Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

Strengthening of nickel -base superalloys for nuclear heat exchanger application

Strengthening mechanisms of nickel-base superalloys have been discussed with the background of the Japanese research and development activities in this field. As candidates for materials of intermediate heat exchangers which will be used for a future programme of nuclear steelmaking systems, two kinds of alloys have successfully been developed in Japan. The designs of these alloys have been reviewed from metallurgical aspects including their composition and creep properties. In addition to the conventional methods to strengthen these alloys, such as solid solution hardening or particle precipitation hardening, a grain-boundary precipitation strengthening due to tungsten-rich ₂ phase in the Ni-Cr-W system, would be expected as a further advanced method.     

Comparison of thermodynamic database models and APT data for strength modeling in high Nb content γ–γ′ Ni-base superalloys

In response to the increasing need for higher operating temperatures in advanced gas turbine engines, new alloying concepts are required to develop novel nickel-base superalloys with enhanced temperature capabilities. Recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys. In order to design, develop and fully exploit this innovative class of superalloys, an understanding of how alloying elements partition to each phase is essential. Using atom probe tomography (APT), compositions of the constituent phases were measured in four high Nb content γ–γ′ Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used in predicting the solid solution strength behavior of the four alloys. The differences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work.     

Comparative thermal fatigue resistance of several oxide dispersion strengthened alloys

The thermal fatigue resistance of several oxide dispersion strengthened (ODS) alloys has been evaluated through cyclic exposure in fluidized beds. The ODS nickel-base alloy MA 754 and ODS iron-base alloy MA 956 as well as four experimental ODS Ni-16Cr-4.5Al base alloys with and without Ta additions were examined. Both bare and coated alloys were subjected to up to 6000 cycles where each cycle consisted of a 3 minute immersion in fluidized bed at 1130°C followed by a 3 minute immersion in a bed at 357°C. Testing revealed that the thermal fatigue resistance of the ODS nickel-base alloys was excellent and about equal to that of directionally solidified superalloys. However, the thermal fatigue resistance of MA 956 was found to be poor. Metallographic examination of tested specimens revealed that, in general, the post-test microstructures can be rationalized on the basis of previous diffusion, mechanical property, and oxidation studies.     

新型镍基高温合金化学成分分析技术研究

研究了新型镍基高温合金中Ｔａ、Ｚｒ和Ｈｆ的分析技术，并探讨了ＩＣＰ－原子发射光谱法中的某些影响因素及其校正。该分析方法用于合金样品分析取得了满意的效果。     

X-ray determination of stresses in alumina scales on high temperature alloys

Abstract

The resistance of alumina scales to cracking and spalling under the influence of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coatings, and bond coats for thermal barrier coatings. However, the relative magnitudes of the stresses and their distribution are often not known. In this study several X-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both during oxidation and after cooling to room temperature. The corresponding stresses are being calculated using appropriate elastic constants. The results include the observations that: (1) Growth stresses are higher in alumina formed on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys (studies have not yet been performed on NiCrAl or CoCrAl alloys). (2) Yttrium additions to do not result in lower growth stresses in alumina scales on FeCrAl alloys even though the additions decrease the amount of lateral scale growth. (3) Growth stresses can be relaxed by plastic deformation of both the alloy and oxide. The implications of these results with regard to alumina adhesion are discussed.     

Mechanical property requirements for aero gas turbine materials

Abstract

The outstanding performance of current military and civil aero gas turbine engines is linked closely to the way in which modern design and manufacturing techniques have become totally integrated with materials designed specifically for operation within the hostile environment of a gas turbine. Advanced titanium alloys are used extensively throughout the compressor and nickel-base superalloys dominate materials application in the turbine. In spite of current achievements, the engine designer is still under severe competitive pressure to improve engine performance still further and this will inevitably lead to even more demanding material requirements. The present paper outlines the continuing trends in engine development and describes the impact these are having on materials technology in general and the mechanical property requirements of nickel-base superalloys in particular.

