DEVELOPMENT OF MoS_2-CONTAINING Ni-Cr BASED ALLOYS AND THEIR HIGH-TEMPERATURE TRIBOLOGICAL PROPERTIES

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Effects of MoS_2 on mechanical and tribological properties of NiCr-based alloys

Ni Cr based alloys with a wide temperature range self lubrication were made by hot pressing the mixture powder of alloyed Ni Cr powder, elemental Mo, Al, Ti and B powders and MoS 2 powder. The mechanical and tribological properties of these alloys when rubbing with Al 2O 3 ceramics and W18Cr4V high speed steel were measured in the temperature range of 20～700 ℃, and the mechanisms of self lubrication and wear resistance were studied. The results showed that the alloy containing 10% MoS 2 has the best combination of mechanical properties, antifriction and wear resistance. Over a wide temperature range from 20 ℃ to 700 ℃, when rubbing with Al 2O 3, its friction coefficient and wear rate are 0.19～0.3 and (1.1～1.5)10 -14 m 3/(N·m), respectively; when rubbing with the high speed steel, those values are 0.18 ～ 0.26 and (0.6～3.2)×10 -15 m 3/(N·m), respectively.

Electro-spark epitaxial deposition of NiCoCrAlYTa alloy on directionally solidified nickel-based superalloy

An 8 mm-high NiCoCrAlYTa coating was epitaxially built-up on a directionally solidified (DS) Ni-based superalloy blade tip by electro-spark deposition.Epitaxial morphologies of the coating and its microstructural characteristics were investigated by means of SEM,XRD and TEM etc.It is observed that the fine column-like dendrites originated from the γ'-particles or γ'-clusters of the DS substrate and are un-continuously coarsened.The β-phase particles precipitate and grow eutectically with the γ-phase.The orientation of fine column dendrites depends on electro-spark deposition processing parameters and the microstructure can be characterized with superfine γ and β phases.

Effect of Rare Earth on Microstructure of Vacuum Melting Ni-Based Self-Fluxing Alloy Coatings

The Ni-based self-fluxing alloy coating containing RE was acquired by the technique of vacuum melting on the hypoeutectoid steel (Fe-0.45%C) matrix. By X-ray diffraction, SEM and EDX, the microstructure and phase structure of section of coating and the microstructure near the interface between coating and matrix were investigated, and the effect of RE on microstructure of coating was also discussed. The results show that the microstructure of the NiCrBSi alloy coating is composed of Ni-based solid solution and a lot of massive, globular and needle secondary phases CrB, Ni_3B, Cr_7C_3, Cr_(23)C_6 among the solid solution. The metallurgical binding between steel matrix and coating is realized. RE makes needle phase of alloy coating vanish. New phases of NiB and Cr_(6.5)Ni_(2.5)Si are precipitated from alloy coating, and secondary phases of alloy coating are sphericized. Consequently, RE also hinders the diffusion of Ni, Cr and Si atoms from coating to matrix and Fe atoms from matrix to coating, holds back the dilution of Fe for NiCrBSi alloy coating, and assures the chemical composition of the alloy coating.

Weldability and Microstructure of Nickel-Silicon Based Alloys

The NiSix based alloy typically has poor weldability due to its lower ductility. A limited amount of work has been performed on the weldability of NiSix based alloys. Therefore, the effect of heat treatment and welding parameters on weldability of the alloys, and the relationship between the weldability and microstructure were studied. The results show that the as-cast Ni-Si-Nb-B alloy （Ni 76. 5%, Si 20%, Nb 3%, and B 0. 5%） could be successfully welded after preheating at 600 ℃. The welding procedure should be performed on the alloys before any heat treatment and a preheating at 600 ℃ should be used. The fusion zone is harder than the matrix due to a large amount of 7 phase and a finer microstructure. The cracks are predominantly intergranular in heat affected zone and associated with the needle-like ）, phase. The heat treatment before welding increases the tendency of cracking in the fusion zone.

