ME 350 – Ch 6, 7 – Metals & Ceramics

ME 350 – Ch 6, 7 – Metals & Ceramics Ch6 - Metals: • Alloys • Binary Phase Diagrams • Metal Types and Designations Ch 7 - Ceramics: • Types • Characteristics

Alloys An alloy = a mixture or compound of two or more elements, at least one of which is metallic • Two main categories: • Solid solutions - An alloy in which one element is in another to form a single‑phase structure • Intermediate phases - When the amount of the dissolving element in the alloy exceeds the limit of the base metal, a second phase forms in the alloy • A = any homogeneous mass of material, in which the grains all have the same crystal lattice structure

Two Forms of Solid Solutions • solid solution - atoms of solvent element are replaced in its unit cell by dissolved element • solid solution - atoms of dissolving element fit into vacant spaces between base metal atoms in the lattice structure • In both forms, the alloy structure is generally than either of the component elements

Two Forms of Solid Solutions

Types of Intermediate Phases • Metallic compounds – consist of a metal and nonmetal, such as Fe3C ( ) • Intermetallic compounds ‑ two metals that form a compound, such as Mg2Pb • In some alloy compositions, the intermediate phase is mixed with the primary solid solution to form a two‑phase structure • Some two‑phase alloys are important because they can be heat treated for much higher strength than

Phase Diagrams A graphical means of representing the phases of a metal alloy system as a function of composition and temperature • A phase diagram for two elements (at atmospheric pressure) is called a

Inverse Lever Rule - Example Solid phase composition: Liquid phase composition: Liquid 2000 1500 1000 α + Liquid Temperature °C Solid phase proportion: Liquid phase proportion: α 0 10 20 30 40 50 60 70 80 90 % A

Nickel-Copper Binary Phase Diagram: • A melt is formed with 35% Ni and 65% Cu, it is slowly cooled from 1300°C to ~1270°C: • initial solid is Ni, initial liquid is Ni • middle solid is Ni middle liquid is Ni • at end solid is Ni

Inverse Lever Rule – Example 2 Solid phase composition: Liquid phase composition: 2000 1500 1000 Temperature °C Solid phase proportion: Liquid phase proportion: 0 10 20 30 40 50 60 70 80 90 % A

Iron-Carbon Phase Diagram γ = α = Fe3C =

Solubility Limits of Carbon in Iron • Ferrite phase can dissolve only about carbon at 723C (1333F) • Austenite can dissolve up to about carbon at 1130C (2066F) • The difference in solubility between alpha and gamma provides opportunities for strengthening by heat treatment

Steel and Cast Iron Defined = an iron‑carbon alloy containing from 0.02% to 2.1% carbon = an iron‑carbon alloy containing from 2.1% to about 4.3% carbon • Steels and cast irons can also contain other alloying elements besides carbon

Eutectic and Eutectoid Compositions composition of Fe-C system = 4.3% C • Phase changes from solid ( + Fe3C) to liquidat 1130C (2066F) composition of Fe-C system = 0.77% C • Phase changes from  to  above 723C (1333F) • Below 0.77% C, called steels • From 0.77 to 2.1% C, called steels

Carbon Content in Steel

AISI-SAE Designation Scheme Specified by a 4‑digit number system: 10XX, where 10 indicates plain carbon steel, and XX indicates carbon % in hundredths of percentage points • For example, 1020 steel contains C • Developed by American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE), so designation often expressed as AISI 1020 or SAE 1020

AISI-SAE Designation Scheme AISI‑SAE designation uses a 4‑digit number system: YYXX, where YY indicates alloying elements, and XX indicates carbon % in hundredths of % points • Examples: 13XX - Manganese steel 20XX - Nickel steel 31XX - Nickel‑chrome steel 40XX - Molybdenum steel 41XX - steel

Stainless Steel (SS) Highly alloyed steels designed for corrosion resistance • Principal alloying element is , usually greater than 15% • forms a thin impervious oxide film that protects surface from corrosion • Nickel (Ni) is another alloying ingredient in certain SS to increase • Carbon is used to strengthen and harden SS, but high C content reduces corrosion protection since chromium carbide forms to reduce available free Cr

Designation Scheme for Stainless Steels • Three‑digit AISI numbering scheme • First digit indicates general type, and last two digits give specific grade within type • Examples: Type 302 – Austenitic SS • 18% Cr, 8% Ni, 2% Mn, 0.15% C Type 430 – Ferritic SS • 17% Cr, 0% Ni, 1% Mn, 0.12% C Type 440 – Martensitic SS • 17% Cr, 0% Ni, 1% Mn, 0.65% C

Cast Irons Iron alloys containing from carbon and from 1% to 3% silicon • Most important is • Other types include ductile iron, white cast iron, malleable iron, and various alloy cast irons • Ductile and malleable irons possess chemistries similar to the gray and white cast irons, respectively, but result from special processing treatments

Cast Iron Chemistries – C & Si

Designations of Aluminum Alloys Alloy group Wrought codeCast code Aluminum  99.0% purity 1XXX 1XX.X Copper alloy 2XXX 2XX.X Manganese alloy 3XXX Silicon alloy 4XXX 4XX.X Zinc alloy 7XXX 7XX.X Tin alloy 8XX.X

Magnesium and Its Alloys • alloys of the structural metals • Available in both wrought and cast forms • Relatively easy to machine • In all processing of magnesium, small particles of the metal (such as small metal cutting chips) oxidize rapidly, and care must be taken to avoid hazards

