Effects of microstructure evolution on the deformation mechanisms and tensile properties of a new Ni-base superalloy during aging at 800℃

Tensile properties at room temperature of a new casting Ni-base superalloy during aging at 800℃ for0-1000 h were investigated.During aging,granular M23C6 carbides presented at grain boundaries and kept growing from dispersed particles to continuous networks.The γ’ phase significantly coarsened,with the morphology of some γ’ phase changed from spherical to rounded cubic shape after 1000 h.Three deformation mechanisms in relation to the γ’ diameter(dγ’) were identified:(ⅰ) weakly coupled dislocations(WCD) connected by anti-phase boundary(APB) traveled in pair across the γ/γ’ structure when dγ’ was small in the under-aged alloys;(ⅱ) strongly coupled dislocations(SCD) with reduced spacing compared to(ⅰ) sheared γ’ phase when dγ’ increased in the over-aged alloys;(ⅲ) dislocations occasionally by-passed γ’ phase when dγ’ was larger than 97 nm after aging for more than 300 h.The alloy obtained the peak strength when 20 h-aged with dγ’=44 nm which was in the transition between(ⅰ) and(ⅱ).The aginginduced variation in yield strength was correlated to the coarsening of γ’ phase using a theoretical model of precipitation strengthening in terms of the formation of APB.The calculated results suggested that the γ’ phase with a volume fraction of 23% contributed more than 61% of the peak-aged yield strength.Observation after fracture revealed that the alloys usually fractured at grain boundaries.High stress concentration around carbides resulted in cracks by carbides self-cracking and the initiation of cavities.The undesirable agglomeration of M23C6 at grain boundaries was harmful to the properties of the overaged alloys.     

Effect of Mo on microstructural characteristics and coarsening kinetics of γ' precipitates in Co–Al–W–Ta–Ti alloys

The solvus temperature,volume fraction,coarsening behavior of Y' precipitates and the partitioning behavior of alloying elements as well as lattice misfit of Y/Y' phases influence the creep behavior of Ni-and Co-base superalloys.However,few investigations about the microstructural characteristics and the coarsening behavior of Y' precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure.Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on Y' solvus temperature,Y + Y' microstructure and Y' coarsening in Co–Al–W–Ta–Ti-base alloys.The results showed that the Y' solvus temperature decreases with the addition of Mo.Mo addition reduces the Y' volume fractions after aging above 1000?C,but results in negligible influence on the Y' volume fractions aging at 900?C.Meanwhile,Y' coarsening is controlled by diffusion in experimental alloys after aging at 900?C and 1000?C,and the kinetics of Y'growth in experimental alloys are consistent with the predictions of LSW theory.     

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。     

Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging

The multi-principal-component concept of high-entropy alloys(HEAs) generates numerous new alloys.Among them,nanoscale precipitated HEAs have achieved superior mechanical properties and shown the potentials for structural applications.However,it is still a great challe nge to find the optimal alloy within the numerous candidates.Up to now,the reported nanoprecipitated HEAs are mainly designed by a trialand-error approach with the aid of phase diagram calculations,limiting the development of structural HEAs.In the current work,a novel method is proposed to accelerate the development of ultra-strong nanoprecipitated HEAs.With the guidance of physical metallurgy,the volume fraction of the required nanoprecipitates is designed from a machine learning of big data with thermodynamic foundation while the morphology of precipitates is kinetically tailored by prestrain aging.As a proof-of-principle study,an HEA with superior strength and ductility has been designed and systematically investigated.The newly developed γ'-strengthened HEA exhibits 1.31 GPa yield strength,1.65 GPa ultimate tensile strength,and 15% tensile elongation.Atom probe tomography and transmission electron microscope characterizations reveal the well-controlled high γ' volume fraction(52%) and refined precipitate size(19 nm).The refinement of nanoprecipitates originates from the accelerated nucleation of the γ' phase by prestrain aging.A deeper understanding of the excellent mechanical properties is illustrated from the aspect of strengthening mecha nisms.Finally,the versatility of the current design strategy to other precipitation-hardened alloys is discussed.     