Effect of solid-solution temperature on the microstructure of Fe-Ni based high strength low thermal expansion (HSLTE) alloy

The influence of solid-solution temperature on the dissolution of carbide precipitates, the average grain size and the microhardness of the austenite matrix in an Fe-Ni based high strength low thermal expansion （HSLTE） alloy was investigated to obtain the proper temperature range of the solid-solution process. The XRD analysis, microstructure observations, and the theoretical calculations showed that the Mo-rich M2C-type precipitates in the Fe-Ni based HSLTE alloy dissolve completely at about 1100℃. The average grain size of the studied alloys increases from 14 to 46 μm in the temperature range of 1050 to 1200℃. The microhardness of the matrix decreases gust for the sake of solid-solution treatment, but then increases later with increasing solution temperature because of the solution strengthening effect.

Microstructure and Wear Resistance of in situ NbC Particles Reinforced Ni-based Alloy Composite Coating by Laser Cladding

The in situ synthesized NbC particles reinforced Ni-based alloy composite coating was produced by laser cladding a precursor mixture of Ni-based alloy powder, graphite and niobium powders on a steel substrate. The microstructure, phase composition and wear property of the composite coating were investigated by means of scanning electron microscopy （SEM）, X-ray diffraction （XRD） and dry sliding wear test. The experiment results show that the composite coating is homogeneous and free from cracks, and about 0.8 mm thick. The microstructure of the composite coating is mainly composed of NbC particles, CrB type chromium borides, 7-Ni primary dendrites, and interdendritic eutectics. CrB phases often nucleate and grow on the surface of NbC particles or in their close vicinity. NbC particles are formed via in situ reaction between niobium and graphite in the molten pool during the laser cladding process and they are commonly precipitated in three kinds of morphologies, such as quadrangle, cluster, and flower-like shape. Compared with the pure Ni- based alloy coating, the microhardness of the composite coating is increased about 38%, giving a high average hardness of HV0.21000, and the wear rate of the composite coating is decreased by about 32%, respectively. These are attributed to the presence of in situ synthesized NbC particles and their well distribution in the coating.

GRAIN REFINEMENT IN BULK UNDERCOOLED Ni-BASED ALLOY

Fluxing of 5 g bulk melt Ni77Si13B10 permits high undercoolings to be attained prior to nu-cleation onset.Investigations of grain refinement in the bulk undercooled alloy as a function ofundercooling,recalescence behavior and cooling rate have been reported.A significant inhomo-geneity of reduction in grain size of a bulk sample is observed,which is caused by the different so-lidification conditions:(1)recalescence process,and(2)the followed plateau in which the heatrelease and extraction rates are equal.It is concluded that the homogeneous refined microstructurecan be achieved if the initial undercooling prior to nucleation,or cooling rate after recalescence isfurther increased.

Effect of Alloying Elements on Microstructure,Martensitic Transformation and Mechanical Properties of Ni-Mn Based Alloys

The microstructural features, shape memory behavior and mechanical properties of Ni-Mn based alloys were investigated by differential scanning calorimetry （DSC）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and thermal cycling test under various stresses. The transformation temperatures shifted toward lower temperatures when adding a third element into the Ni-Mn system. The addition of 10 at. pct Fe increased considerably the mechanical properties exhibiting still high transformation temperatures. However, it was found that in NiMn40Fe10 alloy the martensitic transformation is not thermoelastic in nature. The mechanism of this transformation and the crystallography of Ni-Mn（50-x）-Fex （x=5, 7, 10, 20 at. pct） alloys are presented.

Tribological property and high temperature oxidation behavior of Ni-based alloy

Ni-Cr based alloys were prepared by hot-pressing the mixture of strengthening phases Mo, Al, Ti, and lubricant phase MoS2. The hardness, tribological properties as well as the high temperature oxidation properties were evaluated, The results show that the strengthening phases can improve the mechanical properties of Ni-Cr based alloy obviously, and the wear and friction properties of Ni-based alloy with strengthening phase can be improved. Its friction coefficient and wear rate rubbed with Al2O3 ceramic disk are about 0.4 and 10 -14m3/（N·m）, respectively, and the oxidation process is mainly affected by Cr2O3.

