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.     

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.     

Impression creep behavior of a cast MRI153 magnesium alloy

Creep behavior of a cast MRI153 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress in the range of 360–600 MPa at temperatures between 425 and 490 K. Microstructure of the alloy was composed of α(Mg) matrix phase besides Mg₁₇Al₁₂ and Al₂Ca intermetallic compounds. Stress exponent of minimum creep rate, n, was found to vary between 6.45 and 7. Calculation of the activation energy showed a slight decrease with increasing stress such that the creep activation energy of 115.2 kJ/mol under σ_imp/G = 0.030 decreased to 99.5 kJ/mol under σ_imp/G = 0.040. The obtained stress exponent and activation energy data suggested that the pipe diffusion dislocation climb controlled creep as the dominant mechanism during the creep test.     

Effect of alloying elements on the creep behavior of high Pb-based solders

This study examines the high temperature creep behavior of several Pb-based alloys. All compositions tested were found to follow power-law dislocation creep in the strain rate range of 10⁻⁹-10⁻³ s⁻¹. Both the stress exponent and activation energy were measured from 298 to 473 K to identify the rate controlling mechanism for creep deformation. Creep of 95Pb-5In, 92.5Pb-5Sn-2.5Ag, 93Pb-3Sn-2Ag-2In was rate limited by dislocation climb from the observed stress exponent. A transition in the controlling climb mechanism from pipe diffusion to lattice diffusion was observed around 0.7T_m. Creep of 90Pb-10Sn was, however, rate limited by viscous solute drag rather than dislocation climb due to the greater concentration of Sn in Pb. The enhancement in self-diffusion of Pb was dependent on the degree of solid solution with solute atoms. The outcome of this work identifies variables related to the alloy elements that control creep behavior of Pb-based alloys used in high temperature applications where traditional solders cannot be used.     

Dislocation analysis of die-cast Mg–Al–Ca alloy after creep deformation

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and to explain the creep properties of the Mg–Al–Ca AX52 die-cast alloy at 473 K and stresses from 15 to 70 MPa. TEM observations indicate that dislocations are generated within the primary α-Mg grain in the die-casting process, which consist of both the basal and non-basal segments. The basal segments of dislocations are able to bow out and glide on the basal planes under the influence of a stress, and the jogs follow the basal segments with the help of climb during creep. The creep mechanism for the alloy is deduced as dislocation climb due to the formation of sub-boundaries during creep, while the easy glide of the basal segments of dislocations is controlling the creep rates immediately after the stress application of creep tests.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (σ<30 MPa), both the apparent stress exponent (n_app≈2) and the apparent activation energy (Q_app≈48 kJ mol⁻¹) are low, while at high stresses (σ>34 MPa), these parameters are much higher (n_app=9–15 and Q_app=230–325 kJ mol⁻¹) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y₂O₃ interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.     

Creep mechanisms of U720Li disc superalloy at intermediate temperature

The microstructures of U720Li disc superalloy have been investigated by transmission electron microscopy (TEM) before and after creep test at 725 °C/630 MPa. The evolution of the crept microstructures was marked as three different stages (I, II and III) corresponding to gradually increased strain 0.1%, 5% and 27%, respectively. At stage I, dislocations bypassed secondary γ′ via Orowan loops. At stage II, partial dislocations started to shear secondary γ′, leaving stacking fault (SF) behind and microtwins formed in part of grains. At stage III, grain boundary sliding occurred due to very large strain and increased effective stress. The results indicated that the creep mechanisms of U720Li at 725 °C/630 MPa evolved with gradually increased strain. Orowan looping process combining dislocation slip and climb and partial dislocations shearing precipitates were the main creep mechanisms. It is suggested that decreasing the interparticle spacing of secondary γ′, strengthening secondary γ′ and decreasing stacking fault energy (SFE) of γ matrix may be effective methods to improve the creep property at relatively higher temperatures.     

