Lanzhang Zhou

High temperature oxidation behavior of directionally solidified NiAl–31Cr–2.9Mo–0.1Hf–0.05Ho eutectic alloy

The isothermal oxidation behavior of NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho directional eutectic alloy was investigated with the help of scanning electron microscopy and X-ray diffraction. The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance in the temperature range of 900-1100 degrees C. When the temperature was up to 1150 degrees C, the continuous Al2O3 oxide film ruptured. Trace rare earth element Ho distributed uniformly in the alloy and relatively high level of Al in Cr(Mo) phase are beneficial to the formation of continuous and compact Al2O3 scale. During the oxidation, a phase transformation from theta-Al2O3 to alpha-Al2O3 existed on the surface of oxidation film. It resulted in the abnormal oxidation mass gain happening when the alloy was oxidized at 1000 degrees C or 1050 degrees C.

Microstructural evolution and mechanical properties of new multi-phase NiAl-based alloy during heat treatments

Microstructural evolution and the relationship between microstructure and property during heat treatments in a new NiAl-based alloy (Ni-26.6Al-13.4Cr-8.1Co-4.3Ti-1.3W-0.9Mo, molar fraction, %)) were investigated. The as-cast alloy is composed of NiAl matrix and Cr(3)Ni(2) phase with poor ductility. The Cr(3)Ni(2) phase is distributed as a network along the NiAl grain boundaries. Subsequent heat treatment (1 523 K, 20 h, air cooling+1 123 K, 16 h, furnace cooling) leads to the dissolution of Cr(3)Ni(2) phase and the precipitation of lath-shaped Ni(3)Al phase and alpha-Cr particles, resulting in the improvement of compressive properties and fracture toughness at room temperature. Followed by long-term thermal exposure (1 173 K, 8 500 h), it is found that the residual Cr(3)Ni(2) phase keeps stable while the alpha-Cr particles coarsen and a great mass of lath-shaped Ni(3)Al precipitates are degenerated, which compromises most of the above improvements of mechanical properties through heat treatment.

Microstructure and mechanical properties of NiAl-Cr(Mo)-Hf/Ho near-eutectic alloy prepared by suction casting

Abstract The microstructure and mechanical properties of 0.15 at.% Hf and 0.2 at.% Ho doped Ni-33Al-28Cr-6Mo near-eutectic alloys prepared by conventional casting and suction casting were investigated. The results reveal that the addition of Hf and Ho results in the formation of Ni2AlHf and Ni2Al3Ho phases, respectively, along the NiAl and Cr(Mo) phase interface in intercellular regions. Compared with the conventional-cast alloy, the microstructure of suction-cast alloy is well optimized, characterized by the thin interlamellar spacing, high proportion of eutectic cell area and fine homogeneously distributed Ni2AlHf and Ni2Al3Ho phases. Furthermore, the room temperature mechanical properties of the suction-cast alloy improve significantly, which can be attributed to fine microstructure, uniform distribution of the Ni2AlHf and Ni2Al3Ho phases and solid solubility extension.

铸造镍基高温合金中初生 M C碳化物的退化过程和机理

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During longterm thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+g →M6C+g′, MC+g→M6C + M23C6+ g′ and MC+g→M6C + M23C6+h. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of g′ phase, coarsening of grain boundaries and precipitation of topologically closepacked (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 °C for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the g matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the g matrix, while Ti, W and Mo come from both primary MC and g matrix. The precipitation of h phase is determined by *国家自然科学基金项目51001101和国家能源局项目NY20150102资助 收到初稿日期: 2015-07-17,收到修改稿日期: 2015-11-24 作者简介:孙文,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00399 第455-462页 pp.455-462

Ni-Fe Based Alloy GH984G Used for 700 °C Coal-Fired Power Plants

To meet the requirements of 700 °C ultra-supercritical (USC) coal-fired power plants, a new Ni-Fe based alloy (GH984G) has been developed. GH984G has excellent creep strength, thermal stability and resistance to oxidation/corrosion. Harmful phases were not observed even if after thermal exposure at 650–750 °C for 35,000 h. After exposure at 700 °C for 30,000 h, the yield strength has no obvious change. Its 105 h creep strength at 700 °C is up to 137 MPa and comparable to the level of 617B. The oxidation/corrosion evaluate at 700–800 °C up to 2000 h shows that it can meet the requirements of 700 °C USC coal-fired power plants. Currently, the pipes and tubes with different sizes have been manufactured. Moreover, it is an economic alloy due to the elimination of Co and containing more than 20% Fe. This makes it an interesting alternative for 700 °C USC coal-fired power plants and is now being evaluated at test platform for key components of China.

Precipitation Behaviors of M 23 C 6 Carbides and γʹ Phase in Cast IN617B Alloy During Long-Term Aging

The precipitation behaviors of M23C6 carbides and γʹ phase in cast IN617B alloy during long-term aging was systematically investigated by comparison with previous studying results about wrought IN617B alloy. The results showed that the morphology of M23C6 carbides and the nucleation location of γʹ particles in cast IN617B alloy were distinctly different from those of wrought IN617B alloy due to the difference of processing method. The microstructure observation revealed that the morphology of M23C6 carbides at grain boundaries (GBs) transformed from film-like shape into cellular shape owing to the reduction in the interfacial energy of GBs. Within the grains the bar-like M23C6 carbides and spherical γʹ phase directly precipitated from the γ matrix. Moreover, the growth of M23C6 facilitated the formation of γʹ phase by enriching γʹ-formation elements, and conversely, dense γʹ phase inhibited the coarsening and decomposition of M23C6 by constraining the diffusion of M23C6-formation elements. In addition, the mechanism of inhibiting effect of γʹ phase on the coarsening of bar-shaped M23C6 was discussed.

Corrosive-Wear Properties of Two NiAl Alloys in Sulfuric Acid Solution

The corrosion wear properties of Ni Al- 2.5Ta- 7.5Cr- x(0, 1)B alloys in 5% H2SO4 solution have been investigated at room temperature. The results show that in comparison with Ni Al-2.5Ta-7.5Cr alloy, Ni Al- 2.5Ta- 7.5Cr- 1B alloy exhibited better corrosion resistance to 5% H2SO4 solution with passive current density 0.299(μA·cm-2)and free corrosion potential-0.213 V. For which the key factor is that a passive film could form on the surface of Ni Al- 2.5Ta- 7.5Cr- 1B alloy. The corrosion wear rate of Ni Al- 2.5Ta-7.5Cr-1B alloy decreased 2-8 times compared with Ni Al-2.5Ta-7.5Cr alloy. The abrasion erosion mechanism for Ni Al-7.5Cr-2.5Ta-1B alloy was dominated by the synergistic effect of corrosion wear and protectiveness of passive film. While that for Ni Al- 7.5Cr- 2.5Ta alloy was mainly corrosion wear and abrasive wear.