Results Of Nanoindentation Experiments Research Articles

Role of competition between slip and twinning in nanoscale deformation of sapphire

Insights into the initiation of plasticity under contact loading of $A$ and $M$ planes of sapphire are provided by finite element analysis of the resolved shear stress acting on basal slip and basal twinning systems and by the evaluation of acoustic emission signals associated with the onset of plasticity. The analysis of acoustic emission activity utilizes wavelet-based signal representation in the joint time-frequency domain. The proposed model invoking loading-rate-dependent competition between basal slip and basal twinning predicts different yield-point mechanisms for $A$ and $M$ planes. The predicted mechanisms are consistent with the experimental plasticity initiation patterns. Room-temperature values of critical shear stress of 12.9--13.9 GPa for basal slip and 12.6--14.4 GPa for basal twinning derived from application of finite element analysis to the results of nanoindentation experiments are compared with the values expected from the results of theoretical modeling and extrapolation of high-temperature experimental results.

Study on Brittle-Ductile Transition Mechanism of Ultra-Precision Turning of Single Crystal Silicon

According to the size effect theory established on the concept of geometrically necessary dislocations and results of nano-indentation experiments, a novel brittle-ductile mechanism of ultra-precision turning of single crystal silicon is proposed. The accurate critical chip thickness is firstly calculated on the basis of theoritical analysis. A macro-micro cutting model is created based on the brittle-ductile transition mechanism. Finally, the results of study are testified through experiments.

Shape Memory at Nanometer Scales

ABSTRACTNickel-titanium (NiTi) shape memory alloys undergo relatively large recoverable inelastic deformations via a stress-induced martensitic phase transformation. The nanoindentation experimental results presented in this study are the first to show evidence of discrete forward and reverse stress-induced thermoelastic martensitic transformations in nanometer scaled volumes of material. Shape recovery due to indentation, followed by subsequent heating, is demonstrated for indents depths in the sub 10 nm range via atomic force microscopy. It is also shown that the local material structure can be utilized to modify transformation behavior at nanometer scales, yielding insight into the nature of stress-induced martensitic phase transformations at small scales and providing opportunity for the design of nanometer sized NiTi actuators.

Yield Point Phenomena During Nanoidentation

The yield point phenomenon observed in nanoindentation of metallic materials has been ascribed to the nucleation of dislocations or multiplication and the film fracture on the surface of the metals. The current study has been undertaken to examine the properties of passivating films on Type 316 austenitic stainless steel. A complementary set of ex situ and in situ experiments showed the presence of deformation prior to a discontinuity in loading with both load-depth sensing techniques and imaging the surface topography. The results of nanoindentation experiments showed that the yield point is dependent on the presence of surface passive film. If the materials, which exhibit permanent deformation prior to a discontinuity in loading, and after the surface films are removed by nanowear and depassivated, Type 316 stainless steel shows no sign of yield point effect; the occurrence of excursions is shown in this material to be dependent on surface passive film. It is more likely that the phenomenon is indeed controlled by film fracture and not the rapid generation of dislocation.