1. Metal Alloys: Their Structure and Strengthening by Heat Treatment
Chapter 4

2. Heat Treatment Improves properties of metal alloys
Modifies the microstructure
Improves formability
Improves machinability
Increases strength & hardness
Service performance improved such as in gears (figure 4.1)

3. FIGURE Cross section of gear teeth showing induction-hardened surfaces. Source: TOCCO Div., Park-Ohio Industries, Inc.

4. FIGURE 4.2 Outline of topics described in Chapter 4.

5. Pure Metals & Alloys
In PURE METALS, atoms are all the same type, except for rare impurity atoms
ALLOYS are composed of 2 or more chemical elements, at least one of which is a metal

6. Solid Solutions Solute: the minor element that is added to the solvent
Solvent: the major element
Substitutional solid solutions: the size of the solute atom is similar to the solvent atom (example: brass alloy of zinc & copper)
Interstitial solid solutions: the size of the solute atom is much smaller than that of the solvent (example: steel alloy iron & carbon)

7. Two phase systems A system with 2 or more solid phases (most alloys)
PHASE-a physically distinct and homogeneous portion of the material
Example: water and sand mixture has two phases
Example: lead added to copper in the molten state (4.3a) lead particles dispersed throughout

8. FIGURE (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead–copper alloy. The grains represent lead in solid solution in copper, and the particles are lead as a second phase. (b) Schematic illustration of a two phase system consisting of two sets of grains: dark and light. The green and white grains have separate compositions and properties.

9. Phase Diagrams Pure metals have distinct melting or freezing points
Solidification takes place at a constant temperature
Latent heat of solidification is given off while the temperature remains constant

10. FIGURE 4. 4 (a) Cooling curve for the solidification of pure metals
FIGURE (a) Cooling curve for the solidification of pure metals. Note that freezing takes place at a constant temperature; during freezing, the latent heat of solidification is given off. (b) Change in density during cooling of pure metals.

11. Phase Diagrams Alloys solidify over a range of temperatures
Liquidus-solidification occurs when the temperature drops below
Solidus-solidification is complete
Between liquidus and solidus the alloy is in a mushy or pasty state

12. Phase Diagram Equilibrium or Constitutional Diagram
Shows the relationships among temperature, composition, and phases present in a particular alloy at equilibrium
Equilibrium-the state of a system does not vary over time
Binary Phase Diagram (two elements)

13. FIGURE Phase diagram for nickel–copper alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each have one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals. The second circle shows the formation of dendrites (see Section 10.2). The bottom circle shows the solidified alloy, with grain boundaries.

14. Lever Rule
Used to determine the composition of various phases in the phase diagram
Example: Copper Nickel figure 4.5
At 1288 degrees C, a mixture of solid/liquid
Solid is 42% Cu, 58% Ni
Liquid is 58% Cu, 42 % Ni
The completely solidified alloy is a solid solution because Cu completely dissolves in Ni and each grain has the same composition

15. Mechanical properties of Cu-Ni solid solution
Yield strength
Hardness
Percent elongation
These improve up to a point as Ni is alloyed with pure copper
Zinc also can be alloyed with pure copper but has a maximum 40% solid solubility

16. FIGURE Mechanical properties of copper–nickel and copper–zinc alloys as a function of their composition. The curves for zinc are short, because zinc has a maximum solid solubility of 40% in copper.

17. Lead-Tin System (fig.4.7)
Eutectic Point-the point at which the liquid solution decomposes into the components alpha and beta (Greek Eutektos=easily melted)
Single phases alpha and beta
Two-two phase regions
Alpha + liquid
Beta + liquid
Low temperature eutectic points important for soldering so as to prevent thermal damage to parts during joining.

18. FIGURE 4. 7 The lead–tin phase diagram
FIGURE The lead–tin phase diagram. Note that the composition of the eutectic point for this alloy is 61.9% Sn–38.1% Pb. A composition either lower or higher than this ratio will have a higher liquidus temperature.

