Ferrous

Question 1

What is a phase?

Answer 1

A homogeneous portion of a system that has uniform physical and chemical characteristics. Cannot be separated physically.

Question 2

Binary phase diagrams can help to predict…

Answer 2

phase transformations and the resulting microstructures, which may or may not have equilibrium character

Question 3

What is brass made up of?

Answer 3

Copper and zinc

Question 4

What is a component?

Answer 4

A chemical constituent (element or compound) of an alloy that may be used to specify its composition

Question 5

What is a system?

Answer 5

Two meanings are possible: 1) a specific body of material being considered
2) a series of possible alloys consisting of the same components

Question 6

What is the solubility limit?

Answer 6

The maximum concentration of solute that may be added without forming a new phase

Question 7

What is a heterogeneous system?

Answer 7

Systems composed of two or more phases

Question 8

In metal alloys, microstructure is characterised by…

Answer 8

the number of phases present, their proportions, and the manner in which they are arranged/distributed

Question 9

What does the microstructure of an alloy depend on?

Answer 9

The alloying elements present, their concentrations, and the heat treatment of the alloy

Question 10

Define equilibrium

Answer 10

The state of a system in which the phase characteristics remain constant over indefinite time periods. At equilibrium the free energy is a minimum.

Question 11

Define free energy

Answer 11

A thermodynamic quantity equivalent to the capacity of a system to do work. It is a function of the internal energy and entropy of a system.

Question 12

Metastable state

Answer 12

A non-equilibrium state that may persist for a very long time

Question 13

What is the difference between the states of phase equilibrium and metastability?

Answer 13

For the condition of phase equilibrium the free energy is a minimum—the system is completely stable, meaning that over time the phase characteristics are constant. For metastability, the system is not at equilibrium, and there are very slight (and often imperceptible) changes of the phase characteristics with time

Question 14

What is a phase diagram?

Answer 14

A graphical representation of the relationships among environmental constraints (e.g., temperature and sometimes pressure), composition, and regions of phase stability, typically under conditions of equilibrium.

Question 15

What is a unary phase diagram?

Answer 15

A phase diagram for a pure substance, composition is held constant. Pressure plotted against temperature

Question 16

Solidification

Answer 16

Transformation from liquid to solid

Question 17

ALLOTROPY AND POLYMORPHISM (section 9.06 and 3.06)

Answer 17

TO DO

Question 18

In the binary phase diagram between copper and nickel, the solidus and liquidus lines intersect at the two composition extremes, what do these two points represent?

Answer 18

The melting temperatures of copper and nickel respectively

Question 19

For metallic alloys what do lowercase greek letters commonly represent? (phase diagrams)

Answer 19

Solid solutions

Question 20

Define isomorphous

Answer 20

Having the same structure. In the phase diagram sense, isomorphicity means having the same crystal structure or complete solid solubility for all compositions

Question 21

The phase diagram for the cobalt-nickel system is an isomorphous one. On the basis of melting temperatures for these two metals (Co = 1495 deg. C, Ni = 1455 deg. C), describe and/or draw a schematic sketch of the phase diagram for the Co-Ni system.

Answer 21

Because this phase diagram is an isomorphous one, phase boundaries and phase regions are similar to those for the Cu-Ni phase diagram: there exists a blade-shaped region having intersections of liquidus and solidus lines at for 100 wt% Co and at for 100 wt% Ni. Sketch diagram

Question 22

Polymorphism

Answer 22

The ability of a solid to exist in more than one crystal form

Question 23

What is a triple/invariant point on a P-T phase diagram (unary phase diagram)?

Answer 23

A point where all three phase boundary curves intersect, at this point all three phases coexist in equilibrium

Question 24

What do binary phase diagrams represent?

Answer 24

The relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the microstructure of an alloy

Question 25

What is a tie line (an isotherm)?

Answer 25

A horizontal line constructed across a two-phase region of a binary phase diagram; its intersections with the phase boundaries on either end represent the equilibrium compositions of the respective phases at the temperature in question

Question 26

What are the consequences for isomorphous alloys that have solidified under non-equilibrium conditions?

