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: σ 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 σ-ε Behavior vs. Temperature  Tensile Tests on PolyXtal Iron -200  C -100  C 25  C Strain Stress (MPa)  σ 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 σ-ε 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 σ 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 (ρ 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 σ-ε 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 σ-ε Curve for Steel