Copper Alloys • Strength and hardness of copper is relatively low; to improve strength, copper is frequently alloyed • - alloy of copper and tin (typical  90% Cu, 10% Sn), widely used today and in ancient times (i.e., ) • - alloy of copper and zinc (typical  65% Cu, 35% Zn). • Highest strength alloy is beryllium‑copper (only about 2% Be), which can be heat treated to high strengths and used for springs

Titanium • Coefficient of thermal expansion is relatively low among metals • Stiffer and stronger than Al • Retains good strength at elevated temperatures • Pure Ti is reactive, which presents problems in processing, especially in molten state • At room temperature Ti forms a thin adherent oxide coating (TiO2) that provides excellent corrosion resistance

Zinc and Its Alloys • Low melting point makes it attractive as a casting metal, especially die casting • Also provides corrosion protection when coated onto steel or iron • The term steel refers to steel coated with zinc • Widely used as alloy with copper ()

Refractory Metals • Metals capable of enduring high temperatures - maintaining high strength and hardness at elevated temperatures • Most important refractory metals: • Other refractory metals: • Columbium • Tantalum

Superalloys High‑performance alloys designed to meet demanding requirements for strength and resistance to surface degradation at high service temperatures • Many superalloys contain substantial amounts of , rather than consisting of one base metal plus alloying elements • Operating temperatures often around 1100C (2000F) • Applications: gas turbines ‑ jet and rocket engines, steam turbines, and nuclear power plants (all are systems in which operating efficiency increases with higher temperatures)

Three Groups of Superalloys • Iron‑based alloys ‑ in some cases iron is less than 50% of total composition • Alloyed with Ni, Cr, Co • Nickel‑based alloys ‑ better high temperature strength than alloy steels • Alloyed with Cr, Co, Fe, Mo, Ti • Cobalt‑based alloys ‑  40% Co and  20% chromium • Alloyed with Ni, Mo, and W • In virtually all superalloys, including iron based, strengthening is by precipitation hardening

How to Enhance Mechanical Properties • – adding additional elements to increase the strength of metals • - strain hardening during deformation to increase strength (also reduces ductility) • Strengthening of the metal occurs as a byproduct of the forming operation • - heating and cooling cycles performed on a metal to beneficially change its mechanical properties • Operate by altering the microstructure of the metal, which in turn determines properties

Ch7 - Three Basic Categories of Ceramics • ceramics ‑ clay products such as pottery, bricks, common abrasives, and cement • ceramics ‑ more recently developed ceramics based on oxides, carbides, etc., with better mechanical or physical properties than traditional ceramics • ‑ based primarily on silica and distinguished by their noncrystalline structure

Strength Properties of Ceramics • Ceramics are substantially stronger in than in or in bending • Ceramics fail by much more readily than metals • grain size generally increases strength

Oxide Ceramics Most important oxide ceramic is • also has good hot hardness, low thermal conductivity, and good corrosion resistance

Carbide Ceramics Silicon carbide (), tungsten carbide (), titanium carbide (), tantalum carbide (), and chromium carbide () • WC, TiC, and TaC are valued for their hardness and in applications requiring these properties (e.g., cutting tools) • WC, TiC, and TaC must be combined with a metallic binder such as or in order to fabricate a useful solid product

Nitrides Important nitride ceramics are silicon nitride (Si3N4), boron nitride (BN), and titanium nitride (TiN) • Properties: usually electrically insulating, TiN being an exception • Applications: • Silicon nitride: components for gas turbines, rocket engines, and melting crucibles • Boron nitride and titanium nitride: cutting tool materials and coatings

Glass Ceramics • is the main component in glass products, usually comprising 50% to 75% of total chemistry • It naturally transforms into a glassy state () upon cooling from the liquid, whereas most ceramics crystallize upon solidification • Glass-ceramics are a class of ceramic material that contain % crystalline phase with very small grain size (0.1 - 1μm) and are usually . • They can contain sodium oxide (Na2O), calcium oxide (CaO), aluminum oxide (Al2O3), magnesium oxide (MgO), potassium oxide (K2O), lead oxide (PbO), and boron oxide (B2O3)

Advantages of Glass‑Ceramics • Efficiency of processing in the glassy state • Close dimensional control over final shape • Good mechanical and physical properties • High strength (stronger than glass) • Absence of porosity; low thermal expansion • High resistance to thermal shock • Applications: • Cooking ware • Heat exchangers • Missile radomes

Graphite Form of carbon with a high content of crystalline carbon in the form of layers • Bonding between atoms in layers is covalent and strong, but parallel layers are bonded to each other by weak van der Waals forces • Structure makes graphite anisotropic; properties vary significantly with direction • As a powder it is a lubricant, but in traditional solid form it is a refractory • As a fiber, it is a high strength structural material (e.g., fiber reinforced plastics)

Diamond Carbon is a cubic crystalline structure with covalent bonding between atoms – very high hardness • Applications: cutting tools and grinding wheels; also used in dressing tools to sharpen grinding wheels • Synthetic diamonds fabricated by heating graphite to around 3000C (5400F) under very high pressures

Boron Semi-metallic element in same periodic group as aluminum • Properties: , semiconducting properties, and very high modulus of elasticity) in fiber form • Applications: B2O3 in certain glasses, as a nitride (cBN) for cutting tools, and in nearly pure form as a fiber in polymer matrix composites

Guide to Processing Ceramics • Processing of ceramics can be divided into two basic categories: • Molten ceramics - major category of molten ceramics is glassworking (solidification processes) • Particulate ceramics - traditional and new ceramics (particulate processing)

ME 350 – Ch 6, 7 – Metals & Ceramics