This lab was performed to fully comprehend the temperature dependency of the fracture toughness of the Aluminum and Steel. It was also executed to discover the ductile-brittle transition of each.
The experiment solidified the hypothesis that the 6061 aluminum has no ductile-to-brittle transition do to its FCC crystal structure. The 1018 steel has a ductile-to-brittle transition temperature of -31ºC due to its BCC crystal structure.
Figure 1: Graph of fracture energy vs. temperature for entire class data
From Figure 1 one can determine that steel has a ductile-to-brittle transition temperature at -31ºC requiring a large increase in energy required to fracture over the temperature range. Also it can be determined that aluminum has no ductile to brittle transition due to the almost nonexistent change in fracture energy over the temperature range. Figure 2: Photos of the fracture of each specimen for each testing temperature
From the photos in Figure 2 one can see that the Aluminum fracture remains relatively constant throughout the temperature range. This shows plastic deformation due to its ductile behavior throughout the testing temperatures. The steel on the other hand is brittle at the colder temperatures and from the pictures you can see the grain boundaries are more visible and course. This represents a brittle failure. As the temperature increases, the grains become smoother as seen in the photos, which denote a transition to plastic deformation for a more ductile steel.
The results show that for the 1018 steel the fracture energy increases over the temperature increase, while for the 6061 aluminum the fracture energy stays relatively constant throughout the temperature increase. In the Figure 2 photos, the aluminum is relatively constant on the surface of the fracture. There is a smooth appearance to the grains on the surface of the aluminum, displaying plastic deformation. For the steel, the grains are jagged and rough at the lower temperatures, displaying shearing of the grains, but as the temperature increases the surface becomes smooth like the aluminum. The crystal structure of the 1018 steel is a BCC when the 6061 Aluminum has a FCC. For a BCC crystal structure there is ductile-to-brittle transition due to cleavage in the grains, but in a FCC crystal structure cleavage is nonexistent therefore no ductile-to-brittle transition occurs. In conclusion the 1018 steel has ductile-to-brittle transition.