Mechanical Behavior of Metals and Alloys Focus on Steel and Aluminum
Christopher R. Owen School of Materials Engineering Purdue University 1
Thought or team problem • Consider that we are designing materials that would be used in the development of a pickup truck such as the Ford F-150 • Class question: What properties of materials are important for this design process?
• Make a list for future use
Modulus of Elasticity • Elastic modulus is a measure of a material’s resistance to being deformed elastically. – No permanent deformation – Usually just called “Modulus”
• Defined as stress/strain—initial elastic region only! – Material returns to original form when stress is removed.
• Modulus is constant for a given material. – Other material properties (examples: yield strength and corrosion) will change due to changes in composition and/or thermomechanical processing, but not Modulus of Elasticity! 3
Modulus Differences among Materials • • • • • • • • •
Aluminum = 69 GPa Steel = 207 Gpa Brass = 105 Gpa Lead = 14 Gpa Aluminum oxide = 340 Gpa Wood 8 – 15 Gpa Bamboo = 16 Gpa PVC = 2 Gpa Human bone = 17 Gpa
Modulus and Stiffness • Modulus is a basic property of the material • Stiffness is a measure of the rigidity of the structure. – – – –
Relates to how the structure bends when under a force Depends on shape as well as on modulus High modulus—less deflection under force Low modulus—more deflection under force
• When is stiffness important? 5
Density • Density is defined as Mass per Unit Volume. – More precisely, “volumetric mass density.” – Equation is mass/volume. • Density depends on elemental components. – Cannot change the density of an element. – Each element has a specific density.
– Alloy density changes with alloy composition. – Atoms with different masses make up an alloy.
Typical Density Values for Engineering Materials
• • • • • • •
Aluminum Steels Titanium Copper Magnesium Woods Polymers
2.7 g/cm3 7.9 4.5 8.7 1.4 0.5 to 0.8 0.9 to 2.0 7
Density Questions • How could the difference in density between steel and aluminum impact automotive performance? • Ford is converting to aluminum to improve gas mileage. Do you think this is why other automakers (Audi, Porsche, Jaguar, Tesla, etc.) have already converted to aluminum? • Can a lighter weight vehicle be a high performance vehicle?
And Consider: • If aluminum is better for automotive use because of its lower density, why not – magnesium? – plastics? – wood?
Strength • Strength of a material is its ability to withstand an applied load without failing; may be under tensile or compressive loading. • Strength is often associated with hardness and is related to hardness (but correlations are not always consistent). 10
Definitions • Strength and “stress” are often used interchangeably. • Stress is measured by force/area – Yield stress is the applied load at which a material begins to deform beyond the elastic limit, called plastic deformation. – Steels often show a “yield point” after which the stress decreases and deformation continues. – Most other materials show just a change in the slope of the stress-strain curve when plastic deformation begins. 11
Stress vs. Strain • Engineers define stress as force/original cross sectional area. • Strain is the amount of deformation compared to the original dimension of the material tested. – Strain is dimensionless, usually seen as percent deformation.
• An engineering stress-strain curve for a typical metal is shown on the following slide. – Definitions of areas shown are on the following slide. 12
Uniform Plastic Deformation
• Ultimate tensile strength (UTS), often shortened to tensile strength (TS) is the maximum stress that a material can withstand while being pulled or stretched before failure. • Ductility is a solid material's ability to deform under a tensile or compressive load. • Elongation is the maximum amount a material can be pulled under tension before failure.
Practical stuff • Most often, designers are concerned about tensile strength. • Both tensile yield strength and ultimate tensile strength are important! – After yielding, plastic deformation occurs at a uniform rate. – Plastic deformation cannot be recovered upon removing the applied load! – UTS gives the maximum load that the materials can handle without failure. – Stress beyond UTS causes “necking” which eventually leads to failure. • Necking is demonstrated in the following experiment (slide 16)
Demo: yielding, necking • Here we use a polyethylene tensile specimen to demonstrate yielding in a manual test. • Pull the specimen s l o w l y. – Carefully observe what happens. • Identify yielding • Identify necking
– What causes failure? • Re-test—are the results the same?
