Failures of metals and alloys

What is an alloy

Mixture of components

What does ferrous mean

Includes iron

What temp is alpha ferrite

Below 912 degrees

What is most steel, austenite, alpha ferrite or delta ferrite 

Alpha ferrite 

What unit cell is ferrite

Body centered cubic

What temp range are austenite materials heated to

912-1394 degrees

What unit cell is austenite

Face centered cubic

What temperature is delta ferrite heated to

Above 1394 degrees

What unit cell is delta ferrite 

Body centered cubic 

What is most heat treatments of steel intended for

To produce a mixture of ferrite and cementite for desired properties

What des heat treatment control

Control brittleness and return toughness

What percent’s are high and low carbon steel

Mild steel- 0.13-0.3%, high carbon steel more than 0.6%, carbon steel less than 2%

2 types of surface treatments

Carburising and nitriding, martensite and case hardening

What is carburising

Carbon diffused into steel surface at higher than A₃

What is pearlite

Layers in cementite and ferrite

What is the rough young’s modulus of steel

200GPa

What is eutectoid reaction

Solid to solid at min temperature

What is a eutectic reaction

Liquid to solid transformation at minimum temperature

Stages of austenite to martensite  transformation 

1. Shear displacement in austenite

2. No time for diffusion to occur

3. Steel containing marsenite is stronger but more brittle than pearlite

4. Dislocations are limited in movement as lattice pinned by excess carbon atoms 

4 types of heat treatment

Hardening, annealing, normalising, tempering

What is hardening 

Heated to austenite phase then cooled rapidly in oil or water 

What is annealing

Heated to several hundred degrees then cooled slowly

What is normalising

Heated to austenite phase then cooled slowly

What is tempering

Heated to between 400-600 then cooled

What effects does tempering have alloys 

Increases toughness, reduces brittleness, some harness lost 

What does carburising and nitriding involve

Controlled diffusion of carbon or nitrogen

When carburising and nitriding best

Low carbon steel

How does carburising and nitriding happen

When steel that is surface quenched and tempered comes high carbon marsenite

What produces more strength with carburising and nitriding, carbon or nitrogen

Offer similar strengths

3 ways that carbon and nitrogen are infused

Cyaniding, nitriding, carbo-nitriding

What is cyaniding

Immersing steel in liquid cyanide

What is nitriding

Only nitrogen diffused into steel

What is carbo-nitriding

Carbon oxide and ammonia generated to diffuse carbon and nitrogen into steel surface 

Chemical formula of ammonia 

NH₃

What is marsenite and case hardening as a surface treatment

Selective heating near surface of isolated areas to form marsenite in some areas

How are the specific areas heated in case hardening

Flame or induction heating usually with rotation

How deep is the case depth in case hardening

Few mm

What does carburising do in case hardening 

Impregnates surface with carbon, higher carbon at the surface than at the center

Why is case hardening good

Hard surface and ductile center is useful

What is case depth

Depth of martensite layer in treated steels

What is the center of case hardened steels

Pearlite and ferrite 

How do residual stresses and cracking occur 

During quenching, surface quickly transforms to martensite, center takes longer to cool which places surface under tension so it cracks 

How can mechanical properties of metals be altered

Alloying with metals or non metals

What is an alloy

A solid solution of metal and some amount of other phases that are usually compounds

What is steel alloy

Solid solution of C in iron

What does alloying increase

Strength and hardness

What are interstitial solid solutions

Additional atoms positioned between atoms of main crystal phase

Requirement of interstitial atoms in lattice

Atoms radius must be at least 30% larger than solute atom

What is substitutional solid solutions

Process or result of substituting one atom for another

Requirement for substitutional atoms in the lattice

Less than 15% difference in size to the size of solute atoms

Difference between a compound and a solid solution

Compounds have regular arrangement and are ordered, solid solutions are a solvent with solute placed randomly and are disordered

5 types of metal forming

Bulk forming, forging, drawing, extrusion, rolling

What is bulk forming

Large change in shape on large surface area to volume ratio materials

What is the bit undergoing formation in bulk formation called and what is it like during forming

