Chapter 7: Plastic deformation, Strengthening and Recrystallisation of Metals

Transcription

1 Chapter 7: Plastic deformation, Strengthening and Recrystallisation of Metals

2 What do I need to know? The Mechanism of Plastic formation in single and polycrystalline metals The slip mechanism The slip systems Mechanisms that initiate the strengthening of metals The recovery and HX of cold worked metals

3 Single-crystal Structure only has 1 grain If σ appl > σ ys PLASTIC DEFORMATION (SLIPPING of atoms along slip planes) Slip lines and bands visible on the surface Polycrystalline More than 1 grain: GRAIN- BOUNDARIES σ Grains parallel to slip-systems with the greatest τ deform first Dislocations move on slip planes and the more dislocations created = the accumulation of dislocations at grain boundaries

4 Slip plane Block of atoms is moved over another in the structure Slip occurs during the displacement of dislocations on a sliplevel HDG: {0001; 1010; 1011} FCC: {111} PCC: {110}; {211}; A Slip system {321} is a combination of the slip plane and slip direction N slip systems: HCP = 3; 3; 6, FCC = 12, PCC = 12; 12; 24 THEREFORE: Ductility HCP <<< Ductility PCC/FCC Slip direction Slip occurs in the close-packed direction upon the calculated slip plane of the crystal HCP: <1120> FCC: <110> PCC: <111>

5 Decrease of Grain size (Grain Refinement) d GBA More energy needed for the plastic deformation Therefore d σ acc. Hall-Petch σ ys = σ o + k d V = 4/3πd 3 GBA = 1.5V/d 2

6 Solid Solution Addition of alloying elements Develops lattice deformation and thus the strengthening of Parentmetal Factors that influence the extent of solution 1. Relative grain size 2. Sort distance order 3. Concentration of Alloying elements r alloy < r parent r < 15% r alloy > r parent r < 15% r alloy > r parent r > 15%

7 Work hardening Metallic Strengthening via. Plastic Deformation Accumulation of dislocation, thus DISLOCATION DENSITY increases Tension field occurs between dislocations More tension needed to relieve tension field, therefore an increase in strength % Reduction Plates %Red = (t o t f )/t o Bars/Wire %Red = ( S o S f ) / S o Hot VS Cold Working

8 Ag-Cu alloys are used to manufacture conductors for electrical components. The current manufacturing route produces components with a yield strength of 100 MPa and grains with radii of 2.5µm. A client is interested in purchasing these components but requires a yield strength of 120 MPa. Calculate the grain size that is required to satisfy the client s specification given that the grain size is equal to 5µm at a yield strength of 80 MPa. (2.992µm)

9 The following graph shows the influence of Ni alloying additions on the strength and ductility of Cu. Explain why the ductility decreases and strength increases with an increase in Ni additions.

10 Prove that % cold work for a tensile test can be expressed as %Red = ε/(ε + 1) x100 for a cylindrical test specimen. Assume that the volume of the specimen stays constant during and after the test.

11 An annealing heat treatment can be used to reproduce the properties of the COLD WORKED metal at a temperature at which recovery, HX and grain growth will occur within 1 hour. 1. Recovery The number of point defects decreases (+ and dislocations eliminate one another) and low angle grain boundaries are produced The internal tension is relieved (Hardness and strength decreases slowly) Driving force for recovery is the DECREASE in the INTERNAL ENERGY of structure 2. Recrystallisation Nucleation of new grains and grain growth starts Driving force is the DECREASE in LATTICE ENERGY

12 1. Grain growth Rapid grain growth occurs after HX Grain growth occurs through the decrease in GRAIN BOUNDARY ENERGY since the GRAIN BOUNDARY AREA decreases. (Remember LARGER GRAINS, SMALLER GRAIN BOUNDARY AREA) Time for HX t HX% = C.e (Q/RT)

13 3 Steelspecimens are cold worked to 50, 40 and 30%. After cold work, the specimens are HX fully at 750 C. On set time intervals, the hardness of the specimens are tested. Give a graphical representation of the 3 samples and name the axis of your graph.

14 1. Minimum Cold Work 2. %CW HX Tyd T 1. T HX time T1 > T2 > T3 1. D f and D o f(%cw) 2. Purity HX Temp

15 The following graph shows Rockwell B hardness measurements for Al (KVG) en Zn (HDG) 1. Explain why the hardness of Zn is higher than of Al. 2. Why is the 6 th hardness measurement of Al significantly higher than the average hardness of 54R b?