Hydriding/dehydriding properties of Mg-Ni-based ternary alloys synthesized by mechanical grinding

The Mg-Ni-based ternary alloys Mg2-xTixNi(x=0,0.2,0.4)and Mg2Ni1-xZrx(x=0,0.2,0.4)were successfully synthesized by mechanical grinding.The phases in the alloys and the hydriding/dehydriding properties of the alloys were investigated.Mg2Ni and Mg are the main hydrogen absorption phases in the alloys by XRD analysis.Hydriding kinetics curves of the alloys indicate that the hydrogen absorption rate increases after partial substitution of Ti for Mg and Zr for Ni.According to the measurement of pressure-concentration-isotherms and Van't Hoff equation,the relationship between ln p(H2)and 1 000/T was established.It is found that while increasing the content of correspondingly substituted elements at the same temperature,the equilibrium pressure of dehydriding increases,the enthalpy change and the stability of the alloy hydride decrease.

一种新型Ni-Fe基高温合金的微观组织演化与蠕变行为

采用扫描电子显微镜、透射电子显微镜、电子背散射衍射技术,研究了GH4070P合金在725℃/180 MPa和725℃/150 MPa条件下的蠕变行为,分析了合金在不同条件下蠕变后的微观组织和晶体学特征,揭示了合金蠕变过程中析出相的演变规律及断裂机制。结果表明:在725℃/180 MPa和725℃/150 MPa蠕变条件下,位错通过Orowan机制绕过γ′析出相;725℃/150 MPa蠕变合金由于γ′相的粗化产生位错塞积。蠕变期间,合金晶界应力集中较为明显,产生了蠕变孔洞和微裂纹,最终导致沿晶断裂。

Double oxide film defects in Ni-based super-alloy castings

A new class of defects has been recently discovered,which are regarded double oxide film defects.In this study, a number of Ni-based vacuum-cast test bars were investigated. The fractures in the broken test barswere observed by Scanning Electron Microscopy (SEM). Energy Dispersive Spectroscopy (EDS), as well asWavelength Dispersive Spectroscopy (WDS) were used to characterize the chemical elements of the fracture'ssurface. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiatedthe growth of carbides. This study has added to the evidence that oxide film defects do exist in Ni-based super-alloyin certain vacuum casting conditions. It provides a reference for researchers' further study on the defects, andprovides a possible direction for researchers to improve casting technology so as to remove these defects.

Microstructure and bonding strength of Ni-based alloy coating

A Ni-Cr-B-Si coating technique was developed and successfully applied on austenite grey iron substrate in a conventional resistance furnace under graphite powder protection. The microstructure, phase distribution, chemical composition profile and microhardness along the coating layer depth were investigated. Shear strength of the coating was also tested. Microanalysis shows that the coating is consist of γ-Ni solution and γ-Ni+Ni3B lamellar eutectic, as well as small amount of Cr5B3 particles. Diffusion induced metallurgical bonding occurs at the coating/substrate interfaces, and the higher the temperature, the more sufficient elements diffused, the broader interfusion region and the larger bonding strength, but it has an optimum value. And the bonding strength at the interface can be enable to reach 250-270 MPa, which is nearly the same as that of processed by flame spray. The microhardness along the coating layer depth shows a gradient distribution manner.

Martensitic transformation and shape memory effect in ausaged Fe-Ni-Si-based alloys

It is well known that the morphologies of the α’ martensite formed from the γ phase in ferrous alloys are classified into five types of lath, butterfly, （225）A type plate,lenticular and thin-plate. Among those α’ martensites, only the thin-plate martensite,which is characterized by containing a high density of transformation twins, has a potential of exhibiting a perfect shape memory （SM） effect.Recently the present authors found in Fe-Ni-Si alloys that the thin-plate martensite is formed by the introduction of fine and coherent γ’-（Ni,Fe）3Si particles with a L12 ordered structure in the austenite matrix due to ausaging. In the present study, the SM properties of the ausaged Fe-Ni-Si alloys with the thin-plate martensite are investigated by a conventional bending-test. The effects of the addition of Co to the Fe-Ni-Si alloys on the martensitic transformation and the SM properties are also investigated. It is shown that while the ausaged Fe-Ni-Si ternary alloys exhibit an imperfect SM effect due to reverse transformation from stress-induced thin-plate martensite to austenite, the SM properties are improved by the addition of Co. An almost perfect SM effect is confirmed in the Fe-Ni-Si-Co alloys by heating to 1 100 ℃ after deformation at -196 ℃.

