<i style="">In-Situ</i> Monitoring of Phase Transition and Microstructure Evolution in Ni-Based Superalloys by Electrical Resistivity:Direct Comparison With Differential Scanning Calorimetry and Application to Case Studies

In this study, resistivity measurements are made during continuous heating and cooling on four different Ni-based superalloys of different grain structures and with different phases (i.e., γ′ and carbide). The results are directly compared with differential scanning calorimetry (DSC) profiles to identify the material’s resistivity response. The resistivity measurements have been performed using an electro-thermal mechanical testing (ETMT) system having a capability of heating and cooling a sample at a rate of up to 100 K/s by Joule heating, which is not possible with standard heating methods used in previous in-situ microstructure analysis approaches. By comparing different precipitate variations and thermal histories, γ′ volume fraction and precipitate number density are found to be the most important factors determining the resistivity of the materials. In-situ resistivity measurement was applied to several case studies to show that it can provide microstructural information in complex high temperature experiments.<br/

<i style="">In-Situ</i> Monitoring of Phase Transition and Microstructure Evolution in Ni-Based Superalloys by Electrical Resistivity:Direct Comparison With Differential Scanning Calorimetry and Application to Case Studies

In this study, resistivity measurements are made during continuous heating and cooling on four different Ni-based superalloys of different grain structures and with different phases (i.e., γ′ and carbide). The results are directly compared with differential scanning calorimetry (DSC) profiles to identify the material’s resistivity response. The resistivity measurements have been performed using an electro-thermal mechanical testing (ETMT) system having a capability of heating and cooling a sample at a rate of up to 100 K/s by Joule heating, which is not possible with standard heating methods used in previous in-situ microstructure analysis approaches. By comparing different precipitate variations and thermal histories, γ′ volume fraction and precipitate number density are found to be the most important factors determining the resistivity of the materials. In-situ resistivity measurement was applied to several case studies to show that it can provide microstructural information in complex high temperature experiments.<br/

Overheating of Waspaloy:Effect of cooling rate on flow stress behavior

During service of a gas turbine engine, components may suffer instant overheating which is a concern to safe operations. Effect of short overheating on the tensile properties of a Ni-based superalloy Waspaloy has been studied due to its significant importance for practical applications. The results have shown that a combination of near supersolvus overheating at 1000 °C with very rapid cooling at a rate of 50 K/s is most detrimental case to the tensile properties of the material. This is attributed to the absence of γ' and carbide re-precipitation and growth during cooling period. Microstructure change during overheating and cooling has been deduced using in-situ resistivity measurements. This work provides evidence that multilateral measurements including resistivity can shed light on the failure controlling parameters

Overheating of Waspaloy:Effect of cooling rate on flow stress behavior

During service of a gas turbine engine, components may suffer instant overheating which is a concern to safe operations. Effect of short overheating on the tensile properties of a Ni-based superalloy Waspaloy has been studied due to its significant importance for practical applications. The results have shown that a combination of near supersolvus overheating at 1000 °C with very rapid cooling at a rate of 50 K/s is most detrimental case to the tensile properties of the material. This is attributed to the absence of γ' and carbide re-precipitation and growth during cooling period. Microstructure change during overheating and cooling has been deduced using in-situ resistivity measurements. This work provides evidence that multilateral measurements including resistivity can shed light on the failure controlling parameters

A Comparative Study of High Temperature Tensile and Creep Testing Between Standard and Miniature Specimens:Applicability and Limits

This study concerns the quasi-static and time-dependent mechanical behavior obtained via the miniaturized electro-thermal mechanical testing (ETMT) approach for single crystal (SX) and conventional cast Mar-M-247 superalloy. The experimental outcome was benchmarked against standardized testing procedures. It is found that tensile yielding behavior can be captured accurately by the ETMT approach up to 1100 ºC, provided the appropriate type of thermocouple (T/C) is chosen. Furthermore, creep rupture behavior is underestimated by the miniaturized set-up. High repeatability of the rupture time was obtained for the SX case, whereas a significant scatter was observed for the conventional cast case. The discrepancies are assessed in detail; discussion centers around analytical and practical considerations, such as temperature uncertainty due to parasitic voltage and the choice of T/C, microstructural change as a result of the Joule heating, representative gauge volume, and strain rate non-linearity. Consequently, the applicability and limits of the miniaturized approach are examined critically, and improvements were suggested where appropriate

On the size and orientation effect in additive manufactured Ti-6Al-4V

In this work, the influence of the specimen size and orientation on the strength and ductility of additive manufactured Ti-6Al-4V is analysed and rationalised in a complete framework. First, the mechanical properties are addressed – as a function of surface-type, orientation, and size. Our results show systematic strengthening and a drastic drop in ductility as the material section is reduced. These changes are rationalised and linked to changes in microstructure, chemistry, and surface quality. Strengthening is due to combined changes in α-lath thickness, oxygen enrichment, and prior-β grain size. The microstructural changes are due to the differences in the ratio between contouring and hatching laser strategies as the sample size decreases. Changes in the chemical composition is due to an increase in the surface-to-volume ratio as the sample size decreases. Lost of ductility is due to poor surface quality – smaller sections show rougher surfaces – and grain orientations. Finally, these findings are used to develop a set of conceptual design maps of strength and ductility as a function of the component type, thickness, and orientation

