Minimum In Creep Rate Research Articles

Double minimum creep processing and mechanism for γʹ strengthened cobalt-based superalloy

The double minimum creep, characterized by two creep rate minima, in Co-based superalloy is investigated using a phase-field model coupled with a crystal plasticity model. The constitutive modeling, based on the dislocation slip theory considering dislocation interaction, is applied to simulate microstructural evolution and deformation behavior. Rafting process commences at the beginning of creep until the global minimum of creep rate is reached, demonstrating a strengthening effect from N-type rafts under compressive creep. The high shear strain rate of (111)[01¯1] slip system in the intersections of horizontal and vertical γ channels leads to a slight increase of creep rate after the first local minimum. The evolution of stress field shows that the softening effect is the combined effect of the increase of resolved shear stress and the decrease of hardening stress in the intersections. Further, these changes in stress are primarily caused by the dislocation annihilation and the inhomogeneous plastic deformation. This study indicates that the intermediate local softening stage during creep may be eliminated if the initial inter-distance between γʹ precipitates is decreased.

Understanding creep of a single-crystalline Co-Al-W-Ta superalloy by studying the deformation mechanism, segregation tendency and stacking fault energy

A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950, 975 and 1000°C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the γ′ phase during creep. Density-functional-theory calculations indicate segregation of W and Ta to the stacking fault and an associated considerable reduction of the stacking fault energy. Since solutes diffuse faster at a higher temperature, segregation can take place more quickly. This results in a significantly faster softening of the alloy, since cutting of the γ′ precipitate phase by partial dislocations is facilitated through segregation already during the early stages of creep. This is confirmed by transmission electron microscopy analysis. Therefore, not only the smaller precipitate fraction at higher temperatures is responsible for the worse creep properties, but also faster diffusion-assisted shearing of the γ′ phase by partial dislocations. The understanding of these mechanisms will help in future alloy development by offering new design criteria.

Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy

ABSTRACT The compressive creep deformation behavior of a Co-base single-crystalline superalloy is studied in the high temperature / low stress regime at 950°C/150 MPa. Emphasis is placed on the mechanisms causing the double minimum creep behavior consisting of a local and a global creep rate minimum. The local minimum occurs at ∼0.2% strain with dislocation accumulation at γ/γ′ interfaces after bowing out of dislocations in horizontal matrix channels. Plate-like rafting occurring normal to the applied stress axis in the early creep stages is regarded as the primary reason for the decreasing creep rate until the global minimum takes place at ∼0.8% strain. In the final creep stage following the global minimum the creep rate accelerates due to coarsening and extensive precipitate shearing. The shearing by partial dislocations leads to the formation of numerous stacking faults, nano/microtwins, and occasionally to the formation of small amounts of ordered D019-Co3W precipitates. The reasons for twinning and second phase formation as well as differences of the deformation mechanisms between Co- and Ni-base superalloys during double minimum creep are discussed in detail in this work.

Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?

Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime.

The grain boundary pinning effect of the μ phase in an advanced polycrystalline γ/γ′ Co-base superalloy

A grain boundary pinning phase was introduced into a γ/γ′ Co-base superalloy by the addition of Mo. The intermetallic phase was identified as μ phase (Co,Ni,Cr)7(W,Mo)6 by high-energy X-ray diffraction and energy-dispersive X-ray spectroscopy in the transmission electron microscope. Synchrotron radiation in-situ heating experiments confirmed the stability of the grain structure up to 1050 °C and therefore the grain boundary pinning effect of this phase. Recrystallization heat treatments at different temperatures resulted in varying amounts of μ phase. The grain size analysis revealed that at least 1% of the phase is necessary to maintain a pinning effect during recrystallization. Atom probe tomography confirmed that Mo partitioned predominantly to the γ phase. Since the μ phase and the γʹ phase are competing for W, a lower γ′ volume fraction was formed in comparison to a Mo-free polycrystalline γ/γʹ Co-base superalloy variant. However, the homogeneous distribution of small grains resulted in a higher strength compared to the Mo-free alloy without grain boundary pinning phases. The creep resistance at 750 °C was similar to the Mo-free alloy as well as the polycrystalline Ni-base superalloy U720Li with a creep rate minimum of 1.5 × 10−7 s−1 at 620 MPa.

