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In the present exploration, it was attempted to understand the creep‐fatigue (CF) deformation micromechanisms of alloy CM 247 DS LC by conducting low‐cycle fatigue (LCF) and CF tests employing strain amplitude ranging from 0.6% to 1.0% at T = 850°C in the air and performing extensive electron microscopic examinations. The cyclic life of the alloy lessens for all CF tests conducted at 1 and 5 minute dwell time in comparison to LCF tests. Transmission electron microscopy (TEM) examinations confirmed that during CF tests substructure consists of dislocation loop, mixed dislocations, and γ' rafting, a typical creep deformation signature of nickel‐base superalloys, it also consists of features observed during fatigue deformation such as anti‐phase boundary (APB)‐coupled dislocations inside γ' precipitates and local tangles of dislocations. This confirms that the deformation of CF‐tested specimens is ascribed to the synergistic effect of both creep and fatigue. This fact was further verified by scanning electron microscopic (SEM) examinations. [ABSTRACT FROM AUTHOR]

Dislocation based deformation micromechanisms during low cycle fatigue deformation of nickel base superalloy CM 247 DS LC at 850 °C was investigated by conducting fatigue tests employing constant strain amplitudes for strain ratio (R) values of 0, −1 and carrying out extensive SEM and TEM examinations. Cyclic life of the alloy reduces for all fatigue tests conducted employing R = 0 in comparison with R = −1 owing to sustained mean stress developed during fatigue at R = 0. TEM examinations confirmed that sustained mean stress developed during low strain amplitude fatigue test (Δ ε /2 = 0.5%) using R = 0 condition prevented slip transfer from γ-channels to γ′-precipitates and resulted in the formation of dislocation substructures such as networks, nodes etc. and also promoted dislocation looping around γ′-precipitates. Lower fatigue life at R = 0 is mainly attributed to the development of these types of substructures, which promotes strain localization in both intra as well intergranular regions. Whereas, in specimen fatigue tested (Δ ε /2 = 0.5%) using R = −1 condition, shearing of γ′-precipitates by APB coupled dislocation and formation of stacking faults were observed. The formation and nature of stacking faults were analysed using weak beam imaging technique. Stacking fault formed during fatigue tests using R = −1 condition matured to micro-twins when Δ ε /2 value was increased to 0.8%. The mechanism of formation of these microtwins is discussed in detail. SEM based microstructural and fractographic examinations revealed that mean stress induced creep effect resulted in intergranular crack initiation and grain boundary cavitation during fatigue tests under R = 0 condition and therefore verified the facts revealed in TEM studies. [ABSTRACT FROM AUTHOR]

Abstract In order to meet the 21st century challenges, the need to improve the high temperature load bearing capabilities or creep properties of nickel base superalloys is more important than ever. Thorough understandings of creep deformation micro-mechanisms are essential for improving high temperature performance of these alloys. Therefore, in the present study, creep deformation micro-mechanisms of CM 247 DS LC alloy used for fabricating turbine blades and vanes under relevant service conditions have been explored. The creep deformation behaviour at the investigated temperatures has been divided into three temperature regimes. In the lower temperature regime, a classical three stage creep curve is observed, i.e., primary creep followed by steady-state creep and then an accelerating creep stage leading to failure. While, in the intermediate temperature creep regime a relatively shorter secondary creep region is seen, in contrast, secondary creep stage is almost missing in the high temperature regime. Transmission electron microscopic (TEM) studies revealed that the deformation micro-mechanisms in the low temperature regime are dominated by γ′-precipitates shearing of anti phase boundary (APB) coupled dislocations pair. In the high temperature regime, the deformation is dominated by the extensive dislocation networks formation and severe γ′-rafting. In the intermediate temperature regime, the deformation micro-mechanisms are basically mixture of both low and high temperature deformation micro-mechanisms. The facts revealed by TEM observations were well supported by scanning electron based factographic and microstructural analysis. Highlights • Creep deformation mechanisms of the alloy changing with temperature. • Shearing of γ′ is the dominant creep deformation mode in the low temperature regime. • Dislocation networks and severe γ′-rafting is prevalent in the high temperature regime. • In the intermediate temperature regime, creep deformation is mixture of both. • Formation of N- type rafts prevents dislocations to enter inside γ′-precipitates. [ABSTRACT FROM AUTHOR]

Wear resistance of white cast iron is known to be sacrificed when it is attempted to enhance the impact toughness. In the present study, it is shown that a suitably engineered microstructure can retain/enhance the wear resistance between 2.5 and 3.5 times even when impact energy value is increased from 4 J/cm 2 to 13 J/cm 2 . A novel microstructure is designed based on DICTRA ®™ simulation and is validated after casting and subsequent cyclic annealing. The novelty of this microstructure is the presence of duplex ferritic-austenitic matrix and partially spherodized M 7 C 3 carbides. The wear behaviour of the newly developed grades is compared with standard class-III high chromium white cast iron. The microstructure of the investigated alloys, the role of micro-constituents on the wear mechanism is extensively studied through scanning electron microscopic examination. The abrasive wear resisting mechanisms of the newly developed alloy are ascertained to be resistance to plastic flow leading to formation of humps and the formation of surface fatigue cracks. [ABSTRACT FROM AUTHOR]

High temperature tensile deformation behaviour of directionally solidified nickel base superalloy CM 247 DS was studied by conducting tensile tests in the temperature range 25–955 °C and extensive electron microscopic examinations to understand the influence of different phases on tensile behaviour. Micro-constituents, especially carbides and γ−γ′ eutectics influence the deformation and fracture behaviour of alloy CM 247 DS. Cracking of carbide precipitates was extensively observed during tensile deformation at 25 °C. However, tendency of carbide cracking decreased and more carbide-matrix interface de-bonding were observed with increase in test temperature. [ABSTRACT FROM AUTHOR]

Abstract: High-temperature, high-cycle fatigue and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liners of aero engines are highlighted in this paper. An acoustic emission technique has been utilised to detect the ductile–brittle transition temperature of the bond coat. Results revealed that the ductile-to-brittle transition temperature (DBTT) of this bond coat is around 650°C, which is in close proximity to the value reported for CoCrAlY type of bond coat. The endurance limit in fatigue and the life of TBC coated composite under accelerated creep conditions are substantially higher than those of the substrate material. Fractographic features at high stresses under fatigue was intergranular cleavage whereas those at low stresses were transgranular and ductile in nature. Delamination of the bond coat and spallation of the TBC at high stresses during fatigue was evident. It is recommended that these TBCs should not be put into use above 800°C for more than 1000h of continuous operation as the strength of the TBC superalloy drops to negligible levels. [Copyright &y& Elsevier]