ENGR-45_Lec-18_DisLoc-Strength-2.ppt 1 Bruce Mayer, PE Engineering-45: Materials of Engineering Bruce Mayer, PE Registered Electrical & Mechanical Engineer Engineering 45 Dislocations & Strengthening (2) ENGR-45_Lec-18_DisLoc-Strength-2.ppt 2 Bruce Mayer, PE Engineering-45: Materials of Engineering Learning Goals Understand Why DISLOCATIONS observed primarily in METALS and ALLOYS Determine How are Strength and Dislocation Motion Related Techniques to Increase Strength Understand How can HEATING and/or Cooling change strength and other properties ENGR-45_Lec-18_DisLoc-Strength-2.ppt 3 Bruce Mayer, PE Engineering-45: Materials of Engineering 2-Phase Metal Strengthening Last Time Studied SINGLE-Phase (ONE Xtal Structure) Strengthening 1.Grain Size Reduction 2.Solid Solution Alloying 3.Strain Hardening This Time Examine TWO Phase (2 ) Strengthening by the Formation of Solid Precipitates a.k.a., Precipitation Hardening ENGR-45_Lec-18_DisLoc-Strength-2.ppt 4 Bruce Mayer, PE Engineering-45: Materials of Engineering Strengthen-4 Precipitates Adjustment of Alloy Composition and/or Processing can Produce a (more or less) Uniform Distribution of Small 2 nd Phase Particles (Precipitates) within the Base-Alloy matrix CuAl 2 Precipitates Within an Al Matrix ENGR-45_Lec-18_DisLoc-Strength-2.ppt 5 Bruce Mayer, PE Engineering-45: Materials of Engineering PreCip Strength Mechanism Hard Precipitates are Difficult to shear Ex: Ceramics in Metals (SiC in Iron or Aluminum) Empirical Relation: y ~ 1/S Large shear stress needed to move dislocation toward precipitate and shear it. Side View Top View Slipped part of slip plane Unslipped part of slip plane S Dislocation advances but precipitates act as pinning sites with spacingS. precipitate ENGR-45_Lec-18_DisLoc-Strength-2.ppt 6 Bruce Mayer, PE Engineering-45: Materials of Engineering App PreCip Strengthening Internal wing structure on Boeing 767 Aluminum is strengthened with precipitates formed by alloying. 1.5 m ENGR-45_Lec-18_DisLoc-Strength-2.ppt 7 Bruce Mayer, PE Engineering-45: Materials of Engineering Simulation: Precip Hardening View onto slip plane of Nimonic PE16 45Ni-31Fe-16.5Cr- 3.5Mo-1.2Ti-1.2Al- 1Co Precipitate volume fraction: 10% Average precipitate size: 64 b (b = 1 atomic slip distance) ENGR-45_Lec-18_DisLoc-Strength-2.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering - Behavior vs. Temperature Tensile Tests on PolyXtal Iron -200 C -100 C 25 C Strain Stress (MPa) y and TS DEcrease with INcreasing test temperature. %EL INcreases with INcreasing test temperature. Note Trends ENGR-45_Lec-18_DisLoc-Strength-2.ppt 9 Bruce Mayer, PE Engineering-45: Materials of Engineering - Behavior vs. Temperature Why this Behavior? Increased Vacancy Concentration Mechanism: Vacancies help dislocations move past obstacles -200 C -100 C 25 C Strain Stress (MPa) vacancies replace atoms on the disl. half plane 3. disl. glides past obstacle 1. disl. trapped by obstacle obstacle ENGR-45_Lec-18_DisLoc-Strength-2.ppt 10 Bruce Mayer, PE Engineering-45: Materials of Engineering Post-ColdWork Heat Treatment 1 hour treatment at T anneal... Decreases u, and Increases %El Heat Treating REVERSES the Effects of CW Three Stages 1.Recovery 2.ReXtalization 3.Grain Growth 1 2 3 ENGR-45_Lec-18_DisLoc-Strength-2.ppt 11 Bruce Mayer, PE Engineering-45: Materials of Engineering Recovery Annihilation reduces dislocation density ( d ) Scenario1 Hi-Temp Increases Diffusion Scenario ENGR-45_Lec-18_DisLoc-Strength-2.ppt 12 Bruce Mayer, PE Engineering-45: Materials of Engineering ReCrystallization New crystals are formed that have a much smaller dislocation density Are Small in Physical Size Relative to Existing Xtals Are More Energetically Favorable than CW Xtals 33% cold worked brass New crystals nucleate after 3 sec. at 580C. 0.6 mm ENGR-45_Lec-18_DisLoc-Strength-2.ppt 13 Bruce Mayer, PE Engineering-45: Materials of Engineering ReCrystallization cont All cold-worked crystals are consumed by the New Crystals After 4 seconds After 8 seconds 0.6 mm ENGR-45_Lec-18_DisLoc-Strength-2.ppt 14 Bruce Mayer, PE Engineering-45: Materials of Engineering Grain Growth At Longer Times Some of the NEW Xtals consume OTHER NEW Xtals Grain Boundaries are Hi-Energy Regions, Thus Their Reduction is Thermodynamically Favored After 8 s, 580C After 15 min, 580C 0.6 mm ENGR-45_Lec-18_DisLoc-Strength-2.ppt 15 Bruce Mayer, PE Engineering-45: Materials of Engineering Grain Growth Quantified Many Metals Follow This Grain Growth Reln Where d Grain Size (m) t Time (s) d 0 BaseLine Grain Size at t = 0 (m) K Slope Constant (m n /s) n Power Constant (unitless) ENGR-45_Lec-18_DisLoc-Strength-2.ppt 16 Bruce Mayer, PE Engineering-45: Materials of Engineering Summary Dislocations are Observed Primarily in Metals And Alloys Metal/Alloy Strength Is Increased By Making Dislocation Motion Difficult Techniques to increase strength Decrease Grain Size Solid Solution Lattice Straining Cold Work to Increase Dislocation Density Precipitates to Impede Dislocation Motion ENGR-45_Lec-18_DisLoc-Strength-2.ppt 17 Bruce Mayer, PE Engineering-45: Materials of Engineering Summary cont. Post-CW Heating (annealing) can reduce Dislocation density and increase grain size Anneal Process Time-Phases 1.Recovery 2.ReCrystallization 3.Grain Growth ENGR-45_Lec-18_DisLoc-Strength-2.ppt 18 Bruce Mayer, PE Engineering-45: Materials of Engineering WhiteBoard Work Problem 7.27 - for C46400 (Naval) Brass ~0.16 Parallel (Elastic Recovery) ENGR-45_Lec-18_DisLoc-Strength-2.ppt 19 Bruce Mayer, PE Engineering-45: Materials of Engineering ENGR-45_Lec-18_DisLoc-Strength-2.ppt 20 Bruce Mayer, PE Engineering-45: Materials of Engineering ENGR-45_Lec-18_DisLoc-Strength-2.ppt 21 Bruce Mayer, PE Engineering-45: Materials of Engineering ENGR-45_Lec-18_DisLoc-Strength-2.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering - Curve for Steel