Ru Lin Peng

Residual Stresses in a Nickel-based Superalloy Introduced by Turning

Near-surface residual stress distributions in the nickel-based superalloy Inconel 718 that originate from the machining operation turning are studied. The turning process that is used in the experiments is face grooving which gives quasi-orthogonal cutting conditions. Cutting speed and feed have been varied to investigate their effects on the residual stress state. Tensile residual stresses with a maximum of 1300 MPa were found at the surface that turn rapidly into compressive residual stresses of up to -800 MPa. The depth distributions of the residual stresses are presented and discussed with respect to observations made by optical and transmission electron microscopy.

Identification of Subsurface Deformation in Machining of Inconel 718

There is considerable industrial significance to understand the nature of subsurface deformation under the machined surface for correct prediction of surface properties in machined components based upon the machined conditions and material behaviors that give rise to them. In this study, high speed machining of Inconel 718 was carried with whisker reinforced ceramic cutting tool under different conditions of tool wear, coolant state and cutting parameters. The objective of the present investigation was to determine the effect of both cutting parameters and tool wear on the plastic deformation in the subsurface region of Inconel 718 after the finishing machining with above process conditions. The surface and subsurface region of machined specimens were examined using a high resolution scan electron microscope (HRSEM) and EBSD technique, microhardness measurements were also conducted on the test samples, accordingly plastic strain analysis were carried out.

Stresses and Cracking During Chromia-Spinel- NiO Cluster Formation in TBC Systems

Thermal barrier coatings (TBC) are used in gas turbines to reduce the temperatures in the underlying substrate. There are several mechanisms that may cause the TBC to fail; one of them is cracking in the coating interface due to extensive oxidation. In the present study, the role of so called chromia-spinel-NiO (CSN) clusters in TBC failure was studied. Such clusters have previously been found to be prone to cracking. Finite element modeling was performed on a CSN cluster to find out at which stage of its formation it cracks and what the driving mechanisms of cracking are. The geometry of a cluster was obtained from micrographs and modeled as close as possible. Nanoindentation was performed on the cluster to get the correct Young’s moduli. The volumetric expansion associated with the formation of NiO was also included. It was found that the cracking of the CSN clusters is likely to occur during its last stage of formation as the last Ni-rich core oxidizes. Furthermore, it was shown that the volumetric expansion associated with the oxidation only plays a minor role and that the main reason for cracking is the high coefficient of thermal expansion of NiO.

In Situ X-Ray Diffraction Study of Load Partitioning and Microyielding for the Super Duplex Stainless Steel SAF2507 (UNS S32750)

The deformation behaviour of the super duplex stainless steel SAF2507 (UNS S32750) under successive uniaxial tensile loading-unloading was investigated with respect to load sharing and inter-phase interactions. The steel consists of 58% austenite and 42% ferrite in volume. By insitu X-ray diffraction experiment the evolution of phase-specific stresses with applied load was monitored for three successive loading-unloading cycles with the maximum total strains being 0.34%, 0.75% and 1.63%, respectively. It was found that yielding occurred earlier in the austenitic phase than in the ferritic phase during the first loading cycle. In the followed loading cycles, both phases yielded under larger but similar applied stresses. Due to a similar behavior of the phases in the elasto-plastic regime inter-phase interactions were relatively weak. Low microstresses induced by the plastic straining resulted in somewhat larger stresses in the ferritic phase.

In Situ EBSD During Tensile Test of Aluminum AA3003 Sheet

Miniature tensile-test specimens of soft-annealed, weakly textured AA3003 aluminum sheet in 0.9 mm thickness were deformed until fracture inside a scanning electron microscope. Tensile strength measured by the miniature tensile test stage agreed well with the tensile strength by regular tensile testing. Strain over the microscope field of view was determined from changes in positions of constituent particles. Slip lines were visible in secondary electron images already at 0.3% strain; activity from secondary slip systems became apparent at 2% strain. Orientation rotation behavior of the tensile load axis with respect to the crystallographic axes agreed well with previously reported trends for other aluminum alloys. Start of the fracture and tensile crack propagation were documented in secondary electron images. The region of fracture nucleation included and was surrounded by many grains that possessed high Schmid factors at zero strain. Crystal lattice rotation angles in the grains surrounding the initial fracture zone were higher than average while rotations inside the initial fracture zone were lower than average for strains from zero to 31%. The orientation rotation behavior of the tensile load axes of the grains around the fracture zone deviated from the average behavior in this material.

