This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: 1 4.1 Solidification: metals/alloys, single crystals, solid solutions Chapter 4 Solidification & Crystalline Imperfection Chapter 4 ME 201: Materials Science, Z. Hao 4.2 Crystalline Imperfection: Point, line, planar, and volume 4.3 Experimental Techniques: TEM, SEM, STM … Solidification of Metals ¡ Two Steps ¾ Nucleation: Homogeneous and heterogeneous ¾ Crystals and grain structure Definition: the process in which molten metals or alloys are casted into semi-finished or finished shape. Liquid Æ solid Chapter 4 ME 201: Materials Science, Z. Hao ¾ Crystals and grain structure ¡ Types of grain: Equiaxed and columnar ¡ Industrial casting Two Steps ¡ The formation of stable nuclei in the melt Æ nucleation ¡ The growth of nuclei into crystals and the formation of grain structure Grains Nuclei Chapter 4 ME 201: Materials Science, Z. Hao Liquid Crystals that will Form grains Grain Boundaries Titanium alloy Formation of Stable Nuclei ¡ Two main mechanisms ¾ Homogeneous nucleation ¾ Features ¾ Energy involved Chapter 4 ME 201: Materials Science, Z. Hao ¾ Critical radius versus undercooling ¾ Heterogeneous nucleation Homogeneous nucleation (1) Features ¡ Simplest case ¡ Metal itself will provide atoms to form nuclei ¡ Requires a considerable amount of undercooling: Several hundreds’ degree C below freezing temperature Chapter 4 ME 201: Materials Science, Z. Hao ¡ Several slow moving atoms will bond each other to form nuclei Clusters of atoms Embryo Nucleus Crystal Dissolve Unstable Stable Critical size NO YES Homogeneous nucleation (2) Energy involved ¡ Volume Free Energy ¾ Released by liquid to solid transformation. ¾ Δ G v is change in free it l ¡ Surface Energy ¾ Required to form new solid surface ¾ Δ G s is energy needed to create a surface (retarding Chapter 4 ME 201: Materials Science, Z. Hao energy per unit volume between liquid and solid . ¾ Free energy change for a spherical nucleus of radius r is given by v G r Δ 3 3 4 π create a surface. (retarding energy. ¾ γ is specific surface free energy . γ π 2 s 4 G r = Δ 2 Homogeneous nucleation (3) Energy involved γ π π 2 3 4 3 4 r G r G v T + Δ = Δ + γ π 2 s 4 G r = Δ ¡ Critical radius: r* Since when r=r*, d( Δ G T )/dr = 0 V G r Δ − = γ 2 * ¡ Total free energy Nucleus Chapter 4 ME 201: Materials Science, Z. Hao r* r Δ G- Δ G T r* v G r r Δ = 3 3 4 π Above critical radius r* Below critical radius r* Energy lowered by growing into crystals...
View Full Document
This note was uploaded on 04/14/2008 for the course ME 201 taught by Professor Hao during the Fall '07 term at Old Dominion.
- Fall '07