无机非金属材料工程专业英语 第7章

Fundamentals of Materials Science and Engineering
What should you be able to do after studying this chapter?
Describe dislocation motion from an atom’s perspective. Understand the relationships between dislocation motion and plastic deformation of metallic materials. Define slip system and explain the influence of slip system on the deformability of materials. Explain mechanism of grain boundary strengthening, solid-solution strengthening, strain hardening. Describe recovery and recrystllization in terms of both the alternation of microstructure and mechanical characteristics of the material.
strain field 应变场 Stress field 应力场
Key knowledge points: Deformations mechanisms for metals
1. Historical:

Fundamentals of Materials Science and Engineering

slip plane-----the crystallographic plane along which the dislocation line traverses is the slip 来自百度文库lane.
Macroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations, or slip, in response to an applied shear stress.
Chapter 7
Deformation and strengthening mechanisms
Fundamentals of Materials Science and Engineering
Why study deformation and strengthening mechanism?

2. Basic concepts of dislocations

Plastic deformation corresponds to the motion of large numbers of dislocations. An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line. In an edge dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms. The mechanics of dislocation motion are represented in Fig.7.1

The motion of a screw dislocation in response to the applied shear stresses shown in Fig.7.2b. the direction of movement is perpendicular to the stress direction. For an edge, motion is parallel to the shear stress. However, the net plastic deformation for the motion of both dislocation types is the same. The direction of motion of the mixed dislocation line is neither perpendicular nor parallel to the applied stress, but lies somewhere in between.



Fundamentals of Materials Science and Engineering
The mechanics of dislocation motion
Fig.7.1 Atomic rearrangements that accompany the motion of an edge dislocation as it moves in response to an applied shear stress. (a) The extra half-plane of atoms is labeled A. (b) The dislocation moves one atomic distance to the right as A links up to the lower portion of plane B; in the process, the upper portion of B becomes the extra half-plane. (c) A step forms on the surface of the crystal as the extra-plane exists.
Fundamentals of Materials Science and Engineering



natural ageing自然时效 artifical ageing人工时效 dislocation multiplication 位错繁殖 misorientation 位相差异 equiaxed grains 等轴晶粒 elongated grains 拉长晶粒
Fundamentals of Materials Science and Engineering

Slip 滑移 Slip system 滑移系 Recrystallization 再结晶 solid-solution strengthening 固溶强化 strain hardening 应变强化 strengthening by grain size reduction 细晶强化 annealing 退火 temper 回火 quench 淬火 ageing 时效
Fundamentals of Materials Science and Engineering

Dislocation density --------the number of dislocation in a material is expressed as the total dislocation length per unit volume, or equivalently, the number of dislocations that intersect a unit area of a random section. The units of dislocation density are millimeters of dislocation per cubic millimeter or just per square millimeter.

Fundamentals of Materials Science and Engineering
Important terms and concepts:



Cold working 冷加工 Critical resolved shear stress 临界分剪切应力 Dislocation density 位错密度 Grain growth 晶粒生长 Lattice strain 晶格应变 Recovery 回复 Recrystallization temperature 再结晶温度 Resolved shear stress 分剪切应力
1930s, theory and actually measured discrepancy in mechanical strengths 1950s, dislocation be examined by electron microscope

Fundamentals of Materials Science and Engineering


Fundamentals of Materials Science and Engineering
Fig.7.2 The formation of a step on the surface of a crystal by the motion of (a) an edge dislocation and (b) a screw dislocation. Note that for an edge, the dislocation line move in the direction of the applied shear stress τ; for a screw, the dislocation line motion is perpendicular to the stress direction.
Fundamentals of Materials Science and Engineering
Dislocation motion is analogous to the mode of locomotion employed by a caterpillar ( Fig.7.3)
Fundamentals of Materials Science and Engineering
Fundamentals of Materials Science and Engineering

Slip------- the process by which plastic deformation is produced by dislocation motion is termed slip.

Fundamentals of Materials Science and Engineering
Drawing a carpet in the way of (a) and (b) may help you understand the motion of dislocation better.
Most of metallic products are produced through deforming the metal somehow to obtain desired shape, size and strength. But what factors would influence the process of deformation? You may have an experience that an iron wire will get tougher and it will break finally when it is bended for times. Do you know why? Study on deformation and strengthening mechanism will help us use and develop materials better.