Influence of Dry Cut and Tool Wear on Residual Stresses in High Speed Machining of Nickel-Based Superalloy

Machining induced residual stresses were investigated in Inconel 718 prepared by high speed turning under dry cut condition. The influence of cutting tool wear and the use of cutting fluid were studied. By x-ray diffraction measurements, characteristic residual stress distributions with tensile stresses in the top layer and compressive stresses in the layer below were found in all the investigated samples. The magnitude of surface tensile stresses and size of the tension as well as compression zones varied depending on the cutting condition. The application of cutting fluid for turning using new tool has a minor effect, giving a somewhat larger subsurface compressive zone but reducing the surface tensile stresses. Tool flank wear has shown a much stronger effect. While a flank wear of VBmax=0.15 mm enhanced mostly the surface tensile residual stresses, a severer wear of VBmax=0.3 mm greatly increased the thickness of the subsurface compression zone and at the same time resulted in strong stress anisotropy. Microstructural study by electron channelling contrast imaging shows that the observed influence of tool flank wear or cutting fluid on residual stresses are related to different contributions from increased plastic deformation and cutting heat, which changed with the cutting conditions.

EBSD investigation of the effect of the solidification rate on the nucleation behavior of eutectic components in a hypoeutectic Al-Si-Cu alloy

This article represents a study of the influence of the solidification rate on the crystallographic orientation of eutectic components with respect to the primary α-Al in the tested hypoeutectic alloy. Electron backscattering diffraction (EBSD) patterns were produced from the Al-Si cast specimens that were solidified with different cooling rates and prepared via ion etch polishing as a complementary method after mechanical polishing. The results indicated a strong orientation relationship between the primary α-Al and eutectic Al phase at all cooling rates. It was also found that the silicon eutectic flakes were heterogeneously nucleated in the interdendritic eutectic liquid. The increase of the cooling rate from 2 to 80 mm/min was found to be effective in lowering the intensity of the relationship between the primary α-Al and eutectic Al phases, and changing the misorientation angle clustering between the primary α-Al and eutectic Si phases in the interval from 41–60° to lower angle intervals.

Al2O3 nanoparticle reinforced Fe-based alloys synthesized by thermite reaction

It is very difficult to manufacture oxide nanoparticle strengthened alloys through the conventional casting in the gravity field or even in the space microgravity environment. A thermite reaction process was used to produce molten metal that was then solidified in a graphite mold in super gravity field caused by centrifugal force; we were able to obtain Al2O3 nanoparticle reinforced Fe-based alloys. The formation of Al2O3 nanoparticles was related to the addition of TiO2 xerogel to the thermite mixture, and their uniform distribution in the alloy can be explained by their assembly in (Ni, Fe)Al intermetallics during solidification owing to the low interfacial energy between them.

In-situ Neutron Diffraction Study of the Deformation Behaviour of two High-Manganese Austenitic Steels

In-situ neutron diffraction experiments under tensile loading were carried out to study the micromechanical behaviour of two iron-manganese based steels, a TWIP (twinning induced plasticity) steel with 30 wt% Mn and a TRIP steel (transformation induced plasticity) with 20 wt% Mn. The former was loaded to 31.3% strain and the latter to 20% strain. The 30 wt.% Mn steel had a fully austenitic microstructure which remained stable over the loading range studied, while stress induced austenite to α´- and ε-martensite transformations occur in the 20 wt.% Mn steel which initially contained an α´-martensite in addition to the austenite. The evolution of lattice strains under tensile loading differs between the two steels, reflected their different plastic deformation mechanisms. A stronger grain-orientation dependent behaviour is observed during deformation for the 20 wt.% Mn in contrast to the 30wt.% Mn steel.

On the Development of Grain-Orientation-Dependent and Inter-Phase Stresses in a Super Duplex Stainless Steel under Uniaxial Loading

Microstresses due to intergranular and inter-phase interactions in an austenitic-ferritic super duplex steel (SAF 2507) under uniaxial compressive deformation have been studied by in-situ neutron diffraction experiments. Lattice strains of several hkl planes of austenite respective ferrite were mapped as a function of sample direction at a number of load levels during loading into the plastic regime and unloading. The analysis of the experimental results has shown that during loading both grain-orientation-dependent and inter-phase stresses were generated under plastic deformation that was inhomogeneous at the microstructural level. Residual stresses depending on the grain-orientation and phase have been found after unloading. The results also indicate stronger intergranular interactions among the studied hkl planes of austenite than those of ferrite.