Typical Strength Values • Steel – Tensile strength: 200 – 900 MPa – Elongation to failure: 2 – 20%
• Aluminum – Tensile strength: 40 – 400 MPa – Elongation to failure: 2 – 35%
• Polyethylene – Tensile strength: 5 – 45 MPa – Elongation to failure: 4 – 600% 17
Strengthening Mechanisms • Materials can be strengthened in several ways. – Note in the last slide that the range of strength values varied, sometimes by many times!
• We observed 3 major mechanisms in the previous module on Hardness of Brass. 1. Work hardening 2. Solid solution strengthening (alloying) 3. Grain boundary strengthening 18
1. Work Hardening • Plastic deformation – Caused when a metal is stressed – Occurs when dislocation line defects move through the material, causing atoms to move over oneanother. – Dislocation line displaces one line of atoms at a time, easily formed and easily moved in a pure, relatively defect=free material.
• Deformation occurs by bulk deformation processing (e.g., rolling, forging, extruding) and can be done hot or cold. 19
Work Hardening, Cont’d • During plastic deformation, – Dislocations stress fields interact, raising the yield stress. – Dislocations can tangle, leading to even higher stress needed for further deformation. – Result is a stronger material with higher yield stress and reduced ductility.
2. Solid Solution Strengthening • Alloying introduces atoms of different sizes into the crystal lattice of the host element. – Solid solution means that the foreign atom (the solute atom) enters the lattice and becomes part of the structure. – Causes a distortion (strain) in the lattice. – Makes it more difficult for dislocations to move through the material, thus higher yield stress.
3. Grain Boundary Strengthening • Grain boundary strengthening was the active mechanism in increasing the strength and hardness in the “Hardness of Brass” module. – Dislocations have trouble moving from grain to grain due to different planar alignment of the grains and the presence of the grain boundaries. – Dislocations pile up at grain boundaries, interact with one-another raising stress. – More grain boundaries, hence smaller grains, have a greater effect, thus small grain-size materials are stronger than large grain-size ones. 22
Thermomechanical Processing • Adding thermal processing to deformation allows the material to reduce stresses and soften. – “Annealing” as in the Hardness of Brass module – Dislocations move to reduce stress. • Called “recovery”
– New, small strain-free grains form. • Called “recrystallization”
– Eventually, new grains grow. • Called “grain growth”
• Allows grain size control 23
Thermomechanical Processing Adds two more strengthening mechanisms:
4. Precipitation Hardening Applies to alloys when elements are added that exceed the solubility limit.
5. Transformation Hardening Steels can transform from one phase to another.
4. Precipitation Hardening • Adding solute atoms in concentrations greater than the amount that will dissolve in the lattice (beyond solubility limit) – No longer part of the crystal lattice – Becomes a foreign body, and often a cluster of the foreign atoms. called a precipitate, – Result is strengthening since dislocation motion is blocked at these precipitates, raising the yield stress. 25
Precipitation Hardening: Important in Aluminum Alloy Automotive Development Alloy development: 1960s to 1980s • Basic alloy use (alloy type 2036-T4) • Improve corrosion resistance (6009-T4, 6010-T4, 6016) • Improve formability (6111-T4, 6016-T4)
Alloy development: 1980s to 2000s • More formable alloys for inner panels; improved corrosion resistance (2008-T4) • Better corrosion resistance + improved formability (6022-T4) Bake hardening development: 1980s to Present • Heat treatment after paint is applied—hardens steel and bakes the paint layer • New tempering development to improve paint bake strengthening. 26
5. Transformation Hardening
• Applies only to steels with high C and Mn; requires special heat treatment – Results in a two-phase microstructure called a duplex structure – Called TRIP steels for transformation-induced plasticity – Combination of microstructures gives higher strengths and better formability. – Becomes stronger during the forming process 27
Things to remember: Properties of Metals • Density and modulus – Important in weight and stiffness
• Strength – Depends on processing
• Five major strengthening mechanisms – Deformation and grain boundary strengthening can be recovered through annealing. – Precipitation hardening and transformation strengthening cannot normally be recovered through annealing. 28
Double Check • Go back to your original list of material properties – Note the biggest differences. – Discuss the most important factors.
• How does this discussion relate to the question of steel vs. aluminum in automobiles and trucks?
Mechanical Behavior of Metals and Alloys
This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE #1400619