properties of materials chemical related document for educational information

properties of materials

1. Unit 3- Phase Equilibrium Udari Perera, BSc Eng. (Chemical & Process, Moratuwa), MSc (Research, Monash)

2. Complete solubility 2 T C T C T A T B Line 1 100% A 0 B 0 A 100% B 50% A 50% B Liquid L + α α Liquidus Solidus Wt. % B

3. Complete insolubility 3 T C T C T A T B Line 2 100% A 0 B 0 A 100% B Liquid L + A Wt. % B A + B L + B Line 1 Line 3 T E E C E

4. Partial solubility 4 T C T C T A T B 100% A 0 B 0 A 100% B Liquid L + α Wt. % B α + β L + β T E E C E α β 2 3 5 4 7 6 1

5. Microstructural changes along line 4 5 LIQUID β α T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1

6. Microstructural changes along line 3 6 LIQUID α (hypo-eutectic) LIQUID β α T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1

7. Microstructural changes along line 5 7 LIQUID β (hyper-eutectic) LIQUID β α T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1

8. Microstructural changes along line 1 8 LIQUID α T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1

9. Microstructural changes along line 7 9 LIQUID β T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1

10. Microstructural changes along line 6 10 LIQUID β T A T B Liquid L + α α + β L + β T E E α β 2 3 5 4 7 6 1 α

11. Phase Diagrams Containing Three-Phase Reaction The five most important three-phase reactions that occur in phase diagrams are: Eutectic – a liquid transforms into two solids upon cooling Eutectoid – a solid transforms into two new solids Peritectic – a liquid plus a solid transforms into a new solid Peritectoid – two solids transforms into a new solid Monotecti c – a liquid transforms into a new liquid, and a solid Many combinations of two elements produce more complicated phase diagrams than the isomorphous and simple eutectic systems. Many equilibrium diagrams often show intermediate phases and compounds when either incomplete solubility or compound formation occurs. These new phases are distinguished by the labels “terminal phases” and “intermediate phases”. Their phase diagrams look complex.

12. Phase Diagrams Containing Three-Phase Reaction

13. Complete iron carbon phase diagram

14. Iron–Iron Carbide (Fe–Fe3C) Phase Diagram Thermal arrest lines (in red).

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16. Iron–Iron Carbide (Fe–Fe3C) Phase Diagram Eutectic Point for the iron–iron carbide system : , at 4.30 wt % C and 1147 o C (2097 o F ) for this eutectic reaction, The liquid solidifies to form austenite and cementite phases. Subsequent cooling to room temperature promotes additional phase changes. It may be noted that a eutectoid invariant point exists at a composition of 0.76 wt % C and a temperature of 727 o C. This eutectoid reaction may be represented by Upon heating solid α iron and cementite transform to solid γ phase and upon cooling, the solid γ phase is transformed into α iron and cementite

17. Iron–Iron Carbide (Fe–Fe3C) Phase Diagram Commercially pure iron contains less than 0.008 wt % C and, from the phase diagram, is composed almost exclusively of the ferrite phase at room temperature. Steel contains 0.008 and 2.14 wt % C and most steel contains microstructure consists of both α and Fe 3 C phase. When cooling to RT an alloy this composition range must pass through at least a portion of the γ phase field; distinctive microstructures are subsequently produced, The microstructure that develops depends on both the carbon content and heat treatment. At this moment consider the very slow cooling of steel to maintain the equilibrium state continuously.

18. Iron–carbon Alloys Microstructure Example: An alloy of eutectoid composition (0.76 wt % C) as it is cooled from a temperature within the γ -phase region from 800 o C ( point a). Initially, the alloy is composed entirely of the austenite phase having a composition of 0.76 wt % C and corresponding microstructure. As the alloy is cooled, no changes occur until the eutectoid temperature (727 o C ) is reached. Upon crossing this temperature to point b , the austenite transforms according to The microstructure for this eutectoid steel that is slowly cooled through the eutectoid temperature consists of alternating layers or lamellae of the two phases ( α and Fe3C) that form simultaneously during the transformation with relative layer thickness of approximately 8 to 1.

