure and chemical composition of the dispersoids changes as the dispersoids coarsen at increased temperatures and times.

Abstract: The impact properties of high-pressure die cast Mg-RE alloys were investigated. It was found that, for rare earth contents between 2-4 wt.%, the Mg-La and Mg-Nd alloys performed better than the Mg-Ce alloys in un-notched tests. The notched results appear to be related to the amount of intermetallic. In contrast, the un-notched results indicate that at some compositions the Mg-La alloys out-performed the other alloys when compared to the amount of intermetallic. It was apparent that a lamellar eutectic structure can improve the un-notched impact properties of Mg-RE based alloys even when this is not evidenced in tensile test or notched impact results.

Abstract: It is often assumed that the precipitation sequence and phases in Al-Si-Mg foundry alloys, such as A356 with 7 wt%Si, are similar to those in wrought 6xxx Al-Mg-Si alloys, such as 6063. The foundry alloys have been less extensively studied due to added difficulties in sample preparation, resulting from the high volume fraction of coarse particles of spheroidised eutectic silicon. Recent work has been successful in studying the precipitation sequence in a foundry alloy containing 0.45 wt%Mg. The work highlights some differences and similarities between foundry and wrought alloy precipitation, which may have implications for alloy design and heat-treatment.

Abstract: This paper presents an analysis, based on microstructural observations by transmission electron microscopy, of the influence of chemical composition on creep resistance of high pressure die casting (HPDC) Mg-4RE-0.6Zn alloys. The improved creep resistance, by increasing the Nd/La ratio of the rare earth (RE) mixture, is shown to be associated with the supersaturation of Nd solute in the -Mg matrix, evidenced by the formation of Nd-rich precipitates on dislocations after a thermal ageing treatment. The result indicates that solid solution/precipitation strengthening of the -Mg matrix is important to the creep resistance of HPDC Mg-RE based alloys.

Abstract: The structure of intermetallic phases and planar defects in the as-cast and the solutiontreated microstructures of a Mg-8Y-2Zn-0.6Zr (wt%) alloy are characterized using transmission electron microscopy. The alloy was produced by permanent mould casting and solution treated at 500 °C. It is found that the intermetallic particles in the as-cast microstructure have a monoclinic structure. An appreciable amount of intermetallic particles is still retained along grain boundaries after solution treatments for up to 60 hrs. However, the structure of the retained intermetallic particles changes gradually from monoclinic to hexagonal during the solution treatments. Some planar defects are also detected in the as-cast and the solution-treated (1 hr) microstructures. These defects have characteristic features of stacking faults.

Abstract: The microstructure of Mg-3wt%Nd-0.2wt%Zn-0.4wt%Zr (NZ30K) alloy after solution treatment was investigated and several kinds of interesting phases containing zirconium were found in this study. NZ30K was gravity cast using permanent die casting. After high tempering solution treatment at 540°C, cluster particles were observed inside the grains under optical microscopy. The detailed investigations were carried out on transmission electron microscopy (TEM). Four kinds of precipitates were visible inside grains with different shapes: block-like, globular, short rod-like, long rod-like. The block-like particle was identified as ZrH2 phase and the globular one was Zn2Zr3 phase. The other two were still Zr-containing phases, which could not be identified to any of existed compounds containing Zr. The formation of Zn-Zr compounds would probably be due to relative high ratio of Zr to Zn in the center of grains. ZrH2 would be the results of reaction of zirconium with H element during solution treatment, which probably came into the alloy during melting.

Abstract: The trace additions of Ag to binary Al-Mg alloys promote precipitation of a phase in the under-aged condition. The structure of the phase in an Al-10Mg-0.5Ag (mass%) alloy have been characterised using transmission electron microscopy and electron microdiffraction. An icosahedral quasicrystalline phase, which is identified by the electron microdiffraction patterns, has been found in the Al-10Mg-0.5Ag (mass%) alloy after solution treatment, water quenched and then aged during the time between 20 and 40 min. at 513K. The orientation relationship between the quasicrystalline phase and the a - Al matrix is as follows; i5 // <011>a and i3 // <111>a. A uniform distribution of finer-scale (typically 20 nm), faceted precipitate icosahedral quasicrystalline particles was observed within the core of the grains, but the precipitates became coarser (60-80 nm) approaching the grain boundary precipitate-free zone. The quasicrystalline particles contain elements of Al, Mg and Ag. The quasicrystalline precipitate particles, which are the metastable phase, appear to be the primary strengthening phase in the Al-10Mg-0.5Ag (mass%) alloy aged at 513K.

Abstract: The interaction between vacancies and Sn and Cu solute atoms in an Al-1.7at.%Cu- 0.01at.%Sn alloy was investigated by exploring the effect of incorporating natural ageing into conventional age hardening treatment. It was found that provided the artificial ageing temperature does not exceed a critical value between 160°C and 200°C, a narrow window of natural ageing (3-100 h) will result in a significant acceleration of the age hardening response and no decrease in peak hardness. Transmission electron microscopy showed that this effect reflects a large and rapid increase in number density of Cu GP(I) zones, and, to a lesser extent, of θ". The distribution and number density of θ' are essentially unaffected. Three-dimensional atom probe provided strong evidence that refinement of GP(I) zone distribution is not due to clustering of Cu atoms onto pre-existing Sn clusters. Instead it appears to be caused by a subtle interaction between vacancies, Sn and Cu atoms.