03

  • Introduction

    This week we will start to concentrate on metals.

    This week we will be looking at the crystal structures into which these metals will crystallise: Face Centred Cubic; Body Centred Cubic and Hexagonal Close Packed

    Study Plan

    ● Read through the lecture summary (page 18, 1-2)
    ● Read over week 3 presentation
    ● Read chapter 3.1-3.12 in Callister
    ● Work through questions 1,2 and 3 (a and b sections only) on page 32 of the study pack, and other relevant questions at the end of the chapter in Callister

    Activity : 1. Look at the videos referenced in the study pack
    Activity: 2. View the VMSE on Metallic Crystal Structures and Crystallography

  • CRYSTAL STRUCTURES/MILLER INDICES/DISLOCATIONS: LECTURE SUMMARY


    In the course of this lecture, you will learn that atoms can form into three dimensional structures.
    In the case of metal atoms, these take the form of three main types of UNIT CELL: Face centered cubic (FCC); Hexagonal close packed (HCP) and Body centered cubic (BCC).


    1. The number of atoms per unit cell for each of the main types of arrangement are:
    FCC : 4
    HCP : 6
    BCC : 2

    Unit Cell parameter:

    FCC: 4r/√2
    BCC: 4r/√3
    2. The miller index of a particular plane is determined by reciprocal of the intercept of the plane with the three principal axes: i.e.
    h= 1/x
    k= 1/y
    l= 1/z
    3. There are two main types of dislocation which occur in metals:
    a) Edge dislocation: extra half plane of atoms inserted in the structure,
    b)  Screw dislocation: skewering of the crystal around an axis.
    4. The significance of dislocations are that:
    i)  The observed tensile strength of the metal is much lower than that calculated e.g. steel 100's MPa rather than 1000's MPa.
    ii)  Metals are ductile.
    iii)  The mechanical properties of the metal can be altered by heat treatment.