ME 350 – Lecture 8 – Chapters 4 & 6

1. ME 350 – Lecture 8 – Chapters 4 & 6 Ch 4 - Properties of Materials • Volumetric and Melting Properties • Thermal Properties • Electrical Properties • Electrochemical Processes

2. Density and Specific Gravity • Density = weight per unit volume • Typical units are g/cm3 (lb/in3) • Determined by atomic number and other factors such as atomic radius, and atomic packing • Specific gravity = density of a material relative to density of and is a ratio with no units • Strength‑to‑weight ratio, which is tensile strength divided by density • Useful ratio in comparing materials for structural applications in aircraft, automobiles, and other products where weight and energy are concerns

3. Thermal Expansion • Density of a material is a function of temperature • In general, with increased temperature, density • Change in density is measured by coefficient of thermal expansion  • Change in length per degree of temperature, such as mm/mm/C, in/in/F, or ppm/ C L2 ‑ L1 =  L1 (T2 ‑ T1) where  = coefficient of thermal expansion; L1 and L2 are lengths corresponding respectively to temperatures T1 and T2

4. Thermal Expansion in Manufacturing • Thermal expansion is used to shrink fit expansion fit assemblies • Part is heated to increase size or cooled to decrease size to permit insertion into another part • When part returns to ambient temperature, a tightly‑fitted assembly is obtained • Thermal expansion can be a problem in heat treatment and welding due to thermal stresses (caused by expansion and contraction) that develop in material during these processes

5. Specific Heat • The quantity of heat energy required to increase the temperature of a unit mass of material by one degree ΔQ = where ΔQ = amount of heat energy (joule or calorie); M = its mass (kg or lb); C = specific heat of the material; and ΔT = change in temperature • Volumetric specific heat =  C where  =

6. Thermal Conductivity • Thermal conduction = transfer of thermal energy within a material by purely thermal motions; no transfer of mass • Coefficient of thermal conductivity k. Units: J/s mm C (Btu/in hr F) • Coefficient of thermal conductivity is generally in metals, in ceramics and plastics • Thermal Diffusivity = ratio of thermal conductivity to volumetric specific heat

7. Electrical Resistance • Movement of charge carriers is driven by the presence of a voltage: Ohm's law: V = where I = current, V = voltage, and R = electrical resistance,  • Resistance in a uniform section of material depends on its length L, cross‑sectional area A, and resistivity of the material r : where resistivity r has units of ‑m2/m or ‑m (‑in.) • The reciprocal of resistivity:

8. Electrochemistry • In a water solution, molecules can dissociate into positively and negatively charged ions • They allow electric current to be conducted, playing the same role that electrons play in metallic conduction • Electrolyte - the ionized solution • Electrodes - where current enters and leaves the solution in electrolytic conduction • - positive electrode • - negative electrode • The whole arrangement is called an

9. Electrolysis Example Example of electrolysis: decomposition of water; electrolyte = dilute sulfuric acid (H2SO4); electrodes = platinum and carbon (both chemically inert).

10. Electrolysis in Manufacturing Processes • Electroplating ‑ an operation that adds a thin coating of one metal (e.g., chromium) to the surface of a second metal (e.g., steel) for decorative or other purposes • Electrochemical machining ‑ a process in which material is removed from the surface of a metal part • Production of hydrogen and oxygen gases

11. Ch 6 – Binary 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 binary phase diagram

12. Inverse Lever Rule - Example Liquid 2000 1500 1000 α + Liquid Temperature °C α 0 10 20 30 40 50 60 70 80 90 % A

13. 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

14. Inverse Lever Rule – Example 2 2000 1500 1000 Temperature °C 0 10 20 30 40 50 60 70 80 90 % A

15. Iron-Carbon Phase Diagram γ = α = Fe3C = • Austenite can dissolve carbon up to about • Ferrite phase can dissolve carbon up to about • The difference in solubility between alpha and gamma provides opportunities for strengthening by heat treatment

16. Steel and Cast Iron Defined Iron‑carbon alloy containing from 0.02% to 2.1% carbon: Iron‑carbon alloy containing from 2.1% to about 4.3% carbon: • Both contain other alloying elements besides carbon (Ni, Cr, Mo, Mg, Si, etc.)

17. Carbon Content in Steel

18. Stainless Steel (SS) Highly alloyed steels designed for corrosion resistance • Principal alloying element, usually in concentrations 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

19. 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 ‑copper (only about 2% Be), which can be heat treated to high strengths and used for springs

20. 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 that refers to steel coated with zinc: • Widely used as alloy with copper:

21. Refractory Metals • Metals capable of enduring high temperatures - maintaining high strength and hardness at elevated temperatures • Important refractory metals: • Tantalum • Sometimes referred to as a refractory metal: • Titanium

22. 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)

23. 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

24. 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