Micromechanics of primary creep in Ni base superalloys

The initial stages of creep in nickel base superalloy deserve a special attention since they prevail in most of the service live of critical jet engine components. In nickel base superalloys, a very sharp minimum in creep rate is observed at strains less than 0.5% in a wide range of temperatures: the lower the applied stress the steeper the minimum, often referred to as incubation period. In this paper, the primary creep stage of Ni superalloy is investigated using 3D discrete dislocation dynamics modeling. The simulated volume corresponds to a 3D periodic cell of the cubic arrangement of γ−γ′ NiAl loaded along the (001) direction. Misfit stresses induced by the lattice mismatch (δ = -3.10−3) between the precipitate and the matrix are taken into account through a coupling of the dislocation dynamics code with a finite element code. Frank-Read sources are positioned in the horizontal channel. It is shown that most of the dislocations propagate in the horizontal channels by repeated cross-slip on activated {111} glide planes and a few invade the vertical channels. This behavior is rationalized by the model. In the γ-γ′ interfaces, the zigzagging lines of dislocations straighten out into [100] and [010] directions leading to a dislocation microstructure similar to the ideal misfit dislocation network needed to compensate the lattice mismatch. Calculations of internal stresses in the channels show that this dislocation microstructure induces biaxial planar stresses in the horizontal γ channels leading to an elastic contraction of the sample along the loading direction. Three-dimensional DDD simulations show that the plastic strain induced, in the loading direction, by the movement and multiplication of dislocations is less important than the elastic relaxation it produces along that same direction. Hence, for very low applied tensile stresses, Ni superalloys display an abnormal behavior often referred to as negative creep that can be evidenced experimentally, even in polycrystalline materials. This last observation clearly implies that the first active batches of dislocations, during creep, or static high temperature ageing are mobilized to cancel out the numerous and pervading internal elastic strain fields generated within the crystals by the γ’ precipitates due to their misfit with the γ matrix.

Double minimum creep of single crystal Ni-base superalloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.

High-temperature and low-stress creep anisotropy of single-crystal superalloys

The high-temperature and low-stress creep (1293K, 160MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [001] and [110] directions. In the early stages of creep, the [001]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [110] tensile loading. This may be related to the nature of rafting during [110] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [100] tensile creep.

Mechanical and microstructural observations during compression creep of a short fiber reinforced AlMg metal matrix composite

The constant load compression creep behavior of a short fiber reinforced aluminium-5 wt.% magnesium alloy (AlMg5) containing 15 vol.% Al 2O 3 fibers (Saffil) was investigated. Creep of the matrix alloy and the metal matrix composite (MMC) was studied at 300 °C and 500 °C. Stresses ranging from 10 MPa to 120 MPa result in minimum creep rates between 10 −9 s −1 and 10 −3 s −1. MMC creep is characterized by an initial decrease of the creep rate before a distinct creep rate minimum is reached, while the matrix alloy shows increasing primary creep rates at 500 °C. It is difficult to rationalize the stress dependence of the minimum creep rates by a simple Norton law, because the stress exponent n is not always constant. The orientation of Al 2O 3 fibers with respect to the loading axis is found to affect minimum creep rates. Fiber breakage represents an important damage mechanism. Fiber agglomerates are observed in the initial microstructure which may well affect the entire process of creep.

Evolution of microstructure during creep in gamma Ti–52Al at 1100 K and high applied stresses

Abstract Microstructural changes due to compression creep in gamma Ti–52Al phase at 1100 K and 250 MPa have been investigated using a conventional metallography and transmission electron microscopy. Three distinct microstructural types, the 3D dislocation network, subgrain boundaries and twinning, were found in individual grains after interrupted creep tests. The microstructures always consisted of three fundamental deformation modes: (i) ordinary dislocations, (ii) superdislocations, and (iii) deformation twins that contributed to the overall creep strain from the very beginning of creep. This microstructural evolution and the corresponding metal-type creep behaviour contrasts with the sharp creep rate minima and a lack of superdislocations and deformation twins reported earlier for specimens deformed at 1100 K and applied stresses below 200 MPa. The partitioning of the total strain among fundamental deformation modes is estimated and the link between sharp minima of creep rate and the insufficient activ...

Microstructural Evolution Associated with Creep Rate Minima in Pure- and Near-γ TiAl Intermetallics

Microstructural Evolution Associated with Creep Rate Minima in Pure- and Near-γ TiAl Intermetallics

Microstructural evolution of the martensitic cast steel GX12CrMoVNbN9‐1 during long‐term annealing and creep

The microstructure of four specimens of the martensitic cast steel GX12CrMoVNbN9‐1 crept to fracture at 873 K for 220‐33027 h was quantified by electron microscopy with regard to large M23C6‐particles, small V‐ and Nb‐containing particles, dislocations, pores, and oxidation layers. The laws of time‐dependent coarsening of particles and strain‐controlled growth of subgrains are consistent with those previously established for X20(22)CrMoV12‐1. Using these data the microstructural model of deformation developed for X20(22)CrMoV12‐1 can explain the creep behaviour of GX12CrMoVNbN9‐1. The minimum in creep rate is related to the coarsening of particles and subgrains. At low stresses (< 140 MPa) the fraction of small slowly coarsening V‐ and Nb‐containing particles leads to improved creep resistance. The material resists against pore formation; fracture occurs by necking.

011] Creep in a single crystal nickel base superalloy at 1033K

TEM investigation are performed on [011] single crystals of SRR99 after at 1033 K under a load at 680 MPa. At the creep rate minimum, multiple slip in the matrix occurs on four octahedral slip systems. Octahedral cross slip or cubic slip is not activated. Due to the superposition of coherency and external stressein small matrix channels, deformation is concentratedned in the “roof” matrix channels. During secondary creep, screw interfaial dislocations cross slip in the cube γ/gg' interfaces. In this stage, γ' precipitates are sheared by Shocley super parrials. Common shereing of matrix and γ' particles by Shockley super partials is observed. The high creep rate in the [011] orientation is considered to be caused by the stress concentration in roof matrix channels and the few actiated slip sysems.

Anomalous primary creep behaviour of dilute aluminium-silicon alloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.