The nucleation and growth of η phase in nickel-based superalloy during long-term thermal exposure

The microstructure degradation and subsequent phase transformations in Waspaloy nickel-based superalloy during thermal exposure at 780 °C for 10,000 h were investigated. Two paths of η phase formation in the centre of extra-large γ’ (EL-γ’) following the formation of EL-γ’ were observed: (i) η phase directly precipitated within EL-γ’ when the coalescence of γ’ reached a critical stage; (ii) η phase precipitated at the interface of small size MC carbide and EL-γ’, with both MC and η embedded inside EL-γ’. The phase transformation process including the formation of EL-γ’ were experimentally observed and the formation sequences were schematically suggested. Two criteria of η formation and growth within EL-γ’ were established: (i) stacking faults formation in the nucleation site and (ii) sufficient atom diffusion during nuclei growth. The study of kinetics of η formation through two different paths revealed the critical role of small size carbides in promoting η nucleation and growth. It is concluded that η formation may be suppressed by controlling the size and density of MC carbides during materials processing

The microstructural development of oxide scales on low carbon steels

A doctoral thesis submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The hierarchy of microstructure parameters affecting the tensile ductility in centrifugally cast and forged Ti-834 alloy during high temperature exposure in air

Ductility regression is the main concern in using Ti-834 titanium alloy at temperatures above 500ºC for aerospace applications. The reduction of ductility in titanium alloys at high temperatures is strongly correlated to the exposure time. In the current study the effect of prolonged exposure at 500oC on the tensile ductility of two differently processed Ti-834 alloys was investigated. In order to simulate actual Ti-834 processing routes, forged and centrifugally cast materials were used. The tensile tests were conducted on various specimens exposed at 500ºC for 100, 200 and 500 hours to observe microstructure feature changes. Moreover, the effect of microstructure, microtexture, α-case, α2 and silicide precipitate coarsening during high temperature exposure was studied thoroughly. The cast alloy was found to have a minimum ductility and failed at 1.8% strain after exposure at 500oC/500 hour when the α-case layer was retained during testing, whilst, the ductility of the forged alloy was unaffected. The effects of individual microstructural parameters on the ductility regression in Ti-834 alloy were quantified. The results showed that 7.1% strain differences between the cast and forged alloy are related to microstructural variations including; morphology, lath widths, grain size and shape, grain orientations and microtexture. A total of 9.6 % strain loss was observed in centrifugally cast Ti-834 after aging at 500ºC/500h and quantified as follow; 3.6% due to α-case formation during high temperature exposure, 0.2% due to α2-precipitates coarsening, 4.4% due to further silicide formation and coarsening, 1.4% due to additional microstructure changes during high temperature exposure. Furthermore, silicide coarsening on α/β phase boundaries caused large void formation around the precipitates. A theoretical model supported by experimental observations for silicide precipitation in fully colony and duplex microstructures was established. The element partitioning during exposure caused Al and Ti depletion in the vicinity of the β phase in the lamellae, i.e., αs area. This resulted in lowering the strength of the alloy and facilitated the formation of Ti3(SiZr)2 precipitates. The Al depletion and nano-scale partitioning observed at the αs/β boundaries resulted in easy crack initiation and promoted propagation in the centrifugally cast colony microstructure and reduced the basal slip τcrss. Furthermore, silicides were not formed in αp (high Al, Ti and low Zr areas) in the forged duplex microstructure that promoted superior mechanical performance and ductility over the cast alloy

The Effect of a Two-Stage Heat-Treatment on the Microstructural and Mechanical Properties of a Maraging Steel

Maraging steels gain many of their beneficial properties from heat treatments which induce the precipitation of intermetallic compounds. We consider here a two-stage heat-treatment, first involving austenitisation, followed by quenching to produce martensite and then an ageing treatment at a lower temperature to precipitation harden the martensite of a maraging steel. It is shown that with a suitable choice of the initial austenitisation temperature, the steel can be heat treated to produce enhanced toughness, strength and creep resistance. A combination of small angle neutron scattering, scanning electron microscopy, electron back-scattered diffraction, and atom probe tomography were used to relate the microstructural changes to mechanical properties. It is shown that such a combination of characterisation methods is necessary to quantify this complex alloy, and relate these microstructural changes to mechanical properties. It is concluded that a higher austenitisation temperature leads to a greater volume fraction of smaller Laves phase precipitates formed during ageing, which increase the strength and creep resistance but reduces toughness

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic areas. Here, we propose a concept of ‘Crack Propagation Unit (CPU)’ for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/γ′ channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters

The dislocation behaviour and GND development in a nickel based superalloy during creep