MST/512     

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Inhomogeneous Nickel-Base Superalloys

Recently, it has been shown that shot-peened nickel-base superalloys exhibit an approximately 1% increase in apparent eddy current conductivity at high inspection frequencies, which can be exploited for nondestructive subsurface residual stress assessment. Unfortunately, microstructural inhomogeneity in certain as-forged and precipitation hardened nickel-base superalloys, like Waspaloy, can lead to significantly larger electrical conductivity variations of as much as 4–6%. This intrinsic conductivity variation adversely affects the accuracy of residual stress evaluation in shot-peened and subsequently thermal-relaxed specimens, but does not completely prevent it. Experimental results are presented to demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence. This characteristic independence of frequency can be exploited to distinguish these inhomogeneities from near-surface residual stress and cold work effects caused by surface treatment, which, in contrast, are strongly frequency-dependent.     

微量元素对镍基高温合金微观组织与力学性能的影响

在高温环境中镍基高温合金具有良好的高温强度、抗氧化性能、抗腐蚀性能和抗疲劳性能,被广泛应用于航空航天等领域。镍基高温合金优异的综合性能与其微观组织紧密相关。综述了微量元素B, C, Y, Ce, Hf, Re, Ru, P对镍基高温合金微观组织及其力学性能的影响。针对不同的镍基高温合金,对微量元素的不同作用进行讨论分析。镍基高温合金微观组织及其力学性能与微量元素的含量及其分布有关。添加于镍基高温合金中的微量元素分布在合金基体或者其析出相中,通过偏聚于晶界处或者元素偏析等方式,改变合金的微观组织,从而影响其力学性能。     

Development of a Nickel-base Cast Superalloy with High Strength and Superior Creep Properties

Derived from Russian alloy CHS88U, six experimental Ni-base alloys named as A to F in the Ni-Cr-Co-W-Ti-Al-Hf system are designed, evaluated and processed. One of these alloys, F, shows excellent high temperature tensile strength and ductility with superior creep rupture properties. As predicted by using modeling tools such as PHACOM and NEW PHACOMP, there is hardly the tendency for formation of topologically close-packed phase (TCP) phase in alloy F. Furthermore, through microstructural observation, it is also found that no TCP phase is formed in alloy F after long-time exposure at high temperature. So alloy F has well balance of phase stability and mechanical properties in view of application for gas turbines. It is proved that d-electron approach can be applied for design and development of nickel-base superalloys for gas turbine application.     

人工神经网络法在镍基变形合金蠕变断裂寿命预测中的应用

在试验数据的基础上,利用人工神经网络建立不同成分镍基变形合金的不同温度,外应力与蠕变断裂寿命之间关系模型,并进行网络训练,对合金的蠕变断裂寿命进行模拟.结果表明,模拟结果与实测结果吻合良好,采用人工神经网络方法可以为镍基变形合金蠕变断裂寿命的预测提供一种有效的手段.     

Rafting in single crystal nickel-base superalloys — An overview

Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.     

High-temperature oxidation behaviour of base metal elements in nickel-base alloys

Oxidation of non-precious metals nickel, chromium and copper, nickel-base alloys in an air atmosphere was studied under differential thermal analysis. The results suggest that adding chromium and copper to high-purity nickel metal reduces the amount of oxidation and gives a slower oxidation than in untreated high-purity metals. It is then shown that the activation energy for oxidation in nickel-base alloys is almost the same as that in high-purity nickel metal when alloying elements are added to nickel.     

The structure and interdiffusional degradation of aluminide coatings on oxide-dispersion-strengthened alloys

A study of the effects of the composition and coating processing of oxide- dispersion-strengthened superalloys on the structure and diffusional stability of aluminide coatings was undertaken. Increasing the aluminum content of the substrate resulted in the formation of a more typical nickel-base superalloy aluminide coating structure that is more resistant to spallation during high temperature isothermal exposure. The coating application process also affected the coating stability; a low aluminum outward diffusion type resulted in greater apparent stability. A scanning electron microscopy deep etching and fractography examination technique was used in an attempt to establish the location and kinetics of void formation. Aluminide protective lifetimes are still found to be far short of the mechanical property capabilities of the alloy.     

