第三章凝固与结晶（The third chapter is solidification and crystallization）.doc

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第三章凝固与结晶（The third chapter is solidification and crystallization）.doc

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第三章凝固与结晶（The third chapter is solidification and crystallization）

第三章凝固与结晶（The third chapter is solidification and crystallization） Metal crystallization: microscopic phenomena macro phenomena basic conditions thermodynamic conditions structural conditions energy conditions Formation of third crystal nuclei Nucleation mode In supercooled liquid, the driving force of liquid metal depends on its energy change, and the crystal nucleus forms uniformly in liquid phase - homogeneous nucleation (homogeneous nucleation) In supercooled liquids, the crystals adhere to the surface of other substances to form nuclei - - inhomogeneity and (heterogeneous nucleation) Crystal nucleus - an embryo that can grow spontaneously Uniform nucleation Changes in energy during the formation of 1 embryos Total energy change = driving force + resistance Driving force - system volume, free energy difference, resistance - surface free energy Delta G=, Delta Gv+, Delta Gb =V Sigma Gb+S Sigma =4/3, PI, r^3, r^2, Gb+4, PI, sigma 0rrk r increase ---- Delta G rise --- can not grow up R=rk Delta G= Delta Gmax---- critical state Rrk r increase ---- Delta G drop - spontaneous growth Crystal nucleus: a nucleus with a radius greater than RK 2 critical radius of nucleus G=4/3, PI, r^3, Gb+4, r^2, PI, sigma With the method of RK dG/dr=4 r^2 point, pi pi sigma delta Gb+8 R Order dG/dr=0, rk=-2 sigma / delta Gb Substitute Delta Gb=-Lm, Delta T/Tm, rk= (2Tm, sigma, /Lm) * (1/, Delta, T) As a result, Delta T liters ---rk drop 3 core work G=V, Gb+S, sigma G0--- can not exist stably when 0rrk R=rk, Delta G= Delta Gmax---- critical state Rkrr0, Delta G0 and delta G r l to drop C can exist stably, but the nucleation energy of the system increases, need external energy input, the nucleation energy of A----A= Delta G----r=rk, Delta Gmax-------- A= critical nucleation energy A=, Gmax, =4/3, PI, rk^3, rk^2, Gb+4, PI, sigma Sigma delta Gb / rk=-2 dreams star delta sigma /rk Gb=-2 A=, Gmax, =4/3, PI, rk^2, sigma, =1/3 (4 PI, rk^2 sigma) R A critical =1/3 Delta Gs Conclusion: in the cri