In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) were investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculation were used, namely, EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above the yield stress of Waspaloy) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions on at least two octahedral (111) slip systems for easy dislocation movement. This allowed dislocations to reach the grain boundary without significant geometrically necessary dislocation accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700 °C), the GND accumulation was relatively low due to the insufficient macroscopic stress level. However, the regions near grain boundaries showed high GND density. At 800 MPa/700 °C (above yield at this temperature), in addition to the movement of pre-existing dislocations (SSD and GND) at a higher mobility rate, large numbers of dislocations were generated and moved toward the grain boundaries. This resulted in a much higher GND density but narrower width of high intensity GNDs near the grain boundaries. It is concluded that although GND measurement by means of EBSD can provide great insight into dislocation accumulation and its behaviour, it is critical to consider SSD type which also contributes to the strain hardening of the material

The Effect of a Two-Stage Heat-Treatment on the Microstructural and Mechanical Properties of a Maraging Steel.

Maraging steels gain many of their beneficial properties from heat treatments which induce the precipitation of intermetallic compounds. We consider here a two-stage heat-treatment, first involving austenitisation, followed by quenching to produce martensite and then an ageing treatment at a lower temperature to precipitation harden the martensite of a maraging steel. It is shown that with a suitable choice of the initial austenitisation temperature, the steel can be heat treated to produce enhanced toughness, strength and creep resistance. A combination of small angle neutron scattering, scanning electron microscopy, electron back-scattered diffraction, and atom probe tomography were used to relate the microstructural changes to mechanical properties. It is shown that such a combination of characterisation methods is necessary to quantify this complex alloy, and relate these microstructural changes to mechanical properties. It is concluded that a higher austenitisation temperature leads to a greater volume fraction of smaller Laves phase precipitates formed during ageing, which increase the strength and creep resistance but reduces toughness.The current research was funded by the EPSRC Rolls-Royce Strategic Partnership in Structural Metallic Systems for Gas Turbines (grants EP/H500383/1 and EP/H022309/1). The provision of materials and technical support from Rolls-Royce plc is gratefully acknowledged

Crystallographic Orientation Relationship with Geometrically Necessary Dislocation Accumulation During High-Temperature Deformation in RR1000 Nickel-Based Superalloy

In the current study, it is demonstrated that soft grains along 〈100〉 fiber provided a pure shear condition for easy dislocation movement leading to a relatively low dislocation density. The hard grains along the 〈111〉 fiber, however, were not favorably oriented for slip system activation and caused high dislocation accumulation. It is concluded that the average overall dislocation density does not provide a meaningful value, as it is largely dependent on the original material crystallographic texture, the numbers of hard and soft grains in the electron backscatter diffraction (EBSD) mapped area, and the grain size factor

Microstructural mechanisms and advanced characterization of long and small fatigue crack growth in cast A356-T61 aluminum alloys

Fatigue crack growth-based design is a significant modern engineering consideration for the transportation sector, and its implementation requires accurate characterization and understanding of crack propagation mechanisms with respect to microstructure. To support this goal, long and small fatigue crack growth studies were conducted on widely used A356-T6 cast aluminum alloys in various microstructural conditions. Microstructural variations were created through processing and chemistry means in order to systematically investigate the individual and combined effects of the materials’ characteristic microstructural features on fatigue crack growth at all growth stages. Crack growth mechanisms and failure mode transitions are identified with respect to the eutectic Si morphology/distribution and grain structure by fractographic techniques and electron backscatter diffraction. Crack-microstructure interactions were investigated in depth across all crack sizes, and the respective roles of microstructural features were identified experimentally and further corroborated by numerical models. It is concluded that the eutectic Si phase enhances the alloys’ fatigue crack growth resistance in early growth stages (by transferring stresses off of the α-Al matrix), and progressively decreases due to damage localization. In later growth stages, the eutectic Si phase becomes increasingly detrimental to fatigue crack growth resistance because of its inherently low debonding strength and brittle fracture, as evidenced by the crack selectively following eutectic Si colonies

Process-Structure-Property Relationships in Metals

In this Special Issue of Metals, an open access forum is provided for publishing original papers that the covers direct and effective correlations between a wide range of thermomechanical processing routes and generated microstructure, hence, the final physical and mechanical properties of the materials. The following aspects of the science and engineering of various metals and alloys are covered in this book: • Original research studies that relate to the understanding of the properties obtained following specific processing/heat treatment route (Experimental, theoretical, and simulation modeling). • Understanding the mechanisms involved in microstructure evolution and phase transformation during processing of materials, specifically as they relate to the understanding of final mechanical properties. • Nano/micro/macro structure characterization and chemistry of metals/alloys used in automotive, power generation, nuclear, aerospace, and medical applications. • Micro/macro texture devolvement during thermo-mechanical processing of metals/alloys