Measuring Plasticity with Orientation Contrast Microscopy in Aluminium 6061-T4

Abstract

Orientation contrast microscopy (i.e., electron backscattered diffraction, EBSD) was employed to monitor the plastic strain in loaded tensile samples of aluminium alloy Al6061 in T4 condition. The kernel average misorientation (KAM) is known to be an appropriate parameter in orientation contrast microscopy which has the potential to characterise the plastic strain by monitoring the local misorientations. This technique was applied here to gauge the extent of the plastic zone around a fatigue crack. To establish the magnitude of strain (which can be identified by the KAM parameter), a series of tensile samples were strained in the range of 1% to 25%. KAM maps were compared, and the average misorientations were related to the tensile strain values. The KAM distribution functions for all the strained samples were derived from a large scanned area. In addition, Vickers microhardness tests were conducted for these series of samples. This allowed the comparison of the mesoscopic plastic strain measured by Vickers microhardness with the micro plastic strain locally obtained by KAM. Noise was observed in the average KAM values up to a plastic strain of 1.5%. For the plastic strain exceeding 1.5%, noise no longer dominates the KAM map, and a positive-though not linear-correspondence between plastic strain and KAM was observed. The observed plastic zone at the tip of the fatigue crack by micro-Vickers hardness measurements was about an order of magnitude higher than the plastic zone observed on the KAM maps. In view of the calibration of KAM values on the tensile samples, it could be concluded that in the larger area of the plastic zone, the strain did not exceed the critical value of 1.5%.

Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

“…In this respect, the value of ∆ε pl sum (Equation (8)) should be interpreted as an average (or representative) crack-tip plastic strain range, controlling TMF crack growth. However, the levels of local strain at distinct ∆K values (see Table 3) were found to be of the same order of magnitude as reported in the literature for metals [19,20]. It is acknowledged that TMF lifetimes are also successfully determined as a function of plastic CTOD, with TMF crack growth rates found to be approximately linear with plastic CTOD values [15][16][17].…”

Estimation of the Plastic Zone in Fatigue via Micro-Indentation

“…Such an amount of data points appears to be sufficient to properly characterise the Vickers hardness evolution around the crack tip. Hence, the resolution achieved is up to 57 times higher than in previous works [ 20 , 23 , 25 , 26 ]. The data were linearly interpolated to generate the contour plots.…”

“…The similar behaviour in the three specimens cross-validates the current methodology. The two different slopes were also identified in different materials [ 20 , 23 , 25 , 26 ]. The initial surge just ahead of the crack tip was also detected by Ghodrat and co-authors on a 6061-T4 aluminium alloy [ 20 ].…”

“…By using higher-end microscopes and imaging systems, the quality of the readings can be improved. Nevertheless, by using relatively simple arrangements such as the current set-up, the plastic zone analysis is more accurate than previous studies [ 20 , 23 , 25 , 26 ]. The measurements were made with ImageJ open-source software (version 1.40 g) [ 31 ].…”

“…The volume of material within the plastic zone has been plastically deformed and thus has hardened. Such increment in hardness can be detected with indentations [ 20 ]. Such a method has been used in the past because it is straightforward and is based on direct measurements of a material property.…”