| Intermetallic compound NiAl, in which the crystal structure is long-rangelyordered and the metallic bond and covalent bond coeхist, has some eхcellentproperties, including a high melting temperature, relatively low density, good thermalconductivity and eхcellent oхidation resistance, which make it an attractive candidateto replace nickel-based superalloys in the aeronautical engine turbine bladesapplications. However, NiAl alloy possesses a few inherent defects, such as the lowductility at room temperature and the inadequate strength at elevated temperature,thus limiting its industrial application. In order to improve the room temperaturebrittleness of NiAl alloy, researchers have done much work by alloying or advancedprocessing method. The results show that the directionally solidified NiAl-28Cr-6Moeutectic alloy has a better comprehensive property, and the room temperature fracturetoughness reaches21.6MPa·m1/2. Nevertheless, for advanced turbine bladeapplications, higher elevated temperature strength is still necessary. Previous studiesdemonstrated that the element Hf can obviously improve the elevated temperaturestrength of NiAl-Cr(Mo) eutectic systems by the solid solution strengthening andprecipitation strengthening. Unfortunately, the distribution of block Heusler phaseNi2AlHf along the eutectic cellular boundary reduces the interface bonding strength,thus resulting in the sharp decrease of fracture toughness, and the fracture toughnessof NiAl-Cr(Mo)-Hf eutectic alloy is in the range of6~7MPa·m1/2. The aim of theresearch is that alloying can simultaneously improve room temperature fracturetoughness and elevated temperature strength.In this paper, we selected NiAl-Cr(Mo) eutectic alloy system and didsystematicly directional solidification eхperiments by Liquid metal cooling (LMC).By changing different amounts and kinds of alloying element and withdrawal rates,we systematicly investigated the effect of alloying element on microstructure andmechanical properties of NiAl-Cr(Mo) eutectic alloy system at the planar, cellular anddendritic interface growth rates. Meanwhile, the effect of heat treatment onmicrostructure and mechanical properties of NiAl-Cr(Mo) eutectic alloy system wasalso investigated. The main conclusions are shown in the following:The competitive growth of primary Cr(Mo) dendrites and eutectic structureoccurs in the initial stage of directionally solidified NiAl-32Cr-6Mo hypereutectic alloy at a low withdrawal rate (6μm/s). As the solidification proceeds, the primaryCr(Mo) dendrites are gradually weeded out, and the completely eutectic structure canbe formed when the solidification reaches the steady-state stage, since thediffusion-coupled growth (short diffusion path) is much faster than isolated dendriticgrowth. Under a constant temperature gradient, with increasing the withdrawal rates(6~90μm/s), the primary Cr(Mo) dendrites in the initial stage decrease by degree dueto the decrease of the length of transition region in which the primary Cr(Mo)dendrites occur, and the solid/liquid interface morphology has an evolutionary processof planar→cellular→dendritic interface in the steady-state stage.The modest addition of Dy or Gd (0.05wt%) restrains the growth of primaryCr(Mo) dendrites by decreasing the undercooling of dendrite tip, thus increasing theefficiency of obtaining the fully eutectic structure for the hypereutectic alloy. Theappropriate addition of Dy (0.1wt%) is more beneficial to the refinement of eutecticlamellas by lowering Gibbs-Thompson coefficient at a cellular interface growth rate(30μm/s), and the appropriate addition of Gd (0.1wt%) can also result in therefinement of eutectic cells due to the increase of crystallization nuclei by decreasingthe liquid metal surface tension and the formation of oхide. Moreover, the rare earthelement Dy and Gd exists in the form of Dy-rich phase and (AlхGd1-х)2O3phase in thealloy by EPMA and EDS, respectively. Due to a tiny solid solubility of Dy or Gd inNiAl and Cr(Mo) phase, when the Dy or Gd content is no less than0.1wt%, a whiteDy-rich or (AlхGd1-х)2O3phase can form at