Mechanical Properties of Structural Materials
Mechanical Properties of Structural Materials MSE 200
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This 15 page Class Notes was uploaded by Mrs. Jazmyne Kulas on Thursday October 15, 2015. The Class Notes belongs to MSE 200 at North Carolina State University taught by Yong Zhu in Fall. Since its upload, it has received 34 views. For similar materials see /class/223747/mse-200-north-carolina-state-university in Materials Science Engineering at North Carolina State University.
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Date Created: 10/15/15
MSEZOO Lecture 8 CH 6568 Mechanical Properties I Instructor Yuntian Zhu Objectivesoutcomes You will learn the following P1astic deformation single crystals The slip mechanism and dislocations Slip systems and the critical resolved shear stress Schmid s law Twinning Grain boundary strengthening 339 ttpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Slip Bands and Slip Planes in Single CIVstals Happens in the plastic deformation of single crystals Slip bands in ductile metals occurs in many slip planes Slip occurs in many slip planes within slip bands Caused by dislocation slip glide NC STATE UNIVERSITY Depanmem ofM v m l i Fquot Slip Mechanism The slip occurs due to movement of dislocations MEIEJSWELHHEM V CUJUW A MM MELHUM WQQ WI 39 Depanmem om v m Fn NC SIAIE UNIVERSITV Slip in Cgstals Slip occurs in densely or close packed planes Lower shear stressenergy is required for slip to occur in densely packed planes 6 Close packed plane 9 Nonclosepacked 6 plane DepanmemofM v m l rquot NC STATE UNIVERSITV Slip Systems Slip systems are combination of slip planes and slip direction Burgers vector In FCC crystal slip takes place in 111 planes and lt110gt directions 12 slip systems in fee metals httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Slip Systems in BCC Crystal BCC cr stals are not close acked The sli redominantl occurs in l 10 Which has highest atomic densnv The slip direction in BCC metal is lt1 11gt lanes BCC erAMMoV Hbrass HIUI lll 6XZIZ I chlveW Na 711 111 12x112 uFeK 131 Ill 24x24 httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERStTY Critical Resolved Shear Stress Critical resolved shear stress is the stress required to cause slip in pure metal single crystal Slip begins when shear stress in slip plane in slip direction reaches critical resolved shear stress This is equivalent to yield shear stress httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Schmid s Law stress produced on a slip system is give by Tr OCosiuCoqu Example 69 Depanmem ofM m w The relationship between uniaxial stress and resulting resolved shear Normal to 39 Slip plane A1Area of Slip plane Twinning In twinnin a art of atomic lattice is deformed and forms mirror image of lattice next to it 0 Usually straight boundary Twinning reorient the slip system K N om displacement E 1 1 I III plane 1quotwxrir1imy plunew I i 1 winning direcliun FCC metal httplvwmlmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Effects of Grain Boundaries on Strength Grain boundaries stop dislocation movement and hence strengthen the metals Fine grain size is desirable and hence metals are produced with finer grains rm mwu m m Hm M lu Dislocations piled up Stressstrain curve of single Slip bands in polycrystalline against grain boundaries and polycrystalline copper aluminum grains in stainless Sm NC STATE UlltllEH http lWWN mse ncsu eduresearchzhu Department of Materials Sci 1 Eng Hall Petch Equation 0 Finer the grains superior are the mechanical properties at room temperature HallPetch equation Empirical 12 oy 00 k d httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Effect of Cold Work on Tensile Strength Number of dislocations are increased by cold work Dislocation movements are hindered by both grain boundaries and other dislocations Strain Hardening MWi will lled 1018Annealed Tenai StressStrain curves of 1018 steel 1 h if Strum httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng yicm renew hail lc strength and mi m Elusigulion par31 l 10 3 4U Fl 00 Pununi cold work NC STATE UNIVERSITY Solid Solution Strengthening Addition of one or more metals can increase the strength of metals Solute atoms on case of substitutional solid solution create stress elds around themselves and hinder the dislocation movement Distortion of lattice and clustering of like atoms also impede dislocation movement Example Brass Jewelry httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Nanocrystallinenanostructured Metals Nanocrystalline average grain diameter lt 100 nm 0 Nanostructured average structural feature lt 100 nm 0 Results in high strength and hardness 0 If 1 lt 5 nm elastic modulus drops as more atoms are in grain boundary 0 Negative HallPetch effect might take place 3 important strategies for increasing the strength httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY Survey and homework Example Problems 69 610 Regular Problems Chapter 6 46 47 48 49 63 65 Reading assignment section 6568 httpwwwmsencsueduresearchzhu Department of Materials Sci amp Eng NC STATE UNIVERSITY
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