Early Stages Of Creep Research Articles

Creep properties of a new Re free single crystal Ni-based superalloy, NKH71

The creep of the newly developed Ni-based, Re free, single crystal superalloy NKH71 was examined at 1273 K. The creep properties were compared to those of CMSX-4, a second-generation commercial superalloy containing 3%Re. When the (applied) stress was higher than 160 MPa, the rupture lives of the present alloy were shorter than those of CMSX-4. However, when the stress was below 160 MPa, the rupture lives of the present alloy became longer than those of CMSX-4. It was noted that this lower stress level was more important when Ni-based superalloys were used for engineering land-based gas turbines. TEM and SEM observations showed that NKH71 formed a rafted structure in the early stage of creep. This was because a large number of dislocations existed in the γ / γ ′ interfaces before creep. In contrast, CMSX-4 had very few dislocations before creep. This observation explained why NKH71 was inferior to CMSX-4 under high stress. However under lower stress, the γ / γ ′ rafted structure changed to a more complex morphology in CMSX-4, while that of NKH71 tended to remain unchanged. This conservation of the rafted structure lead to the good creep resistance of NKH71 under a lower stress. In brief, the new alloy was shown to have good creep resistance even without Re element.

Evolution of grain contacts in a granular sample under creep and stress relaxation

This article deals with the characterization, using an acoustic technique, of the mechanical behavior of a dry dense granular medium under quasistatic loading. Ultrasound propagation through the contact-force network supporting the external load offers a noninvasive probe of the viscoelastic properties of such heterogeneous media. First the response of a glass bead packing is studied in an oedometric configuration during creep and relaxation tests. Quasilogarithmic increases of sound velocities are found in both mechanical tests. A model based on the mechanics of microcontacts between rough grains adequately reproduces our experimental results, especially for the evolution of elastic modulus. Another main experimental finding is that collective grain rearrangements within the packing also play a crucial role at the early stage of creep and relaxation.

Effect of stress on microstructural evolution and mechanical properties of 12Cr3W3Co steel during aging and short-term creep

The dual-phase (martensite and delta ferrite) microstructure in 12Cr3W3Co steel was obtained by the method of reheat treatment in the duplex phase (δ+γ) region. The coarsening process of martensite lath and precipitation behavior of M23C6 and Laves phase particles in this steel were investigated during aging and short-term creep under different stresses at 600°C for 1100h. The results showed that large numbers of Cr-rich M23C6 carbides formed in martensite matrix and Laves phase prefered to precipitate inside delta ferrite at the early stage of creep. The number of Laves phase precipitates inside delta ferrite in crept specimens was larger than that in aged specimen and the size increases with the stress increase in crept specimens. Martensite lath structure tended to grow and the recombination of lath boundaries under stress caused the disappearance of some lath boundaries leaving a row of M23C6 carbides in the martensite matrix. The short-term creep mechanical behavior was studied by analyzing steady-state creep rate and stress exponent. Reletively low steady-state creep rate and high stress exponent were found and these were attributed to the solid solution strengthening and precipitation strengthening.

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.

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.

Microstructural evolution of delta ferrite in SAVE12 steel under heat treatment and short-term creep

This research focused on the formation and microstructural evolution of delta ferrite phase in SAVE12 steel. The formation of delta ferrite was due to the high content of ferrite forming alloy elements such as Cr, W, and Ta. This was interpreted through either JMatPro-4.1 computer program or Cr{sub eq} calculations. Delta ferrite was found in bamboo-like shape and contained large amount of MX phase. It was surrounded by Laves phases before creep or aging treatment. Annealing treatments were performed under temperatures from 1050 Degree-Sign C to 1100 Degree-Sign C and various time periods to study its dissolution kinetics. The result showed that most of the delta ferrite can be dissolved by annealing in single phase austenitic region. Dissolution process of delta ferrite may largely depend on dissolution kinetic factors, rather than on thermodynamic factors. Precipitation behavior during short-term (1100 h) creep was investigated at temperature of 600 Degree-Sign C under a stress of 180 MPa. The results demonstrated that delta ferrite became preferential nucleation sites for Laves phase at the early stage of creep. Laves phase on the boundary around delta ferrite showed relatively slower growth and coarsening rate than that inside delta ferrite. - Highlights: Black-Right-Pointing-Pointer Delta ferrite ismore » systematically studied under heat treatment and short-term creep. Black-Right-Pointing-Pointer Delta ferrite contains large number of MX phase and is surrounded by Laves phases before creep or aging treatment. Black-Right-Pointing-Pointer Formation of delta ferrite is interpreted by theoretical and empirical methods. Black-Right-Pointing-Pointer Most of the delta ferrite is dissolved by annealing in single phase austenitic region. Black-Right-Pointing-Pointer Delta ferrite becomes preferential nucleation sites for Laves phase at the early stage of creep.« less

