FORMATION OF MESOSTRUCTURES AT THE SURFACE OF FERRITIC STEEL AND A NICKEL MONOCRYSTAL UNDER INCREASING LOAD - AN IN-SITU AFM EXPERIMENT

The surface structure development of loaded ferritic polycrystalline s teel and a monocrystalline nickel sample was studied. In situ investig ations were carried out using a tensile testing module designed for at omic force microscopes (AFM). Prior to investigation the unloaded stee l and nickel samples were polished mechanically and electrolytically, respectively. The grain size of the polycrystalline steel was measured to be about 3 mu m. At each level of deformation up to fracture (from epsilon = 0 up to epsilon approximate to 10% for steel and epsilon ap proximate to 45% for nickel) a fixed area of about 80 x 80 mu m(2) was examined by AFM in contact mode. On loading the polycrystalline sampl e the surface roughness increases and visible mesoscopic structures of about 25 mu m in diameter appear. The fractal dimension df of the sur face topography was analyzed and it was found that df starts to increa se at the yield strength and finally ends in a plateau. Investigation of the nickel monocrystal showed the formation of well-characterized s lip bands and non-regular substructures beyond a short elastic region. The development of the fractal dimension is similar to that of the po lycrystalline steel sample. The results obtained are discussed in the framework of mesomechanics.