Ni-Mn基磁性形状记忆合金弹热制冷效应的研究现状

弹热制冷不仅具有大而可逆的温度变化,而且其驱动方式更为简单,同时兼具环保、高效、节能的特点,平衡了成本、制冷量、效率以及可行性,有望成为替代传统蒸汽压缩冷却技术的候选之一。本文综述了Ni-Mn基磁性形状记忆合金弹性热效应的研究进展,从掺杂合金元素、热处理方法以及制备工艺等多个方面,总结了近年来对Ni-Mn基合金弹性热效应进行改进优化的研究成果。然而,目前Ni-Mn基合金弹热制冷材料的研究仍处于探索阶段,实现其实际应用还需进一步深入研究。未来的研究重点应集中在降低驱动能耗、提升疲劳寿命和循环稳定性、提高绝热温变以及优化传热效率等方面。

时效处理对Fe-Ni基奥氏体合金耐点蚀性能的影响

针对Fe-Ni基奥氏体合金在740℃时效处理后的析出相随时效时间的演变进行探究;并通过电化学腐蚀试验以及Mott-Schottky分析,研究了时效处理对合金耐腐蚀性能的影响。结果表明:740℃时效处理可以促进γ′沉淀强化相[Ni_(3)(Al,Ti)]在合金中的析出;随时效时间的增加,γ′析出相的密度和尺寸增加;同时,时效处理会增加合金表面点缺陷数量,降低合金钝化膜致密性,从而导致合金耐点蚀性降低;通过恒电位阳极极化处理能够有效降低合金表面点缺陷数量,提高合金耐点蚀性。

Effects of Salt Quenching Temperatures on Microstructure and Creep Properties of a PM Ni-based Superalloy

By means of the microstructure observation and creep properties measurement, an investigation has been made into the influence of the salt quenching temperatures on the microstructure and creep property of FGH95 superalloy. The results shown that, after full heat treatment, a high volume of g￠ phase and some granular carbide dispersedly precipitate in the matrix. Thereinto, as the molten salt temperature decreases from 650℃to520℃, the size of fine g￠ phase in the alloy decrease gradually and the amount of carbides increase in the alloy. And the alloy quenched in molten salt at520℃possesses better creep resistance due to the fact that there are more granular carbides precipitating in the alloy to enhance the grain strength. During creep, the deformation features of the alloy are that the configurations of stacking fault and slipping dislocations are activated in the alloy.

Effect of Heat-Treatment Process on Properties of Rare Earth Mg-Based System Hydrogen Storage Alloys with AB_3-Type

The effect of heat-treatment process on the properties of Mm0.8Mg0.2(NiCoAlMn)3.5 hydrogen storage alloy was discussed . The electrochemical properties such as cycling stability, activation property, and the plateau voltage of the alloy which was heat-treated in various temperatures and times had different changes during the cycle process, the optimum heat-treatment conditions of this alloy were determined by this work.

Effect of molybdenum on the microstructure and corrosive wear resistance of laser clad Ni-based coatings

Five kinds of Ni-based coatings with 0 wt% , 2. 5 wt% , 5.0 wt% , 7.5 wt% and 10. 0 wt% molybdenum were prepared on 45CrNi steel plates by using laser cladding technique. The effect of Mo on the microstructure of Ni-based coatings was investigated by using scanning electron microscopy. The corrosive wear resistance and the corrosion resistance of five coatings were tested. The results show that the corrosive wear resistance of the coating with 5.0 wt% Mo is better than those of other coatings. During the corrosive wear process, the corrosion and wear effects are combined. The corrosive wear resistance is closely related to the microstructure of the coating.