一种镍基单晶合金的组织演化与蠕变行为

通过蠕变曲线测定及组织形貌观察,研究了一种镍基单晶合金的蠕变行为和变形特征.结果表明:单晶合金在试验的温度和应力范围内,对施加应力和温度有明显的敏感性.由所得数据测算出合金的蠕变激活能和应力指数.蠕变初期在施加温度和应力场的作用下,立方γ′相逐渐转变成与施加应力轴方向垂直的N型筏状结构.稳态蠕变期间,合金的变形机制是位错攀移越过筏状γ′相,由于高温蠕变稳态阶段形成的N型γ′相筏状组织厚度较小,位错易于攀移,因而合金具有较大的应变速率.蠕变后期,由于塑性变形,在近断口处筏形γ′相转变成与应力轴方向呈45°角的形貌,合金的变形机制是位错剪切筏状γ′相.     

Surface recrystallization of a single crystal nickel-base superalloy

The recrystallization behavior of a single crystal nickel-base superalloy was investigated by shot peening and subsequent annealing. Two kinds of recrystallization microstructures, which are intensively dependent on the annealing temperature, are shown in the nickel-base superalloy after shot peening and subsequent annealing. Surface recrystallized grains are obtained when the superalloy is anparticles occurs. Cellular recrystallization is observed after annealing at lower temperatures. Cellular structures induced by high diffusivity of the shot-peened alloy annealed at 1050℃ accords with the Johnson-Mehl-Avrami-Kolmogorov equation. The low Avrami exponent is caused by the inhomogeneous distribution of stored energy, the decreasing of stored energy during recovery, and the strong resistance of boundary migration yb γ' particles.     

Influence of solid/liquid interfaces on the microstructure and stress-rupture life of the single-crystal nickel-base superalloy NASAIR 100

The [001] oriented single crystals of nickel-base superalloy NASAIR 100 with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces were prepared, respectively, and their microstructure and stress-rupture behavior at 1050 °C were investigated in both as-cast and solution heat-treated conditions. It was found that in as-cast single crystals of NASAIR 100, microsegregation and γ/γ′ eutectic produced in the solidification process increased, γ′ size decreased, and γ′ shape tended progressively to be cuboidal, with the successive transition of the S/L interface from planar to cellular, then to coarse-dendritic, and finally to fine-dendritic morphology. Furthermore, the solution temperature required to dissolve all as-cast γ′ and most of the γ/γ′ eutectic increased with the aforementioned successive transition of S/L interfaces. The reprecipitated γ′, after solution heat treatment (SHT), was usually fine and cuboidal. However, some W-rich phase was present in the heat-treated dendritic single crystals. Both the planar and the cellular single crystals of NASAIR 100 exhibited no superiority in stress-rupture life, irrespective of the heat-treatment conditions. Instead, the single crystals with dendritic morphology possessed excellent stress-rupture lives, after heat treatment at 1320 °C for 4 hours, followed by air cooling (AC). Perfect γ′ rafts with high-average aspect ratios formed during the stress-rupture tests of dendritic single crystals; in contrast, irregularly coarsening structures appeared in both the planar and cellular single crystals. The microstructure and solution behavior were illustrated in detail. Furthermore, the microstructural factors to affect the high-temperature stress-rupture life of the single crystals of NASAIR 100 were also analyzed.     

The effects of environment on the elevated temperature fatigue behavior of nickel-base superalloy single crystals

The effects of air and vacuum on the fatigue behavior of a nickel-base superalloy, Mar-M200, in single crystal form were investigated. Between 800° and 1400°F fracture is entirely in the Stage I mode in air and vacuum, and fatigue life is unaffected by environment. At 1700° F in both environments, fracture is predominantly in the Stage II mode and fatigue life in air is greater than that in vacuum. At both temperatures, fatigue cracking in air is internally initiated, whereas in vacuum cracking is generally initiated at the specimen surface. Identical fatigue lives in air and in vacuum between 800° and 1400° F are attributed to the fact that internally initiated cracks in air are actually propagating in a high vacuum, surface cracking being inhibited by dynamic oxidation of emerging surface slip offsets. The subsurface portion of the Stage I fracture surface produced in air tests and all of the Stage I fracture produced in vacuum tests shows a dimpled structure, whereas the Stage I fracture surface produced while the crack propagation is in air shows a cleavage appearance. At 1700° F, bulk oxidation of surface initiated cracks interferes with the plastic blunting mechanism of Stage II crack growth normally observed at this temperature, internally initiated cracks causing ultimate failure. Shorter lives in vacuum are thought to result from enhanced Stage II surface crack propagation. Formerly with Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.     

