Effect Of Residual Stress On The Hardness Of Metallic Glasses Using Finite Element Simulation Of Indentation Experiment

The mechanical reliability of bulk materials and coatings is strongly affected by their residual stress, mostly introduced by mechanical and thermal processing. Thus, in order to achieve a reliable engineer design, the level and sign of residual stress in materials must be determined.Since 80^th last century, micro/nanoindentation technology becomes an effective method to investigate the mechanical properties of materials, especially on the relationship between hardness and residual stress in materials. It is found that the hardness of materials is independent of residual stress, when the significant pile-up is found around the surface of indentation. For most engineering materials, the pile-up around the indentation is so common that the application of using the measurement of indentation to determine the residual stress is limited.Recently, a new material named bulk metallic glasses (BMGs) has been developed. Metallic glasses perform better in the mechanical, chemical and physical fields, because of its special microstructures. The application of metallic glasses is very wide, including mechanical engineering, communication, aerospace, automobile, chemical industry, sports equipment and even military industry.In this paper, we have performed the indentation experiment on the specimen of bulk metallic glasses subjected to pure bending and find that the residual stress affects the hardness of metallic glasses strongly. This result shows the possibility of using the measurement of indentation to determine the residual stress on new type high strength materials such as bulk metallic glasses.We have used the finite element software to simulate the axisymmetric indentation experiment and compare the relationship between hardness and residual stress of metallic glasses with that of aluminum alloy 8009 studied earlier. It is found that if the depth of indenter is same, the span of maximal load with changing residual stress is larger, when the indentation is made on metallic glasses. It is found that because of the high yielding strength of metallic glasses, pile-up forms with difficulty and the contact area changing with residual stress on the metallic glasses is much smaller than alloy 8009. For the materials with low E/σ_y such as metallic glasses, the hardness is sensitive to the residual stress. However, for most traditional engineering materials with high E/σ_y such as alloy 8009, the hardness is almost independent of the residual stress.We have made the 3D model to simulate the indentation experiment with uniaxial stress with gradient, which is studied rarely before. The relationship between hardness and residual stress of metallic glasses obtained from 3D model fits the experimental observation very well. By analyzing the indentation from 3D simulation, we can also explain the unsymmetry of indention which is observed in experiment clearly.