ESG 332 Structure of Materials Ch. 7; Deformation and Strengthening Mechanisms

creep resistance

Resistance to high temperature deformation

Annealing

the reheating of a metal after cold-working to allow for it to return to its pre-cold-worked state. -the metal is reheated above the re-crystallization temperature (this temperature depends on the type of metal/composition) -annealing is used to either return a metal to its original ductility before cold-working in order to continue cold-working, or as a final processing step to produce specific grain sizes or mechanical properties.

Strengthening Method: Precipitation Strengthening

-Hard precipitates are difficult to shear (i.e. the addition of ceramics to metals) -aluminium can be strengthened with precipitates formed through alloying.

Anisotropy in Stress

-anisotropy can be induced in metals with isotropic grains that are equiaxed and randomly oriented by ROLLING -rolling out a metal will affect the grain shape and orientation and create different characteristics corresponding to the different directions.

Strengthening Method: Cold Working (Strain Hardening)

-cold working is deformation of a metal at room temperature (for most metals) -common methods of cold working involve decreasing the cross section area of a metal, such as rolling, drawing, forging and extrusion. COLD WORK INCREASE: -increased yield strength (sigma y) -increased tensile strength (TS) -ductility (%EL or %AR) decreases -after being cold-worked, metals are typically annealed to encourage re-crystallization and grain growth

Dislocation in Metals

-dislocation motion is easiest in metals -non-directional bonding -close-packed directions for slip

Dislocations IRL

-dislocations are mostly observed in metals -strength of metals can be increased by making dislocation motion difficult -Ways to increase strength: 1) decrease the grain size 2) use solid solution strengthening 3) use precipitate strengthening 4) cold working the metal -heating (annealing) can reduce the dislocation density and increase grain size, DECREASING the strength

Metallic Deformation & Strengthening Summary

-dislocations are primarily observed in metals and alloys -strength is increased by making dislocation motion difficult (i.e. cold working) -cold-worked metal that is heat treated can have recovery, recrystallization and grain growth, altering its properties.

Properties of Cold Working

-ductility decreases, there reaches a point where a metal cannot be cold-worked anymore without breaking -point defect density increases -dislocation density increases -cold-worked metal must be annealed at a certain point for further cold-working

Recovery (cold working)

-recovery occurs at low temperatures -point defects are created -recovery does not change the grain structure of a metal but rather changes the dislocations (opposite sign dislocations annihilate each other, point defects annihilate each other) -dislocations annihilate and form perfect atomic planes or shifted atoms can move to new slip planes

What causes dislocations?

-shear stress. sigma=force/area

Influence of Grain Size on Metallic Properties

-smaller grains: strong and tough at LOW temperatures -larger grains: good creep resistance at high temperatures

Strengthening Method: Formation of Solid Solution

-the addition of impurity atoms can distort a lattice and generate lattice strain. These strains in the lattice can act as barriers to prevent dislocation motion. (both smaller and larger impurity atoms create lattice strains, but both oppose slip in different ways) -alloying metals tends to increase strength through the formation of solid solution, where the added alloy atoms concentrate in regions of tensile strain. These atoms REDUCE the mobility of dislocations and INCREASE strength. *alloying increases yield strength and tensile strength.

Recrystallization (cold working)

-with the application of heat (activation energy in the form of heat) strained grains from cold-working become strain free grains as new grains grow -the driving force (energy) for the recrystallization process is the stored strain energy of the dislocations created when cold-working. *recrystallization growth temperature is around 1/3-1/2 the absolute melting point of the metal (also known as the temperature at which 50% of the cold-worked material will recrystallize in one hour.

Cold working and annealing affects during phase

1) cold work (puts metal into a high energy state) Ductility decreases Conductivity decreases Internal stress increases Tensile strength increases (hardness?) 2) recovery Internal stress decreases Conductivity increases Ductility increases Tensile strength remains the same 3) Recrystallization and grain growth: Conductivity increases Ductility increases Internal stress decreases (all stress energy created is used in recrystallization) Tensile strength decreases

Factors affecting recrystallization temperature

1) degree of cold work (how much the metal is previously worked/strained) 2) the initial grain size of the metal 3) the purity or composition of the metal

Methods of Strengthening Metals:

1) reduction of grain size 2) formation of solid solution 3) precipitation strengthening 4) cold working (strain hardening)

Shear Stress

A stress that occurs when forces act in parallel but opposite directions, pushing parts of a solid in opposite directions -shear stress is required for plastic deformation -"slip" process of dislocation motion begins when shear stress (represented by symbol tao) on the slip plane and direction reach the critical value (tao critical)

Ductility (%EL or %AR)

Ductility measures the amount of plastic deformation a material can withstand before failure (breaking). Ductility itself is a measure of how much strain a given stress produces. %EL: this is the percent ELONGATION, which is how much a metal elongates (ductility is the property of something being able to be drawn out into a wire) as its stressed. %AR is the percent REDUCTION IN AREA. As a metal is being deformed or stretched, the area of the section under stress will stretch and elongate, reducing its cross sectional area.

Strengthening Method: Reduction of Grain Size

Grain boundaries are barriers to slip, therefore barrier strength increases with increasing variety/randomness of orientation and with smaller grain size. Smaller grain size=more barriers preventing slip

Brittle vs. Ductile Materials

Highly Ductile Metals: -can display significamt strain before fracturing -%EL and %RA are both >(or equal to) 25% Brittle Materials: -display very little strain before failure -%El<(or equal to) 5% at fracture.

Dislocation Motion

In METALS: -plastic deformation occurs by slip and edge dislocation (when an extra half-plane of atoms slides over the adjacent plane's half plane of atoms. Essentially an edge dislocation moving across a crystal structure -if dislocations cannot move, then plastic deformation will not occur -slip direction is the same as the Burger's Vector direction

tensile strength

Tensile strength is the resistance to breaking under tension, it specifies the point where steel goes from elastic (temporary) to plastic (permanent) deformation. Tensile strength show how much tensile stress can be applied before a material fails (either ductile or brittle failure)

dislocation density

The total dislocation length per unit volume of material (total dislocation length)/(unit volume)

Stress-Slip Requirements in Single Crystals

To cause slip in single crystals as a result of stress, the stress required is dependent on: 1) crystal structure (BCC, FCC, HCP...) 2) atomic bonding characteristics 3) temperature of deformation 4) orientation of active slip planes with respect to shear stress

yield strength (σy)

Yield strength is the maximum stress that can be applied before a material begins to change plastically (permanently)

Dislocation in Covalent Ceramics

i.e. (Si, diamond) -motion is difficult -directional (angular) bonding is present

Dislocation in Ionic Ceramics

i.e. NaCl -motion is difficult -atoms avoid nearest neighbors with like signs (packing structure makes this very difficult)