Numerical simulation of γ/γ′ microstructural evolutions induced by TCP-phase in the MC2 nickel base single crystal superalloy

The influence of topologically close packed-phase (TCP-phase) precipitation on the γ/γ′ microstructure evolution of a single crystal superalloy has been studied. Four representative configurations of needle-shaped μ-phase particles observed in the MC2 alloy after creep deformation at 1050 °C/160 MPa have been selected.Finite element modeling was performed on the selected configurations using a crystal plasticity framework and a viscoplastic approach.The local calculated stress/strain fields in the vicinity of TCP-phase precipitates, especially stress concentration, allow an interpretation of the observed microstructural evolutions (γ′-precipitate orientation and thickness).Moreover, our numerical simulation brings a fruitful contribution to the understanding of the phenomenon often observed beyond needle-like μ-phase: the formation of a distorted γ/γ′ microstructure in lieu of the expected γ/γ′-rafted structure.     

The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys

A new in-house designed series of Ni based superalloys with stepwise increased Re and Ru additions has been investigated, to systematically determine the influence of Re and Ru on γ/γ′-microstructure and high temperature creep properties. Improved creep resistance and thus also a higher alloy temperature capability of up to 87 K/at.% was found for additions of Re. Additions of Ru revealed a lower temperature capability improvement of up to 38 K/at.% for low Re-containing second generation alloys. However, in third and fourth generation alloys with higher Re-contents, no significant influence of Ru on creep rupture strength was observed. The creep properties are discussed with respect to the γ′-volume fraction, γ′-size and γ′-coarsening rate, as well as the γ/γ′-lattice misfit and the γ/γ′ partitioning coefficient of the different Re and Ru containing alloys. The presented data shows, that these microstructure parameters are strongly influenced by additions of Re, but only marginally by additions of Ru. A further influence on creep rupture strength is given by the solid solution hardening of the γ-matrix, which is discussed based on solid solution hardener concentrations either experimentally derived or calculated from ThermoCalc data.     

Coupling phase field with creep damage to study evolution and creep deformation of single crystal superalloys

A phase-field model coupling with elastoplastic deformation and creep damage has been built to study the microstructural evolution and deformation behavior for Ni-Al single crystal alloy during the whole creep processing.The relevant experiments were conducted to verify the model validity.The simulation results show that under the tensile creep at 1223 K/100 MPa,cubic γ'phases coarsen along the direction parallel to the axis of tensile stress during the first two creep stages;and spindle-shaped and wavy γ'phases are formed during tertiary creep,similar to the experimental results.The evolution mechanism ofγ'phases is analyzed from the perspective of changes of stress and strain fields.The "island-like" γ phase is observed and its formation mechanism is discussed.With the increase of creep stress,the directional coarsening of γ'phase is accelerated,the steady-state creep rate is increased and the creep life is decreased.The comparison between simulated and experimental creep curves shows that this phase-field model can effectively simulate the performance changes during the first two creep stages and predict the influence of creep stresses on creep properties.Our work provides a potential approach to synchronously simulate the creep microstructure and property of superalloys strengthened by γ'precipitates.     

Evolution of precipitated phases during prolonged tempering in a 9%Cr1%MoVNb ferritic-martensitic steel: Influence on creep performance

Ferritic-martensitic steels of the 9%Cr1%Mo type have been extensively used in power plant components, heat exchangers, piping and tubing, etc., due to an excellent combination of properties such as creep resistance, toughness and resistance to oxidation at high temperatures. In these steels the stabilizing role of MX carbonitrides (M = Nb, V; X = C, N) is one of the main factors responsible for the resistance under creep conditions. The control of precipitation and coarsening of MX phases during prolonged, high temperature tempering or post-weld heat treatment is then a key point to obtain the desirable microstructure and hence, to achieve high temperature resistance under service conditions.In the present contribution we report the evolution of the precipitated phases during heat treatment at 780 °C for increasing times in the range 40 min to 7 h for an ASTM A213 T91 steel. The Nb and V contents in solid solution were determined as a function of the time of treatment and maxima were observed for 5 and 5.66 h, respectively. Creep tests to rupture were also conducted at 600 °C - 190 MPa for as-treated specimens. A maximum creep rate was observed to occur in coincidence with the maximum values of Nb and V contents in solid solution. We suggest possible relationships between the observed second phase evolution and the creep resistance behavior.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Numerical simulation of mechanical controlling parameters for Type IV cracking on the welding joints of martensitic heat-resistant steel

The maximum principal stress, von Mises equivalent stress and equivalent creep strain in the welding joint of martensitic heat-resistant steel (9Cr1MoVNb) are simulated by finite-element method (FEM) under the condition of 600°C and applied stress of 80 MPa. The results show that the maximum principal stress and von Mises equivalent stress are high on the curved points of two sides of the groove face near the fine-grain heat-affected zone (HAZ). The creep strain mainly concentrates in the fine-grain HAZ; the maximum creep strain locates in the bottom of fine-grain HAZ of specimen. The stress triaxiality in the fine-grain HAZ is maximum, and creep cracking occurs because of the intensive constrain of base metal and weld. The simulation result is good in agreement with those of crack initiation site and propagation path by using the stress triaxiality as the mechanical controlling parameter of weld joint of martensite heat-resistant steel. Therefore, it is reasonable that the stress triaxiality is used for analysis initiation and propagation of Type IV cracking in the fine-grain HAZ.     