Creep rupture behaviour of modified 9Cr-1Mo heat-resistant steel strengthened with different mechanisms

The creep rupture behaviours and microstructural changes of a modified 9Cr-1Mo heat-resistant steel were investigated at 853 K. Analysis of creep results suggests that dislocation climb is the dominant deformation mechanism with true stress exponent of 5 under the present conditions. Based on the microstructural analysis, strengthening contributions from M₂₃C₆ carbides and MX carbonitrides were clarified. The M₂₃C₆ carbides can promote grain boundary strengthening by exerting Zener pinning forces, whereas MX carbonitrides can enhance the creep strength by interacting with mobile dislocations to induce threshold stress. Besides, softening of the steel is related not only to the decrease of dislocations, but also the coarsening of precipitates and substructures. The value of creep damage tolerance factor is close to 6.6, which further confirms that the creep damage is mainly attributed to the microstructural degradations, such as the coarsening of precipitates and substructures and decrease of dislocations.     

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

Creep behaviour of single crystal nickel base superalloy

Abstract

The creep activation energy and structure constant at the different creep stages have been calculated, and the microstructures have been observed by SEM and TEM. The results showed that the internal stress σo decreased with an increase in temperature. Over the stress and temperature range, there are different activation energies, time exponents, and structure constants at different creep stages. The change in microstructure has an influence on creep resistance in this superalloy (Ni-6.0AI-7.0Ta-8.5Mo, wt-%). It is shown that the dislocation climb is the major deformation mechanism during tensile creep stages I and II, but during the tertiary stage, the creep resistance decreased as a result of dislocations shearing into the γ′ rafts. Creep fracture occurs mainly by the cavities and microcracks produced at the γ′/γ phase interface due to the interaction of multislips.     

Ti65合金的初级蠕变和稳态蠕变

使用透射电镜(TEM)研究了Ti65合金在600~650℃、120~160 MPa条件下的蠕变变形行为及其微观变形机制。结果表明：初级蠕变变形机制主要由受攀移控制的位错越过α₂相的过程主导;稳态蠕变阶段蠕变机制主要由受界面处扩散控制的位错攀移的过程主导,且应力指数为5~7。在初级蠕变阶段α₂相与位错的相互作用是α₂相对合金高温强化的主要方式,在稳态蠕变阶段沿α/β相界分布的硅化物阻碍位错运动与限制晶界滑移是硅化物对合金强化的主要方式。     

Creep deformation behaviour of AZ31 magnesium alloy over wide range of temperature and stress

Abstract

The creep deformation behaviour of coarse grained AZ31 magnesium alloy was examined in the temperature range from 423 to 673 K (0·46–0·73T_m) under various constant stresses covering low strain rate range from 4×10^?9 to 2×10^?2 s^?1. Most shape of the creep curve was typical of class II behaviour. However, only at low stress and low temperature, the shape of the creep curve was typical of class I behaviour. At very low stress at 673 K, the stress exponent for the secondary creep rate was ～2. At low stress level, the stress exponent was ～3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ～5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute atmosphere breakaway concept.     

Effects of stacking fault energy on the creep behaviors of Ni-base superalloy

Cobalt in a 23 wt.% Co containing Ni-base superalloys was systematically substituted by Ni in order to study the effects of stacking fault energy (SFE) on the creep mechanisms. The deformation microstructures of the alloys during different creep stages at 725 °C and 630 MPa were investigated by transmission electron microscopy (TEM). The results showed that the creep life increased as the SFE decreased corresponding to the increase of Co content in the alloys. At primary creep stage, the dislocation was difficult to dissociate independent of SFE. In contrast, at secondary and tertiary creep stages the dislocations dissociated at γ/γ′ interface and the partial dislocation started to shear γ′ precipitates, leaving isolated faults (IFs) in high SFE alloy, while the dislocations dissociated in the matrix and the partials swept out the matrix and γ′ precipitates creating extended stacking faults (ESFs) or deformation microtwins which were involved in diffusion-mediated reordering in low SFE alloy. It is suggested that the deformation microtwinning process should be favorable with the decrease of SFE, which could enhance the creep resistance and improve the creep properties of the alloys.     

