Mechanism of α-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

It is commonly accepted that the drop in mechanical properties of INCONE1718(IN718) after long time exposure at high temperature is a result of coalescence ofγ", γ and the transformation ofγ′ to δ-Ni3Nb phase. Besides the γ′, γ and δ,the precipitation of B.C.C. α-Cr is another factor that affects the mechanical properties of IN718. In this paper, the Thermal-Calc software is used to reveal the thermodynamic condition of α-Cr formation. The calculated sections of the pseudo ternary diagrams of Cr-Nb-IN718 and Cr-Fe-IN718 explain why α-Cr particles do not precipitate dispersedly in γmatrix but in the vicinity of δ phase. The selected area diffraction (SAD) of TEM confirms the crystallography relationship of α-Cr with δ is (010) δ// (-110) α-Cr, [100] δ//[111] α-Cr° In combination of TTT diagram with the Thermal-Calc and TEM results, the mechanism of α-Cr formation in IN718 is suggested.     

Mechanism of a-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

IN718is the worldwide most used nickel-basesuperalloy because of its unique mechanical propertiesand excellent technological behavior.Alloy IN718ismainly strengthened by phases y"-NijNb with orderedBCT DO22structure and Y-Ni3Al with FCC L12structure.Of these two phases,y"contributes majoreffect in strengthening because of its large lattice misfitwith matrix.But the instability of main strengtheningphase y"at high temperature has limited the highestservice temperature of IN718below650°…     

Phase formation in a Ni-50Cr HVOF coating

A complex, fine scale microstructure of non-equilibrium phases is obtained by HVOF deposition of Ni-50Cr alloy due to the rapid cooling experienced by the splats which make up the as-deposited coatings. XRD analyses indicated that the as-deposited coatings consisted predominantly of a single fcc γ-Ni phase. Two small peaks suggested the presence of NiO and/or NiCr₂O₄ at the limit of detection (∼ 5%). Shoulders on the main γ-Ni peaks were interpreted as evidence of a second, lower Cr content γ-Ni phase. Characterization of the oxide content of the as-deposited coatings by X-ray diffraction, image analysis of backscattered electron images, and electron probe microanalysis yielded conflicting results due to the size of the microstructural features present relative to the spatial resolution of these techniques. Due to the nature and feature size of the non-equilibrium oxide phase(s), direct measurement of the oxygen content by EPMA was found to be the most accurate technique. Heat treatment of an as-deposited coating at 650 °C in vacuum resulted in coarsening of the microstructural features, and an approach towards a mixture of equilibrium phases consisting of γ-Ni, α-Cr, and Cr₂O₃. Evidence was also seen in the XRD pattern of an intermetallic σ phase that has previously only been reported in thin films of Ni-Cr alloys.     

铂改性铝化物涂层的热生长层内应力研究

研究了镍基高温合金热障涂层系统中铂改性铝化物粘结层在空气中非连续高温氧化生成的Al2O3层内应力状态及相应的粘结层微观结构。利用Raman光激发荧光谱技术,发现铂铝粘结层在900℃氧化初期生成了θ-和α-Al2O3,而在1100℃氧化时,表面则形成连续致密的α-Al2O3层。通过α-Al2O3的光激发荧光谱偏移量,计算得到了热障涂层中热生长层的内压应力略高于3.0GPa。铂改性铝化物涂层表面Al2O3"背脊"处的内应力相对较低,同时由于没有陶瓷层的压制,生成的Al2O3起伏较大,并发生局部的Al2O3脱落。随氧化时间延长,由于Al元素沿晶界扩散较快,导致更多的γ′-Ni3Al在粘结层晶界处形成,粘结层中基本相β-NiAl向γ′-Ni3Al转变,改变了粘结层本身的热膨胀系数,引起热生长层中内应力变化。     

Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

The effects of laser shock processing (LSP) and warm laser shock processing (WLSP) on the microstructure of surface hardening layer and high-cycle fatigue performance at room temperature and high temperature (600 ℃) of IN718 alloy were investigated.It has been revealed that the grain refined hardening layer with greater residual compression stresses,higher fraction of coincidence site lattice (CSL) boundaries and dislocation densities was formed in WLSP-treated alloy than in LSP-treated alloys.Moreover,microtwins included γ" phase/high density dislocation complex was found in the surface of WLSP-treated alloy.These characters caused the significant enhancement of the medium value fatigue strength of WLSP-treated alloy at room temperature and elevated temperature.Apparently,the microtwins included y" phase/high density dislocation complex formed in the surface hardening layer of LSP-treated alloy has more complicated steric structure and more stable at elevated temperature than γ" phase/low density dislocation complex formed in LSP-treated alloy,leading to the slow recovery process.Therefore,the surface hardening layer in the WLSP-treated alloy remained more ideal strengthen-ing effect under high-cycle fatigue at elevated temperature than that in LSP-treated alloy.This resulted in the much longer fatigue crack initiation incubation and longer high-cycle life of WLSP-treated IN718 alloy under cycling load at 600 ℃.This discovery provides a new cognition of fatigue resistance by WLSP treatment of precipitation strengthening superalloy.     

