Metals and Alloys Lecture Notes PDF

METALS AND ALLOYS Lecture 8  Terminology Precious metals= high position in the normal voltage series more inert => more resistant to corrosion => a higher => “noble quality” (chemically more stable) -Au, Pt, Pd, Ra, Ru, Ir, Os +/- Ag(it is considered noble, bu...

METALS AND ALLOYS Lecture 8  Terminology Precious metals= high position in the normal voltage series more inert => more resistant to corrosion => a higher => “noble quality” (chemically more stable) -Au, Pt, Pd, Ra, Ru, Ir, Os +/- Ag(it is considered noble, but not precious) - Resist oxidation, tarnish and corrosion during heating casting and soldering  Terminology Precious metals= the intrinsic value of the metal. -noble metals= precious metals, but precious metals = /  noble ones. Non-noble metals= reactive metals, which are combining easily with the surrounding environment. The carat/karat and the fineness. The gold containing of an alloy is expressed by the carat or the fineness. The carat = a relative contain of Au from an alloy; a carat is equal with the 24th part of the total weight. The pure gold has 24 carats (24K). The fineness = the number of pure gold parts per one thousand alloy parts. The title of pure gold is 1000. - 1977, ADA accepted the expression in percentages  Methods of processing cold working: deep drawing – a metal disc transformed into a cup; stamping– a metal cup is transformed into a dental crown heat working: melting + casting melting + soldering melting + welding thermal treatments – for improving the properties of an alloy which is cold worked galvanoplating and pulverization– to obtain metallic casts/models  Classification According to chemical composition: - elements in a descending order of the percentage - Au-Ag-Pt (Au 78%, Ag 12%, Pt 10%) Noble alloys with high percentage of gold; with low percentage of gold; based on Ag-Pd; based on Pd. Non-noble alloys based on Ni-Cr based on Co-Cr based on titanium based on iron (stainless steels)  Classification ADA (SUA – 1981, changed in 1984): -according to chemical composition in weight percentages of noble metals from alloy High noble alloys - HN contain of noble metals > 60% and in Au ≥ 40% Noble alloys - N contain of noble metals ≥ 25% and in Au unspecified Predominantly base alloys - PB contain of noble metals < 25% and in Au unspecified Ag = non-noble(according to ADA)  Classification ADA specification #5 for high noble alloys: Type I – Au 85% - soft – for inlay Type II – Au 75% - medium – for inlay/ onlay Type III - Au 70% - hard – for onlay, crowns, dental bridges Type IV – Au 65% - extra hard – for crowns, dental bridges, partial denture frameworks According to casting temperature: normal T˚C high T˚C Crystal structure  Crystal structure of the metal through cooling in the liquid state, metal solidifies with crystals formation the atoms have a specific arrangement into the crystal repeated spatial crystals’ structure – a three-dimensional crystal lattice of the metal (a) cubic structure (b) faced-centered cubic structure (c) body-centered cubic structure  when cooling a metal, groups of its atoms pass into the solid state => crystals are formed the first formed crystals = nuclei (centers) of crystallization (a) around the nuclei come other atoms => the system is growing forming grains (c) crystals grow as dendrites = a three-dimensional branched network structures emanating from the central nucleus (b) the crystallization continues till the dendritic crystals will contact with each other, having the same dimension in each direction, but without a perfect shape (cube, sphere, etc.) The nuclei structure can be achieved by: 1. Homogeneous nucleation – is stimulated by the rapid cooling of the metal - high cooling rate = more centers of crystallization= smaller grains - rapid cooling of the molten metal or alloy after casting = quenching 2. Heterogeneous nucleation – is stimulated by the presence of metal’s impurities = agents of crystallization (metals with high melting temperature or oxides) - Many impurities = many centers of crystallization = smaller grains  Grain boundaries = the atoms aren’t in favorable positions to bond with the crystal’s atoms from the adjacent grain grain boundaries form natural barriers against movements of dislocations, but favour the fractures’ propagation the concentration of grain boundaries increases as the grain size decreases (small grain) Reducing the dimension of grains involves small grains and small grain boundaries and has following effects: - increasing of the yield strength (Young’s modulus >) - reducing of ductility - reducing of the fractures’ propagation = increasing of the fracture toughness Big and few grains involves big grain boundaries and has following effects: - reduced Young’s modulus - high ductility - favours the fractures’ propagation =decreased fracture toughness Practical applications: - Fine grain (small and many grains) through heterogeneous nucleation = partial denture frameworks (hard to be deformed, because they have the Young’s modulus and the fracture toughness ↑)  Crystal structure of alloys Alloy = a mixture of 2,3 or more metals which form in the liquid state a solution during cooling the metals’ atoms can randomly remain into the corners of each crystal = solid solution  Solid solution metals are soluble one in another It can be: =>irregular solid solution (the atoms of 2 metals have random positions into the crystal lattice) =>regular solid solution (the atoms of 2 metals have specific / regular positions into the crystal lattice) => interstitial solid solutions (the atoms of the one metal have positions within the crystal lattice, and the atoms of the second metal have positions outside the crystal lattice in to the interstices of the network. One of the metals has the atoms’ rays bigger than the other metal.  Insoluble solid solution - the atoms of each metal form crystals without interfering with the atoms of the other metal - the metals aren’t soluble in the solid state -there certain zones (phases) into alloy formed from the A metal and others formed from B metal -the alloy with 2 or more phases is susceptible to electrolytic corrosion, especially if the metals have electric potentials different from each other.  Partially soluble solid solution the metals are partially soluble in the solid state the atoms of a metal are more soluble into the crystal lattice of the other metal if: -they have the same structure of the crystal lattice - they have closer atomic radius - they have the same valence  Intermetallic compounds the atoms of the 2 metals have high affinity one for another, so they can form the grains of an intermetallic compound intermetallic compounds are very stable, few imperfections of crystals and thus a reduced potential of dislocation brittle alloys with reduced ductility Practical application: Impurities may determine modification of the microscopic structure of the alloy - Impurities soluble into solid state = alloys with fine grain - Impurities insoluble into solid state = deposit at the grains’ margin = defects within the structure - Gaze inclusions = defects within the structure – favour the fractures - Air inclusions onto the prosthetic restoration surface = roughness = favour the corrosion, tarnish  Deformation: elastic plastic = hardening homogenization heat treatments  Elastic deformation occurs due to tensile forces along the crystal lattice = slip of atoms from equilibrium positions with an interatomic distance deformation is proportional with the value of external F and is limited by the yield point of the metal removal of the F ext = the atoms will recover to their initial positions the alloys should be stiff (Y’s modulus ↑) for practical applications  Plastic deformation. Hardening = atoms’ slip from equilibrium position to a new position into the crystal lattice, when the yield point is exceeded - movement of dislocation - ductility - easier if distances between the initial and final position are smaller - ductile metals with face/body- centered cubic structures - alloys with coarse grain = ductile - intermetallic compounds = brittle ( the positions of the atoms are specific and not interchangeable)  The process of dislocation continues and leads to distortion of the crystal lattice ductility is decreasing increases the yield strength (becomes more stiffer) Hardening of the alloy through “cold” working And accumulation of some internal stresses within metal If they are released step by step may produce : -distortions with the loss of clasp adaptation - or are used for orthodontic appliances  Release of internal stress through “annealing” heat treatment = heating the metal at a temperature of recrystallization = 1/2 or 1/3 of melting T, 1 hour the metal becomes again ductile and can be worked  Hardening through heat treatments = ageing (grain growth) = slow cooling of the alloy molten into oven for 30 min at 3500C -applied especially for Au alloys type III and IV to increase the yield strength and stiffness for dental bridges and partial denture frameworks  Thermal treatments of homogenization After quench (rapid cooling of the alloy), the grains will have a heterogeneous composition (metal with melting T > it will crystallize rapidly, forming the center of the grain, and the other one will form the periphery of the grain) Increases the susceptibility to corrosion Heating of the alloy at recrystallization T Homogenization of the composition and Reducing the risk of corrosion Imposed conditions: price biocompatibility mechanical properties handling adhesion  Biocompatibility Corrosion and/or tarnish strength Side-effects to the patients: – Oral galvanism – Irritations – Allergic reactions – the most frequent (alloys based on Ni) More frequent for alloys with 60% of noble metals Alloys based on Cu – lichen planus Side-effects to the dental technician: – Be – carcinogen, toxic acute bronchitis, conjunctivitis – Cr – suppress synthesis of hemoglobin – Al – Alzheimer disease – Ni – pulmonary reactions, carcinogen  Mechanical properties Stiffness: – Design of the prosthetic restoration – Modulus of elasticity of the alloy – Important for: dental bridges with few elements post and core partial denture framework  Mechanical properties Ductility – cast clasps (a degree of ductility, not to be brittle) – Inlay – burnish (soft and ductile alloys) Hardness – An indicator of easiness in mechanical working – The hardness is strongly related to yield strength – Ductile alloys = less hard – It can be increased through hardening heat treatments  Casting Range of melting (eg. 950-10000C) – solidus temperature 9490C = crystallization T˚C – liquidus temperature 10000C = melting T˚C – casting at a T above liquidus T – melting T below solidus T – T solidus > with 150-2000C than sintering T of dental ceramic – narrow melting range = homogeneous crystallization  Casting Density of the alloy – high density = high weight – easy casting, low risk of defects – non-noble alloys have < density Coefficient of thermal contraction – high values – high risk for undersized prosthetic restorations  Adhesion With the aesthetic material for metal-ceramic, metal-acrylic, metal- composite alloys With the lutting cement – classic techniques with glassionomers which adhere to non-noble metals and to hard dental tissues  Noble dental alloys Properties: => high corrosion strength => biocompatibility  Noble alloys Indications Type I – Au 85% - soft –inlay Type II – Au 75% - medium –inlay/ onlay Type III - Au 70% - hard –onlay, crowns, reduced dental bridges, post and core Type IV – Au 65% - extra-hard – crowns, post and core, extended dental bridges, partial denture framework The role of the elements from an alloy composition Cu (copper) – increases the stiffness (Type III and IV) – reduces the melting T - over 16% – favours tarnish and corrosion - there isn’t into metal-ceramic alloys (MCA) Zn – prevents the absorption of O2 by the Ag during casting - reduces alloy’s porosity during casting - increases hardness, reduces ductility Ru (ruthenium) or In(indium) (traces) = centers of crystallization (fine grain) Properties - homogenization heat treatment after casting followed by - hardening heat treatment for type III and IV with Cu > 12% - alloys with homogeneous solid solutions - Pt and Pd increase the melting range – increase the risk of heterogeneity – it needs a homogenization heat treatment - casting T - 900-10000C (reduced) - thermal contraction -1,4% - compatible with sulphate based investments - reduced hardness– easiness for mechanical working  Medium and Low-Gold alloys Role of elements: -Pd –counteracts the tendency of Ag to tarnish -Cu –allows homogenization heat treatments Properties -Au below 20% - silver (more Pd and