Work hardening in metallic polycrystals (all content)

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Contents

Introduction

This practical introduces the effect of work hardening in polycrystal metals, in the form of bending a copper rod.

To have an appreciation of why and how work hardening happens in copper bar, it is necessary to understand the creation of dislocations, their gliding and interactions. In other words how the microstructure of  metallic polycrystals changes during the deformation process

It would be helpful to be aware of major deformation processes and have a clear understanding of plastic deformation (including work hardening), which can be found in the Introduction to Deformation Processes TLP.

The metal will be brought to the room and volunteers will be able to see this effect themselves.

Scientific background

In real metals there are a lot of defects such as vacancies, impurities atoms, dislocations, grain boundaries etc. Dislocation is a half-plane in crystalline structure; this is often termed an extra-plane. Dislocations can move by gliding or climbing. If dislocations could only move by gliding on a single slip plane, their motion would be constrained and they would soon become impeded by obstacles, such as other dislocations or fine precipitates.

Dislocations often meet other dislocations, in various configurations. They may amalgamate to form new dislocations, annihilate each other, be repelled or cut through each other.

When two dislocations intersect, jogs form in both. These are short sections with length and direction equal to b of the other dislocation. If a jog lies in a slip plane (when it is sometimes called a kink), it can glide with the rest of the dislocation, but if it lies out of the slip plane, it may be sessile (unable to glide, i.e. not glissile) – see below. Such defects can reduce dislocation mobility after straining.

Formation of jogs by intersection of two edge dislocations. The jog in b2 is undetectable, whereas that in b1 is sessile (unable to glide)

In pure materials the main source of resistance to the motion of dislocations other than the lattice resistance is from other dislocations that intersect the glide plane.

Dislocation Density, ρ ~10¹⁰ m^-2 (10 μm spacing) when “Annealed”, ~10¹⁶ m^-2 (10 nm spacing) when “Cold-worked”

Forces between Dislocations can promote Annihilation & Alignment, leading to Polygonisation

Dislocation Motion is assisted by Cross-slip and Climb (the latter being Faster at High Temperature)

Intersection of Dislocations during Glide creates Jogs, which may be Sessile (unable to glide)

Dislocation Multiplication can occur, eg at a Frank-Read Source

In general, Plastic Straining Raises the Density of Dislocations, but Reduces their Mobility, leading to “Work Hardening”

IT resources

Various effects can raise ρ. Dislocations can be created at free surfaces, grain boundaries and within grains.

Consider a dislocation line fixed (pinned, e.g. by sessile jogs) at both ends. When a shear stress τ is applied, a force F = τ b acts normal to the line.

Since the dislocation line is pinned at both ends, it bows outwards into a loop, which eventually surrounds the pinning points. Opposing segments annihilate on meeting, forming a loop and recreating a short, pinned dislocation. This can repeat, generating a new loop each time. This is a Frank-Read source.

Frank-Read Source

As deformation continues, ρ tends to rise and dislocation mobility to decrease (as tangles, jogs etc form), making it harder to deform the material.

Dislocation simulation video

This is often termed work hardening.

Equipment and facilities

For the experiment we will need a commercial purity copper bar (rod) after annealing. In this stage the material is ductile and soft and can be easily deformed by hand.

Description of the demonstration

At first copper rod will be deformed by bending. At this stage, the volunteer will not need to apply too much forces. Metal will bending easily.  

After then another volunteer will try to straighten out the bent rod, to return it to the original shape. You will see that the force required for this much more.

This is happening because of dislocations are created and they intersect with another ones. Therefore their mobility decreases and deformation becomes difficult.

Bent Cu rod

Bent Cu rod alongside original unbent rod

Video of the demonstration

Academic consultant: Bill Clyne (University of Cambridge)
Content development: Elina Dinislamova
Photography and video: Steve Penney, Jess Gwynne
Web development: Lianne Sallows and David Brook

This DoITPoMS LDP was funded by the UK Centre for Materials Education and the Department of Materials Science and Metallurgy, University of Cambridge.