Self-consistent Constitutive Modeling of the Creep Damage Behaviour of Nickel-base Directionally Solidified Superalloys with Different Grain Orientations

A self consistent creep damage constitutive model is developed for nickel-base directionally solidified superalloys. Grain degradation and grain boundary voiding are considered. The model parameters are determined from the creep test data of single crystal and directionally solidified superalloy with a special grain orientation. The numerical analysis shows that the model creep damage behaviours of nickel-base directionally solidified superalloys with difFerent grain orientations are in good agreement with the experimental data.     

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000?h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.     

Hot corrosion tests with the NRIM burner rig

Salt-induced high temperature corrosion of superalloys has been studied with a burner rig. Results of investigations into the effects of alloying. composotion on hot corrosion behaviour of nickel-base superalloys, the effects of testing condtions on hot corrosion behaviour of a high-tungsten alumina-forming nickel-base superalloy and hot corrosion of a CoCrAlY-coated superalloy are reported.     

CRACK GROWTH IN A TITANIUM ALUMINIDE ALLOY UNDER THERMAL MECHANICAL CYCLING

Abstract— The fatigue crack growth behavior in a titanium aluminide (Ti₃Al) alloy under thermo-mechanical loading as well as under elevated temperature conditions was investigated. The thermal mechanical fatigue crack growth behavior appears, in a general sense, to follow the same trends observed in similar data obtained in tests on nickel-base superalloys. However, crack growth in Ti₃Al appeared to be influenced by blunting of the crack tip due to creep in addition to a cyclic-dependent contribution together with time dependent or environmental enhanced degradation. This complex phenomenon in Ti₃Al is unlike that in nickel-base superalloys where crack growth was found to be due to a linear combination of cycle and time dependent contributions. Thus, the linear cumulative modelling technique is not applicable to the tested Ti₃Al.     

Gamma prime (γ′) precipitating and ageing behaviours in two newly developed nickel-base superalloys

Two cast nickel-base superalloys with superior creep rupture lives at high temperatures and high stresses were developed. In the present study, the microstructural characteristics of γ′-precipitates and the hardening behaviour were studied by electron microscopy and micro-hardness testing. The alloys were solution-treated at 1553 K for 2 h and air-cooled followed by step ageing heat treatments. Ageing up to 300 h at 1143, 1223 and 1293 K was carried out to examine the effects of ageing temperature and ageing time on the growth of γ′-precipitates. The growth kinetics of γ′-precipitates was analysed. The experimental evidence reveals that the morphology of γ′ does not change in shape during heat treatments. The γ′ is usually cubic in both Alloy B and Alloy C. Growth of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion of solute atoms despite the high volume fraction of γ′ and complex compositions in these two alloys. The activation energies for the growth are evaluated as 272 and 277 kJ mol^-1 for Alloy B and Alloy C, respectively, which correlate well to coarsening of γ′ in other commercial and developed superalloys. The hardness increases to a peak value at about 20 to 60 h for Alloy B and at about 10 to 20 h for Alloy C during ageing. The hardness of the alloys hardly decreases and still maintains a high value at high temperatures after passing the hardness peak. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hardening and phase stability in rapidly solidified Al–Fe–Ce alloys

Rapidly solidified (RS) Al–Fe–Ce alloys were prepared by melt spinning. The phases present and the thermal stability, at temperatures up to 500 °C, were then followed by X-ray analysis, chemistry, hardness and thermal analysis techniques. The results obtained indicated that the alloys studied have enhanced mechanical properties compared to commercial aluminium alloys and castings of the same alloy compositions, and the RS alloy also exhibit good stability up to about 300 °C; a result of stable second phase particles. It is suggested that these results indicate that there are two mechanisms responsible for the hardening and stability of the RS alloys: solid solution strengthening at lower temperatures, and semicoherent particles formed from supersaturated solid solution at higher temperature. The maximum hardness, after 2 h ageing occurred at about 300 °C. At higher temperatures the dispersed phase became incoherent with a dramatic loss in hardness.