Influence of precipitate morphology on intermediate temperature creep properties of a nickel-base superalloy single crystal

The influence of γ’ precipitate morphology on the-creep behavior of the single crystal nickel-base superalloy NASAIR 100 at 760-°C was investigtated. As-heat treated crystals with cuboidal γ’ particles and crystals given an additional pre-rafting treatment to form a continuous lamellar structure were creep tested at stress levels which produced rupture lives ranging from 40 to 2500 hours. At high applied stresses, the crystals with cuboidal γ’ had both lower minimum creep rates and longer rupture lives than the crystals with lamellar γ. At lower stress levels, the initially cubic γ’ material maintained a lower crep rate, but exhibited a similar rupture life compared to the pre-rafted crystals. Examination of the microstructures which developed during creep indicated that dislocations could shear the semi-coherent γ’ rafts relatively easily compared to the coherent cuboidal γ’. In tests at lower applied stresses, slow directional coarsening of the initially cuboidal γ’ resulted in the development of a lamellar structure similar to that in the pre-rafted material, such that the rupture lives of the two materials were similar.     

一种单晶合金的高温蠕变行为及其变形特征

通过测定一种单晶镍基合金的高温拉伸蠕变曲线及位错运动的内摩擦应力σ0,建立了综合蠕变方程,计算出稳态蠕变期间的表观蠕变激活能及相关参数.结果表明：在蠕变期间,位错运动的内摩擦应力σ0,随外加应力的提高略有提高,随温度的升高而明显降低.蠕变后期,由于缩径使样品不同位置承受不同的有效的应力,导致筏状γ＇相具有不同的粗化特征,在近断口处,载荷的有效应力增大,使筏状γ＇相扭曲且粗化加剧.界面位错网对形变硬化和回复软化具有协调作用,并减缓位错切入γ＇相,因此有利于合金蠕变抗力的提高.     

Deformed microstructure of the single-crystal superalloy NASAIR 100 at 1050 °C

The microstructure of the [001] oriented single-crystal superalloy NASAIR 100, solidified respectively with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces, deformed at 1050 °C and 160 MPa, was investigated with the attempt to reveal the variation of γ′ structures in the necked zones of creep specimens and, furthermore, to explain that the dendritic single crystals constantly possessed superior stress-rupture lives. During the stress-rupture tests, it was found that directional coarsening of γ′ precipitates occurred and that γ′ rafts perpendicular to the tensile load axis, [001] orientation, formed. The γ′ rafts in the necked zones of tensile creep specimens were thickened significantly and were no longer perpendicular to, but gradually inclined to, the tensile load axis. Additionally, the γ′ rafts were much thicker and showed less perfect morphology in both the planar and cellular single crystals than in the dendritic single crystals. From the observation of dislocation configurations, it was evident that the shearing of γ′ precipitates was the main movement of dislocations when passing the γ′ particles. At the primary creep stages of both the planar and cellular single crystals, the slip of dislocations was inhomogeneous. However, the shearing of γ′ precipitates in the dendritic single crystals was homogeneous, resulting in higher creep resistance.     

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.     

A fractographic study of Stage I fatigue cracking in a nickel-base superalloy single crystal

An electron fractrographic study was made of opposing crystallographic (Stage I) fatigue fracture surfaces of specimens of a single crystal nickel-base superalloy, low carbon MAR-M200, that were tested at room temperature. In regions near the crack initiation site, features on both surfaces are the same. In regions at some distance from the initiation site significant differences in the type and extent of microfeatures were observed. For a crack propagating in an upward direction, irregular markings were observed on the upper fracture surface and regularly spaced slip offsets and slip band cracks were observed on the lower surface. These observations are explained by a consideration of the elastic stress field and the resultant glide forces on all possible slip systems surrounding a crack growing at an angle to the principal stress direction. Additionally the results are used to support a previously proposed model for Stage I fatigue crack propagation.     

