Time-dependent characteristics of incipient plasticity in nanoindentation of a Ni3Al single crystal

Effect of the Spherical Indenter Tip Assumption on the Initial Plastic Yield Stress

“…In this case, the deformation is purely elastic prior to the first pop-in; if the indenter tip is unloaded before the first pop-in, atomic force microscopy (AFM) images show no indent on the specimen surface, whereas, if unloading occurs after the pop-in, a residual indent is observed (Chiu & Ngan, 2002;Schuh & Lund, 2004).…”

“…When a pop-in event is caused by the sudden onset of crystal plasticity, whether through dislocation source activation (Bradby & Williams, 2004;Schuh et al, 2005) or homogeneous dislocation nucleation (Bahr et al, 1998;Chiu & Ngan, 2002), the maximum stress at the yield event is generally interpreted as the maximum shear stress in the body (Minor et al, 2006). This maximum shear stress, MAX, at the first pop-in load, PCRIT, is generally estimated from elastic contact theory (Johnson, 1999) as…”

“…The most common view is that the manufacturing process produces an approximately spherical cap on the apex of the indenter tip. The radius of the assumed spherical tip is generally obtained from the tip manufacturer, AFM, scanning electron microscopy (SEM), or by a Hertzian fit to the elastic load-displacement data (Chiu & Ngan, 2002;Constantinides et al, 2007;Gerberich et al, 1996;Gouldstone et al, 2000).…”

Time-dependent incipient plasticity in Ni3Al as observed in nanoindentation

“…The time-dependent characteristics of incipient plasticity in Ni 3 Al during nanoindentation in the subcritical load regime were investigated statistically. The waiting time for incipient plasticity to occur at constant load was found to follow a Poisson-like distribution, with the peak shifting toward zero holding time as the load increased and eventually becoming an exponential distribution when the load was close to a critical value.…”

Size Dependence of Incipient Dislocation Plasticity inNi3Al

“…Incipient plasticity is also easily observable in nanoindentation on annealed, bulk crystalline materials as a load drop or displacement jump [4], and recent experiments show that the statistical distributions of the strain-burst loads in SiC [5] and Ni 3 Al [6] obey characteristics of homogeneous nucleation at room temperature. Nanoindentation experiments on annealed Fe-3%Si [4], Ni 3 Al [6,7], and Al [8] have also shown that, even if the applied stress is initially in the elastic range, yielding may occur rather suddenly after a certain waiting time, implying that these materials cannot indefinitely sustain GPa-level stresses in the elastic range [9].…”