Alloy systems

Question 1

What is an alloy system?

Answer 1

describes all possible permutations of alloys, phases, compositions and temperatures for mixing of two or more element

Question 2

What are alloy components?

Answer 2

chemical elements that make up the alloy . Number of components described by type of alloy or alloy system, for instance, binary alloy - 2 components, ternary alloy - 3 components

Question 3

When are useful alloys only formed?

Answer 3

A useful alloy is only formed when the elements in question are mutually soluble in the liquid state forming a single liquid that is uniform in composition throughout

Question 4

What are three conditions that can occur when a alloy system solidifies from liquid state?

Answer 4

When this solution solidifies one of the following conditions is likely:
– Insoluble in the solid state ‐ they separate out as particles of the two pure metals
– Complete or partial solubility in the solid state ‐ in the former case a single solid solution will be formed, whilst in the latter a mixture of two different solid structures will result.
– An intermetallic compound ‐ as solidification proceeds the two metals give rise to an intermediate phase, or intermediate compound having distinct chemical formula.

Question 5

What is alloy composition?

Answer 5

(concentration) – an alloy’s composition is described by presenting the concentration of each component in weight% or atomic%

Question 6

What is alloy constitution described by?

Answer 6

• Phases present

* Weight fraction of each phase • Composition of each phase

Question 7

What is a phase?

Answer 7

A portion/region of material that has uniform physical and chemical characteristics

A phase can exist in any state of matter – solid, liquid, gas

Question 8

Usually a solid solution (or two sold solutions) exist in an alloy system, along with other phases: (5)

Answer 8

– Eutectic/Eutectoids
– Peritectic/Peritectoids
– Metastable phases (martensite in steels) – Intermediate Phases
– Intermetallic compounds
• Electron compounds
• Size factor compounds

Question 9

What is liquidus?

Answer 9

a line of phase boundary on an equilibrium diagram which limits the bottom of the liquid field.

Question 10

What is solidus?

Answer 10

a line or phase boundary on an equilibrium diagram which limits the top of the solid field.

Question 11

What is the two phase (liquid-solid) field?

Answer 11

The region between the solidus and liquidus

Question 12

What is the y axis of a phase diagram?

Answer 12

Temperature, TA, TB

Question 13

What is the x axis of a phase diagram?

Answer 13

Composition, A, B

Question 14

What is the solubility limit?

Answer 14

Maximum concentration for which only a single phase solution exists.

Question 15

What are components?

Answer 15

The elements or compounds which are present in the alloy (e.g., Al and Cu)

Question 16

What are phases?

Answer 16

The physically and chemically distinct material regions that form (e.g.,  and ).
‐ a single phase (a solution, or a pure material) is said to be homogenous;
‐ a mixture is heterogeneous

Question 17

What is a binary system?

Answer 17

2 components:

Question 18

What is an isomorphous system?

Answer 18

complete solubility of one component in another

Question 19

What is the tie line/isotherm?

Answer 19

connects the phases in equilibrium with each other

Question 20

Why are alloys of eutectic composition attractive for casting and as filler metals in soldering and brazing?

Answer 20

because they have the lowest discrete melting point of the given alloy system, and solidify to a relatively high strength structure

Question 21

What is complete solid solubility? (3)

Answer 21

implies complete solubility (mixing and interchangeability) of the atoms of alloying species.
Two or more type of atoms have to be able to exist in the same crystal structure.
Atom sizes and valence electron structure of the species or elements need to be similar

Question 22

What is partial solid solubility?

Answer 22

results when there is a saturation limit for one of the alloying species in the other, and this saturation limit depends on temperature.
So as the temperature is lowered and solubility decreases a two-phase material forms from the initial single phase.

Question 23

What is insolubility?

Answer 23

means that the materials are so different in nature (atomic size, valence electron structure, etc…) that they are insoluble – ie. separate into two distinct elemental phases