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Lab 3: Properties of Metals

June 23 2000

The mechanical properties of metals can be controlled by the addition of point defects by addition of point defects such as substitution and interstitial atoms. In metals point defects disturb the atomic arrangement in the lattice and interfere with the movement of dislocations.
An alloy results when a sufficient number of defects are added. It changes the mechanical properties of a material, such as yield strength, hardening, ductility, electrical conductivity, and thermal conductivity [1].

Figure-2 shows Copper (pure metal) has a yield strength of approximately 80Mpa, whereas Figure-3 shows Brass (copper and zinc alloy) has a yield strength of approximately 100Mpa. Thus Brass has higher yield strength as the result of alloying.

In addition, ultimate tensile strength versus percentage elongation (Figure-4) comparision for brass and copper specimens shows that percentage elonationgation for hard brass is higher than that of hard copper. Thereby proving that alloying increases the ductility of materials.

Furthermore, Figure-5 that illustrates ultimate tensile strength versus hardness trend shows that brass has a higher hardness value of 85.3 (Rockwell 15-T) than copper, which has a hardness value of 77. By this means showing that alloying hardens materials.

Mild steel displays double yield point (sharp upper and lower yield point) in Figure-1, because small carbon interstitial atoms clustered around the dislocations obstruct the slip, and raise the yield point. As the applied stress increases, the dislocations begin to slip at a higher yield point. However, once dislocations move away from the clusters of carbon atoms, they continue to move rapidly at a lower stress. In copper, brass and aluminium there are no interstitial atoms to resist dislocation movement, therefore they do not show any sharp upper and lower yield points.

Annealing is heat treatment designed to eliminate the effects of cold working. When the metal is first heated (recovery) below the recrystallisation temperature the additional thermal energy permits the dislocations to move and form a polygonised structure. However, the dislocation density does not change, thus the mechanical properties of the metal are relatively unchanged. Recovery, however, reduces or eliminates residual stress. Comparing the stress-strain curve for untreated brass and brass annealed at 300C in figure #$%$# shows that the mechanical properties of brass are only slightly changed. The treated brass has a slight increase in ductility and a slight decrease in strength.
Recrystallisation occurs by the nucleation and growth of new grains containing few dislocations. When a metal is heated above the recrystallisation temperature, rapid recovery eliminates residual stress and produces the polygonised dislocation structure. Then small new grains nucleate at the cell boundaries of the polygonissed structure, eliminating most of the dislocations. Thus the annealed metal has a lowered strength but higher ductility as the result of reduced dislocations. This trend is obserbed in figure 9, when comparing brass annealed at 400C and untreated brass.
At even higher annealing temperatures, recovery and recrystallisation occr rapidly, producing a fine recrystallised grin structure. The grain grows consumeing the smaller grains. This makes the maerail even less stronger but more ductile. This trend is obsered when comparing brass anneled at 600C and the brass specimens at the other levels.

The partially annealed steel does not have double yield points, thus the residual stress have been eliminated. This indicates that the specimen only went through recovery, hence it was partially annealed.

The results in the lab are fairly accurate in some instances.

Possible sources of errors in the lab occur as a result of inaccurate calibration of machines, tampered metal specimens, and improper test methods. However the results in this lab are fiarly accurate, since the calculation of the modulus of elastisity gave a percentage difference of around 5%.