<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

<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

What is the role of rhenium in single crystal superalloys?

Rhenium plays a critical role in single-crystal superalloys –its addition to first generation alloys improves creep life by a factor of at least two, with further benefits for fatigue performance. Its use in alloys such as PWA1484, CMSX-4 and Rene N5 is now widespread, and many in this community regard Re as the “magic dust”. In this paper, the latest thinking concerning the origins of the “rhenium-effect” is presented. We start by reviewing the hypothesis that rhenium clusters represent barriers to dislocation motion. Recent atom probe tomography experiments have shown that Re may instead form a solid solution with Ni at low concentrations (< 7 at.%). Density functional theory calculations indicate that, in the solid solution, short range ordering of Re may be expected. Finally, Re has been shown to diffuse slowly in the γ-Ni phase. Calculations using a semi-analytical dislocation climb/glide model based upon the work of McLean and Dyson have been used to rationalise the composition-dependence of creep deformation in these materials. All evidence points to two important factors: (i) the preferred partitioning of Re to the γ phase, where dislocation activity preferentially occurs during the tertiary creep regime and (ii) a retardation effect on dislocation segments at γ/γ′ interfaces, which require non-conservative climb and thus an associated vacancy flux

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 role of boron on improving ductility in a new polycrystalline superalloy

AbstractThe role of boron in promoting ductility at high temperature in a prototype nickel-based superalloy designed for industrial gas turbines is studied. Both a boron-containing and boron-free variant are tested in tension at 750 °C, with further in-situ tests carried out using scanning electron microscopy (SEM), to clarify the mechanism of ductility improvement. The improvement in ductility is observed to be greater at the lowest investigated strain rate, where the grain boundary character plays a significant role on the mechanical properties; no ductility improvement was observed at the highest investigated strain rate. The in-situ tests were also performed at 750 °C and revealed directly the greater susceptibility of the grain boundary morphology in the boron-free case to fracture and – in the boron-containing case – the mechanism of ductility enhancement. The findings are supported further by high-resolution electron backscattered diffraction (HR-EBSD) strain mapping which confirms that the distribution of elastic strain and geometrically necessary dislocation (GND) content are influenced markedly by boron addition. The mechanism through which boron indirectly enhances the mechanical properties at elevated temperatures is discussed

Role of AMPA Receptor Cycling in Synaptic Transmission and Plasticity

AbstractCompounds known to disrupt exocytosis or endocytosis were introduced into CA1 pyramidal cells while monitoring excitatory postsynaptic currents (EPSCs). Disrupting exocytosis or the interaction of GluR2 with NSF caused a gradual reduction in the AMPAR EPSC, while inhibition of endocytosis caused a gradual increase in the AMPAR EPSC. These manipulations had no effect on the NMDAR EPSC but prevented the subsequent induction of LTD. These results suggest that AMPARs, but not NMDARs, cycle into and out of the synaptic membrane at a rapid rate and that certain forms of synaptic plasticity may utilize this dynamic process