热处理对Inconel718合金组织及力学性能的影响

针对Inconel 718合金的不同用途,分析研究了4种常用热处理工艺对Inconel 718合金组织和力学性能的影响。结果表明:固溶温度超过1020℃时,奥氏体晶粒显著长大。合金中主要析出相有MC、δ、γ’和γ″相。δ相沿晶界分布,1025℃固溶时呈颗粒状少量析出;950℃固溶时呈块状大量析出;直接时效时呈网状不连续分布。同时,δ相对合金的晶粒度影响较大,且其析出数量和形态决定了合金的韧塑性,γ″、γ’相的析出量和尺寸与晶粒尺寸决定了合金的强度变化。     

Alloy phase transformations driven by high temperature corrosion processes

Heat-resistant alloys survive high temperature exposure by growing protective oxide scales. This process involves the selective oxidation of an alloy constituent and necessarily leads to changed alloy compositions in the subsurface regions. This paper examines several examples of phase transformations driven by selective oxidation processes. Depending on the alloy system, the phase transformations concerned involve either the formation of a new phase or the dissolution of an existing one. For example, aluminium depletion from γ-TiAl leads to formation of Z-Ti₅₀Al₃₀O₂₀ as a result of simultaneous inward oxygen diffusion. As another example, selective oxidation of aluminium from α-Cr+β-NiAl alloys leads to formation of γ-Ni at the alloy surface. Inward diffusion of nickel then causes an α+β→γ+β phase transformation. In the case of depletion causing phase dissolution, the examples of metal silicide dissolution in Ni-Si and Co-Si alloys and chromium carbide dissolution in Fe-Cr-C alloys are shown to occur under diffusion control during selective formation of either SiO₂ or Cr₂O₃.     

Precipitation behavior of hot-extruded alloy 718 during isothermal treatment

The precipitation and dissolution behavior of hot-extruded alloy 718 was investigated during isothermal treatment at temperatures from 600°C up to 1150°C at different times from 0.3h to 72h. Analysis was conducted using hardness measurement and an electron microscope. It was observed that the γ/γ′ phases were precipitated further during isothermal exposure at 600°C-700°C. When the isothermal treatment was carried out at 800°C, the δ phase was precipitated at the expense of the γ′ phase, which resulted in a slight decrease of hardness with increasing holding time. Above exposure temperature of 900°C, a rapid decrease in hardness occurred within the holding time of 0.3h, resulting from the dissolution of the γ phase and a sharp decrease in the amount of the γ′ phase. Rapid grain growth also affected the decrease in hardness above 900°C.     

Distribution of Phosphorus and Its Effects on Precipitation Behaviors and Tensile Properties of IN718C Cast Superalloy

The effect of phosphorus on the precipitations of γ",γ' and δ phases and associated tensile properties in IN718C alloy are investigated in this study.It is revealed that P atoms are dissolved in the grain interior to a relatively high degree and hence influence the precipitation behaviors in the grain interior and improve the tensile strength of IN718C alloy.γ" and γ' phases did not precipitate in the alloy without P addition during air cooling,while γ" and γ' phases precipitated in the grain interior during air cooling in the alloys with P addition,and the amounts of γ" and γ' phases increased with increasing P content.Therefore,the Vickers micro-hardness in the as-cast state increased gradually with increasing P content.In double-aging state,the sizes of γ" and γ' phases in the alloys with P addition were larger than that in the alloy without P addition,while the sizes were invariable when the P content(wt%)was higher than 0.015.Therefore,the micro-hardness and tensile strength of IN718C alloy treated by double aging increased first and then kept invariable with increasing P content.The precipitations of δ phases both in the grain interior and on grain boundaries were inhibited by P markedly.The inhibitory effect of P on δ phase enhanced gradually with increasing content of P,but the plasticity increased first and then decreased.What is more,the crack tended to propagate into the matrix around the particles(Laves phases and NbC carbides)in the alloys without P addition at the beginning of the tensile fracture,while it tended to propagate along the interfaces between the matrix and those particles in the alloys with P addition,which resulted from the synthetical effect of P on γ" γ' and δ phases.     