Grain boundary transformation character in Fe-N austenite

The morphology and crystallography of the isothermal transformation of Fe-N austenite at 225 °C were characterized by transmission electron microscopy and scanning electronic microscope. Fe-N austenite was produced by through-nitriding thin pure iron sheets at 640 °C. Lath-shaped and periodically distributed (γ-austenite + γ′-Fe₄N + α-Fe) microstructures formed at the prior austenite grain boundaries, this kind of microstructure was shown to be bainite in nature and kept a Greninge-Troiano orientation relationship between the γ-austenite (γ´-Fe₄N) and α-ferrite. The formation mechanism of the grain boundary microstructure was shown to be the preferential precipitation of the proeutectoid γ′-Fe₄N at the grain boundary, followed by the transformation of the N-depleted austenite into α-ferrite and γ′-Fe₄N, thus forming a periodic γ-γ′-α-γ′-γ-γ′-α-γ′-γ-γ′ arrangement.     

Evolution and analysis of γ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.     

Phase transformation of 316L stainless steel from wire to fiber

In this work, quantitative crystalline phase analysis of 316L stainless steel from wire to fiber using a multi-pass cold drawing process was studied using the Rietveld whole XRD profile fitting technique. The different diameters of the fibers: 179, 112, 75, 50, 34, 20, and 8 μm, were produced from an as-received wire with a diameter of 190 μm. The crystalline phases were identified using MDI Jade 5.0 software. The volume fractions of crystalline phases were estimated using a Materials Analysis Using Diffraction software. XRD analysis revealed that the crystal structure of as-received wire is essentially a γ-austenite crystalline phase. The phase transformation occurred during the 316L stainless steel from wire to fiber. Three crystalline phases such as γ-austenite, α′-martensite, and sigma phase of the fine fiber were observed. A cold drawing accelerates the sigma phase precipitates, particularly during the heat treatment of the fiber.     

Numerical simulation of mechanical controlling parameters for Type IV cracking on the welding joints of martensitic heat-resistant steel

The maximum principal stress, von Mises equivalent stress and equivalent creep strain in the welding joint of martensitic heat-resistant steel (9Cr1MoVNb) are simulated by finite-element method (FEM) under the condition of 600°C and applied stress of 80 MPa. The results show that the maximum principal stress and von Mises equivalent stress are high on the curved points of two sides of the groove face near the fine-grain heat-affected zone (HAZ). The creep strain mainly concentrates in the fine-grain HAZ; the maximum creep strain locates in the bottom of fine-grain HAZ of specimen. The stress triaxiality in the fine-grain HAZ is maximum, and creep cracking occurs because of the intensive constrain of base metal and weld. The simulation result is good in agreement with those of crack initiation site and propagation path by using the stress triaxiality as the mechanical controlling parameter of weld joint of martensite heat-resistant steel. Therefore, it is reasonable that the stress triaxiality is used for analysis initiation and propagation of Type IV cracking in the fine-grain HAZ.     

一种4.5%Re镍基单晶合金在980℃蠕变期间的变形与损伤机制

通过蠕变性能测试和组织形貌观察,研究了一种Re含量为4.5%Re(质量分数,下同)的镍基单晶合金的高温蠕变行为、变形和损伤机制。结果表明,4.5%Re合金在980℃/300MPa的蠕变寿命为169h。蠕变初期,合金中立方γ′相转变为垂直于应力轴的N型筏状结构。稳态蠕变期间,合金的变形机制为位错在基体中滑移和攀移越过筏状γ′相。蠕变后期,合金的变形机制为位错在基体中滑移和剪切进入筏状γ′相。由于γ基体通道较窄,位错在基体通道中滑移所需的阻力较大。剪切进入γ′相的110超位错可由{111}面交滑移至{100}面,形成K-W锁,从而抑制位错的滑移和交滑移,这是合金具有较好蠕变抗力的主要原因。主/次滑移位错的交替开动,可致使筏状γ′相扭曲,并促使裂纹在筏状γ/γ′两相界面萌生;裂纹沿垂直于应力轴方向扩展,直至断裂,这是合金的蠕变断裂机制。     

Microstructure evolution in a 3%Co modified P911 heat resistant steel under tempering and creep conditions

Microstructure evolution was studied in a 3%Co modified P911 heat resistant steel during creep tests at 873 and 923 K to failure, which occurred in 4103 and 4743 h, respectively. The tempered martensite lath structure consisted of packets, blocks and laths. The average spacing of high-angle boundaries and the mean transverse lath size were about 1.9 μm and 360 nm, respectively. Various second phase particles precipitated upon tempering. Fine M(C, N) carbonitrides with an average size of about 30 nm were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M₂₃C₆ carbide particles (average size 120 nm) were located at internal boundaries. The tempered martensite lath structure was rather stable upon aging for about 4 × 10³ h. The boundary precipitates of M₂₃C₆ and Laves phases, which appeared during creep tests, exerted a high pinning pressure on low-angle lath boundaries and high-angle packet/block boundaries. The growth of martensite structural elements during the tests correlated with the coarsening of second phase particles. Quantitative relations of pinning and driving pressures for low- and high-angle boundaries are discussed.     