Ag) -Comparable with type III and IV -Problem: high variation of properties within the class Indications - extended dental bridges over abutments/implants - post and core Noble alloys based on Ag-Pd Properties -same properties with high noble alloys Type III -hardening heat treatment through slow cooling - Ag and Pd have affinity for O in the liquid state – porous castings - lower density than of Au alloys – defects during casting -Ag – increases ductility, decreases hardness and corrosion strength -Ag-Pd ratio 3:1 to reduce the risk of corrosion Indications : various according the alloy (different features): - inlay, crowns, post and core, reduced dental bridges Noble alloys based on Pd-Ag Properties - same with high noble alloys Type IV Advantages - high yield strength - better corrosion strength than those based on Ag-Pd - non-toxic - cheap price Disadvantages - high % of Ag – ceramic’s dyeing, reducing of the adhesion - high % of Pd – susceptible alloy to gas absorption – porosities Indications - same as for MCA (but without Ag, with: Cu, Ga, Au, Co, In)  Non-noble alloys Properties:  don’t contain Au, Ag, Pt, Pd  more reduced biocompatibility  contraction  high casting T  more reduced ductility, hardness  Alloys based on Co-Cr The role of elements: Co 55 - 65% Cr no more than 25%, till 30% Co and Cr = solid solution if Cr is below 30% The higher the percentage % of Cr is, the higher corrosion strength is – good biocompatibility Mo no more than 4% C or Ti (5%) Ni between 0 - 30% - increases ductility and the risk to allergic reactions, reduces hardness Properties: - Low density –casting defects, but not so visible - melting T0C (1300-14000C) –casting through induction and phosphate-bonded materials (investment material) - High hardness- mechanical working, hard to polish it - Stiff – advantage for thin connectors, disadvantage for clasps - Reduced ductility –especially when contaminated with C during casting (electrical arch or oxyacetylene flame)- the clasps can easily fracture Indications - partial denture frameworks  Alloys based on Ni-Cr The role of components : Ni 45 - 88% - allergen Cr 10-20% -assures corrosion strength Co between 0 - 30% Mo no more than 4% Be – gives liquefaction of the molten mixture, oxides layer, but carcinogenic Al, Mn, Si, Cu, Ga, Fe  Properties: – Intermediate hardness between Au type IV and Co-Cr – Reduced ductility than Au type IV reducing of the metal coping at 0.3mm (MCA) extended pontic adhesive bridges – Phosphate-bonded materials – Viscous in the molten state – semi or automatic spin – Low density – light casting defects Indications - porcelain-fused to metal restorations - polymeric- to metal restorations - metallic adhesive bridges - dental bridges with more than 3 elements  Titanium and titanium alloys Composition extracted Ti has 4 degrees of purity according to its content in O2 (0.18-0.40%) and Fe (0.2-0.5%) pure Ti – industrial = cpTi in dentistry it can be used as: - cpTi – pure comercial Ti (contain O2 between 0-0.5%) - Ti alloy – Ti, Al 6%, Vanadium 4% in solid state = crystal structure = α-hexagonal phase through heating above 8820C α-hexagonal phase transforms into β- body-centered cubic phase  Composition α-hexagonal phase has high affinity for O, N, C – forms interstitial stable solutions β-body-centered cubic structure has smaller affinity and can be stabilized with Mo, Ni (niobium) or V (vanadium) -O2 is soluble into Ti – cpTi = monophasic alloy (α) -Ti alloy (Ti-6Al-4V) = biphasic alloy (with α and β grains) -The Ti-6Al-4V alloy is more mechanically resistant than cpTi  Properties -cpTi -White, low density, lower modulus of elasticity than Co-Cr, ductile, excellent corrosion strength -Ti-6Al-4V -Modulus of elasticity is much higher than cpTi, according to ratio modification between phases -Higher fatigue resistance -Corrosion strength is explained through passivation Casting - Melting T 16700C –very high thermal shrinkage - increased chemical reactivity of liquid Ti –vacuum or inert gas atmosphere (argon) casting -Ti reacts with the investment material – formation of a layer of ά- case at the surface = high hardness, ductility and biocompatibility chemically instable -Removal of this layer (100-200µm) mechanically, chemically, electrochemically