1. The American University in Cairo Mechanical Engineering Department MENG 426: Metals, Alloys & Composites Interactive MENG 426 Lab Tutorials Experiment (7) Precipitation Strengthening Prepared by Eng. Moataz M. Attallah Fall 2002

2. Precipitation Strengthening  Precipitation Hardening  Form small particles of a second phase in the structure  Increase strength and hardness by blocking dislocation motion

3. Ppt. Hardenable Alloy System

4. Precipitation Hardening  Limited solid solubility  Alloys that exhibit complete solid solubility have only one solid phase  Decreasing solid solubility with temperature  Composition less than maximum solid solubility α + β α α + L L C PH Temperature

5. Precipitation (age) hardening  Solution treatment -  alloy is heated into  temperature range to dissolve all B. Quenching retains B in solution (supersaturated)  Soft & ductile condition  Aging - holding solution treated alloy at a temperature at which fine precipitates of  are formed  Strengthened condition T (deg C) Percentage BPure A Liquid L +    

6. 1-Solution Treatment  Single phase alpha region  Hold to dissolve any beta  Quickly cool to room temperature  Diffusion too slow for beta to form  Supersaturated alpha phase  Not at equilibrium α + β α α + L L CoCo Temperature  

7. 2-Precipitation Heat Treatment  Heat to an intermediate temperature  Diffusion is faster  Fine dispersion beta phase begins to form  Phase coarsens with time at temperature  Process referred to as “ aging ”  Cool to room temperature α + β α α + L L CoCo Temperature    

8. Energy during ppt. hardening

10. Aging Response  Peak hardness is a function of time and temperature  Low temperatures gives a higher peak hardness but at a longer aging time  Peak hardness corresponds to low ductility  Aluminum alloy  0.9% Si, 4.4% Cu, 0.8% Mn, 0.5% Mg

11. Natural Aging  Some aluminum alloys experience appreciable precipitation hardening at room temperature  Necessary to refrigerate after solution treatment and quench  Application: rivets  Driven while soft  Age harden in place

12. Aging Curves

13. Temper designations