An evaluation of the rate-controlling flow process in Newtonian creep of polycrystalline ice

Using experimental data and theoretical calculation for Newtonian creep in polycrystalline ice, it is demonstrated that unlike most other materials, in which the rate-controlling flow process is edge dislocation climb under saturated condition, the rate-controlling flow process of polycrystalline ice is dislocation glide along the basal plane under a constant dislocation density. The dislocation density during Newtonian creep of ice is determined by the initial state instead of the magnitude of the Peierls stress. The transition stress (threshold) from power-law creep to Newtonian creep is controlled by the dislocation density instead of the Peierls stress. The activation energy of the Newtonian creep is similar to that of the self-diffusion due to the requirements of the diffusion of protons during dislocation glide.     

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.     

Dislocation network formation during creep in Ni-base superalloy GTD-111

The Ni-base superalloy GTD-111 is used as a blading material in the first stage blades of high power gas turbines. The creep-rupture properties of the cast superalloy were studied over a wide range of temperatures and stresses. The observations of dislocation structures during steady-state creep confirmed that the creep mechanism was different in the high and low stress regions. The results showed that in the high stress region, shear mechanisms including stacking fault formation and anti-phase boundary creation were operative and in the low stress region, a by-passing mechanism occurred by either looping or dislocation climb and glide. With increasing exposure time in the high-temperature low-stress region, dislocations formed networks at γ–γ′ interfaces, as well as inside γ′ particles. The transition in the mode of dislocation–γ′ precipitate interaction from shearing to by-passing was found to depend on creep conditions (stress and temperature) and microstructural characteristic of the alloy. The present paper provides microstructural evidence by means of transmission electron microscopy for a high temperature by-passing mechanism operating in the superalloy GTD-111.     

Ni-25Cr-20Co合金时效组织结构及性能研究

在Thermo-Calc热力学软件模拟计算基础上,采用光学显微镜、扫描电子显微镜、能谱检测和透射电子显微镜研究了Ni-25Cr-20Co合金在长期时效过程中析出相的变化情况及对性能的影响,理论分析了γ′相颗粒粗化对合金拉伸变形过程中第二相与位错交互作用机制的影响。结果表明:经750℃时效后合金中析出MC、M_(23)C_6和γ′相,γ′相的体积分数约为16%。长期时效后,γ′相颗粒的平均尺寸与时间t符合LSW理论,受溶质原子扩散及γ/γ′界面能的影响。时效后合金的拉伸强度明显增加,随时效时间的延长,拉伸强度逐渐降低。随γ′相的粗化,拉伸变形过程中第二相与位错交互作用的机制由位错热攀移机制→位错切割机制→Orowan绕越机制转变为位错热攀移机制→Orowan绕越机制→位错切割机制。     

Uniaxial creep behavior of nanostructured, solution and dispersion hardened V-1.4Y-7W-9Mo-0.7TiC with different grain sizes

Nanostructured vanadium (V) alloys are expected to exhibit high performance under neutron irradiation environments. However, their ultra-fine or refined grains cause significant decrease in flow stress at high temperatures due to grain boundary sliding (GBS), which is the major concern for their high-temperature structural applications such as future fusion reactors. The contribution of GBS to plastic deformation is known to depend strongly on grain size (GS) and may give more significant influence on long-time creep test results than on short-time tensile test results. In order to improve the creep resistance through elucidation of the effect of GS on the uniaxial creep behavior of nanostructured V alloys, a solution and dispersion hardened V alloy, V-1.4Y-7W-9Mo-0.7TiC (in wt%), with GSs from 0.58 to 2.16 μm was developed by mechanical alloying and HIP processes, followed by annealing at 1473-1773 K, and creep tested at 1073 K and 250 MPa in vacuum. It is shown that the creep resistance of V-1.4Y-7W-9Mo-0.7TiC increases monotonically with GS: The creep life for the alloy with 2.16 μm in GS is as long as 114 h, which is longer by factors of 2-30 than those for the other finer grained alloys and by two orders than that for coarse-grained V-4Cr-4Ti (Nifs heat2, GS: 17.8 μm) that is a primary candidate material for fusion reactor structural applications. The minimum (steady state) creep rate decreases with increasing GS as ?_s ∝ (1/?)³, where ?_s is the steady state creep rate and ? is the grain diameter. The observed superior creep resistance of V-1.4Y-7W-9Mo-0.7TiC is discussed in terms of GS effects on dislocation glide/climb, GBS, and strain hardening capability enhanced by solution and dispersion hardening.