OS05W0273 AFM observation of evolution of slip deformation by high cycle fatigue in low carbon steels

抄録

AFM(Atomic Force Microscopy) provides three-dimensional digitized images of surface morphological features and thus is one of the most useful tools that analyze the mechanisms of fatigue. AFM is used to study the surface slip features in polycrystalline low carbon steels tested under the cyclic load at high cycle fatigue range. In general, excluding the case of intensely high stress concentration or high strength materials which have high sensitivity to defects, following the formation of extrusions and intrusions along persistent slip bands (PSBs) occurred at a free surface, a stage I fatigue crack initiates. For the improvement in the fatigue strength, to reveal the difference of slip features depending on the strengthening method, non-hardening steel (base), solid-solution hardening steel by each of Ni/Si/Cu (Ni-s, Si-s, Cu-s) and precipitation hardening steel by Cu (Cu-p) are submitted. The all steels have ferritic-pearlitic structures with an average grain size of about 45 μm. The fatigue specimens are plates with 1mm thick. Strain controlled fatigue tests at constant strain that yielded fatigue lives of 10^5-10^6 cycle and a stress ratio R=-1 were performed on the out-plane bending machine. Interrupting the fatigue test properly, periodic AFM observations carried out on original specimens. AFM discriminated difference of slip features. On base steel slip bands were observed early at 10^3 cycle (N/N_f=0.5%) and distinct extrusions and intrusions grew tall linearly. Then the growth was saturated at 10^4 range, but the crack had not been generated. Much more damage was required for the crack initiation. The growth rate was about 0.02 nm/cycle and the highest extrusion was about 600 nm high. The crack initiation did not occur only at grown-up intrusions but also at root of extrusions. On Ni-s steel a distinct intrusion didn't occur, but whole roughness increased. On Si-s steel some low and planar stairs-like slips generated, because of prevention of cross slips by Si. On Cu-s steel rare and sharp extrusions occurred. On Cu-p steel extrusions ion was late at 10^4 range.