STRAIN ESTIMATION BY ELECTRON BACKSCATTERED DIFFRACTION

The quality of the Kikuchi patterns obtained for example by electron b ackscattered diffraction is altered when the dislocation density incre ases in strained materials, and consequently if can theoretically be c orrelated to the strain for monotonic deformation pathes. For that pur pose, in a first step, it is necessary to define a qualify factor in t he Hough space or with the Burns method currently used when the Kikuch i patterns are automatically analysed. Then, and it is the most import ant point, the quality factor must be linked to the strain through a ' 'calibration'' curve independent of crystallographic orientation. The main objective of this study is to test the reliability of this method in the ''simple'', case of the hardness test. This test is simulated using the finite element method and the obtained results are compared to those measured by electron backscattered diffraction. Several ''cal ibration'' methods are defined. The first one consists in defining a c alibration curve from the simulation and then verifying it for some ot her tests with different loads inducing different strain scales. Two o ther experimental methods have been also applied. In the first case, i t is assumed that the behaviour of an anisotropic polycrystalline mate rial can be represented by a I calibration curve defined from single c rystals with the same orientation and deformed in channel die with sev eral strains. In the second case, the ''calibration'' curve is determi ned from a torsion test that is very interesting since the relationshi p between the plastic strain and the sample radius is theoretically li near, Because of some difficulties in the numerical analysis such as t he choice of parameters in the rheological and the friction laws usefu l for the calculation or the choice of the analysed subsurface (the st rain gradient is very important in the sample thickness), it appears d ifficult to use the finite element method to determine a ''calibration '' curve for the hardness test. On an other hand, the results obtained from the two experimental ''calibration'' methods give similar result s. They are also close to those obtained with the simulation. However, and taking into account in particular the remark concerning the choic e of the subsurface, it becomes difficult to evaluate the precision of strain measurements. To estimate exactly this precision, it is necess ary to modify the comparative procedure by choosing for example other laboratory tests where the strain is more evenly distributed in the sa mple and so to prevent any ambiguity for the comparison with the strai ns measured by electron backscattered diffraction.