The figure below shows stress-strain curves of various materials at relatively l

Question

The figure below shows stress-strain curves of various materials at relatively low strain levels. We will
focus on pure Al and Al alloy 7075-T6 (primary alloying element is ~5% Zn).

a. Find the elastic modulus (E) and yield strength (0.2) of Al alloy 7075-T6.

b. The figure also shows the stress-strain curve of annealed pure Al (heated at relatively high temperature for a period of time). How will the grain size change compared to before annealing? How would it affect the strength of the material? (consider coarsening process)

551 MPa Aluminium alloy 7075-T6 Molybdenum Mild Steel..._- BS EN 10087 11SMn30 Strong Grey Cast Iron BS EN1561 EN-GJL-350 275 MPa-/- Magnesium alloy Weak Grey Cast Iron BS EN1561 ENGJL 150 Pure Aluminium-Annealed 0,012 0,016 0,028 0,032 0,004 0,008 0.024 0,028 Strain

Explanation / Answer

a. Elastic modulus is given as the ratio of true stress Vs Strain plot. From graph it is evident that slope of the plot wiil be stress (275MPa) /strain (0.0045) = 61 GPa. Yield strength is the stress value at which the plot starts being nearly parallel to strain (x-axis). COrresponding value in the graph got Al7075-T6 is approximately 500MPa.

b. After annealing as a process of heat treatment, the material properties of material improves due to stress relieving and relaxation acroos the grain boundaries. Further grains refine and grain size decreases. Decreasing grain size requires more load/force to migrate the grains across grain boundaries and hence the load/stress required for deformation increases with decreased grain size. It also have been theoratically by Hall Patch relationsgip that strength is inversly proportional to square root of grain diameter.   

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