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
Precipitation Strengthening Precipitation Hardening Form small particles of a second phase in the structure Increase strength and hardness by blocking dislocation motion
Ppt. Hardenable Alloy System
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
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 +
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
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
Energy during ppt. hardening
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
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
Aging Curves
Temper designations