Grains and their formation - Fundamentals

• Engineering
• Further Education (Key Stage 5)

Grain Structures of Metallic Materials

Lecturer: Kyle Smith B.Eng(Hons) Eng Mngt w.DPP, ProfMICME, GradIMMM (IOM3), HND (Mfg. Eng.) 

Lesson Introduction

Lesson Aim: 

Provide an introduction to grain structure of metallic materials

Lesson Objectives:

1. By the end of the lesson you will be able to explain the key feature of metallic grains including; Formation, growth, boundaries and grain size.
2. Be able to illustrate the process of grain formation through the use of diagrams or sketches and you will also label the key features of metallic grains.
   

RECAP - State any of the three atomic structures (unit cells) found in metallic materials

ICE - 2 - WATER - 2 ICE: Please submit your image of your flow diagram.

Metallic grains - Atomic Scale

Single Crystal - have infinite periodicity

Polycrystal - have local periodicity

A polycrystalline solid or polycrystal is comprised of many individual grains or crystallites. Each grain can be thought of as a single crystal, within which the atomic structure has long-range order. 

In an isotropic polycrystalline solid, there is no relationship between neighbouring grains. Therefore, on a large enough length scale, there is no periodicity across a polycrystalline sample.

Grain Formation

Metals start as solid > heat applied to form liquid (reaching melting point) 

Liquid metal cools (freezes -similar to ice crystals forming) - These form 'Dendrites' like Christmas trees

Futher Reading

https://www.doitpoms.ac.uk/tlplib/solidification_alloys/dendritic.php

Grain Structure

Grain

Any single phase metal is polycrystaline - made up of millions of small crytals or 'Grains'

Grain Boundary

This is where single grains meet up - boundaries have their own properties. The first of these is that they have low bond density because of the surface energy associated with it and the more open the structure of the boundary can allow diffusion to occur at a faster rate.

Pure copper etched in DIC

Microstructure can only be assessed by microscope (stereo microscope, light microscope using reflected light, digital microscope or scanning and transmission electron microscope). Usually, the size of characteristics observed range from parts of millimeters down to micrometers and even nanometers. Microstructural observations are used for a wide range of investigations, for example, to determine grain sizes, check for defects, target preparation in microelectronics, welds of all kinds and failure analysis.

Group task 

Drawing of Ferrite & Pearlite - Low Carbon Steel Microstructure

Recall two things that you have learned from todays lesson.

Tell me one thing you liked about the lesson and one thing you want to know more about

How do you feel after todays lesson?

😒🙁😐🙂😃