I'm working on the mechanical properties of AM60 (Mg alloy). Compression test were carried out and the fracture surface of the sample were examined using SEM. Could any one help me with interpreting the SEM image of the fracture surface?
Hello Mohammad. Fracture phenomena in your image is very interesting. In my opinion and in a first glance, the relatively "clean" edges of the tearing regions show low ductility of your alloy. I recommend you to complete the analysis with a grain size study. Have you got the strain-stress charts?. Some researchers have studied the amount of porosity in the alloy. I hope these comments help you.
http://www.sciencedirect.com/science/article/pii/S0921509306005338
From the fractograph, i feel it is cleavage fracture. you can see that the surface of the fracture is completely flat, which is possible only in case of 100% cleavage fracture.
You need a tensile or shear stress for crack formation and growth. Under uni-axial compression, the failure can occur at an angle to the loading axis. What I see is shear failure. The inhogenities at the grain boudaries can initiate void formation, from which shear cracks can initiate, depending on the ductility of the material. In ductile materials, the stresses can be dissipated. In brittle materials, shear cracks can initiate and propagate. Hope this helps.
Mohammad
Most unexplained or unexpected failures of cast materials are the result of bifilms, the double oxide films which are folded in by turbulence during stirring and pouring. These double films have no bonding between the films, so they act as cracks. The fracture surface follows the unbonded interface, leaving fragments of the original oxide films on both of the opposed fracture surfaces. The appearance resembles cleavage, but the multilayer aspects of the fracture surface strongly suggest multiple bifilms as a result of extremely poor casting conditions.
An analysis of the 'platelets' in the surface should reveal MgO, but may also reveal other intermetallics. Most second phases which precipitate during freezing form on oxide bifilms as favoured substrates. These will lie on the wetted undersides of the surface oxide film on the fracture, but the oxides may be so thin that the signal from the underlying intermetallic will be strong or even dominant.
In my opinion (and collegues) it is the effect of smearing when mirror fractures form, effect often observed on fatigue fractures .
Thank you every body, I made use of the answers which was given from different viewpoints.
Mr Campbell: I have done an EDS analysis on the sample before compression test which shows 26.91 %w O in sample. The analysis map shows a well distribution of Oxygen. However, I did not carry out an XRD analysis to determine whether MgO exists or not. Can I say that bifilms are formed in my sample?
Mr J. Fernando Guillen G : Grain size analysis revealed that the grain size of the sample were almost 230nm which predicts an increased strength and a decreased ductility, as you said.
Thank you Mr Karol Jozwiak, Srivatsa Kulkarni and Kuntimaddi Sadananda.
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