0Cr15Ni70Ti3AlNb合金平衡析出相热力学计算

采用Thermo-Calc热力学计算软件,模拟计算了0Cr15Ni70Ti3AlNb合金的平衡析出相和凝固过程。结果表明:0Cr15Ni70Ti3AlNb合金的平衡析出相主要包括γ、γ′、MC、M23C6;合金凝固过程中Ti和Nb属于正偏析,Cr、Fe、Si等属于负偏析。Al和Ti是影响0Cr15Ni70Ti3AlNb合金中γ′相析出温度和最大析出含量的主要因素。MC相的析出温度和最大析出含量主要由C元素决定。M23C6相的析出温度主要由Cr元素决定,M23C6相的析出量主要由C元素控制。0Cr15Ni70Ti3AlNb优化合金成分(质量分数)为C(0.03%~0.05%)-Al(0.7%~0.9%)-Ti(2%~2.25%)-Nb(0.75%~0.95%)-Cr(13.5%~15.5%)。研究结果可为合金的成分设计和热处理工艺制定提供理论指导。     

Microstructural stability,hardness and compressive behaviour of directionally solidified intermetallic Ni3Al-based alloy with γ/γ′-β structure

Microstructural stability, hardness and compressive behaviour were investigated in directionally solidified (DS) Ni–20.2Al–8.2 Cr–2.44 Fe (at.%) alloy. The as-grown lamellar γ(Al)/γ′(Ll₂)-β(B2) structure of DS alloy is found to transform to a γ/γ′-α(A2) structure during annealing in the temperature range 1023–1173 K. This annealing is connected with precipitation of spherical α-Cr particles within the lamellae and lath-shaped α-Cr particles in the interlamellar γ/γ′-region. The size of the lath-shaped α-Cr precipitates depends on the annealing temperature. However, these precipitates are found to be relatively stable with respect to their size and morphology during annealing for 100–600 h at given temperature. Annealing at temperatures higher than 1210 K stabilises β-lamellae in the microstructure. The volume fraction of transformed lamellae is found to follow ΔV_l∝t^1/n law with n=4 and the activation energy for lamellae transformation is determined to be Q=236 kJ/mol. The kinetics of lamella transformation is proposed to be governed by diffusion along disordered regions bounding growing ordered domains of γ′-phase around the lamellae. The peak hardness and γ/γ′-region microhardness values are reached after 100 h annealing at 1023 K. The compressive yield stress increases with increasing temperature, reaching a peak value at about 1000 K, and rapidly decreases at higher temperatures. The quasi-steady strain-hardening rate decreases with increasing temperature in the temperature range 773–1273 K. Both yield stress and strain-hardening rate depend on strain rate.     

扫描间距对选区激光熔化IN718合金微观组织的影响

通过选区激光熔化(SLM)技术制备了IN718合金,利用X射线显微镜、X射线衍射仪、扫描电镜和电子背散射衍射等技术分析了IN718合金的致密性、相组成和晶粒形态以及取向关系。结果表明:SLM成形IN718合金的物相组成为面心立方结构的γ-Ni相与体心四方结构的γ″相,晶粒沿构建方向呈柱状晶。当激光扫描间距为100 μm时合金的致密度达到最高,当激光扫描间距为90 μm时,合金沿构建方向形成强<001>织构。     

Nb、Al含量对GH706合金组织及力学性能的影响

研究了降Nb提Al对GH706合金组织和力学性能的影响。当合金中Al含量由0.35%提高至1.26%(质量分数,下同),Nb含量由2.88%降低至1.98%后,可抑制晶内主要强化相γ″-Ni3Nb相的析出,促进γ′-Ni3Al相的析出,显著降低合金的室温屈服强度,但明显改善了合金的室温冲击性能。同时提Al降Nb还抑制了晶界强化相η-Ni3(Ti,Nb)相的析出,使得富Nb的Laves相及富Nb、Cr的Cr2Nb相析出,弱化了高温下的晶界强度,极大地恶化了合金的持久性能。     

激光熔覆Fe基非晶合金涂层裂纹分析

使用激光熔覆技术在304不锈钢基材表面熔覆铁基非晶合金涂层时产生了裂纹,通过对裂纹周围元素、硬度和裂纹易萌生处析出物的定性分析,研究了涂层裂纹成形机理。结果表明,涂层主要有α-Fe、α-Cr、碳化物(M₇C₃、M₅C₂)、硅化物(CrSi₂、FeSi)等物相。裂纹主要由热影响区较大的热应力以及C、Si元素偏析而成的高熔点及高硬度的碳硅化物造成,裂纹带附近组织较为疏松且存在较多的缝隙和孔隙,同时硬质相结合不牢固,使得裂纹带附近硬度相较于同一水平其他无缺陷处的要低。     