19. Iron–carbon Alloys Microstructure Photomicrograph of a eutectoid steel showing the pearlite microstructure consisting of alternating layers of α -ferrite (the light phase) and Fe 3 C (thin layers most of which appear dark). 470 This microstructure, is called pearlite . The pearlite exists as grains, often termed colonies; within each colony the layers are oriented in essentially the same direction, which varies from one colony to another. Mechanically, pearlite has properties intermediate between those of the soft, ductile ferrite and the hard, brittle cementite. The thick light layers are the ferrite phase, and the cementite phase appears as thin lamellae, most of which appear dark. Many cementite layers are so thin that adjacent phase boundaries are so close together that they are indistinguishable at this magnification and, therefore, appear dark.

20. Iron–carbon Alloys Microstructure The alternating α and Fe3C layers in pearlite form due to the composition of the parent phase [austenite (0.76 wt % C)] is different from that of either of the product phases [ferrite (0.022 wt % C) and cementite (6.70 wt % C)], and the phase transformation requires that there be a redistribution of the carbon by diffusion Schematic representation of the formation of pearlite from austenite; direction of carbon diffusion indicated by arrows.

21. Hypoeutectoid Alloys Consider a composition C to the left of the eutectoid, between 0.022 and 0.76 wt % C; this is termed a hypoeutectoid (“less than eutectoid”) alloy. At about 875 o C, point c, the microstructure consists entirely of grains of the γ phase About 775 o C (point d) both α and γ phases coexist. Most of the small α particles form along the original γ grain boundaries. The compositions of α and γ phases is about 0.020 and 0.40 wt % C respectively. The composition of the ferrite phase changes with temperature along the α - ( α + γ ) phase boundary, line MN, becoming slightly richer in carbon. Cooling from point d to e, just above the eutectoid but still in the α + γ region, produces an increased fraction of the α phase and a microstructure. At this point, the compositions of the α and γ phases are s 0.022 wt % C, and 0.76 wt % C respectively.

22. Hypoeutectoid Alloys As the temperature is lowered just below the eutectoid, to point f, all the γ phase that was present at temperature T e (and having the eutectoid composition) transforms into pearlite No change happens to the α phase that existed at point e in crossing the eutectoid temperature—it is normally present as a continuous matrix phase surrounding the isolated pearlite colonies. Hence, the ferrite phase is present both in the pearlite and as the phase that formed while cooling through the α + γ phase region. The ferrite present in the pearlite is called eutectoid ferrite , whereas the other, which formed above Te, is termed proeutectoid (meaning “pre- or before eutectoid”) ferrite, Proeutectoid ferrite and pearlite—which appear in all hypoeutectoid iron–carbon alloys that are slowly cooled to a temperature below the eutectoid

23. Hypoeutectoid Alloys 0.38-wt% C The spacing between the α and Fe3C layers varies from grain to grain

24. Hypoeutectoid Alloys Determine the fraction of pearlite (Wp) and proeutectoid W α for C’ o point using leaver rule. Fractions of both total α (eutectoid and proeutectoid) and cementite are determined using the lever rule and a tie line that extends across the entirety of the a + Fe3C phase region, from 0.022 to 6.70 wt % C.

25. Hypereutectoid Alloys Consider an alloy of composition C1. At point g, only the γ phase is present. Upon cooling into the γ + Fe 3 C (point h), the cementite phase begins to form along the initial γ grain boundaries. This cementite is called proeutectoid cementite ( which forms before the eutectoid reaction). The cementite composition remains constant (6.70 wt % C) as the temperature changes. However, the composition of the austenite phase moves along line PO toward the eutectoid. As the temperature is lowered through the eutectoid to point i , all remaining austenite of eutectoid composition is converted into pearlite Hence, the resulting microstructure consists of pearlite and proeutectoid cementite

26. Hypereutectoid Alloys Photomicrograph of a 1.4 wt % C steel having a microstructure consisting of a white proeutectoid cementite network surrounding the pearlite colonies. 1000*. The proeutectoid cementite appears light as it has much the same appearance as proeutectoid ferrite

27. Isothermal Transformation Diagrams: Pearlite Consider again the iron–iron carbide eutectoid reaction Pearlite is one microstructural product of this transformation Temperature plays an important role in the rate of the austenite-to-pearlite transformation.