Influence of microstructure and its evolution on the mechanical behavior of modified MAR-M247 fine-grain superalloys at 871°C

Abstract This study added Re to Mar-M247 fine-grain superalloy and investigated the influence of microstructure and its evolution on the mechanical behavior at 871 °C. Mar-M247 and modified Mar-M247 alloys consisted of dual precipitation of primary and secondary γ ′ particles in the γ matrix. An increase in Re content led to a reduction in the primary γ ′ phase with the γ ′ phase becoming finer; however, the addition of Re did not have an obvious influence the secondary γ ′ phase. Tensile and creep tests (871 °C/379 MPa) showed that both tensile and creep strength increased with an increase in Re content up to a maximum of 3 wt%. In the early creep stage, the addition of Re improved creep resistance by increasing the strength of the γ matrix and decreasing the inter-particle spacing. In the later creep stage, the linear fraction of the γ ′ raft in stress axial direction increased following the addition of Re, resulting in improved creep resistance. Observation of microstructure evolution indicated that only secondary γ ′ phase coarsened directionally into a rafting structure and the primary γ ′ phase was unaffected by creep. An interrupted creep tests verified that the γ ′ rafting structure initiated in the primary creep stage and completed in the secondary creep stage. The addition of excessive quantities of Re, such as 5 wt%, resulted in the formation of needle-like P phase, which damaged the tensile and creep properties. During tensile and creep tests, cracks initiated and propagated along grain boundary (GB) in alloys containing 0–3 wt% Re; and propagated along both the GB and P/γ interface in an alloy containing 5 wt% Re. In conclusion, optimal results were obtained for fine-grain Mar-M247 following the addition of 3 wt% Re.

Investigations on the growth kinetics of Laves phase precipitates in 12% Cr creep-resistant steels: Experimental and DICTRA calculations

The growth kinetics of Laves phase precipitates (type Fe 2W) in the early stage of creep (650 °C for 10,000 h) in two 12% Cr ferrite–martensitic steels has been investigated. In one alloy the Laves phase formed on tempering, while in the second alloy the Laves phase precipitated during creep. Kinetic simulations were performed using the software DICTRA. The particle size of the Laves phase was measured on transmission electron microscopy samples. The equilibrium phase fraction of the Laves phase was reached in the first thousand hours. Simulations of particle growth showed good agreement with the experimental results. Competitive growth between M 23C 6 and the Laves phase showed that M 23C 6 carbides reached their equilibrium after 12 days, whereas the Laves phase reached equilibrium after 3 months. Simulations of the influence of the interfacial energy and addition of Co, Cu and Si on Laves phase precipitation are presented.

Dislocation motion in the early stages of high-temperature low-stress creep in a single-crystal superalloy with a small lattice misfit

Dislocation configurations at different creep stages (1100 °C and 137 MPa) in a superalloy TMS-75(+Ru) were studied in transmission electron microscopy (TEM) and the movement path of these creep-produced dislocations could be fully illustrated. Due to the small value of γ/γ′ lattice misfit, these dislocations cannot glide in the horizontal γ matrix channels by cross slip, but they mainly move by climbing around the γ′ cuboids. In the primary stage, the dislocations first move by slip in the γ-matrix channels. When they reach the γ′ cuboids, they move by climbing along the γ′ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, away from the deposition orientation of 〈110〉 to the misfit orientation of 〈100〉. The velocity of the reorientation is lower and a perfect γ/γ′ interfacial dislocation network cannot be formed quickly. This factor results in a large creep rate of the alloy during the secondary creep stage. The path for dislocation motion during the early creep stages consists of the following sequences: (i) climbing along the γ′ cuboid surface, (ii) deposition onto the (001) γ/γ′ interfacial plane, and (iii) reorientation from the 〈110〉 direction to the 〈100〉 direction.

Recovery and Precipitate Analysis of 9 Pct Cr-1 Pct MoVNb Steel during Creep

The effect of tempering temperature and creep exposure on the microstructure of a modified 9Cr steel was investigated. Creep-interrupted specimens, including the grip portion, were investigated precisely using mainly X-ray and inductively coupled plasma (ICP) spectroscopy. After saturation of precipitation due to creep exposure, the amount of extracted residue decreased once and then increased within a short period (dip). Chemical analysis showed that during the dip, the precipitates temporarily dissolved into the matrix and precipitated again. The size of the Cr23C6 increased gradually during creep, but the growth rate was relatively small, as compared to the Ostwald ripening. The size of the VN particles in the specimens tempered at 800 °C in the early stage of creep was very fine, approximately 20 nm, and tended to decrease further with the progress of creep. The size variations of the precipitates and the dip were explained from the annihilation or migration of precipitation sites, i.e., dislocations and boundaries, during creep. Transient creep for the specimens tempered at 500 °C was controlled by a reduction of the mobile dislocation density. On the other hand, transient creep for 800 °C was due to precipitation hardening of fine VN particles with the progress of creep, which was supported by the increase in both the lattice strain and the activation energy with creep.