Billet, it is hot and malleable

What is forging 

Compressive method where two opposing dies are used to shape material to the die 

What is drawing

Work material drawn through an orifice to created necessary diameter

What is extrusion

Bulk material is pushed through an opening ( a die) that is shaped to produce a desired cross section 

What is rolling

Rollers to squeeze larger sheet material through and compress it to a desired thickness of a sheet

3 types of second stage sheet forming

Bending, deep drawing, shearing

What can be said about surface area to volume ratio in metal and sheet forming

Higher in sheet forming

What is bending

Physically bending sheets into shapes, usually at simple angles

What is deep drawing

Used to punch a flat sheet into a concave die, sheet takes cup like shape of the die

What is shearing

Cutting of sheets into required dimensions

What temperature does hot forming happen at

0.5-0.75 of melting temperature

Pros of hot forming 

More deformation without cracking, force required is lower, little work-hardening 

Cons of hot forming

Accuracy is harder to predict, some materials may oxidise, components of forming machine have shorter life

Pros of cold forming

Tolerance of final dimensions can be smaller, surface finish generally better, energy requirements for machinery is lower, work hardening can produce harder product 

Cons of cold forming

Must be ductile or material cracks, annealing may be required to prevent cracking, higher forces required, work hardening not wanted 

3 properties of alluminium alloy

Low melting point, resistant to corrosion, high thermal and electrical conductivity

Uses of aluminium alloy

Transportation industry, cans and foil, electrical applications

What 2 classes can aluminium alloy be classed into

Wrought or cast

What is a wrought alloy

Shaped by plastic deformation

What do many cast aluminium alloys contain and why

Silicon which gives lower melting point, good fluidity and castability

How can properties of cast aluminium be controlled

Dispersion strengthening, solid solution strengthening and solidification rates

What materials can be added to aluminium alloy to change grain size and eutectic structure

Titanium, magnesium, boron

What do copper and zinc permit in an aluminium alloy

Age hardening of alloys

Properties of magnesium alloy in comparison to aluminium alloy 

Lighter, comparable corrosion resistance and specific strength 

Poor magnesium alloy properties

Low elastic modulus, poor fatigue and creep resistance

Uses of magnesium alloy

Aerospace, sporting equipment, housing for electronics, transportation

What can be said about ductility of magnesium vs aluminium and how can it be changed

Magnesium is less ductile than aluminium but alloying magnesium increases ductility 

How can magnesium and copper alloys be strenghtened

Heat treatment

How can corrosion resistance in magnesium alloy be increased

Alloying or anodising

What alloy is denser than steel

Copper alloy

Out of aluminium, magnesium and copper alloys, which has lowest strength, higher creep and fatigue

Copper alloy

Strong properties of copper alloys

Very ductile, good corrosion resistance, very electrically and thermally conductive

What happens to copper alloy when in solid solution by what elements

Remain single phase despite addition of large amounts of alloying elements like brass and bronzes which gives high strength and ductility

Why are copper and beryllium alloys used as age hardening alloys

High strength and stiffness with non sparking qualities

What is precipitation hardening

Heat treatment process that increases yield strength of malleable metal alloys 

How is precipitation hardening different to tempering

Heated at an elevated temperature for a long time, if heated for several hours, precipitates and nucleate grow 

What is stainless steel made up of

Iron alloys containing about 20% chromium and 10% nickel

Why are chromium and nickel added to stainless steel

Corrosion resistance and more ductile

Why is corrosion resistance useful

The material can be used in chemical industry and areas where aggressive environments are 

Why are nickel and cobalt alloys used

Corrosion protectio, high temperature resistance, high melting points and strength

What are monels

Alloys used for strength and corrosion resistance in salt water

What can happen to aged monels 

Hardened to doubled strength 

What is a super alloy

Nickel, iron nickel and cobalt alloy with high strength at elevated temperatures, corrosion resistance and creep resistance up to 1000 degrees

What are applications of super alloys

Blades for turbines, jet engines, heat exchangers

What elements do some super alloys contain

Titanium, carbon, chromium, aluminium

Properties of titanium alloys

Excellent corrosion resistance, high temperature properties, high strength, low density

What is allotropic

Hexagonal closed packed at low temperatures, BCC at high temperatures