南京航空航天大学《材料科学基础》Ch4 二元相图及其合金的凝固.ppt

Ch4 二元相图及其合金的凝固 4.1 相图的表示和测定方法 4.2 相图热力学基本要点 4.3 二元相图分析 4.4 二元合金的凝固理论 4.5 高分子合金概述 4.3 二元相图分析 和单组元不同，二元合金系有成分变化，因此在恒压条件下二元合金相图需要用两根坐标轴表示,其纵坐标表示温度，横坐标表示成分。设合金由A、B两组元组成，横坐标一端表示纯组元A ，而另一端表示纯组元B ，则二元合金相图由该平面坐标系中一系列曲线组成。 一、匀晶相图 1.定义: 二组元液态、固态均无限互溶的合金系,其相图就是匀晶相图.(Cu-Ni、Nb-Ti、Cr-Mo、Sb-Bi、Au-Ag、Cd-Mg、Pt-Rh) In both substitutional and interstitial solid solutions the overall atomic structure is virtually unchanged. Examples of solid solution alloys include Copper- Nickel, Gold- Silver all whom have an F.C.C structure. Molybdenum- Tungsten is an example of an solid solution with a B.C.C structure. A complete solid solution diagram looks like this. When the system plots in the purple field it is all liquid, in the yellow field it is a mixture of liquid and solid, and in the blue field it is entirely solid. The upper bound of the liquid+solid field is called the liquidus, and the lower bound is called the solidus. Sometimes solid solutions can have maxima or minima between the end members. These are no problem: the solid and liquid compositions will still slide down the liquidus and solidus curves. Treat each section as a simple solid solution. The only complication: what happens if a system exactly matches the maximum or minimum? In that case, it simply crystallizes a solid of that composition. Tin and lead are an example of a system with a minimum. Solder has a composition at the minimum point. This ensures that the solder has the lowest possible melting point, and that a solid of uniform composition crystallizes. The Lever Rule The equilibrium diagram for a solid solution alloy that we have just been dealing with contains two distinct phases, liquid and solid solutions. Between the liquidus and solidus lines these two phases exist together in equilibrium and hence the area between the curves is known as the two phase region. If a horizontal line is drawn through the two phase region, such a line is called a tie lin