ch11.1-S06 - Chapter 11 Phase Transformations structure...

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MY2100 – Ch 11, Part 1, Slide 1 Chapter 11 - Phase Transformations A phase transformation is a change in the number and/or character of the phases that make up the microstructure of a material. Why is this an important topic for us to study?? structure processing properties Performance
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MY2100 – Ch 11, Part 1, Slide 2 ISSUES TO ADDRESS... Transforming one phase into another takes time. Fe γ (Αυστενιτε 29 Ευτεχτοιδ τρανσφορματιον Χ ΦΧΧ Φε 3 Χ (χεμεντιτε 29 α (φερριτε 29 + (ΒΧΧ 29 How does the rate of transformation depend on time and temperature? How can we slow down the transformation so that we can engineer non-equilibrium structures? Are the mechanical properties of non-equilibrium structures better than those obtained under slow cooling?
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MY2100 – Ch 11, Part 1, Slide 3 Materials Processing How do these questions relate to materials and steel, in particular? In this chapter, we learn that it is possible to change the mechanical behavior of a metal significantly without changing its composition , but by changing its processing. For example, in a steel, we can fix the carbon concentration but change the heat treatment, and thereby achieve tremendously different mechanical behavior. We are controlling phase transformations in a way that allows us to control the microstructure and ultimately, the properties of a material .
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MY2100 – Ch 11, Part 1, Slide 4 Stages in phase transformations There are two stages to most phase transformations: Nucleation Growth Nucleation is the stage where atoms diffuse and collect together to form a new phase (form small particles called nuclei). For example, in the pearlite reaction, carbon must diffuse and concentrate from a random solution of 0.76 wt% carbon in austenite to 6.7 wt% carbon in order to form Fe 3 C. Growth of the new phases occurs only after they nucleate. Growth also requires diffusion. Growth occurs because the material can reduce its energy by reducing the amount of interfacial area between phases. (Few, big particles have less interfacial area than many, small particles.)
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MY2100 – Ch 11, Part 1, Slide 5 Kinetics of phase transformations At a given temperature, a plot of the fraction of material that has transformed vs time has the general shape (S curve) shown here. Initially, large numbers of tiny nuclei form and then grow.
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