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Introduction to metals Dr Ambreen Azam
- 1. Introduction to Metals Part 1 DR AMBREEN AZAM MSC DENTAL MATERIALS (UK)
- 2. Learning objectives Discuss structure of metals Understand cooling curves and practical implications of phase diagrams Define & Classify alloys Differentiate between types of alloys Discuss coring
- 3. What are metals? Third biggest family of materials Hard lustrous, crystalline in nature with metallic primary bonds Opaque, good conductors of heat and electricity
- 4. Metals and Alloys in Dentistry
- 8. Structure of Metals Molten metal or alloy is cooled it solidify …..crystallization Nuclei formation 3 dimensional Crystal grow either (dendrites & spherulites) called grains Grain boundaries ….area between two grains Grains are equiaxed Atoms within each grain are arranged in regular three dimensional lattice either cubic, face centered cubic (FCC), Body centered cubic (BCC)
- 9. Structure of Metals Imperfections in crystal structure are called dislocations or defects Metals try to achieve a perfect structure depending upon atomic radius and charge distributions on the metal atoms Whenever metals get stressed these defects or dislocations move across the lattice till they reach the grain boundary This plane of movement is called slip plane These defects help in formation of wires Grain boundaries resist these movements Finer grains have more grain boundaries and vice versa
- 10. Movement across slip planes
- 11. Structure of Metals A fine grain structure can be achieved by rapid cooling of the molten metal or alloy following casting. This process, often referred to as quenching Forms many nuclei of crystallization resulting in a large number of relatively small grains Slow cooling….less nuclei.. larger grains Refined grain structure by seeding the molten metal with an additive metal …..nuclei
- 12. Shaping of metals and alloys Done by either Casting Heating the material until it becomes molten then poured into an investment mould Cold working Mechanical shaping of the metal at relatively low temperatures, taking advantage of the high values of ductility and malleability Amalgamation Mixing of alloys with mercury to form a plastic mass packed into a tooth cavity whilst still in the plastic state …Amalgam fillings
- 13. Cold working Ductility…extension…..applied tensile force Malleability… Compression…applied compressive force At stress greater than yield stress these cold working changes the micro structure Dislocations concentrate at the grain boundaries with change in grain shape The grains no longer equiaxed but become fibrous harder and stronger with a higher value of yield stress Ductility or malleability is decreased Cold working is sometimes referred to as work hardening Mechanical work at elevated temperature can change shape without altering grain shape and size
- 14. (a) Below the recrystallization temperature – produces a fibrous grain structure. (b) (b) Above the recrystallization temperature – retains an equiaxed grain structure.
- 15. Cold working in dentistry 1. The formation of wires, in which an alloy is forced through a series of circular dies of gradually decreasing diameter 2. The bending of wires or clasps during the construction and alteration of appliances. 3. The swaging of stainless steel denture bases (reducing diameter by cold working) Cold working beyond limit can cause fracture May cause the formation of internal stresses within a metal object If gradually relieved they may cause distortion which could lead to loss of fit of, for example, an orthodontic appliance A low temperature heat treatment referred to as stress relief annealing reduces these internal stresses without effecting the mechanical properties
- 16. Alloys Mixture of two or more metals in all possible combination. According to the number of constituting metals alloys can be Binary, ternary, quaternary… etc. Phases in an alloys due to different alloying metals 1. Homogeneous 2. Physically distinct 3. Mechanically separable portion of a system
- 17. Alloying While alloying metals usually show mutual solubility, one within another. When the molten mixture is cooled to below the melting point a solid solution is formed (solvent and solute) This solid solution can take one of these forms Substitutional solid solution (SSS) with Random/disordered or ordered arrangement of atoms Interstitial Solid Solution Gold-Copper Carbon-Iron
- 18. Qualities of Substitutional solid solution (sss) 1. Same space lattice type. 2. Same valence. 3. Same size or Atomic size difference must be less than 15 %; other wise, interstitial S.S. may form. 4. No chemical affinity; other wise, intermetallic compounds may be formed Gold-Copper
- 19. Qualities of Interstitial Solid solution The solute atoms are present in random positions (interstices) between the atoms in the crystal structure of the solvent metal. Requires that the solute atoms be much smaller in diameter than the solvent atoms The interstitial solid solution of carbon in iron is important, since it forms the basis for the family of carbon steels Carbon-Iron
- 20. Advantages of solid solution 1. Due to difference in atomic size localized distortion of the lattice or dislocation movement becomes more difficult strength, Hardness increases and ductility is decreased this effect is termed Solution hardening 2. Atoms of different atomic radii within the same lattice form a mechanical resistance to the movement of dislocations along slip planes 3. Being one phase system (homogenous structure), the resistance to tarnish and corrosion is high.
