ALUMINUM ALLOYS

Strengthening Mechanisms in Heat Treatable Aluminium Alloys

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TABLE OF CONTENTS

OVERVIEW1

DISLOCATION THEORY1

SOLID SOLUTION HARDENING2

Solid Solution Hardening in non-heat Treatable Alloys4

Solid Solution Hardening in Heat Treatable Alloys4

PRECIPITATION5

Precipitate nucleation and growth6

Grain Boundary Precipitation7

Grain interior precipitation8

Precipitation hardening8

MICROSTRUCTURE11

Re-crystallisation12

Texture13

Overview

In the broader sense, the heat treatment of the aluminium alloys refers to any cooling and heating operations. These operations are performed in order to change the metallurgical structure, mechanical properties or residual state of stress. When the term of heat-treating is used for aluminium alloys, it is restricted to particular operations that are used in order to increase the hardness and strength of the alloys. These alloys are mainly heat-treated aluminium alloys in order to differentiate from the alloys in which not any major strengthening is achieved by cooling and heating.

The viable heat-treatable alloys are, with certain expectations, based on quaternary systems relating to the solutes that are involved in the strength development; that is achieved by means of precipitation. The alloys whose hardness and strength can be considerably improved by heat treatment consist of wrought alloy series of 2xxx, 6xxx, and 7xxx and casting alloy series of 2xx.0, 3xx.0 and 7xx.0

Dislocation Theory In the science of materials, a dislocation is an irregularity or crystallographic defect, within the structure of crystal. The dislocation's presence strongly influences several material properties (Kane, 2000, p.1295). Vito Volterra initially developed the dislocation theory in the year 1905. Some forms of dislocation can be considered as being resulted by the termination of the atom's plane in the centre of a crystal. In this particular case, the surrounding planes of the atom are not straight; however, the plane bends around the terminating plan's edge so that the structure of crystal is ordered perfectly on each of the side (Hull, 2001, p.87). The apt analogy for this particular case is with the stack of paper, if a half paper piece is inserted in the paper stack, then only at the half of the sheet, the defect in the stack will be noticeable. There are two main types of dislocation: screw dislocations and edge dislocations (Juan, 1996, p.233). The intermediate dislocations between them are mixed dislocations. Scientifically, dislocations are the form of topological defect, often called as soliton. The scientific theory explains that why dislocations work as stable particles; they are capable to be moved but they maintain their individuality as they move (Ghoniem, 2000). The annihilation is the process that takes place when two dislocations with opposite directions are brought with each other and cancel each other. However, a single dislocation mainly cannot expire on its individual basis. The metal's ability to deform its plasticity is dependent(Juan, 1996, p1367) upon the easiness of dislocation action under applied external stress . The hardening of metal includes the obstructing dislocation action. The dislocation action can be obstructed in several ways, so are the strengthening mechanisms of the metals (Hirth, 1968).

Solid Solution Hardening

Solid solution hardening is the form of alloying that cab be utilised in order to improve the pure metal's ...