Creep-rupture behavior of a directionally solidified nickel-base superalloy

The creep-rupture behavior of the directionally solidified (DS) nickel-base superalloy DZ17G has been investigated over a wide stress range of 60 to 950 MPa at high temperature (923 to 1323 K). In this article, the detailed creep deformation and fracture mechanisms at constant load have been studied. The results show that all creep curves exhibit a short primary and a dominant accelerated creep stage, which results in higher ductility of DS superalloy DZ17G compared to the conventionally cast alloy. From the creep parameters and transmission electron microscopy (TEM) observations, it is suggested that the dominant creep deformation mechanism has a change from gamma prime particles shearing by matrix dislocations in high stress region to dislocation climb process in low stress region. It is found that the fracture mode of DS superalloy DZ17G is transgranular, and it is controlled by the propagation rate of creep cracks initiated at both surface and inner microstructure discontinuities. The creep rupture data follows the Monkman-Grant relationship under all the explored test conditions.     

Yielding and deformation behavior of the single crystal superalloy PWA 1480

Interrupted tensile tests were conducted to fixed plastic strain levels on (001) oriented single crystals of the nickel-base superalloy PWA 1480. Testing was done in the range from 20 to 1093 °C, at strain rates of 0.5 and 50 pct/min. The yield strength was constant from 20 to 760 °C, above which the strength dropped rapidly and became a strong function of strain rate. The data could be represented very well by an Arrhenius-type equation, which resulted in three distinct temperature regimes. The deformation substructures could also be grouped in the same three regimes, indicating that there was a fundamental relationship between the deformation mechanisms and the activation energies. At low temperatures, the activation energy for yielding was zero, and the deformation was dominated by γ′ shearing by pairs of 111a/2(110) dislocations. At high temperatures, the true activation energy for yielding was calculated to be 500 kJ/mol, which is indicative of a diffusion-controlled process, and deformation was dominated by γ′ by-pass. Intermediate temperatures exhibited transitional behavior. No currently available precipitation hardening model could adequately describe the behavior observed in the low temperature regime, due to the observation that penetration into the precipitate was not rate-limiting at all temperatures. In the high temperature regime, the functional form of the Brown-Ham by-pass model fit the data fairly well. The results of this study also demonstrated that the initial deformation mechanism was frequently different from that which would be inferred by examination of specimens which had been tested to failure.     

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.     

Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale. Formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University     

Calculation of the internal stresses and strains in the microstructure of a single crystal nickel-base superalloy during creep

The evolution of internal stresses and strains in the microstructure of a single crystal nickel-base alloy during annealing and during creep in [001] direction has been calculated using a visco-plastic model. Two limiting conditions are considered: an “overloading” case where the internal stresses reach the critical resolved shear stress of the whole γ′ volume and an “underloading” case where the critical resolved shear stress of the γ′ precipitate is reached only at distinct areas. During creep deformation a triaxial stress state evolves in the microstructure and large pressure gradients are built up. The influence of an initial coherency misfit is shown to be negligible after short times of creep. The calculations allow the prediction of flow patterns in the microstructure, creep-induced lattice parameter changes, type and arrangement of interfacial dislocations and of the dependence of the stationary strain rate on the cube or plate morphology of the γ′ phase.     

Effect of orientation on crystallographic cracking in notched nickel-base superalloy single crystal subjected to far-field cyclic compression

The effects of crystallographic orientation on the fatigue crack initiation and growth under far-field cyclic compression are discussed in single crystals of nickel-base superalloy MAR-M200. Results indicate that cracking occurs primarily due to planar slip on the {111}-type planes. Crystallographic cracking can occur on two or more slip planes simultaneously, but through-thickness cracks are not observed. In addition, it has been shown that the threshold stress intensity for crack initiation shows a strong dependence on orientation. The threshold stress intensity for crack growth in cyclic compression is 5 to 10 times the threshold for crack growth in cyclic tension.