- 21. Types of Alloys according to solubility 1. Solid-Solution alloy Complete solubility Homogenous No chemical affinity 2. Solid solution alloys will make intermetallic compounds, complete solubility, homogenous, chemical affinity 3. Solid solution alloy Partial or no solubility Heterogeneous (Eutectic alloy)
- 22. 1- Solid Solution Complete Solubility between solvent and solute Gold Noble but soft Copper strong but low tarnish and corrosion resistance + = AuCu Strong and high resistance to tarnish and corrosion Clinical significance: Gold alloy metal crown
- 23. 2- Intermetallic compound Complete Solubility between solvent and solute Silver Precious Tin Weak and low melting temperature + = Ag3sn Brittle and can resist corrosion Clinical significance: Amalgam restoration
- 24. 3- Partial or no solubility (Eutectic alloy) There is a limit to the solubility of the two metals Heterogeneous structure. So, poor corrosion resistance. Strength and hardness surpass those of the constituent metals Brittle Heterogeneous structure. So, poor corrosion resistance Clinically used in soldering procedures
- 25. Soldering
- 26. Points of comparison Soluble Solid solution Intermetallic Compound Eutectic Solubility in the liquid state Solubility in the solid state Phases Complete Solubility Complete solubility Complete Solubility Partial or no solubility Homogenous Homogenous Heterogenous
- 27. Points of comparison Solid solution Intermetallic Compound Eutectic Cooling curve Properties As parents: Strength Hardness Ductility Melting range Melting range Low Melting Point No similarity to parents: Strength Hardness Brittle similarity to parents: Strength Hardness Brittle
- 28. Points of comparison Solid solution Intermetallic compound Eutectic Resistance to tarnish and corrosion Uses Metallic restorations High High Very low Amalgam restorations Soldering
- 30. Cooling curve for the solidification of an alloy
- 31. What is a phase diagram It is a collection of cooling curves of all possible combinations of the alloys system It shows temperature on the vertical axis verses composition on the horizontal axis Role of phase diagram: •The liquid phase [above the liquidus line]. •The liquid + solid phase [between the liquidus and the solidus line]. •The solid phase [below the solidus line]. •It is a map showing what phases are present at certain temperature and composition •It determines the chemical composition of each phase •It determines the appropriate melting temperature at which the alloy must be cast, as the alloy must be cast at 100C above its melting range (when the alloy is surly in its liquid state).
- 32. Temperature Time Composition 1555°C 960°C L L L L L L + S L + S L + S L + S L + S S S S S S S 100% Ag 0% Pd 0% Ag 100% Pd 80% Ag 20% Pd 70% Ag 30% Pd 50% Ag 50% Pd 30% Ag 70% Pd Equilibrium phase diagram of solid solution Liquidus line Solidus line
- 33. Eutectic alloy Phase diagram
- 34. What is coring? Non uniform composition of the crystal during solidification The center (core) of the crystal is rich in the metal of higher Tm The periphery of the crystal is rich in the metal of lower Tm Will result in heterogeneous structure with lower corrosion resistance Causes of coring 1. Great difference between Tm of constituent metals. 2. Rapid rate of cooling To prevent coring homogenization is performed It is a solid state (heat treatment) to eliminate Coring and formation of heterogeneous structure
- 36. What is Different ? Cored Structure Heterogeneous The crystal is not uniform in composition. Strength & hardness Ductility & corrosion resistance Homogenized Structure Homogenous The crystal is uniform in composition. Strength & hardness Ductility & corrosion resistance
- 37. Heat treatment of gold-copper alloy (Au/Cu) AuCuAuCu3
- 38. Annealing Relief of internal stresses Need less time because it a fibrous structure. Prolonged heating will lead to grain growth Homogenization Reverse the effect of coring Need more time because it is a cast structure. Prolonged heating has no effect
- 39. Increase amount of nuclei of crystallization Cold working Solution hardening (Alloying) Order hardening (heat treatment of gold copper system) Methods of altering the mechanical properties of alloy
- 40. Solid State Reaction Annealing Homogenization Heat treatments of gold-copper system
- 41. References Mccabe Appiled dental materials 9th Edition Phillips science of dental materials 12th Edition.
Editor's Notes
- 1-Primary 2-secondary 3-tertiary 4-quaternary 5- quinary 6-senary 7-septenary 8-octonary 9-nonary 10-denary 12-duodenary