Xishan Xie

Interfacial segregation and cosegregation behaviour in a nickel-base alloy 718

Abstract The Auger analyses provided clear evidence of sulfur and phosphorus segregation to grain boundaries and carbide/matrix interfaces. The degree of segregation was different for the two elements. Sulfur was preferentially segregated at the carbide/matrix interface while phosphorus was mainly distributed on grain boundaries. It was also found that metal elements, such as Mo and Nb also segregated to grain boundaries, these grain boundary segregation results show the existence of the attractive interactions of phosphorus with metal elements of Mo and Nb at grain boundaries, especially the co-segregation of Mo–P. It did not find any interaction of sulphur with metal elements at grain boundaries.

Investigation of the abnormal effects of phosphorus on mechanical properties of INCONEL718 superalloy

Abstract The effects of phosphorus on mechanical properties of the world’s most widely used superalloy INCONEL718 was examined. The results indicate that P has little effect on the tensile properties of INCONEL718 at room temperature, 500 or 650°C, but it increases stress rupture life and ductility. Phosphorus also improves the creep property and crack propagation resistance of INCONEL718. Further microstructure analysis by means of optical, SEM and TEM does not reveal the obvious effect of P on grain size and precipitation of INCONEL718. Some theoretical discussions were held aiming at further understanding the role of P in INCONEL718. It was suggested that the interaction of P–GB–M (metal element) complex might be responsible for this.

Experimental Investigation and Thermodynamic Calculation on Phase Precipitation of INCONEL 740

A novel nickel-based superalloy INCONEL 740 was under development for application in ultra-supercritical superheater tubers above 750°C. The precipitation behaviors of η phase and γ′ particles of the alloy were investigated through experimental study and phase computation. Experimental results showed that η phase formed a Widmanstatten pattern structure following long-term exposure at elevated temperatures and that the coarsening of γ′ particle follows a cube rate law: r−3 ∝ t. Thermodynamic calculation results showed that Al and Ti had an important effect on the precipitation behavior of γ′ and η phases. Two suggested novel modified alloys, wherein the Al and Ti contents were modified, were designed and melted for the experimental study. The preliminary results indicated that the modified alloys exhibited higher structural stability following long-term exposure at 750°C till 5000h.

Molecular dynamics simulation on phosphorus behavior at Ni grain boundary

Phosphorus is commonly regarded as a common impurity and detrimental element in most Fe-base and Ni-base alloys. To study the problem in detail, it is feasible to use computer simulation techniques, which can investigate events on a much finer space and time scale than can be reached with presently available experimental techniques. The study of fracture and embrittlement at the atomic scale often used molecular dynamics (MD) and quantum-chemical methods. The most important and difficult step of molecular dynamics simulation is the determination of interatomic potential. As the first step of studying on the behavior of phosphorus in Ni-Cr-Fe system, the goal of this paper is to study the behavior of phosphorus in Ni grain boundary.

The appearance of magnesium and its effect on the mechanical properties of Inconel 718 with low sulfur content

The appearance of magnesium and its effect on the mechanical properties of Inconel 718 with low sulfur content were investigated. The results show that magnesium has almost no effect on the tensile properties of Inconel 718 at room temperature, 500°C and 650°C. When the content of sulfur is lower than 10 ppm, magnesium can decrease the stress rupture life of Inconel 718 at 650°C, 686 MPa. The fractography and microstructure analysis by means of scanning electron microscopy (SEM) reveal that magnesium has little effect on the rupture method and microstructure of Inconel 718, i.e. grain size and δ-phase, etc. The Auger research shows that there is no segregation of magnesium at grain boundaries. The main appearance method of magnesium is Mg(O,S).

Superalloys for Advanced Ultra-Super-Critical Fossil Power Plant Application

Superalloys are world-wildly used not only for aerospace but also for chemistry, oil & gas and power engineering application. In recent years the 700 °C level Advanced UltraSuper-Critical (A-USC) technology with high thermal efficiency is developing in the world to reduce the coal consumption and pollution emissions. Any kind of ad‐ vanced ferritic and austenitic heat-resisting steels can not meet 700 °C A-USC technology requirement. In this case high quality Ni-base superalloys must be adopted for 700 °C A-USC technology. The research and development of Ni-Fe and Ni-base superalloys such as HR6W, GH2984, Haynes 230, Inconel 617/617B, Nimonic 263, Haynes 282, Inconel 740 and 740H are reviewed in this chapter.

Effects of the shape and size of rectangular inclusions on the fatigue cracking behavior of ultra-high strength steels

The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy (SEM). The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.

Stress Relaxation Behavior Comparison of Typical Nickel-Base Superalloys for Fasteners

In the present work, stress relaxation behavior of three typical nickel-base Superalloys: Alloy 718, Waspaloy and AEREX 350, were investigated in a temperature range of 600–800 °C for up to 10 h. The effects of stress relaxation parameters on behavior were analyzed and stress relaxation characteristics of the three superalloys are compared and mechanisms revealed by FESEM and TEM observation. The study results show that the stress relaxation property of Alloy 718 is very sensitive to temperature. It is very stable at 650 °C, but decreases extremely with temperature increasing to 750 °C as a result of serious microstructure degeneration. Moreover, stress relaxation stability is related with initial stress, and appropriate increase of initial stress under normal service temperature can increase the stress relaxation limit of Alloy 718. The stress relaxation resistance of Waspaloy decreases with increasing temperature. The increase of initial stress and initial strain is beneficial for stress relaxation resistance of Waspaloy, but the influencing degree is related with temperature. Furthermore, Waspaloy with heat treatment A (1020 °C × 4 h/AC + 845 °C × 4 h/AC + 760 °C × 16 h/AC) shows better stress relaxation resistance than that with heat treatment B (1080 °C × 4 h/AC + 845 °C × 24 h/AC + 760 °C × 16 h/AC). In addition, stress relaxation stability of AEREX 350 is the best among the three superalloys in the temperature range of 600–800 °C on the whole. The combined effect of γ′ phase and η phase guarantee the stress relaxation property of AEREX 350.