Primary creep in single crystal superalloys: some comments on effects of composition and microstructure

Creep at low temperatures in current single crystal superalloys is characterised by a large but orientation dependent primary creep strain followed by prolonged secondary creep at a rate closely linked to the primary creep strain. The processes of incubation, nucleation and propagation are described and the linked effects of alloy composition and microstructure are discussed in relation to these processes. It is shown that the incubation period is relatively insensitive to the alloy composition but that heat treatments which vary the γ′ size can have a larger effect on the early stages of creep. The key to high primary creep appears to be the mobility of the dislocation ribbons nucleated and this is sensitive to the alloy composition. Fault energies in both the γ and the γ′ are important in determining the form of the dislocation ribbons. Early results suggest that ruthenium may have a beneficial effect reducing the primary creep of fourth generation alloys.

Influence of grain-boundary morphology on creep of a wrought Ni-base superalloy

This paper elucidates the effects of grain-boundary serrations on the creep of the superalloy ‘modified Waspaloy’, heat treated to have either planar or serrated grain boundaries. Planar or serrated grain boundaries are produced by either air cooling or controlled furnace cooling, respectively, from the solution heat treatment. Tensile creep at 705–800 °C and 250–690 MPa exhibits limited primary creep following the instantaneous strain, with the creep life dominated by a continuously increasing strain rate. At higher stresses and lower temperatures serrations increase the tertiary creep strain. At low stresses (≤300 MPa) and high temperature (800 °C) serrations cause: a lower strain rate during the early stage of creep; delayed onset of tertiary creep; and, in contrast to samples with planar grain boundaries, exhibit no dynamic recrystallization during creep. Based on previous research connecting serrations with grain-boundary coincidence, the creep behaviour is suggested to be influenced by serrations reducing the local grain-boundary misorientation which reduces the ability to absorb dislocations and lowers grain-boundary diffusivity, thereby improving resistance to both dislocation and Coble creep, and reducing the propensity for dynamic recrystallization.

Microstructure evolution and its influence on deformation mechanisms during high temperature creep of a nickel base superalloy

The interaction of dislocation with strengthening particles, including primary and secondary γ′, during different stages of creep of Rene-80 was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During creep of the alloy at 871 °C under stress of 290 MPa, the dislocation network was formed during the early stages of creep, and the dislocation glide and climb process were the predominant mechanism of deformation. The density of dislocation network became more populated during the later stages of the creep, and at the latest stage of the creep, primary particles shearing were observed alongside with the dislocation glide and climb. Shearing of γ′ particles in creep at 871 °C under stress of 475 MPa was commenced at the earlier creep times and governed the creep deformation mechanism. In two levels of examined stresses, as far as the creep deformation was controlled by glide and climb, creep curves were found to be at the second stage of creep and commence of the tertiary creep, with increasing creep rate, were found to be in coincidence with the particles shearing. Microstructure evolution, with regard to γ′ strengthening particles, led to particles growth and promoted activation of other deformation mechanisms such as dislocation bypassing by orowan loop formation. Dislocation-secondary γ′ particles interaction was detected to be the glide and climb at the early stages of creep, while at the later stages, the dislocation bypassed the secondary precipitation by means of orowan loops formation, as the secondary particle were grown and the mean inter-particle distance increased.

Effect of creep deformation on oxidation behavior of nickel‐based alloy with Re‐based diffusion barrier coating

AbstractOxidation and creep behaviors of a Ni‐Mo‐based alloy (Hastelloy‐X) with a diffusion barrier coating consisting of duplex, inner Re‐based alloy, and outer β‐NiAl layers were investigated at 1243 K in air with an external tensile stress of 22.5 MPa. For comparison the alloys, as‐received, heat treated, and with the Ni‐aluminide coating, were oxidized under creep deformation. Creep rapture time for the diffusion barrier‐coated alloy was longer than those for the bare alloy and with the β‐NiAl‐coated alloy. After creep deformation to a strain of 3.5% for 190 h, it was found that the Re‐based alloy layer has few cracks and flaws and the β‐NiAl layer has the similar structure and composition before and after the creep test. The external scale mainly consisted of θ‐Al2O3 at the early stage of creep, and with further oxidation the external scale became a duplex layer, inner, equi‐axed α‐Al2O3, and outer, plate‐like θ‐Al2O3, which exfoliated significantly. It was concluded that the Re‐based alloy layer acts effectively as a barrier against inward Al diffusion and outward diffusion of alloy elements under creep deformation.

