The evolution of dislocation structures in individual bulk grains in copper during strain path changes was studied with a new in situ synchrotron technique which combines high angular resolution with fast three-dimensional reciprocal space mapping. Deformed copper contains regions with vanishing dislocation density called sub-grains bounded by dislocation rich walls. With the new technique reciprocal space maps, consisting of sharp peaks arising from the sub-grains superimposed on a cloud of lower intensity arising from the dislocation walls, were obtained, which allows properties such as sub-grain volume fraction to be quantified. The studied strain path changes were tension-tension sequences. Polycrystalline copper sheets were pre-deformed in tension to 5% strain, and tensile samples were cut with varying angles between the first and second loading axis. The second tensile deformation up to additional 5% strain was performed in situ while mapping a selected X-ray reflection from one particular bulk grain with high angular resolution. The reciprocal space maps were analyzed with a recently developed fitting method, and a correlation was found between the evolution of the sub-grain volume fraction and the degree of strain path change the sample was subjected to.The Effect of Strain Path Change on Subgrain Volume Fraction Determined from in situ X-ray Measurements. C.Wejdemann, H.F.Poulsen, U.Lienert, W.Pantleon: IOP Conference Series - Materials Science and Engineering, 2009, 3[1], 012003