Chemistry Comes Alive! C C Alive! Table of Contents Index Textbooks

Simulation of Dislocations in Metals

An array of soap bubbles is used to simulate a close-packed arrangement of atoms in a metal crystal. When stress is applied, the bubbles form grains of close-packed arrays in different orientations, and the dislocations move through the system. Close-ups indicate that the bubbles move one at a time, leading to a stepwise displacement of the defect.

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Keywords

crystal lattice, solid state structure, solids and liquids, metals, bonding


Multimedia

Introduction

_Play movie (22 seconds, 1.3 MB)

 
An aquarium pump is used to blow small uniform soap bubbles through a syringe needle. A slider is used to apply stress to the array. Bubbles in a raft form grains of close-packed arrays with various orientations relative to each other.

Demonstration

_Play movie (38 seconds, 2.2 MB)

   
As the raft is stressed with the slider, dislocations move into and out of the grains and their boundaries. Defects can be seen zipping through the raft as the raft is stressed. Defects move one bubble at a time, gradually displacing the defect. The non-directional attraction between soap bubbles models the non-directional attraction between metal atoms, and a close-packed array is formed.


Discussion

Unlike covalently bonded substances, where the orientations of atoms maximize overlap of the valence orbitals, metallic bonds are non-directional. This means that the atoms will arrange themselves to maximize the number of nearest neighbors. Thus, many metal crystals have close-packed arrangements. In both the hexagonal and cubic closest packing lattices, each atom is surrounded by a hexagonal arrangement of six atoms (as illustrated in this demonstration), as well as three atoms above and three atoms below the hexagonal plane.

All crystal lattices have defects. When stress is applied, the number and nature of the defects changes, and the defects can move through the lattice, as illustrated in these demonstrations. Another topic "Heat Treatment of a Metal Bobby Pin" shows how metals can be treated to alter the number of dislocations, and illustrates how the number of defects influences the physical properties of the metal.

Additional still images for this topic

Demonstration Notes: Warnings, Safety Information, etc.


Exam and Quiz Questions

1. Describe how defects move through the system when the array is "stressed."

2. Examine a portion of the array of bubbles with no defects nearby. How many nearest neighbors does a bubble have? (That is, how many other bubbles are touching it?) Imagine additional layers of bubbles above and below the layer in the demonstration. Now how many nearest neighbors would a bubble in the middle layer have?

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