Introduction to Materials Engineering: CH9

This paragraph introduces the topic of phase diagrams within the field of materials engineering, focusing on the equilibrium state resulting from the combination of two elements. It discusses how the composition in weight percent of copper and nickel, along with temperature, determines the number of phases formed, their compositions, and quantities. The concept of phase equilibria and solid solubility is explained using sugar in water at 20 degrees Celsius as an example, highlighting the solubility limit and the transition between single-phase and multi-phase mixtures. The paragraph also explains the difference between components and phases in an alloy system, using the aluminum-copper alloy as an example to illustrate distinct material regions with different crystal structures.

The paragraph delves into the effects of temperature changes on phase equilibria, using a sugar-water system as a base example. It explains how altering the temperature can change the number of phases present, moving from a two-phase region of liquid plus solid to a single-phase region of syrup at different temperatures. The Hume Rothery conditions for solid solubility in a simple system like nickel and copper are discussed, emphasizing the importance of similar crystal structures, electronegativities, and atomic radii for high mutual solubility. The paragraph introduces the concept of a binary system, focusing on temperature and composition while disregarding pressure, and describes the phase diagram for copper-nickel, highlighting the regions of single and multiple phase fields.

This section teaches how to interpret phase diagrams to determine the phases present, their compositions, and the weight fractions of each phase. It introduces the concept of isomorphous, where complete solubility of one component in another results in a single phase across the entire composition range. The paragraph explains the steps to determine the phases present at a given temperature and composition, using the phase diagram for copper-nickel as a reference. It also introduces the lever rule, which is used to calculate the weight fraction of each phase in a two-phase region, with examples provided to illustrate the calculations.

The paragraph discusses the microstructural changes that occur during the cooling of a copper-nickel alloy, starting from a high temperature with a liquid phase containing 35% nickel. As the alloy cools, alpha precipitates form with compositions that change due to the decreasing solubility of nickel in alpha. The concept of coring is introduced, explaining the difference between equilibrium cooling and the faster cooling rates typically experienced in real-world scenarios, which result in varying compositions of the solidifying alpha phase.

This section introduces binary eutectic systems, characterized by a special composition with a minimum melting temperature, exemplified by solder materials like silver-tin. The eutectic system is described with three single-phase regions and a eutectic composition line. The paragraph explains the eutectic reaction at a specific composition and temperature, leading to the formation of alpha and beta phases with distinct compositions. An example using a lead-tin alloy is provided to illustrate how to determine the phases present, their compositions, and the relative amounts of each phase at a given temperature.

The paragraph explores the microstructural developments in eutectic systems, starting with the cooling process from a liquid state and the formation of alpha and beta precipitates. It describes the microstructure at different tin compositions, including the formation of a lamellar eutectic structure at the eutectic composition, characterized by alternating layers of alpha and beta phases. The paragraph also explains how the cooling rate affects the size of the lamellar structures, with faster cooling leading to finer structures due to limited diffusion time.

This section introduces more complex phase diagrams involving intermetallic compounds and peritectic transformations. Intermetallic compounds are characterized by fixed compositions and appear as lines on the phase diagram. The paragraph explains the difference between eutectic, eutectoid, and peritectic transformations, providing examples from the iron-carbon and copper-zinc systems. It highlights the importance of understanding these transformations for the properties and microstructures of alloys.

The paragraph focuses on the iron-carbon phase diagram, detailing the eutectic and eutectoid transformations. It describes the formation of pearlite, a lamellar structure of alternating soft ferrite and hard iron carbide, which gives a polished sample a beautiful mother-of-pearl appearance. The paragraph also explains the microstructures of hypo-eutectoid and hypereutectic steels, detailing the formation of pro-eutectoid ferrite and the conversion of austenite to pearlite, and how to calculate the weight percentages of different phases in the steel.

In conclusion, the paragraph summarizes the importance of phase diagrams in determining the number and types of phases present in an alloy system, given its temperature and composition. It emphasizes the impact of the cooling rate on the microstructure of an alloy, which can deviate from equilibrium due to faster rates in practical applications. The paragraph recaps the key transformations, including eutectic, eutectoid, and peritectic, and their significance in materials engineering.