Chapter_4_1

Chapter_4_1 - 1 4.1 Solidification: metals/alloys, single...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the 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.

Page1 / 5

Chapter_4_1 - 1 4.1 Solidification: metals/alloys, single...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online