Stacking faults of metastable γ″ phase precipitated in an Ni-15Cr-8Fe-6Nb alloy

Abstract

The stacking faults of a metastable γ″ phase precipitated in a nickel base superalloy, a modified JIS NCF 3 type alloy (X–750M), are investigated by TEM and an X-ray diffraction method. The γ″ precipitates are circular shaped plates at the early stage of aging and they become elliptic or irregular shaped plates at the latter stage of aging at temperatures up to 1033 K. Contrast which suggests the existence of stacking faults on {112}_γ″ planes can be seen in many of the large γ″ precipitates extracted from the specimens aged at 1033 K for 36 ks or more. It is clear that the values of γ″ ?γ lattice mismatch increase with increasing aging time from the measurement of lattice constants of the γ and the γ″ phases. The formation of stacking faults on {112}_γ″ planes in the large γ″ precipitates is due to the movement of an a/6 <disp-formula><graphic href="splitsection1-m1.tif"/></disp-formula> partial dislocation introduced by γ″ ?γ coherency strain. Since a part of the stacking sequence has a similar crystal structure to that of stable δ phase precipitates in a γ″ phase, the formation of stacking faults in the γ″ precipitates is considered to be favourable for their stabilisation.     

Mechanism of Bainite Nucleation in Steel, Iron and Copper Alloys

During the incubation period of isothermal treatment(or aging) within the bainitic transformation temperature range in a salt bath (or quenching in water) immediately after solution treatment, not only are the defects formed at high temperatures maintained, but new defects can also be generated in alloys, iron alloys and steels. Due to the segregation of the solute atoms near defects through diffusion, this leads to non-uniform distributions of solute atoms in the parent phase with distinct regions of both solute enrichment and solute depletion. It is proposed that when the Ms temperature at the solute depleted regions is equal to or higher than the isothermal (or aged) temperature, nucleation of bainite occurs within these solute depleted regions in the manner of martensitic shear. Therefore it is considered that, at least in steel, iron and copper alloy systems, bainite is formed through a shear mechanism within solute depleted regions, which is controlled and formed by the solute atoms diffusion in the parent phase.     

Stability of Several Oxide Dispersion Strengthened Alloys and a directionally solidified y/y′ -α eutectic alloy in a thermal gradient

Thermal gradient testing of three oxide dispersion strengthened alloys (two Ni-base alloys, MA 754 and MA 6000E, and the Fe-base MA 956) and the directionally solidified eutectic alloy, γ/γ′-α, have been conducted. Experiments were carried out with maximum temperatures up to 1200°C and thermal gradients on the order of 100°C/mm. The oxide dispersion strengthened alloys were difficult to test because the thermal stresses promoted crack nucleation and growth; thus the ability of these alloys to maintain a thermal gradient may be limited. The stability of individual fibers in γ/γ′-α was found to be excellent; however, microstructural changes were observed in the vicinity of grain boundaries. Similar structures were also observed in isothermally annealed material; therefore thermal gradients do not affect the microstructure of γ/γ′-α in any significant manner.     

室温压缩AZ91镁合金显微组织及β-Mg17Al12相析出动力学

对常温压缩粗镁直接熔炼AZ91镁合金时效处理后的组织及β-Mg17Al12相析出动力学进行研究。结果表明:AZ91镁合金在常温压缩过程中出现大量的孪晶,为β-Mg17Al12相的析出提供了大量的形核基底;时效时β-Mg17Al12相优先在晶界、孪晶界析出,尤其易在孪晶与晶界、孪晶交接处析出并长大,且孪晶内析出的β-Mg17Al12相与α-Mg基体保持一定的位向关系;时效时间越长,析出的β-Mg17Al12相越多,温度越高,析出定量β-Mg17Al12相所需时间越短;结合实验数据,由JMAK方程计算得到AZ91镁合金析出β-Mg17Al12相激活能为23.8～37.9kJ/mol。     

Microstructure of the Ductile Phase in Intermetallic Compound Ni_(50)Al_(20)Fe_(30)