Phase-Field Simulation on the Formation and Collapse Processes of the Rafted Structure in Ni-Based Superalloys

In Ni-based superalloys, the rafted structure is known to form in the early stage of creep and to get into wavy morphology in the final stage of creep at elevated temperatures. This rafting phenomenon is essentially related to the anisotropic relaxation of the lattice misfit between the � and � 0 phases due to the creep strain under the external stress. In this study, in order to simulate comprehensively from the formation to collapse processes of the rafted structure by the phase-field method, a new idea that the anisotropy increases with simulation time is employed in the calculation of the elastic strain energy in alloy. This idea corresponds to the phenomenon that creep strain increases with creep time. The results are in good agreement with the microstructural change observed in practical Ni-based alloys. [doi:10.2320/matertrans.MBW200709]

Development of γ phase stacking faults during high temperature creep of Ru-containing single crystal superalloys

An unusual deformation mode involving the formation of intrinsic stacking faults in the γ matrix of experimental Ru-containing γ– γ′ superalloys with high Co and Re contents during high temperature creep at 950 °C/290 MPa has been observed. The morphology, distribution and dependence of these stacking faults on alloy chemistry has been investigated along with their formation mechanism. Additions of Re and Co substantially decrease the stacking fault energy of the γ matrix. The observed stacking faults in the γ matrix form by the dissociation of a/2〈1 1 0〉 matrix dislocations with Burgers vectors perpendicular to the loading direction in the early stages of creep. The dependence of creep properties on elemental additions that influence stacking fault energy is discussed.

High‐temperature dislocation plasticity in the single‐crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single-crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a gamma/gamma'-microstructure consisting of gamma'-cubes (L12 phase, 80 vol.%) separated by thin gamma-channels (face-centred cubic). <100> tensile creep tests were performed at 980 and 1020 degrees C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high-temperature creep. It was found that in the early stages of creep, a(0)/2<011> dislocations form within the gamma-channels. Later on, dislocation networks form and gamma' cutting processes with a(0)/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high-temperature low-stress creep regime.

Effect of initial preform porosity on solid-state foaming of titanium

Titanium foams were produced by the expansion of pressurized argon pores trapped within a preform during a previous powder-consolidation step. Compared with creep expansion at 903 °C, superplastic expansion (induced by a 830–980 °C cycling around the allotropic temperature of titanium) increases foaming rate and final porosity. The pore size and fraction in the preforms were varied by using a range of initial titanium powder sizes and argon pressures. As initial preform porosity increases from 0.06 to 2.7%, foaming rate increases in the early stages of creep and superplastic foaming. However, at a later stage, foaming ceases prematurely for preforms with high initial porosity, as pores connect to the surface, allowing the escape of the pressurized argon. Preforms with 0.40% initial porosity result in foams with an optimal combination of high foaming rate, high final porosity (up to 47%), tailorable open or closed porosity, and Young's modulus as low as 23 GPa.

The effect of particles on dynamic recrystallization and fabric development of granular ice during creep

AbstractThe mechanical behavior and microstructural evolution of laboratory-prepared, particle-free fresh-water ice and ice with 1 wt.% (~0.43 vol.%) silt-sized particles were investigated under creep with a stress level of 1.45 MPa at −10°C. The particles were present both within the grains and along the grain boundaries. The creep rates of specimens with particles were always higher than those of particle-free ice. Dynamic recrystallization occurred for both sets of specimens, with new grains nucleating along grain boundaries in the early stages of creep. The ice with particles showed a higher nucleation rate. This resulted in a smaller average grain-size for the ice with particles after a given creep strain. Fabric studies indicated that ice with particles showed a more random orientation of c axes after creep to ~10% strain than the particle-free ice.

Cataclastic deformation of garnet: a record of synseismic loading and postseismic creep

Microstructural evidence for episodic deformation of the uppermost plastosphere related to failure of the schizosphere in major seismic events has been previously reported from the Sesia Zone, Western Alps, with inferred extraordinarily high stresses and strain rates for the early stages of creep during stress relaxation at temperatures of about 300–350°C. In some of these rocks, crystals of the high strength mineral garnet embedded in a quartz matrix are intensely fragmented. Detailed optical, electron microprobe and scanning electron microscopic study, combined with electron backscatter diffraction techniques, suggests that fragmentation of garnet took place quasi-instantaneously at very high peak stresses. Subsequent deformation during stress relaxation caused displacement of the garnet fragments by frictional sliding, controlled by dislocation creep in the surrounding quartz matrix. In a final stage, after restoration of a near-lithostatic pore fluid pressure, the garnet fragments were pulled apart and the opening cracks were filled by quartz and feldspar precipitated from the pore fluid. This record appears to be consistent with the proposed model of synseismic loading and postseismic creep, and the related pore pressure changes.