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Archive		   Lab Project 3  

Precipitation Hardening by Second Phase Particles on an Alloy of Composition Cu - 10%Ni - 5% Sn

Team Members: Casteel,  A. Grevstad, M. Moore

Objective:

The proposed project objective is to study of precipitation hardening by second phase particles on an alloy of composition Cu - 10%Ni - 5% Sn. This will be accomplished by studying two physical quantities, hardness and second phase particle size as a function of heat treatment time and furnace temperature.

Plan and Procedure:

Five samples of Cu - 10%Ni - 5%Sn will be solutionized at 850 °C for 180 minutes and water quenched. Each sample will be heat treated as described in table 1.

Brinell Hardness will be measured for each sample and plotted as a function of heat treatment time and temperature.

Selected samples (see Table 1) will be polished and etched in preparation for diffraction (WAXS). Intensity versus diffraction angle (2q) will be plotted for the samples. This plot will display primary composition peaks as well as "satellite" peaks representing second phase particle presence. These satellite peaks will be used calculate average second phase particle size for each sample. This will be plotted versus hardness to obtain a relationship between both quantities.

Sample Temperature Time (min) XRD
1 0 0 x
2 325 10
3 350 10 x
4 375 10
5 400 10
6 325 50
7 350 50 x
8 375 50
9 325 100
10 350 100 x
11 375 100
12 325 200

Expectations:

Precipitation hardening is often employed in copper alloys to improve mechanical properties such as yield strength and hardness. This change in properties is the result of the formation of small finely dispersed (10-100 nm) second phase particles in the alloy. These particles induce lattice strain in the copper matrix that restricts dislocation flow. This motion restriction results in the aforementioned enhanced mechanical properties.

Formation of these particles occurs when a solid solution alloy is quenched to room temperature to result in a metastable supersaturated solution. The alloy is then heated to and held at a temperature in the two Ð phase field that has appreciable diffusion. It is at this point that second phase particles begin to form; this is the onset of precipitation hardening. The age hardening temperature is maintained for a determined time period before the alloy is cooled to room temperature, ending the process.

The number and size of second - phase particles to obtain maximum mechanical enhancement is a function of time and temperature. If furnace time is too great for a specific temperature, mechanical properties will begin decline after a maximum is reached; this is known as overaging.