Answer 26

The distribution of the two elements within the grains is non-uniform, a phenomenon termed segregation. The center of each grain, which is the first part to freeze, is rich in the high-melting element (e.g., nickel for this Cu-Ni system), whereas the concentration of the low-melting element increases with position from this region to the grain boundary. This is termed a cored structure, which gives rise to less than the optimal properties. As a casting having a cored structure is reheated, grain boundary regions melt first because they are richer in the low-melting component. This produces a sudden loss in mechanical integrity due to the thin liquid film that separates the grains. Furthermore, this melting may begin at a temperature below the equilibrium solidus temperature of the alloy.

Question 27

How can coring be eliminated?

Answer 27

By a homogenisation heat treatment carried out at a temperature below the solidus point for the particular alloy composition. During this process, atomic diffusion occurs, which produces compositionally homogeneous grains.

Question 28

Why are alloys generally stronger than pure metals?

Answer 28

Because impurity atoms that go into solid solution typically impose lattice strains on the surrounding host atoms. Lattice strain field interactions between dislocations and these impurity atoms occur, and, consequently, dislocation movement is restricted.

Question 29

What is a eutectic reaction?

Answer 29

When a liquid phase cools directly into two solid phases or vice versa

Question 30

What is the eutectic temperature?

Answer 30

The temp. at which eutectic reaction occurs

Question 31

What is the eutectic composition?

Answer 31

The composition at which eutectic reaction occurs

Question 32

What is a microconstituent?

Answer 32

An easily identifiable part of a microstructure with a characteristic structure

Question 33

What is a primary phase?

Answer 33

Solid phase that forms above the eutectic temperature

Question 34

Hypereutectic

Answer 34

Having composition above (to the right of) the eutectic composition

Question 35

Hypoeutectic

Answer 35

Having composition below (to the left of) the eutectic composition

Question 36

What do solvus lines represent?

Answer 36

The solid solubility limit of metal B in the phase rich with metal A

Question 37

What are solidus lines?

Answer 37

Boundaries between regions with only a solid phase(s) and regions with containing liquid

Question 38

What are liquidus lines?

Answer 38

Boundaries between regions with both liquid and solid phases and regions with just liquid

Question 39

What happens when liquid with the eutectic composition cools below the eutectic temperature? (Use binary phase diagram of Lead and Tin to explain, where alpha is Pb rich and beta is Sn rich)

Answer 39

Eutectic alpha and beta phases are formed, however they have different compositions than that of the eutectic liquid. Since the alpha phase is Pb rich, it can't hold much tin, Sn diffuses out of it and into the beta phase. Pb also diffuses out of the beta region so that both phases reach their respective compositions.

Question 40

During atomic diffusion, why do thin layers form between the different phases?

Answer 40

So the two metals only have to diffuse over a short distance

Question 41

Explain the change in microstructure as a hypoeutectic liquid cools below the eutectic temperature (Use binary phase diagram of Lead and Tin to explain, where alpha is Pb rich and beta is Sn rich)

Answer 41

Primary alpha forms by rejecting Tin, meaning the Tin diffuses out of the alpha phase and into the liquid. This increases the wt% of Sn in the liquid. As the temperature lowers, the chunks increase in size, enriching the liquid with even more Tin. Just before the eutectic reaction happens, the liquid has been enriched with Tin to the eutectic composition. As the material cools below the eutectic temperature, the eutectic liquid solidifies into alternating platelets of eutectic alpha and beta phases. The primary alpha chunks are still present below the eutectic temp., they have the same composition as the eutectic platelets of alpha.

Question 42

What is pearlite?

Answer 42

A combination of two phases: ferrite and Fe3C (cementite/iron carbide)

Question 43

Why is the volume fraction ratio between ferrite and cementite approximately the same?

Answer 43

Their densities are almost identical

Question 44

Why does pro-eutectoid alpha or pro-eutectoid cementite form at the triple points of grain boundaries?(Iron Carbon Phase Diagram)

Answer 44

They are regions of high energy so it is easier for a new phase to form there

Question 45

Why is steel with a carbon wt% with more than 1.2% rarely used in practical applications?

Answer 45

Above 1.2%, the steel becomes brittle and not very workable