19. Iron-Carbon System Steel (up to 2.11% C)
Commercially pure iron (up to .008% C)
Cast iron (up to 6.67% C) most (<4.5% C)
Iron-iron carbide phase diagram up to 6.67% C because Fe3C is a stable phase

20. Ferrite Ferrite. Alpha ferrite. α-ferrite BCC iron
Delta ferrite. δ-ferrite. Stable only at high temperatures thus no practical engineering use
Soft and ductile. Magnetic at room temperature

21. FIGURE 4. 8 The iron–iron-carbide phase diagram
FIGURE The iron–iron-carbide phase diagram. Because of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams.

22. Austenite Gamma iron, γ-iron
Polymorphic transformation from BCC to FCC
W.R. Austen,
More solid solubility than ferrite
Denser than ferrite
Its single phase FCC structure is ductile at high temperatures (good formability)
Large amounts of Ni and Mangenese can be dissolved in FCC iron

23. Cementite Carbide is another name 100% iron carbide, Fe3C 6.67% Carbon
Hard
Brittle
Can include other alloying elements such as chromium, molybdenum, mangenese.

24. FIGURE The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of carbon (by weight) on the lattice dimensions for martensite is shown in (d). Note the interstitial position of the carbon atoms. (See Fig. 1.7.) Note also the increase in dimension c with increasing carbon content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism.

25. Pearlite
At 727 ⁰C, austenite transforms into alpha ferrite and cementite
Eutectoid reaction-single solid phase is transformed into two other solid phases
This eutectoid steel is called pearlite because at low magnification it resembles mother of pearl

26. FIGURE Schematic illustration of the microstructures for an iron–carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727°C (1341°F).

27. FIGURE Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite and the darker regions are carbide. Magnification: 2500.

28. Cast Irons
A family of ferrous alloys composed of iron, carbon (ranging 2.11% to 4.5%), and Silicon (up to 3.5%)
Gray cast iron or gray iron
Ductile cast iron, nodular cast iron, or spheroidal graphite cast iron
White cast iron
Malleable iron
Compacted Graphite iron

29. FIGURE Phase diagram for the iron–carbon system with graphite (instead of cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.8.

30. Gray Iron
Graphite exits largely in the form of flakes. When broken the fracture path along the graphite flakes has a gray, sooty appearance
Negligible ductility
Weak in tension/strong in compression
Dampens vibrations
Used for machine tool bases & machinery structures

31. 3 types of gray iron
Ferritic (fully gray iron) consists of graphite flakes in the alpha-ferrite matrix
Pearlitic- has a structure of graphite in a matrix of pearlite. Still brittle but stronger than ferritic iron
Martensitic-obtained by austenitizing a pearilitic gray iron and rapidly quenching to produce a structure of graphite in a martensite matrix. Very hard

32. Ductile (Nodular) Iron
Graphite is in a nodular or spheroid form (fig.4.13b)
Small additions of magnesium and/or cerium to the molten metal before pouring
Somewhat ductile and shock resistant

33. Malleable Iron
Obtained by annealing white cast iron in an atmosphere of carbon monoxide and carbon dioxide at between 800⁰ and 900⁰ C for up to several hours.
Cementite decomposes into iron and graphite
Rosettes or clusters of graphite (fig.4.13c) in a pearlite or ferrite matrix
Ductility, strength, shock resistance similar to nodular iron
Malleable comes from Latin malleus “it can be hammered”

34. Compacted-graphite iron
Graphite is in the form of short, thick, interconnected flakes
Mechanical and physical properties intermediate between flake-graphite and nodular-graphite cast irons

35. FIGURE 4. 13 Microstructure for cast irons. Magnification: 100
FIGURE Microstructure for cast irons. Magnification: 100. (a) Ferritic gray iron with graphite flakes. (b) Ferritic ductile iron (nodular iron), with graphite in nodular form. (c) Ferritic malleable iron; this cast iron solidified as white cast iron, with the carbon present as cementite, and was heat treated to graphitize the carbon.