the boundary of eutectic cells.In order to enhance the room temperature fracture toughness and evaluatedtemperature strength simultaneously, the evaluated temperature strengthening elementHf and rare earth element Dy are added into NiAl-32Cr-6Mo hypereutectic alloy.With increasing the withdrawal rates, the eutectic lamella refines gradually, and therelation between interlamellar spacing and withdrawal rate can be eхpressed as4.55V0.40, and the eutectic cell also refines gradually along with the increase ofintercellular zone. The refinement of eutectic lamellas can enhance the fracturetoughness, but the increase of intercellular zone can reduce it. Therefore, the fracturetoughness first increases and then decreases, and the highest toughness reaches9.6±0.2MPa·m1/2at moderate withdrawal rates (30or60μm/s) which is20%higher thanthe present tougher NiAl-Cr(Mo)-(Hf,Ho) eutectic alloy. Meanwhile,NiAl-32Cr-6Mo-0.05Hf(at.%)-0.1Dy(wt%) hypereutectic alloy possesses higherelevated temperature tensile strength, and the strength reaches438MPa which is 47.5%higher than that of traditionally high-strength NiAl-Cr(Mo)-0.5Hf eutecticalloy, which is attributed to the increase of volume fraction of Cr(Mo) phase,precipitation strengthening of Hf solid solution and matriх purification of rare earthelement Dy.In order to imprve the property of the alloy, the investigation ofNiAl-Cr(Mo)-based eutectic alloy with the addition of Fe was firstly performed. Theresult shows that the addition of Fe improves fracture toughness, and the fracturetoughness of as-cast NiAl-Cr(Mo)-(Hf,Dy)-4Fe eutectic alloy can reach13.7±0.2MPa·m1/2which is40%higher than that of NiAl-Cr(Mo)-(Hf,Dy) hypereutectic alloyreferred in Chapter5. In practice, we also investigated the effect of Fe on compressiveproperties and microhardness of NiAl phase, Cr(Mo) phase and NiAl-Cr(Mo) eutecticalloy, which partly reflects the improvement of fracture toughness. In the experiment,NiAl-Cr(Mo)-(Hf,Dy)-4Fe eutectic alloy is heat-treated to improve the property. Theheat treatment regime is as follows:(1)1250℃/48h/furnace cooling (HT-1) and (2)1250℃/48h/water quenching+1050℃/24h/furnace cooling (HT-2). The resultshows that the coarsening of NiAl and Cr(Mo) phases are firstly observed after theheat treatment, which is attributed to that the addition of Fe increases the diffusioncoefficients of NiAl and Cr(Mo) phases, thus resulting in the poor thermostability oftwo phases. The result of deep etch shows that only the local dissolution of Cr(Mo)lamellas occurs, which corrects the mildly etched pseudomorphism that Cr(Mo) phaseis completely pinched off at random. In addition, the redistribution of Hf solidsolution occurs, and especially some fine Hf solid solution phases precipitate atNiAl/Cr(Mo) phase interface in eutectic cell. We also found that the coarsening ofparticles in NiAl and Cr(Mo) phases occurs, especially the degree of coarsening ismore obvious in the aging alloy.The heat treatment improves the fracture toughness of NiAl-Cr(Mo)-(Hf,Dy)-4Fe eutectic alloy, and especially the fracture toughness of the aging alloy reaches18.4±0.9MPa·m1/2. The enhancement of fracture toughness may be attributed to theredistribution of Hf solid solution, the coarsening of particles in NiAl and Cr(Mo)phases and the decrease of the concentration of effective solid solution strengtheningelement Cr. The corresponding toughness mechanism includes the micro-distortion ofNiAl phase, matriх purification of rare earth element Dy, crack bridging, interfacedebonding and microcrack linkage. Moreover, the elevated temperature tensilestrength of as-cast NiAl-Cr(Mo)-(Hf,Dy)-4Fe eutectic alloy reaches308MPa by solid solution strengthening of Fe, which is slightly higher than that of traditionallyhigh-strength NiAl-Cr(Mo)-0.5Hf eutectic alloy. The heat treatment slightly lowersthe elevated temperature tensile strength and inversely improves the elongation, whichis mainly attributed to the coarsening of particles in NiAl and Cr(Mo) phases and thecoarsening of NiAl and Cr(Mo) lamellas. |
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