The microstructure of the single hot extruded and annealed Ni50Al20Fe30Y0.003 intermetallic compound alloys has been examined by means of high resolution electron microscopy (HREM). In these extruded and annealed alloys. the ductile phase is of a mixture of the disordered fcc γ matrix and or dered γ' precipitates. This fact well interprets the reason why the degree of annealing treatment can influence the strength and ductility of these alloys. The HREM observation revealed directly that there was some strain concentration at γ'-γ interfaces, due to the presence of more iron atoms in these two phases. The fixed orientation relationship between the γ phase and γ' precipitates was identified to be {001}γ{00 }γ' and <100 >γ < 100 > γ'     

Anomalous crystal structure of γ" phase in the Mg-RE-Zn(Ag) series alloys:Causality clarified by ab initio study

The crystal structure of the single-unit-cell thickness γ " phase,as a key strengthening phase in Mg-REZn(Ag) series alloys,has been extensively studied,and several structural models have been proposed in the past two decades.However,these reported models,and even the lattice constants at the same proposed structure,are scattered severely,which has led to considerable confusion and not available for further mechanical property simulation and prediction of Mg alloys containing this phase.In this study,by using first-principles calculations,the crystal structure of y" phase is clarified,resolving the discrepancies among different experiments,and its intrinsic mechanical properties have also been studied for the first time.It is verified that the γ " phase contains quasi-five atomic layers,instead of the previously reported tri-layer,and surprisingly,its crystal structure has many variants,which would change with the alloy composition.Besides,with the help of the simulated selected area electron diffraction(SAED) patterns,it is found that the atoms in the central layer remain partially ordered distribution,and this ordered extent primarily depends on the atomic ratio of RE:Zn(Ag) and the solute content in an alloy.That is,the ordered extent increases with decreasing the atomic ratio of RE:Zn(Ag) and/or increasing solute content of alloy,and vice versa.Ag and Zn dissolved in the γ" phase would produce almost opposed mechanical anisotropy for the γ " phase under the identical crystal structure,and the addition of Ag shows more efficient on increasing the shear modulus of γ" phase.     

Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

The microstructures and age hardening behaviours of a series of Mg–Al–rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, Al₄MM and Al₁₂Mg₁₇, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al₄MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al₁₂Mg₁₇ partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of Al₁₂Mg₁₇ precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al₁₂Mg₁₇ formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al₁₂Mg₁₇ also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.

MST/3382     

Phase stability under irradiation

Irradiation of metals and alloys with neutrons, electrons, heavy ions, or γ-rays may introduce up to 10⁸ J/mol of energy in the form of atomic displacements. This energy, which is in the form of vacancies, self-interstitials, and cores of displacement cascades is then available to produce a range of phase changes and microstructural alterations which are not observed under thermal conditions. There exist numerous mechanisms to convert part of this displacement energy into microstructural change, including irradiation-induced solute segregation, Frenkel pair recombination at the particle: matrix interface, irradiation disordering or amorphization, and recoil resolution of atoms from precipitates. In addition, the cores of displacement cascades may act as precipitate nucleation sites and Frenkel pair recombination may trigger spinodal-like instabilities.

The theory of these mechanisms is developed in some detail, and is followed by a systematic review of experimentals observations of irradiation-altered phase stability. The observations include enhanced nucleation on displacement cascades, precipitation induced by solute segregation to defect sinks, nucleation of wrong phases, disordering and amorphization, Frenkel pair recombination driven precipitate, and inverse Ostwald ripening.     

位错对高体积分数SiCP/2024Al时效行为的影响

采用粒径为1μm的SiC颗粒,用挤压铸造法制备出体积分数为45 %的SiC_P/2024Al复合材料,研究了位错对高体积分数SiC_P/2024Al时效行为的影响。结果表明,复合材料中的高密度位错可以湮灭大量的淬火空位,这在一定程度上抑制了GP区的析出。但是,高密度位错的存在降低了其它析出相的热扩散激活能,促进了析出相形核;还能为原子的管道扩散提供通道,促进了溶质原子的扩散,加速析出相的长大,在宏观上表现为对时效行为的促进,使峰时效提前。高密度的位错为强烈依赖于位错等缺陷形核的θ'和S'相提供许多优先形核的场所,使复合材料中的形核密度增加,同时使析出相的尺寸减小,所以复合材料中的析出相呈现细小弥散的分布特点。      

