The role of composition control on the structure property relationships in Mg-Nd-Zn alloys

镁合金是理想的重量关键应用，其中重量的减少与能源消耗的减少相关。然而，商业上可行的镁合金的性能概况不足以满足许多设想的应用。通过控制合金成分和二次相的分布来设计镁合金对于充分发挥镁合金的潜力至关重要。有报道称，在稀土含量相对较低的情况下，Mg-RE合金（RE=稀土）中添加Zn可以改变合金的组织和力学性能。然而，为了在一系列加工条件下了解RE:Zn比对微观结构和由此产生的力学性能的影响，研究人员已经系统地研究了在Mg中具有高溶解度的RE添加物，如Gd和Y。Nd是一种低成本、低溶解度的稀土添加剂，通过在Mg-Nd和Mg-Nd-RE合金中形成有效阻碍塑性变形的细尺度颗粒来增强镁合金的强度。在相对较低的Nd含量下，Zn的存在可以通过对微观组织的改性来提高合金的力学性能。本研究的最终目标是通过改变合金成分和控制热处理，开发出具有可调机械性能的低成本镁合金结构应用系统。通过在晶界处形成固态沉淀和金属间颗粒来设计微观结构和由此产生的机械性能的能力，将允许开发具有增强强度和延展性的协同组合的合金。将研究一系列不同Nd: Zn比的合金，利用原位x射线衍射和随后的非原位电子显微镜来了解凝固过程中的显微组织演变。金属间相、它们的形成温度和它们的体积分数将被用来改善目前可用的Mg- nd - zn相图中富Mg的一面。热处理在改变凝固过程中形成的金属间相和在这种热处理过程中形成新的固态析出物方面的作用将借助电子显微镜进行研究。这将提供进一步的步骤，在此过程中，微观结构和由此产生的机械性能可以被修改。铸件和热处理合金的机械性能将在室温和高温下进行评估，以了解合金成分、金属间相和热历史对这些性能的作用。

Mg alloys are ideal for weight critical applications, where the reduction in weight correlates with a reduction in energy consumption. However, the property profiles of commercially viable magnesium alloys are not sufficient of many of the applications envisaged. The design of Mg alloys through control of both the alloy composition and distribution of secondary phases is important in realizing the full potential of these alloys. It has been reported that the addition of Zn to Mg-RE alloys (RE=rare earth) results in modification of the microstructures and the resulting mechanical properties at relatively low RE content. However, to present only RE additions with high solubility in Mg, such as Gd and Y, has been investigated systematically to understand the role of RE:Zn ratio on the microstructure and resultant mechanical properties in a range of processing conditions. Nd is a lower cost and low solubility RE addition that has shown to enhance the strength of Mg alloys through formation of fine scale particles that hinder plastic deformation effectively in Mg-Nd and Mg-Nd-RE alloys. It is expected that the presence of Zn will enhance the mechanical properties through the modification of microstructure at a relatively low Nd content.The ultimate goal of this investigation is to develop a lower cost Mg alloy system for structural applications with tunable mechanical properties through modification of the alloy composition and controlled heat treatments. The ability to design the microstructure and resultant mechanical properties through formation of solid-state precipitation along with intermetallic particles at the grain boundaries and would allow the development of alloys with a synergistic combination of enhanced strength and ductility.A range of alloys with varying Nd: Zn ratios will be investigated to understand the microstructural evolution during solidification using in situ X-ray diffraction and subsequent ex situ electron microscopy. The intermetallic phases, their formation temperature and their volume fraction of these phases will be used to improve the Mg rich side of the Mg-Nd-Zn phase diagram that is currently available. The role of heat treatments on modifying the intermetallic phases formed during solidification and formation of new solid-state precipitates during such heat treatments is to be investigated with the aid of electron microscopy. This would provide for a further step during which the microstructure and the resultant mechanical properties could be modified. The mechanical properties of the cast and the heat treated alloys will be evaluated both at room and at elevated temperature to understand the role of alloy composition, intermetallic phases and the thermal history on such properties.