Alloying effects on the oxidation behaviour of shot-peened Co-Ni base Superalloys

The effect of shot-peening on the oxidation mechanism in Co-Ni alloys with different Co-Ni ratios and varying Cr and Al contents were investigated at 850 °C for 200 h. A characterization method using isotopic oxygen tracing combined with focused ion-beam secondary ion mass spectrometry (FIB-SIMS) was performed to study the oxidation mechanism in both conditions. Multi-layered oxides in both peened and un-peened conditions consisted of chromia-rich scale on the outermost surface and a protective alumina-rich layer as the internal scale. Internal oxidation to different depths occurred within the alloys with the formation of alumina particles within a γ\u27-depleted zone. The morphology of the sub-surface oxides differed between the two surface conditions examined. Oxidation performance was quantified by measuring cross-sectional damage depths of the outer scale (chromia) and the internal oxidation depth (oxide fingers and y’-depleted depth). Oxidizing environments can promote recrystallization in the near-surface of the shot-peened conditions. The sub-surface oxide penetration for the shot-peened condition occurs to a more uniform depth and is associated with the grain boundaries of small recrystallized grains. This work forms part of an ongoing investigation to determine the effects of shot-peening in this alloy system with the oxidation performance to equivalent polished material at 850 °C

Transport mechanisms during the high-temperature oxidation of ternary γ/γ′ Co-base model alloys

Over a decade ago, γ′-strengthened Co-base alloys were introduced as potential replacement for conventional Ni-base Superalloys. Insufficient resistance against high-temperature oxidation restricts the number of possible applications. The present study contributes to the understanding of elementary mechanisms such as material transport during extensive oxide scale formation on γ/γ′ Co-base alloys to explain their inferior oxidation behaviour. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections. By means of electron backscattered diffraction (EBSD), the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16O2- and 18O2-containing atmospheres proved counter-current material transport. The combination of focused ion beam (FIB) and secondary ion mass spectroscopy (SIMS) visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale. An elaborate review of available transport paths for oxygen is given during the discussion of results. All experimental findings were combined to a coherent explanation of the inferior oxidation resistance of this relatively new class of high-temperature materials at temperatures above 800 °C

The strain fields present during the bending of ultra-high strength steels

Ultra high strength steels (UHSS) have an ultimate tensile strength of greater than 1GPa. Typically, their ambient temperature elongation is less than 10% and as a result, they are rarely used in stamping applications. However, the continuous demand for the weight reduction of structures built for the transport sector means that such materials are attractive because they can be used for parts with thinner cross-sections while maintaining required in-service performance. One way to overcome the ambient temperature ductility of these materials is to roll-form them, particularly with emerging flexible roll forming technology. Using numerically-controlled actuators, the rolls on each stand are designed with sufficient degrees of freedom to form parts that curve, vary in depth and width along their lengths. This makes flexibly roll-formed parts attractive to the transport, particularly the automotive, sector. Roll forming deforms a material through incremental, localised bending, which is known to suppress the necking response, resulting in deformations that are higher than in stretch deformation. Recent work, such as Le Maoût, Thuillier & Manach, Eng. Frac. Mech., Vol. 76, p.1202 (2009), focussed on the development of ductile fracture models to explain failure but their validation was limited to load displacement and surface strain data. This work aims to characterise the strain field during bending more comprehensively. Using the digital image correlation technique, the macroscopic strain distribution in UHSS in the thickness of the sheet and the strain partitioning in its microstructure is measured during bending. The data provides a detailed explanation of the strain distribution during bending

Effects of oxygen-related damage on dwell-fatigue crack propagation in a P/M Ni-based superalloy : from 2D to 3D assessment

Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725 oC. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior

Scanning electrochemical cell microscopy : a versatile method for highly localised corrosion related measurements on metal surfaces

The development of tools that can probe corrosion related phenomena at the (sub)microscale is recognized to be increasingly important in order to understand the surface structural factors (grain orientation, inclusions etc.) that control the (electro)chemical stability (corrosion susceptibility, pitting, passivity etc.) of metal surfaces. Herein we consider the application of scanning electrochemical cell microscopy (SECCM), a relatively new member of the electrochemical droplet cell (EDC) family, for corrosion research and demonstrate the power of this technique for resolving structure and activity at the (sub)microscale. Hundreds of spatially-resolved (2 μm droplet size) potentiodynamic polarization experiments have been carried out on the several hours timescale and correlated to complementary structural information from electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) in order to determine the effect of grain orientation and inclusions on electrochemical processes at low carbon steel in neutral solution (10 mM KNO3). Through this approach, it has been shown unequivocally that for the low index planes, anodic currents in the passive region (an indicator of corrosion susceptibility) are greatest on (101) planes compared to (100) and (111) planes. Furthermore, individual sub-micron MnS inclusions have been probed and shown to undergo active dissolution followed by rapid repassivation. This study demonstrates the high versatility of SECCM and the considerable potential of this technique for addressing structure-activity problems in corrosion and electromaterials science

