MATTER home

Materials Science on CD-ROM User Guide

Aluminium Alloys: Strengthening

Version 2.1

Graeme Marshall, Alcan International Ltd. Paul Evans, Alcan International Ltd. Andrew Green, MATTER

Assumed Pre-knowledge

Before starting this section, it is important that you have a basic understanding of binary phase diagrams. It is also suggested that you have referred to the MATTER module Aluminium Alloys: Systems.

You should also be familiar with the following terms: density, thermal conductivity, electrical conductivity, specific heat, melting point, tensile strength, tensile ductility, elastic modulus, hardness.

Module Structure

This module comprises 4 sections:

• Solid Solution Strengthening
• Age Hardening I - Precipitation
• Age Hardening II - Strengthening
• Strain Hardening

Solid Solution Strengthening

The substitution of solute atoms for aluminium ones distorts the crystal lattice, hinders dislocation mobility and hence strengthens the alloy. This section looks at how the properties of aluminium can be improved by solid solution strengthening.

The first page explains that in order to be effective solid solution strengtheners, alloy additions must satisfy 2 criteria:

1. high RT solid solubility;
2. atomic misfit to create local compressive or tensile strains.

An exercise on the second page shows how solute atoms of different sizes interact with dislocations. The next page goes on to compare the atomic radii of the most important alloying elements in aluminium. The final page of this section shows how the proof stress is affected by adding different amounts of solute.

Having completed this section, the user should be able to:

• describe why the addition of foreign atoms can result in solid solution strengthening;
• list the 2 criteria that determine the effectiveness of any solute towards solid solution strengthening;
• describe how solute atoms of different sizes and dislocations may interact to minimise their total combined energy;
• identify the most important solid solution strengthening addition for aluminium;
• explain why solid solution strengthening increases with temperature and how this can have an important effect on processing.

Age Hardening I - Precipitation

The strongest aluminium alloys (2xxx, 6xxx and 7xxx) are produced by age hardening. In this section the user can learn how a fine dispersion of precipitates can be formed by appropriate heat treatment. The strengthening that results from this is covered in the following section, Age Hardening II - Strengthening.

The section starts with a review of the major requirement for an alloy system to respond to this form of hardening, namely a significant decrease in solid solubility of one or more of the alloying elements with decreasing temperature. This is followed by an overview of the 3 main stages of heat treatment, i.e. solution heat treatment, quenching and ageing. The formation of a non-equilibrium supersaturated solid solution by quenching is contrasted to the type of equilibrium structure that might form if the (hypothetical) alloy were slowly cooled.

The remainder of the section is primarily concerned with the controlled decomposition from a supersaturated solid solution.

A general model for decomposition is given, followed by details of the precipitation sequences in 4 specific alloy systems: Al-Cu, Al-Cu-Mg, Al-Mg-Si and Al-Zn-Mg. The Al-Cu system is used as the main example of decomposition, i.e.

or, more fully:

The unit cells of the q'', q' and q phases are provided in a 3D format which the user is able to rotate around the x, y and z axes.

Having described the types of metastable precipitates that might form during ageing, the section then goes on to look at the thermodynamic and kinetic bases for their formation in preference to the direct precipitation of the equilibrium phase.

The kinetics of metastable precipitate formation are also illustrated using time-temperature-transformation (TTT) diagrams. An example of such a diagram is used in an exercise designed to help the student understand some of the key concepts relating to precipitate formation. This in turn leads to the concept of metastable solvuses.

The section is rounded off by a look at the formation of precipitate free zones (PFZs) and includes an exercise in which the user is asked to minimise the width of a PFZ in a hypothetical alloy by manipulating the heat treatment.

Age Hardening II - Strengthening

Having studied the formation of precipitates in the previous section, this section goes on to look at how they actually contribute to the overall strengthening of the alloy. The 3 main mechanisms are:

• Coherency strain hardening;
• Chemical hardening;
• Dispersion hardening

Coherency strain hardening results from the interaction between dislocations and the strain fields surrounding GP zones and/or coherent precipitates. The software illustrates how an optimum precipitate spacing exists at which the strengthening effect is maximised.

Chemical hardening results from the increase in applied stress required for a dislocation to cut through a coherent (or semi-coherent) precipitate. This in turn depends on a number of factors, including (i) the extra interfacial area - and hence energy - between precipitate and matrix; (ii) the possible creation of an anti-phase boundary (APB) within an ordered precipitate and (iii) the change in separation distance between dissociated dislocations due to different stacking fault energies of matrix and precipitate. The 3 contributions are illustrated by interactive graphics.

Dispersion hardening occurs in alloys containing incoherent precipitates or particles - i.e. typically those that have been overaged. This hardening results from the increased shear stress required for dislocations to by-pass these obstacles. Orowan bowing is one mechanism by which this might be achieved.

Having considered the various hardening mechanisms, the ageing curve (plot of yield stress versus ageing time) is presented as a 'sum' of the different individual contributions. The section is rounded-off by an exercise that allows the student to check they have understood the key concepts covered.

Strain Hardening

Wrought alloys that do not respond to age hardening (e.g. 1xxx, 3xxx, 5xxx) are usually strengthened by strain hardening. This generally involves cold-working at ambient temperatures, at which the multiplication of dislocations occurs at a faster rate than they are destroyed by dynamic recovery.

This section covers the basic principles of strain hardening and then compares the strain hardening behaviour of 99.5% Al with that of an Al-5%Mg alloy. After

having completed this section, the user should be able to:

• explain the phenomenon of strain hardening in terms of dislocation interactions;
• describe how the dislocation density increases with strain, quoting typical values;
• draw typical stress-strain curves for 99.5% Al and Al-5%Mg and relate these curves to their respective microstructures;
• explain why adding solute (e.g. Mg) to aluminium inhibits dynamic recovery, promotes a more uniform dislocation structure and a higher rate of work hardening.
• explain why an extended yield point occurs in Al-5%Mg.

Bibliography

The student is referred to the following resources in this module: