FABRICATION OF A Ni3Al/Al2O3UNIDIRECTIONAL COMPOSITE BY PRESSURE CASTING

A gas pressure liquid metal infiltration technique for producing fiber reinforced Ni₃Al/Al₂O₃ and Ni₃Al/Al₂O₃ composites has been described. Composite bars of 9.5 mm diameter, 150 mm long were produced. It was found necessary to add 0.5 at.%Ti to both Ni₃Al and NiAl alloys to obtain adequate wetting. There was a strong tendency for liquid metal infiltration along one side of the fibers causing a shift of the fibers to the opposite side. No interface reaction zone was observed at optical magnifications up to 1000X. A composite of Ni₃Al containing 33 volume percent of FP fibers fractured at 890 MPa which is approximately 3 times the yield strength of the matrix IC-50(21.73A1-0.34Zr-0.IB-Balance Ni at.%) + 0.5 at. %Ti.     

Nickel Enriched Diffusion Layers on a NiAl Alloy

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. Formation of Ni₅Al₃, Ni₂Al, Ni₃Al phases which influence the extent of martensitic transformation in NiAl have been studied up to now with controversial results. We have investigated (using SEM and local elemental analyses) the microstructure of nickel enriched surface layers on a Al-79 wt.% Ni alloy. The layers were prepared by diffusion annealing and subsequently given two different heat treatments: at 930°C outside the Ni₅Al₃ region and at 500°C within the Ni₅Al₃ region of the phase diagram. In the specimen which was only diffusion annealed separate islands of Ni₅Al₃ phase elongated in the direction of the concentration gradient could be recognized within the nickel enriched surface layer. In the samples additionally annealed at 500°C, a well defined continuous layer of the Ni₅Al₃ phase situated 0.4 mm below the specimen surface was found. In the samples annealed at 930°C, isolated Ni₃Al precipitates were observed. Their number and size gradually increased with increasing nickel content.     

Ni3Al金属间化合物多孔材料的制备及抗腐蚀性能

使用粉末冶金模压成形和无压反应烧结方法制备出Ni3Al金属间化合物多孔材料,研究了反应过程中Ni3Al金属间化合物多孔材料的体积膨胀、孔结构参数、组织形貌,以及在KOH溶液中的抗腐蚀性能.结果表明:烧结后Ni3Al金属间化合物多孔材料发生了显著膨胀,最大孔径和开孔隙度都随着温度的升高而升高,当温度到达750℃时体积膨胀...     

Rapidly Solidified and Annealed Powders of Ni3Al

Rapidly solidified powders of stoichiometric Ni₃Al (B,Ti) were characterized, both as received and after short anneals. Powders generally exhibit spherical morphologies; deviations arise from particle collisions. In the as-received state the stoichiometric Ni₃Al exhibits both lamellar and dendritic structures but the Ni₃Al (B,Ti) contains only dendrites. Only small compositional variations exist across lamellae or dendrites. The as-received powders are only partially ordered. Annealing homogenizes the microstructure and produces strongly ordered structures in which most particles develop large grains. Hardness decreases during annealing. No cracks were found around microhardness indentations on any powders, indicating that Ni₃Al exhibits ductility under compression.     

Processing and Characterization of Fiber Reinforced Intermetallic Matrix Composites

A series of intermetallic matrix composites reinforced with Al₂O₃ based fibers were fabricated by pressure casting. The Al₂O₃ based fibers used were DuPont's 20 μm diameter Fiber FP and PRD-166 fiber, Mitsui's 10 μm diameter Almax fiber, and Saphikon's 125 μm diameter single crystal Al₂O₃ fiber. The intermetallic matrices employed were alloys based on Ni₃Al, NiAl, Fe₃Al, Ti₃Al+TiAl, and Nb₂Al+NbAl₃. Optical, scanning and transmission electron microscopy were used to investigate the microstructure of the composites and the fibers. Tensile testing was conducted to determine the Weibull mean strength of the fibers in the as-received and heat treated conditions. The effect of fiber gage length on the Weibull mean strength of the PRD-166 and Fiber FP was evaluated. Indentation tests were performed to determine the effect of alloying additions on the fiber/matrix bond strength in shear in Saphikon fiber reinforced Ni₃Al composites.     

Recycling process of WC-Co cermets by hydrothermal treatment

Hydrothermal extraction process of Co binder phase from WC-Co cermet was investigated in order to establish a novel recycling system of WC-Co cermet scraps. When the cermet chips were hydrothermal-treated in hydrochloric acid above 110^∘C, Co binding phase was efficiently extracted and the cermet chips were disintegrated into relatively large fragments. After hydrothermal treatment, WC sintered body become very brittle and pulverized easily by ball milling and the mean particle size of thus obtained WC particle became similar to that of virginal WC particle. The recycled WC powder was a little easier to undergo oxidation than the virginal WC powder, so that the mechanical properties of recycled WC-Co cermets were degraded. However, the degradation of mechanical properties was prevented only by drying the WC powder more carefully. This hydrothermal process will be one of the recycling systems for WC-Co cermets.     

Study on preparation and mechanical property of nanocrystalline NiAl intermetallic

Nanocrystalline NiAl materials were fabricated using mechanical alloying and hot-pressing sintering technique. The crystal structural and microstructure of milled powders during mechanical alloying, and the microstructure and mechanical properties of bulk NiAl intermetallic were characterized. The results show that B2 ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion via the gradual exothermic reaction mechanism during mechanical alloying. Scanning electron microscope image confirmed that the powder particles were flat and flake shape in the early stage of milling, but changed to a spherical shape with the crystallite size about 30 nm after the milling. After sintering, the crystal structure of nanocrystalline NiAl intermetallic was assigned to B2 order NiAl phase with the average crystallite size about 100 nm. The nanocrystalline NiAl intermetallic exhibited prominent room temperature compressive properties, such as the true ultimate compressive strength and the fracture strain were 2143 MPa and 32.2%, respectively. The appearances of vein-like patterns on the fracture surface of NiAl intermetallic materials indicated that the fracture mechanism could be characterized as ductile fracture. It can be concluded that higher sintering density and nanocrystalline of NiAl intermetallic were benefited for the improvement of mechanical properties.     

Effects of Minor Element Additions to the Nanocrystalline FeAl Intermetallic Alloy Obtained by Mechanical Alloying

Chemical, microstructural, and mechanical properties of nanocrystalline FeAl intermetallic alloys with Li, Ce, and Ni additions have been assessed. Mechanical alloying and sintering procedures were used to produce and consolidate the alloys. The sintering procedure was based on room temperature uniaxial pressing followed by annealing in air of the pressed specimens. The mechanically alloyed powders have a microstructure consisting of micrometer-size particles that contain FeAl intermetallic nanocrystals. The three minor additional elements form a solid solution with the B2 intermetallic structure of the FeAl alloy. Densification greater than 90% has been obtained. The hardness values are higher than those obtained from specimens produced with conventional casting procedures. High resolution transmission electron microscopy images showed clusters of less than 5 nm with well-defined structure corresponding to Fe₃Al.     

Microstructural characterization of P/M Ni3Al consolidated by HIP

Rapidly solidified powder of Ni₃Al doped with boron was produced by inert gas atomization and consolidated by hot isostatic pressing (HIP). Morphology and microstructure of the powder were studied. From the particle morphology, it could be deduced that the solidification time was similar at least to the time necessary for complete fragmentation of the liquid. The powder showed a two-phase microstructure that was finer the smaller the particle size. The presence of dendrites of NiAl (β) phase was consistent with the diagram proposed by Schramm and not with the traditional diagram of Singleton et al. The microstructure of the material consolidated at 1100°C and 1200°C was studied. A monophasic structure was observed after HIP, and no relevant microstructural differences were seen between the two temperatures used.     

Boron in polycrystalline Ni3Al-mechanism of enhancement of ductility and reduction of environmental embrittlement

A model is proposed to explain on a unified basis the role of boron in enhancing ductility and reducing environmental embrittlement in polycrystalline Ni₃Al. The grain boundaries in strongly ordered Ni₃Al have a porous structure with crack-like microcavities, which can open up under small stresses. Furthermore, atomic hydrogen, generated by the reaction of environmental moisture with Al, can diffuse to the tips of the microcavities under stress and can cause embrittlement. It is argued that strong bonding between interstitial B atoms and Ni atoms in B-doped Ni₃Al reduces the strength of directional bonding between Ni and Al atoms in the interior of the grains. When two such grains with weakened Ni---Al bonding meet each other, the atoms near the grain boundary can relax easily and close up the microcavities. As a result, the ductility is enhanced in B-doped Ni₃Al. The environmental embrittlement is also reduced, because a small amount of environmental hydrogen cannot nucleate microcracks. Numerous experimental observations have been explained with the proposed mechanism.     

颗粒弥散和核-壳共存的TiCN基金属陶瓷的制备

为了制备具有良好综合力学性能的TiCN基金属陶瓷，研究了烧结温度对TiCN-HfN陶瓷微观结构和力学性能的影响，构建了颗粒弥散和核-壳共存的微观结构模型，揭示了材料的致密化机制、增硬机制、增韧补强机制。结果表明:在1 500℃下所制备的TiCN-HfN材料具有颗粒弥散与核-壳共存的微观结构，其中弥散的颗粒为HfN，核为TiCN，壳主要为(Ti, Hf, Mo)CN固溶体；材料具有较好的性能，其相对密度为99.7%、硬度为20.6 GPa、抗弯强度为1 682.5 MPa、断裂韧度为8.5 MPa·m1/2；其致密化机制主要为颗粒和金属液相填充到烧结颈实现致密化，增硬机制主要为致密化和颗粒钉扎强化增硬，增韧补强机制主要为颗粒弥散和颗粒钉扎增韧、骨架结构和颗粒钉扎增强。      

Ti添加对WC-Ni3Al硬质合金微观组织与力学性能的影响

为提高WC-Ni₃Al硬质合金的力学性能，采用放电等离子烧结制备Ti掺杂的WC-Ni₃Al硬质合金，并研究不同Ti添加量对WC-Ni₃Al硬质合金微观组织和力学性能的影响。结果表明: Ti的添加减小WC-Ni₃Al块体样品中反应生成的少量Al₂O₃的尺寸，并且使Al₂O₃的分布更加均匀。一方面，小尺寸的Al₂O₃与原位生成的(Ti, W)C协同提高WC-Ni₃Al块体样品的硬度；另一方面，适量Ti的添加还提高WC-Ni₃Al硬质合金的断裂韧度，这是由于原位生成的(Ti, W)C与WC有较好的界面结合，增加对裂纹扩展的桥接与偏转作用。当添加3%(质量分数)的Ti时，WC-Ni₃Al硬质合金获得了优异的力学性能，硬度和断裂韧度分别为(19.29±0.18) GPa和(13.14±0.24) MPa·m^1/2。     

Precipitation phenomena in MC-Ni-Co cermet systems

Ti(CN)-Ni-Co, WC-Ni-Co, and TaC-Ni-Co cermets were prepared, in order to understand the effect of carbide dissolution during the heating and sintering stages of the precipitation phenomena. Magnetic properties, such as magnetic saturation (4) and coercive force (H _c), were measured and related to the observed changes in the microstructure. There is evidence in the TaC-Ni-Co system that early precipitation occurs during the heating stage. But they do not influence the final microstructure. Most of the dissolved carbides tend to re-precipitate on the growing particles during sintering while the remaining solutes in the melt precipitate in the binder during cooling, thus affecting the coercive force of cermets. The TEM analysis identified the precipitates as TiNi(TiCo), Ti(CN) in the Ti(CN)-Ni-Co cermet, Co₃W, WC in the WC-Ni-Co cermet, and Co₂Ta, TaC in the TaC-Ni-Co cermet.     

新型三元硼化物基金属陶瓷的研究

用反应液相烧结法在1280℃下真空烧结成功制备出三元硼化物基金属陶瓷(TBBC),其硬度可达92.4HRA,密度为8.2g/cm³,热膨胀系数为8.0×10^-6/K.用扫描电镜、X射线衍射和能谱对其微观结构进行了分析,结果表明:此种金属陶瓷是由三元硼化物基硬质相(MoFeCrNi)₃B₂和铁基粘结相α-Fe组成,硬质相和铁基粘结相分散均匀,硬质相占有相当大的比重,约占体积比的80％.磨损实验表明此种金属陶瓷具有良好的耐磨性.     

