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Materials Science and Engineering: An Introduction

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Unformatted text preview: 2nd REVISE PAGES EQA 7.10 Strain Hardening - 191 O O 000 O 0 assists: assess: O O m 0 O 0“ 110-“ atoms by a smaller substitutions] impurity atom. O O O O (b) Possible locations of smaller ------------ impurity atoms relative to an O O O O O O edge dislocation such that there w O O O O O O liiiiifiiiiifii‘iéilfia (in) strains. atom imposes compressive strains in its vicinity (Figure 7.18a).'lhese solute atoms tend to diffuse to and segregate around dislocations in a way so as to reduce the overall strain energyithat is, to cancel some of the strain in the lattice surrounding a dislo- cation. To accomplish this, a smaller impurity atom is located where its tensile strain will partially nullify some of the dislocation’s compressive strain. For the edge dislo- cation in Figure 7.1%, this would be adjacent to the dislocation line and above the slip plane. A larger impurity atom would be situated as in Figure 7.1813. The resistance to slip is greater when impurity atoms are present because the overall lattice strain must increase if a dislocation is torn away from them. Fur- thermore, the same lattice strain interactions (Figures 7.17b and 7.18.5) will exist between impurity atoms and dislocations that are in motion during plastic defor- mation.Thus, a greater applied stress is necessary to first initiate and then continue plastic deformation for solid-solution alloys, as opposed to pure metals; this is evi- denced by the enhancement of strength and hardness. 7.10 STRAIN HARDENING strain hardening Strain hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Sometimes it is also called work hardening, or, because the temperature at which deformation takes place is “cold" relative to cold working the absolute melting temperature of the metal, cold working. Most metals strain harden at room temperature. It is sometimes convenient to express the degree of plastic deformation as per- cent cold work rather than as strain. Percent cold work (% CW) is defined as Percent cold work— dependence on a _ original and ACW _ ( deformed cross- sectionai areas AID—Ad ) x 100 (7.8) [l where A0 is the original area of the cross section that experiences deformation, and Ad is the area after deformation. 0 O 000 O O sensesLearner O O m 0 O on host atoms by a larger substitutional impurity atom. O O Orin O O (b) Possible locations of larger .- impurity atoms relative to an O O O O edge dislocation such that there is partial cancellation of O O O O O O impurity—dislocation lattice strains. (a) (b) ...
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