Determination of residual stresses and mechanical properties using neutron, X-ray diffraction, micro- and nanoindentation techniques

Sammanfattning: The existence of residual stresses in engineering materials can significantly affect subsequent lifetime by augmenting or impeding failure. Consequently, for an accurate assessment of engineering lifetimes, there is a need to quantify residual stresses. Furthermore, knowledge of the origin of these stresses in conjunction with mechanical properties such as hardness and fracture toughness, among others, can be used to improve functionality by tailoring the microstructure through processing. In this work, neutron, x-ray diffraction, micro- and nanoindentation techniques were used for residual stress determination and mechanical characterization of WC-Co functionally graded composites, a Co-based Haynes® 25 alloy weld, compressed steel and compacted Fe-brass powders. The neutron and x-ray diffraction techniques were used to assess residual strains and stresses while the instrurnented indentation techniques were used to determine hardness, fracture toughness and elastic modulus. In each of these engineering materials, valuable insight relating to the overall mechanical performance was obtained.X-ray diffraction was used to determine thermal residual stresses that develop in a functionally graded WC-Co composite, commonly used as tool bits. Microstresses in the graded zone were attributed to the thermal mismatch between WC and the Co phase. The compressive macrostresses were determined to be a result of the compositional gradient. Micro- and nanoindentation experiments were used to determine hardness as a function of depth in two WC-Co functionally graded materials (FGMs). A relationship between hardness and Co phase content was established and explained for the two graded and five homogeneous samples.An experimental and simulation study of residual stresses was made in the vicinity of a gas tungsten arc weld in a Co-based Haynes® 25 alloy used in a satellite component. The experimental measurements were made by neutron diffraction on the recently commissioned Spectrometer for Materials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory, USA and the simulation used the implicit Marc finite element code. Comparison between experiment and theory showed general agreement.Strain pole figures representative of residual intergranular strains were determined from an -2.98 % uniaxially compressed austenitic stainless steel sample. The neutron diffraction measurements were made on SMARTS, using an Euler cradle to obtain spectra over a range of sample orientations. The measurements were compared with predictions from an elasto-plastic self-consistent model and found to be in reasonable agreement. The model was also used to assess the sensitivity of the strain distribution in the deformed sample to the initial texture.Neutron diffraction was used to measure residual stresses in a powder metallurgical green body manufactured by high speed compaction from Fe and 15 wt.% brass powders. The tests were performed on SMARTS with the aid of radial collimators configured to measure spatially resolved strains in the axial and radial directions in a cylindrical specimen. Furthermore, sharp (Berkovich) and spherical (Hertzian) indenters were used for instrumented indentation experiments to determine the hardness and elastic modulus.