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M₂₃ C₆ (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Microstructure and High‐Temperature Strength of Monocrystalline Nickel‐Base Superalloys

The progress in the further development of monocrystalline nickel‐base superalloys as the most advanced turbine‐blade materials has been the result of combined efforts in alloy development and microstructural refinement. Some aspects of these developments are reviewed. The questions of microstructural stability with respect to the formation of brittle topologically closed‐packed (TCP) phases and, in particular, directional coarsening, i.e., γ/γ′ rafting under service‐near high‐temperature creep conditions are addressed. Since γ/γ′ rafting is usually accompanied by creep acceleration, attempts have been made to avoid rafting by modifying the microstructure by appropriate thermal/mechanical treatments. These attempts were not successful. On the other hand, it could be shown that pre‐rafting in compression, leading to the formation of rafts parallel to the stress axis in the case of alloys with negative γ/γ′ lattice mismatch, enhances both the isothermal high‐temperature creep and fatigue strengths. According to current understanding, there exists no optimum γ/γ′ lattice mismatch in the case of negative mismatch but, at best, an optimum compromise between low‐ and high‐temperature behavior. It is speculated that a more suitable situation could be found in alloys with positive lattice mismatch.     

Microstructural modelling of grain-boundary stresses in Alloy 600

The stress distribution in a random polycrystalline material (Alloy 600) was studied using a topologically correct microstructural model. The distributions of von Mises and hydrostatic stresses, which could be important factors when studying the intergranular stress corrosion cracking, at the grain vertices were analysed as a function of microstructure, grain orientations and loading conditions. The grain size, shape, and orientation had a more pronounced effect on stress distribution than the loading conditions. The stress concentration factor was higher for hydrostatic stress (1.7) than for von Mises stress (1.5). Hydrostatic stress showed more pronounced dependence on the disorientation angle than von Mises stress. The observed stress concentration is high enough to cause localized plastic microdeformation, even when the polycrystalline aggregate is in the macroscopic elastic regime. The modelling of stresses and strains in polycrystalline materials can identify the microstructures (grain-size distributions, texture) intrinsically susceptible to stress/strain concentrations and justify the correctness of applied stress state during the stress corrosion cracking tests.     

Investigation and improvement on structural stability and stress rupture properties of a Ni–Fe based alloy

The structural stability and stress rupture properties of a Ni–Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied. Investigation on the structural stability of the existing alloy GH984 shows that the most important changes in the alloys are γʹ coarsening, the γʹ to η transformation and the coarsening and agglomeration of grain boundary M₂₃C₆ during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 °C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that η phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of γʹ phase is related to Fe content. Adding B and P was suggested to stabilize M₂₃C₆ and increase grain boundary strength. Based on the research presented and analysis of the data, a modified alloy was developed through changes in composition. For the modified alloy, η phase is not observed and M₂₃C₆ is still blocky and discretely distributes along grain boundary after thermal exposure at 700 °C for 20,000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 °C A-USC.     

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimens for creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composites, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinations from 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behavior based on microstructural observations are discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 125–128, January–February, 2008.     

Microtwinning and other shearing mechanisms at intermediate temperatures in Ni-based superalloys

In Ni-based superalloys, microtwinning is observed as an important deformation mechanism at intermediate temperature and low stress and strain rate conditions. Current knowledge concerning this unusual deformation mode is comprehensively reviewed, and fundamental aspects of the process are further developed using state of the art experimental and modeling techniques. The nature of microtwins and the microtwinning dislocations at the atomic level have been determined using High Angle Annular Dark Field Scanning Transmission Electron Microscopy imaging. The results unambiguously confirm that the operative twinning dislocations are identical Shockley partials a/6〈1 1 2〉, and that they propagate through the γ′ precipitates in closely-separated pairs on consecutive {1 1 1} planes. The rate-limiting process of the microtwinning deformation mechanism is the diffusion-controlled reordering in γ′-phase. It is shown that reordering requires very simple, vacancy-mediated exchange between Al and Ni atoms. The energetic aspect of the vacancy-mediated exchanges is studied for the first time using ab initio calculations. The concept of reordering as a rate-limiting process is generalized and shown to be relevant for other, previously reported deformation mechanisms in superalloys such as a〈1 1 2〉 dislocation ribbons, and superlattice intrinsic and superlattice extrinsic stacking fault formation. Other diffusion phenomena associated with microtwinning, such as segregation of heavy elements, is also discussed and supported by experimental evidence. The influence of the γ/γ′ microstructure on microtwinning deformation mode is also discussed in light of observations and phase-field dislocation modeling results.