Indentation Plastometry for Study of Anisotropy and Inhomogeneity in Maraging Steel Produced by Laser Powder Bed Fusion

This work concerns the use of profilometry-based indentation plastometry (PIP) to obtain mechanical property information for maraging steel samples produced via an additive manufacturing route (laser powder bed fusion). Bars are produced in both “horizontal” (all material close to the build plate) and “vertical” (progressively increasing distance from the build plate) configurations. Samples are mechanically tested in both as-built and age-hardened conditions. Stress–strain curves from uniaxial testing (tensile and compressive) are compared with those from PIP testing. Tensile test data suggest significant anisotropy, with the horizontal direction harder than the vertical direction. However, systematic compressive tests, allowing curves to be obtained for both build and transverse directions in various locations, indicate that there is no anisotropy anywhere in these materials. This is consistent with electron backscattered diffraction results, indicating that there is no significant texture in these materials. It is also consistent with the outcomes of PIP testing, which can detect anisotropy with high sensitivity. Furthermore, both PIP testing and compression testing results indicate that the changing growth conditions at different distances from the build plate can lead to strength variations. It seems likely that what has previously been interpreted as anisotropy in the tensile response is in fact due to inhomogeneity of this type

Grain Boundary Serration in Nickel Alloy Inconel 600: Quantification and Mechanisms

The serration of grain boundaries in Inconel 600 caused by heat treatment is studied systematically. A new method based on Fourier transforms is used to analyse the multiple wave-like character of the serrated grain boundaries. A new metric -- the serration index -- is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and non-serrated boundaries. By considering the variation of the serration index with processing parameters, a causal relationship between degree of serration and solution treatment/cooling rate is elucidated. Processing maps for the degree of serration are presented. Two distinct formation mechanisms arise which rely upon grain boundary interaction with carbides: (i) Zener-type dragging which hinders grain boundary migration and (ii) a faceted carbide growth-induced serration

Alloy Design for Additive Manufacturing:Early-Stage Oxidation of Nickel-Based Superalloys

This body of work aims to inform alloy design for additive manufacturing by investigating the early-stage oxidation behavior of Ni-based superalloys processed by laser-powder bed fusion. The oxidation of 14 Ni-based superalloys—some novel and some heritage—at 1000 °C for 24 hours is studied through thermo-gravimetric analysis. The mass gain, oxide layer thickness, oxide scale composition, and depletion γ' zone size are measured. The influence of the alloy composition on these variables is assessed in order to elucidate how increasingly processable and oxidation resistant alloys can be developed. The alloy compositions with Al content greater than 9 at. pct form continuous Al2O3 scales at 1000 °C and display markedly lower parabolic rate constants, mass gain, oxide layer thickness, and γ' depletion zone size. The alloys of lesser Al content have reduced oxidation resistance and formed oxide scales of predominantly Cr2O3. Alloys with Ti content of 2.7 at. pct and greater formed Ti-rich oxide phases in their oxide scales as well as TiN subscale. A trade-off between alloy processability and oxidation resistance is identified, dictated by the deleterious effect of Al content on the ductility dip and the benefit of Al for oxidation resistance. A property space along the pareto front is highlighted which is ideal for having oxidation resistance and processability

Alloy Design for Additive Manufacturing:Early-Stage Oxidation of Nickel-Based Superalloys

This body of work aims to inform alloy design for additive manufacturing by investigating the early-stage oxidation behavior of Ni-based superalloys processed by laser-powder bed fusion. The oxidation of 14 Ni-based superalloys—some novel and some heritage—at 1000 °C for 24 hours is studied through thermo-gravimetric analysis. The mass gain, oxide layer thickness, oxide scale composition, and depletion γ' zone size are measured. The influence of the alloy composition on these variables is assessed in order to elucidate how increasingly processable and oxidation resistant alloys can be developed. The alloy compositions with Al content greater than 9 at. pct form continuous Al2O3 scales at 1000 °C and display markedly lower parabolic rate constants, mass gain, oxide layer thickness, and γ' depletion zone size. The alloys of lesser Al content have reduced oxidation resistance and formed oxide scales of predominantly Cr2O3. Alloys with Ti content of 2.7 at. pct and greater formed Ti-rich oxide phases in their oxide scales as well as TiN subscale. A trade-off between alloy processability and oxidation resistance is identified, dictated by the deleterious effect of Al content on the ductility dip and the benefit of Al for oxidation resistance. A property space along the pareto front is highlighted which is ideal for having oxidation resistance and processability