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Materials Science and Engineering: An Introduction

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Unformatted text preview: 2nd REVISE PAGES E Q A 190 - Chapter 7 I Dislocations and Strengthening Mechanisms slip and increase the strength of the material. Boundaries between two different phases are also impediments to movements of dislocations; this is important in the strengthening of more complex alloys. The sizes and shapes of the constituent phases significantly affect the mechanical properties of multiphase alloys;these are the top- ics of discussion in Sections 10.7, 10.8, and 16.1. 7.9 SOLID-SOLUTION STRENGTHENING Another technique to strengthen and harden metals is alloying with impurity atoms that go into either substitutional or interstitial solid solution. Accordingly, this is solid-solution called solid-solution strengthening. High-purity metals are almost always softer and Sirengthenillg weaker than alloys composed of the same base metal. Increasing the concentration of the impurity results in an attendant increase in tensile and yield strengths. as in- dicated in Figures 7.160 and 7.16!) for nickel in copper; the dependence of ductiln ity on nickel concentration is presented in Figure 7.16c. Alloys are stronger than pure metals because impurity atoms that go into solid ‘ solution ordinarily impose lattice strains on the surrounding host atoms. Lattice strain field interactions between dislocations and these impurity atoms result, and, conse- quently, dislocation movement is restricted. For example, an impurity atom that is smaller than a host atom for which it substitutes exerts tensile strains on the sur~ rounding crystal lattice, as illustrated in Figure 7.17a. Conversely, a larger substitutional 180 - 25 60 400 16° 7‘; .3 A A E 3 e;- 140 20 E J; v E: 50 ‘59 a a a g 120 if: m h *5 33 300 E s 15 E E 2 a 100 g I9 40 fl r 80’ __1° 200 30 60 O 10 20 30 4O 50 0 10 20 30 40 50 Nickel content (M%J Nickel content twt%l (a) (b) 60 .__. 50 .E N E 39 j; 40 .9 in 2 L” 30 20 0 10 20 3D 40 50 Figure 7.16 Variation with nickel content of (a) tensile MCI‘E' Gunter“ (“1%) strength, (15) yield strength, and (c) ductility (%EL) for (c) copper-nickel alloys, showing strengthening. ...
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