Nanoscale electrochemical visualization of grain-dependent anodic iron dissolution from low carbon steel

The properties of steels and other alloys are often tailored to suit specific applications through the manipulation of microstructure (e.g., grain structure). Such microscopic heterogeneities are also known to modulate corrosion susceptibility/resistance, but the exact dependency remains unclear, largely due to the challenge of probing and correlating local electrochemistry and structure at complex (alloy) surfaces. Herein, high-resolution scanning electrochemical cell microscopy (SECCM) is employed to perform spatially-resolved potentiodynamic polarisation measurements, which, when correlated to co-located structural information from electron backscatter diffraction (EBSD), analytical scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), reveal the relationship between anodic metal (iron) dissolution and the crystallographic orientation of low carbon steel in aqueous sulfuric acid (pH 2.3). Considering hundreds of individual measurements made on each of the low-index planes of body-centred cubic (bcc) low carbon steel, the rate of iron dissolution, and thus overall corrosion susceptibility, increases in the order (101) < (111) < (100). These results are rationalized by complementary density functional theory (DFT) calculations, where the experimental rate of iron dissolution correlates with the energy required to remove (and ionise) one iron atom at the surface of a lattice, calculated for each low index orientation. Overall, this study further demonstrates how nanometre-resolved electrochemical techniques such as SECCM can be effectively utilised to vastly improve the understanding of structure-function in corrosion science, particularly when combined with complementary, co-located structural characterisation (EBSD, STEM etc.) and computational analysis (DFT)

Effects of oxidation on fatigue crack initiation and propagation in an advanced disc alloy

Powder metallurgy Ni-based superalloys are widely used for aeroengine turbine disc application due to their exceptional strength properties at elevated temperatures, good fatigue and creep performance as well as excellent corrosion and oxidation resistance. However, oxygen enhanced fatigue crack initiation and intergranular propagation at elevated temperatures in air is commonly observed in aeroengine turbine disc superalloys under dwell fatigue testing conditions [1-7], and this phenomenon is usually ascribed to either decohesion/reduction in cohesion strength of grain boundary (GB) due to dynamic embrittlement [8, 9] or GB oxide cracking caused by stress assisted grain boundary oxidation (SAGBO) [5, 10-12]. Although the influence of oxygen on fatigue crack initiation and propagation has been intensively studied, the underlying mechanism for the oxygen-assisted fatigue failure process is still not clear due to the complex composition of disc alloy and the interaction between environmental attack and mechanical load. In this study, fatigue tests were conducted on the Low Solvus, High Refractory (LSHR) alloy designed by NASA for turbine disc application, with a particular focus on studying the influence of the formation of GB oxides on fatigue crack initiation and propagation processes

A novel experimental set-up for in-situ microstructural characterization during continuous strain path change

Strain path change is a typical phenomenon during continuous stamping operations of sheet metal for a variety of applications including automotive body parts. During stamping, a punch continuously deforms a metal sheet to produce a desired geometry while following various strain path transitions depending on overall design of the stamping process. The strain path change can potentially alter the expected forming limit of the material. Previous researchers investigated the effect of changing strain path by loading sample in two distinct steps. Typically, between the steps the sample is unloaded before being re-loaded in the new strain path. This practice reflects the key challenge in elucidating this strain path dependent deformation, which is the ability to control the strain path change in a single deformation stage in an experimental set-up. In this work, a novel testing rig and specimen geometry that is capable of changing the strain path of a sample continuously without unloading the specimen were conceptualised, modelled and subsequently manufactured. Using this apparatus, the specimen was deformed in the uniaxial strain path in the first step before being deformed biaxially without unloading in between the steps. Thus, the apparatus ensures that the sample undergoes a continuous strain path change without unloading between the steps. The size of this mechanical test rig permits it to be placed inside a scanning electron microscope (SEM) chamber in order to study strain path transition in-situ to highlight strain localization and related microstructural changes in real time. Utilizing this test set-up, strain path change and corresponding strain values along each strain path were evaluated. The changes in material microstructure were concurrently investigated using in-situ SEM and electron back scattered diffraction (EBSD) analysis