can induce problems at the cervical, occlusal or proximal adaptation - it needs specific investment materials - porosity - difficult mechanical working – due to ↓ thermal conductivity, Ti has the tendency to weld itself to the instrument Indications -prefabricated - implantology -oro-maxillo-facial surgery - prosthetics (CAD-CAM) Alloys for metal-ceramic technique (MCA=metal ceramic alloys)  Problem: achievement of a stronger metal-ceramic bond than cohesive forces of ceramics = a thin layer of oxides at the alloy’s surface MCA indicated for PFM restorations: - Noble alloys high Au alloys Low-Au alloys (Au-Pd, Au-Pd-Ag) alloys based on Pd (Pd-Cu, Pd-Ag) - Non-noble alloys - alloys based on Ni-Cr +/- Be; - alloys based on Co-Cr -cpTi alloys -Ti alloys Imposed conditions: 1. Physical 2. Chemical 3. Mechanical 4. Biological 1. Physical conditions: - solidus T of MCA > with min.150-2000C than sintering T of the ceramics (850-11000C) -generally, MCA are compatible with ceramics with low sintering T -Ti alloys are the only compatible with ceramics with a higher sintering range T (14000C) - Coefficient of thermal expansion MCA slightly > or = with that of the ceramic masse 2. Chemical conditions: - Strong bond between MCA and ceramics - a layer of oxides at the metal’s surface (Sn, In, Zn, Ga, Fe sau Be) – about 0.5-1% from MCA composition - corrosion strength - avoids the crevice corrosion which it will undermine the ceramic - indicated for Au alloys or with passivation - The capacity of MCA not to change the colour of ceramics - Ag – change of ions with Na ions from ceramics 3. Mechanical conditions: - High modulus of elasticity (90-220 GPa)= stiff - prevents transmission of occlusal stress towards the ceramic masse (less resistant) - important for reducing the dimensions of the metallic substructure - Hardness – necessary for mechanical working and polishing in those zones where the ceramic doesn’t cover the alloy  4. Biological conditions: - non cytotoxic - non irritant for the tissues - doesn’t contain Ni or Be  MCA composition Alloy Composition (%) Based on gold Au 74-88; Pt 0-20; Pd 0-16; Ag 0-15; Sn 0-3; In 0-4; Zn 0-2; Fe 0-0,5; Ta 0-1. Au-Pd Au 45-68; Pt 0-1; Pd 22-45; Sn 0-5; In 2-10; Ga 0-3; Zn 0-4 Au-Pd-Ag Au 42-62; Pd 25-40; Ag 5-16; Sn 0-4; In 0-6; Ga 0-2; Zn 0-3 Pd-Cu Au 0-2; Pt 0-1; Pd 66-81; Sn 0-8; In 0-8; Ga 3-9; Cu 4-20; Zn 0-4 Pd-Ag Au 0-6; Pt 0-1; Pd 50-75; Ag 1-40; Sn 0-9; In 0-8; Ga 0-6; Zn 0-4; Mn 0-4 Ni-Cr Ni 59-74; Cr 11-22; Mo 10 Ni-Cr-Be Ni 70-80; Cr 12-15; Be 0,6-2 Co-Cr Co 54-65; Cr 24-32 cpTi cpTi grad 2 şi 4 Ti alloys Ti-6Al-4V sau Ti-Nb1-Al  MCA properties Modulus Hardness Yield of Density Alloy Vickers strength Advantages Disadvantages elasticity (g/cm3) (VHN) (MPa) (GPa) -expensive -very good bond -low Y’s modulus Based on with the ceramic 182 90 448 18,3 -high density gold -doesn’t change the -metal caping of ceramic’s colour 0.5mm Same Au-Pd 220 124 572 13,5 same -colour change - high Y’s modulus Au-Pd-Ag 218 110 439 13,8 -replaced with alloys based on Pd - low Y’s modulus -Contraindicated for Pd-Cu 425 96 1145 10,6 -medium hardness bridges with more than 3 elements -the higher Y’s modulus for noble -colour change Pd-Ag 242 138 531 11,1 alloys -cheap -difficult casting -high Y’s modulus -shrinkage Ni-Cr 257 207 400 8,7 -minimal thickness -unpredictable bond of the metal caping with ceramics  Used criteria to select the best suited dental alloys according to the clinical indication Indication Important criteria Less important Indicated alloys criteria Crowns Casting easiness Stiffness Au type II Accuracy Mechanical strength Au-Pt Biocompatibility Au-Pd Tarnish and corrosion Pd-Cu strength Pd-Ag Hardness Ni-Cr Extended Same as above, plus: Ductility Au type III bridges (more Mechanical strength Au-Pt than 3 unit) High modulus of elasticity Au-Pd Stiffness Pd-Ag Easy soldering Ni-Cr Movable Same as above Ductility Au type IV partial denture Co-Cr frameworks PFM crowns Same as for crowns, plus: Accuracy Pd-Cu High modulus of elasticity Hardness Pd-Ag Adhesion with the ceramic Ni-Cr Not to dye the ceramic Au-Pd ± Ag Extended PFM Same as for bridges, plus: Accuracy Ni-Cr bridges Adhesion to ceramic Ductility Pd-Ag 62 Not to dye the ceramics Hardness Au-Pd ± Ag