materials science and engineering --- my2100 chapters 1 and 2 metals and metal structures key...

Materials Science and Engineering --- MY2100

Chapters 1 and 2

Metals and Metal Structures Key Concepts

Major Engineering Alloy Systems The Design Process Metal Structures and Terms

(Polymorphism, Solutions, Phase, Boundaries, Phase Morphology)


Generic Iron-Based Metals


Generic Copper-based Metals

Generic Nickel--based Metals


Generic Aluminum-based Metals


Generic Titanium-based Metals


Product/Process Realization

Design Concept(ApproximateGeneric Data)

Design Concept (Handbook and Supplier Data)

Prototype(Material Verification

Testing)

First Article(Quality Assurance)


Types of Material Properties

Structure-Insensitive—Properties which are nearly independent of microstructure:• Density, Young’s Modulus, thermal expansion,

specific heat

Structure-Sensitive—Properties which vary strongly with processing and detailed alloy composition• Yield and tensile strength, ductility, fracture

toughness, creep and fatigue strength


Hierarchy of Structure (Metals)

Feature Typical Scale (m)Engineering Structures .003 m – 1000 m

Aggregates of grains (crystals) 0.01 mm – 10 mm

Individual grains & phases (Size and shape)

0.1 m – 1000 m

Grain and phase boundaries 0.01 m

Atomic arrangements

(crystal structure; solutions)

0.1 nm – 1 nm

Subatomic structure 0.1 nm


Crystal Structures of Selected Pure Metals

Face Centered Cubic (FCC)• Aluminum (Al)

• Copper (Cu)

• Nickel (Ni)

Close packed hexagonal (CPH)• Titanium

• Magnesium

Body Centered Cubic (BCC)• Iron (Fe)

• Chromium (Cr)

• Tungsten (W)


Polymorphism

Polymorphism “Many Forms” Some metals (and ceramics) change crystal

structure as they are heated or cooled. Behavior can affect:

• Forming operations

• Heat treatment and residual stresses

• Service characteristics

Can be “manipulated” through cooling rate and alloy chemistry

Examples include austenitic stainless steels and + titanium alloys


Polymorphism in Alloys

Austenitic stainless: FCC form stabilized with Ni

Ti-6Al-4V: BCC form stabilized with aluminum


Solid Solutions

Solute atoms can dissolve in a solid solvent metal in two ways:• Interstitial Solution

o Solvent atoms fit within interstitial voids of the solvent crystal

o Examples: C in iron; O in titanium

• Substitutional Solutiono Solvent atoms replace atoms on solvent latticeo Examples: Ni in iron; Cu in aluminum


Solid Solutions--II


Material Phases

Phase-A region of material with uniform chemical and physical properties

Pure, Liquid Water

Water + Ice

Ice

Salt Water Solution

Oil/Water Emulsion

(1 Phase)

(2 Phases)

(1 Phase)

(1 Phase--Solution)

(2 Phases)


Grain Boundaries Separate regions of different crystal orientation Low bond density (energy ~ 0.5 J/m2) Open Structure (fast diffusion/impurity segregation) Influence mechanical properties

• Low TIncrease strength & ductility

• High T Decrease strength


Phase Boundaries Interfaces which separate regions of

different chemical and physical properties. Coherent boundaries

• Difference in chemical composition but not crystal structure

• Very low energy (~0.05 J/m2)• May have coherency strain due to differences

in lattice spacing.(increases boundary energy)


Semi-Coherent and Incoherent Boundaries

Semi-Coherent• Difference in composition and small difference

in crystal structure

• Higher energy--more effective strengthening

• Periodic dislocations compensate for differences in atomic spacing

Incoherent• Difference in composition and large difference

in crystal structure

• Energy similar to grain boundary


Semi-Coherent and Incoherent Boundaries-II


Shapes of Grains & Phases

Soap Bubbles

Processing, composition and energy relationships determine shape

“Equilibrium” shape• Flat faces joined at 120o

• “Bubble Raft” analogy


Phase Morphology

gb cos2

materials science and engineering --- my2100 chapters 1 and 2 metals and metal structures key...

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