INFLUENCE OF ASSOCIATED AND SEPARATE PRECIPITATION OF γ″ AND γ′ ON STRUCTURE STABILITY IN INCONEL718 AND MODIFIED ALLOYS

The precipitation behavior in Inconel 718 and modified alloys has been studied by means ofTEM.The structure of associated precipitation and compact morphology of γ″+γ′ were ob-tained by modifying the contents of Al,Ti and Nb.Experimental results show that the com-pact morphology of γ″+γ′ has the best structure stability at higher temperatures.Instead ofthe transformation γ″→δ in alloy 718 the dissolution of strengthening phases in modified alloyleads material degradation.     

一种改型Inconel718合金的组织与性能的研究

采用化学相分析、显微组织分析和显微硬度测试等方法,研究了一种改型镍基高温合金718A在标准热处理状态的显微组织和力学性能。试验结果表明,原型718合金和改型718A合金的主要析出相皆为γ″、γ′、δ、MC相。在标准热处理状态下,改型718A合金中强化相γ′和γ″的量较原型718合金明显增多,而对于组织稳定性不利的8相的析出量则较718合金少。此外,改型718A合金的显微硬度与原型合金相比有了明显的提高,因此具有广阔的应用前景。     

Cu过渡层对激光增材制造TC4/IN718双金属结构组织性能的影响

钛合金和镍基合金各具有独特性能,二者的双金属结构在航空航天领域具有广泛的应用前景。由于2种金属的物理与化学性能差异较大,易在结合区域产生开裂。基于直接激光沉积（DLD）技术,分别制备了无过渡层和Cu过渡层的TC4/IN718双金属结构。测试分析结果表明,无过渡层双金属结构,结合区形成大量Ti-Ni脆性相,导致裂纹敏感性提高,产生开裂;Cu过渡层的双金属结构,在TC4与IN718之间形成Cu过渡区,限制TC4与IN718直接结合,降低裂纹敏感性,无裂纹等冶金缺陷产生,由于冶金反应与元素扩散的发生,在Cu过渡区中形成了少量Ti-Cu与微量Ti-Ni化合物相;化合物相在双金属结构中的产生,使Cu过渡区维氏硬度最高约为5801.6MPa,但远低于无过渡层双金属结构的维氏硬度（最高约为8281 MPa）。     

The Effect of δ Phase on the Mechanical Properties of an Inconel 718 Superalloy

The relationship between the δ phase content and the mechanical properties of Inconel 718 superalloy is still uncertain in the scientific literature. This study investigated the effects of the amount of δ phase and of the grain size on the mechanical properties of an aged γ-matrix with γ′ and γ″ precipitates. The material in as-received condition in the form of a forged bar was solution-treated in different conditions and aged according to UNS7718 standard. The microstructures were characterized using optical, scanning, and transmission electron microscopy. Hardness and tensile tests were also conducted. After solution treatment, γ′ and γ″ phases are dissolved and δ phase volume fraction is reduced to a minimum amount only observed by TEM, resulting in an increase of the grain size and a decrease of hardness and strength. After aging, the precipitation of γ′ and γ″ occurs and the amount of δ phase increases. The volume fraction of δ phase varying from 0.30 to 1.38% and the grain size varying from ASTM 7 to 5 do not have a significant effect on the tensile properties and hardness.     

Brittle-to-ductile transition of a multiphase intermetallic alloy based on NiAl

The microstructure and mechanical behaviors of a multiphase β+γ+γ′+α-Cr Ni–Al–Cr alloy have been investigated. This alloy exhibits only 1% tensile ductility despite the presence of the continuous ductile phases. It is noted that both fractograph and BDTT are typically dependent on strain rate, with a two orders of magnitude increase in strain rate resulting in approximately a 150 K increase in BDTT. An asymmetry between the compressive and tensile yield strength was also observed at low temperature. SEM observation of the side surface of the fractured specimen indicated that the phase boundary seemed to be weaker than the fracture strength of the constituent phases at temperature below the BDTT; the random orientation relationship between the β phase and the γ/γ′ phases may result in difficulty in strain transfer from soft phase to hard phase, which, in turn, contributes to crack initiation at phase boundary.