The growth mechanisms of θ’ precipitate phase in an Al-Cu alloy during aging treatment

The plate-shaped θ’(Al2 Cu) precipitate acts as one of the primary strengthening phases in Al-Cu alloys.The interface,especially the semicoherent interface,between Al-Cu solid solution(αAl) and θ’ phase contains a lot of clues about phase transformations.Thus,these interfacial structures in an Al-Cu alloy after high-temperature and longtime aging have been analyzed in detail using atomic-scale high-angle annular dark-field scanning transmission electron microscopy and first-principles calculations in this work.It was found that the lateral growth of θ’ precipitates is subjected to a combination of several major mechanisms under this aging condition.Except for some common intermediate phases,two novel and striking structures were observed on the interface,which implies two alternative atomic diffusion mechanisms for θ’ precipitate growth.For one condition,the atomic diffusion from αAl to θ’ phase transformation adopts an interstitialcy mechanism based on additional Al atoms.For the other condition,the diffusion is carried out through Al atoms.Both mechanisms are distinctly different from the previous theory based on direct diffusion of Cu atoms.The first-principle calculations also confirm that these newfound diffusion processes are ene rgetically favored.     

Zn对Mg-6Al合金凝固的溶质偏析及组织的影响

采用液态淬火方法得到了Mg-6Al和Mg-6Al-0.5Zn合金固液共存界面试样,表征了溶质Al在这两种合金中的凝 固偏析特征,研究了Zn对Mg-6A1合金凝固溶质偏析和凝固组织的影响.结果表明:Al元素在Mg-6A1合金中的近平衡分配系数是0.33,偏析比为2.17,最大偏析率为0.43.Zn的加入使Mg-6A1合金...     

Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickelbased single crystal superalloy was investigated.The thermal exposure of samples after the full heat treatment was carried out at 1000℃,1100℃ and 1140℃ for 100 h and 200 h.The γ' coarsening,γ' rafting and γ channel widening were observed in samples after thermal exposure.When the thermal exposure time was constant,the morphology of γ' phase in the alloy evolved significantly with increasing aging temperature.The interracial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature.When the higher exposure temperature was used,enla rgement of the defect pores was observed in samples,the microcracks were more likely to initiate and propagate at the corner of these pores.After aging at 1000℃ for 100 h,the creep life at 1140℃/137 MPa was slightly longer than that of heat-treated sample,which could be attributed to the slightly coarsened γ' phase,homogenization of refractor elements.In contrast,the creep life of sample exposed at 1140℃ for 100 h was greatly decreased.The decrease of creep life was dominated by the rafting of γ'phase,the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.     

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.     

Complex precipitation pathways in multicomponent alloys

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials.     

Re对一种新型镍基高温合金组织稳定性的影响

为了解Re对一种新型镍基高温合金组织稳定性的影响,采用扫描电镜观察了合金经长期时效后的样品微观组织,并利用透射电镜分析长期时效后析出的新相.实验结果表明,质量分数为1%～4%Re的合金经热处理后,组织由γ相、γ’相和碳化物MC、M23C6组成,当Re含量达到4%时,晶内析出了针状的M23C6.经长期时效后,随Re含量增加,合金中γ'相长大速率下降,合金的组织稳定性恶化,900℃超过1 000 h时效的含4%Re合金中出现针状σ相.因此,所研究合金中Re的含量应低于4%.     

Investigation on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys with various Zn/Mg ratios

The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150℃ have been investigated by using tensile tests,optical metallography,scanning electron microscopy,transmission electron microscopy and atom probe tomography analyses.With increasing Zn/Mg ratios,the ageing process is significantly accelerated and the time to peak ageing is reduced.T'phase predomi-nates in alloys of lower Zn/Mg ratios while η'phase predominates in alloys with a Zn/Mg ratio over 2.86.Co-existence of T'phase and η'phase with a large number density is beneficial to the high strength of alloys.Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture.A strength model has been established to predict the co-strengthening effect of T'phase and η'phase in Al-Zn-Mg-Cu alloys,including the factors of the grain boundary,solid solution and precipitation.