Effect of Ion-Plated Yttrium on the Oxidation Behavior of Nickel Aluminide Intermetallic Compound

The Ni₃Al specimens were coated with yttrium (Y) by ion plating method. Post heat treatment was performed after Y ion plating to improve adherence of the Y-coating layer to the substrate. Performance of the Y-coated Ni₃Al was evaluated by isothermal oxidation and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y ion plating altered the oxidation kinetics of Ni₃Al significantly. The test results suggest that the corrosion resistance of Ni₃Al can be improved by a proper method of Y-deposition.     

板状WC晶粒WC-(Co-Ni)硬质合金的组织和性能

采用真空烧结法制备了板状WC晶粒WC-(Co-Ni)硬质合金,通过XRD、SEM、EDS等手段研究了Ni/(Ni+Co)比对硬质合金组织和性能的影响规律。结果表明:随着Ni/(Ni+Co)比的增大,硬质合金显微组织中板状WC晶粒的比例逐渐减少,硬质相颗粒的尺寸逐渐增大且平均长厚比逐渐减小。当Ni/(Ni+Co)比过大时,硬质合金中硬质相颗粒出现了团聚现象,使其力学性能显著降低。当Ni/(Ni+Co)比为0.3和0.5时,WC-(Co-Ni)硬质合金的综合力学性能较高,这与其硬质相颗粒较细和平均长厚比较大有关。当Ni/(Ni+Co)比为0.5时,WC-(5Co+5Ni)硬质合金具有较优的综合力学性能,其抗弯强度、硬度和断裂韧性分别为2 448 MPa、90.0HRA、21.2 MPa·m~(1/2)。     

Analysis of the microstructure of Cr–Ni surface layers deposited on Fe3Al by TIG

A series of Cr–Ni alloys were overlaid on a Fe₃Al surface by tungsten inert gas arc welding (TIG) technology. The microstructure of the Cr–Ni surface layers were analysed by means of optical metallography, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that when the appropriate TIG parameters were used and Cr25–Ni13 and Cr25–Ni20 alloys were used for the overlaid materials, the Cr–Ni surface layers were crack-free. The matrix of the surface layer was austenite (A), pro-eutectoid ferrite (PF), acicular ferrite (AF), carbide-free bainite (CFB) and lath martensite (LM), distributed on the austenitic grain boundaries as well as inside the grains. The phase constituents of the Cr25–Ni13 surface layer were γ-Fe, Fe₃Al, FeAl, NiAl, an Fe–C compound and an Fe–C–Cr compound. The microhardness of the fusion zone was lower than that of the Fe₃Al base metal and Cr25–Ni13 surface layer.     

Sintered porous cermets based on TiB2 and TiB2–TiC–Mo2C

TiB₂ powder, with different binders (Ni and Ni/Mn), after milling were cold compacted (300 MPa) and sintered in H₂ at 1300 and 1350°C for 1 h. To improve the sintering behaviour, TiC/Mo₂C alloy carbide was added and the milled charge along with the same binders (Ni and Ni/Mn) was cold compacted and sintered under similar conditions. Sintered density, porosity, transverse rupture strength (TRS), grain size and lattice parameter of binder and hard phases were measured. Better densification was observed with Ni/Mn binder as compared to Ni binder for either hard phase based systems. Maximum value of TRS was noted for TiB₂–TiC–Mo₂C–40 wt.% Ni/Mn cermet. Melt exudation was observed for either hard phase based systems with Ni binder.     

The effect of segregation and partial order on the thermodynamics of (1 1 1) antiphase boundaries in Ni3Al

Thermodynamic properties of thermal and a-thermal (1 1 1) antiphase boundaries (APB) in Ni₃Al are computed from first-principles. The effect of off-stoichiometry, partial disordering and segregation are evaluated and a rough estimate of the vibrational contribution to the antiphase boundary energy is given. Although the vibrational effect is found to be small, the configurational effects are large, so that at non-zero temperature and in off-stoichiometric Ni₃Al the antiphase boundary energy (APBE) may be only half of that in perfectly ordered stoichiometric Ni₃Al at zero temperature. This result points to a discrepancy between electronic structure calculations and experimental measurements.     

Abnormal precipitation behavior of Ni3Al phase

Evolution of Ni₃Al phase in a nickel-base directionally solidified superalloy has been investigated during creep rupture testing (1253 K/150 MPa). Besides the splitting, rafting and Ostwald ripening cases, many finer Ni₃Al particles have been firstly observed to precipitate from the rafted Ni₃Al particles. A simple qualitative interpretation has been proposed.