Ch 10 - CHAPTER 10 PHASE TRANSFORMATIONS ISSUES TO ADDRESS...

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1 ISSUES TO ADDRESS. .. Transforming one phase into another takes time. CHAPTER 10: PHASE TRANSFORMATIONS How does the rate of transformation depend on time and temperature? How can we slow down the transformation so that we can engineering non-equilibrium (stronger) structures? CHBE213 – Dan Samborsky PHASES - Austenite , γ (gamma): face-centered cubic, a solid solution of carbon and/or alloying elements in FCC iron (carbon ranges from 0 - 2%) - Cementite , Fe 3 C (iron carbide) : orthorhombic, a chemical compound of iron and carbon, very hard and brittle. First - some definitions dealing with steels - Ferrite, α (alpha) , body-centered cubic, a solid solution of carbon and/or alloying elements in FCC iron (carbon ranges from 0 ~ 0.025%) [stable at temperatures below ~ 912 o C, non-magnetic >770 o C] - Ferrite, δ (delta) , body-centered cubic, a solid solution of carbon and/or alloying elements in FCC iron (carbon ranges from 0 ~ 0.1%) [stable at temperatures between 1394 and 1538 o C] MICROSTRUCTURES - Austentite, Ferrite, Cementite (from last slide) - Pearlite (coarse and fine) ( α +Fe 3 C): a mixture of ferrite and cementite consisting of alternate platelets with the thickness of ferrite being about seven times the thickness of cementite. (alternating layers of ferrite (88 wt%) and cementite (12%)) (alternating layers of ferrite (88 wt%) and cementite (12%)) - Bainite (upper and lower) ( α +Fe3C): a mixture of ferrite and cementite in which the carbides are in very fine rod-like or needlelike forms resulting in an acicular appearance. MICROSTRUCTURES (cont) - Spheroidite (coarse and fine) ( α +Fe3C): a mixture of ferrite and cementite and/or alloy carbides in which the carbides are in spherical form. - Martensite : a supersaturated solid solution of carbon in distorted body-centered cubic iron. The iron lattice is commonly referred to as body-centered tetragonal (BCT). - Tempered martensite ( α +Fe3C): a mixture of ferrite and cementite in which the carbides are very finely dispersed in a submicroscopic spherical form. Table 10.2 Summary of Microstructures and Mechanical Properties for Iron-Carbon Alloys Reaction rate is a result of nucleation and growth of crystals. The higher the under cooling (below the critical temperature) the stronger the nucleation force. The higher the temperature, the faster diffusion takes place. Example - freezing water NUCLEATION AND GROWTH
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2 10.3 - 10.4 Phase Transformations and Kinetics (will not be covered in class or exams) - diffusion dependant (time and temperature required) - diffusionless (“martensitic”, shear transformation) Nucleation and diffusion drive the formations of different phases It should be noted that any change in structure which is diffusionless is called a “martensitic” change.
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Ch 10 - CHAPTER 10 PHASE TRANSFORMATIONS ISSUES TO ADDRESS...

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