Solidification and Grain Size Strengthening

1. Solidification and Grain Size Strengthening

2. Solidification and Grain Size Strengthening • Stages of Solidification • Nucleation: occurs when a small piece of solid forms in the liquid and must attain a minimum critical size before it is stable • Growth: occurs as atoms from the liquid are attached to the tiny solid until no liquid remains • Both conditions are met when the free energy of the particular phase is lower

3. Solidification Terminology • To get the solidification process started, the liquid phase must be undercooled, cooled to a temperature below the freezing point. • Once a nucleus forms, it can proceed to grow as fast as the latent heat of solidification and specific heat can be carried away. Controlled by: • thermal conductivities • relative masses • shapes of the melt, the solid, and mold

4. Growth of Interfaces depends on Concentration Gradient Temperature Gradient undercooling Growth Rate Types of Interfaces Planar Cellular Dendritic Equiaxed Growth Interfaces

5. Schematic Illustration of Solidification Morphologies

6. Solidification Time • Chvorinov’s Rule ts: Solidification time B: Mold Constant V: Volume of the Casting A: Surface Area of the casting in contact with the mold

7. Cooling Curves

8. Casting or Ingot Structure

9. Solidification Defects • Shrinkage - the degree, size, and location of shrinkage defects is dependent on the type of solidification of the alloy. For castings, can be corrected with risers. • Microshrinkage - commonly occurs in dendritic and equiaxed growth types. • Gas Porosity - metal dissolve considerable gas, higher concentration in liquid than solid. Thus, on solidification, gas bubbles form and are trapped by surrounding solid metal.

10. Control of Solidification • Inoculation or Grain Refining Practice - introduction of nuclei to encourage rapid solidification (reduce and minimize undercooling) • Directional Solidification - control the growth rate and temperature gradient to manipulate growth interface. • Use of chills in castings produce finer grain structure • Specialized solidification techniques to create single crystals castings

11. Atom Movement in Materials • Diffusion is the movement of atoms to produce a homogenous, uniform composition • Applications • Heat Treatment of Metals • Manufacturing of Ceramics • Solidification of Materials • Curing of Cement and Refractories • Manufacture of Electronic Components

12. Stability of Atoms • Imperfections and normal atoms are not stable or at rest • Atoms possess thermal energy that allow from atom movement Co is a material constant Q is the activation energy R is the gas constant T is temperature

13. Diffusion Mechanism

14. Diffusion Mechanisms

15. Activation Energy • A diffusing atom must squeeze past the surrounding to reach its new location

16. Rate of DiffusionFick’s First Law • Diffusion Coefficient or Diffusivity is dependent on temperature from Arrhenius rate equation

17. Composition ProfileFick’s Second Law • Applications • Heat Treatment of Steels • Plating and Metallic Coating

18. Diffusion and Material ProcessingGrain Growth

19. Diffusion and Material ProcessingDiffusion Bonding

20. Diffusion and Material ProcessingSintering

21. Solidification and Solid Solution Strengthening • Mechanical properties of materials can be controlled by the addition of point defects, particularly substitutional and interstitial atoms. • In addition, the introduction of point defects changes the composition of the material and influences the solidification behavior. • Definition of a Phase • has the same structure or atomic arrangement throughout • has roughly the same composition and properties throughout • is a definite interface between the phase and any surrounding or adjoining phases

22. Illustration of Phases and Solubility

23. Unary Phase Diagram

24. Unlimited Solubility • Regardless of the ratio of A and B, only one phase is produced by mixing them together. • Example: Cu-Ni alloy system

25. Limited Solubility • A is soluble in B, only one phase is found. However, if excess A is added to B and either A precipitates or A-B compound forms, then A has limited solubility in B. • Example: Cu-Zn alloy system

26. Conditions for Unlimited Solid Solubility in Metals and Some Ceramics • Atoms of the metals must be of similar size, with no more than a 15% difference in atomic radius. • Metals must have the same crystal structure. • Atoms of the metals must have the same valence. • Atoms of the metals must have about the same electronegativity.

27. Solid Solution Strengthening

28. Effect of Solid Solution Strengthening

29. Isomorphous Phase Diagrams

30. Phase Compositions

31. Lever Law

32. Relationship Between Strength and the Phase Diagram

33. Equilibrium Solidification of a Solid Solution Alloy

34. Cooling Curve

35. Solidification Simulation “Growth of Solutal Dendrites: A Cellular Automation Model and Its Quantitative Capabilities”, L. Beltran-Sanchez and D.M. Stefanescu, Metallurgical and Materials Transactions A, Volume 34A, Feb. 2003, p. 367-382

36. Nonequilibrium Solidification of Solid Solution Alloys

37. Effect of Nonequilibrium Solidification • Microsegregation - centers of dendrites (typical growth condition) are rich in solute. Lower mechanical properties result. Effect reduced by homogenization heat treatment. • Macrosegregation - similar to microsegregation but on a large scale. Effect reduced by hot working. • Zone Refining

38. Effect of Freezing Range on Shrinkage