粉末状态对超音速喷涂合成TiC-Ni涂层组织和性能的影响

以Ti粉、Ni粉和石墨为原料通过PVA制粒和混合制粒制备两种喷涂粉末.研究表明:制粒方式在超音速火焰喷涂对涂层组织和耐磨性能有很大影响.PVA制粒粉末进行喷涂,涂层的相组成为TiC、Ni和少量Ti与Ni的氧化物,组织致密具有典型的层状涂层结构特征,涂层耐冲蚀磨损性能较强;混合制粒由于在火焰气流中缺乏SHS反应的条件,粉末喷涂后涂层中含有大量的Ti和Ni的氧化物和少量的粘结相Ni,具有较多的孔洞涂层组织疏松,耐冲蚀磨损性能较差.     

Investigation of the properties of nickel-boron/nano-diamond composite coatings upon pre/post heat treatment

The study has explored the physical and chemical properties of nickel-boron/nano-diamond composite coatings upon pre/post heat treatment. The nano-composite coatings were produced by electroless plating. In fact, the effects of using the nano-diamond additives (0 g l⁻¹, 0.1 g l⁻¹, 0.3 g l⁻¹, 0.5 g l⁻¹, 0.7 g l⁻¹) were investigated in pre/post heat treatment. According to the results, the micro-hardness of the nickel-boron/nano-diamond composite coating produced in the bath containing 0.5 g l⁻¹ nano-diamond additive reached to 1005 HV 0.05 upon post-heat treatment. Also, the corrosion resistance of the nickel-boron/nano-diamond composite coatings was modified by using the nano-additive or heat treatment. But, adding nano-diamond particles reduced the ductility and wear resistance of the nickel-boron coatings. We also found, modification of the properties (ductility and wear resistance) of the composite coatings was achieved by using the heat treatment.     

Effects of aluminum content and mechanical nano-alloying time on the preparation of titanium-aluminum composite coatings on magnesium-lithium alloy

A titanium-aluminum composite coating was successfully fabricated on magnesium-lithium alloy substrate by mechanical nano-alloying treatment (SMNAT). The effects of aluminum content and mechanical nano-alloying time on the microstructure and properties of titanium-aluminum composite coatings were investigated. The experimental results showed that the ratio of titanium-aluminum powder has a significant effect on the preparation of the coating. Low aluminum content and high aluminum content in the mixture powder resulted in the formation of the coatings with inhomogeneous thickness. The titanium-aluminum ratio of 3 : 1 was considered to be optimal for fabricating the coating with homogeneous thickness under the selected milling parameters. The hardness of top surface layer on titanium- aluminum coatings decreased from 292.0 HV 0.01 to 121.4 HV 0.01 with the increase of aluminum content. Additionally, the prolongation of treatment time facilitated to increase the thickness and density of the coatings, resulting to enhance their hardness and corrosion resistance.     

Effect of heat treatment on ultrasonic velocity in ductile cast iron

Abstract

The measurement of the ultrasonic velocity is a common method in the foundry industry for the evaluation of the nodularity in ductile iron castings. Practical experience has shown that heat treatment can reduce the ultrasonic velocity compared to the as cast condition. Using ductile iron samples with different heat treatments in order to vary the ferrite and pearlite content respectively confirmed this decrease in the ultrasonic velocity compared to the as cast state. Further investigations showed that with all the heat treatments applied, irrespective of their effect on the microstructure, the density was decreased. The decrease in density correlated with the decrease in ultrasonic velocity for all heat treatments. The mechanisms involved in the reduction in the density are discussed.     

Effect of multi pulse current treatment on microstructure and microhardness of resistance spot welded dual phase steel DP590

In this paper, microhardness and microstructural characteristics of dual phase DP590 steel resistance spot weld joint under two different welding conditions (i. e. single pulse and multi pulse) were studied. It was observed that applying multi pulse currents resulted in quasi-equiaxed grains of tempered martensite in fusion zone (FZ) of the weldment. The refinement of microstructure in fusion zone using multi pulse current treatment resulted in reduced hardness of the weld joint.     

Formation of amorphous and nanocrystalline phases in high velocity oxy-fuel thermally sprayed a Fe–Cr–Si–B–Mn alloy

High velocity oxy-fuel (HVOF) thermal spray was used to deposit a Fe–Cr–Si–B alloy coating onto stainless steel (1Cr18Ni9Ti) substrate. Microstructures of the powder and the coating were investigated by X-ray diffraction (XRD), scanning election microscopy (SEM), transmission election microscopy (TEM) and differential scanning calorimeter (DSC). The coating had layered morphologies due to the deposition and solidification of successive molten or half-molten splats. The microstructures of the coating consisted of a Fe–Cr-rich matrix and several kinds of borides. The Fe–Cr-rich matrix contained both amorphous phase and nanocrystalline grains with a size of 10–50 nm. The crystallization temperature of the amorphous phase was about 605 °C. The formation of the amorphous phase was attributed to the high cooling rates of molten droplets and the proper powder compositions by effective addition of Cr, Mn, Si and B. The nanocrystalline grains could result from crystallization in amorphous region or interface of the amorphous phase and borides by homogeneous and heterogeneous nucleation.     

Effect of heat treatment on the corrosion resistance and mechanical properties of selective laser melting Ti6Al4V alloy

The present work shows that the effect of several heat treatments on the corrosion resistance and mechanical properties of Ti6Al4V processed by selective laser melting (SLM). The microstructure of Ti6Al4V alloy produced by selective laser melting exhibited bulky prior β columnar grains, and a large amount of fine acicular martensites α′ were observed inside the prior β columnar grains. The acicular martensitic α′ were transformed to a mixture of α and β after heat treatment, and the grain size increases with the increase of heat-treated temperature. The results of 3.5 wt% NaCl solution electrochemical corrosion test showed that the heat-treated samples possess a higher corrosion resistance than the as-received sample. Among of them, the sample after heat-treated at 730 °C exhibited best corrosion resistance and excellent fracture strain. The sample heat treated at 1015 °C showed worst mechanical properties due to the formation of Widmanstätten structure.     

Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel

Post-weld heat treatment (PWHT) is commonly adopted on welded joints and structures to relieve post-weld residual stresses; and restore the mechanical properties and structural integrity. An electrolytic plasma process (EPP) has been developed to improve corrosion behavior and wear resistance of structural materials; and can be employed in other applications and surface modifications aspects. In this study the effects of PWHT and EPP on the residual stresses, micro-hardness, microstructures, and uniaxial tensile properties are explored on tungsten inert gas (TIG) welded AISI-4140 alloys steel with SAE-4130 chromium–molybdenum alloy welding filler rod. For rational comparison all of the welded samples are checked with nondestructive Phased Array Ultrasonic Testing (PAUT) and to ensure defect-free samples before testing. Residual stresses are assessed with ultrasonic testing at different distances from weld center line. PWHT resulted in relief of tensile residual stress due to grain refinement. As a consequence higher ductility but lower strength existed in PWHT samples. In comparison, EPP-treated samples revealed lower residual stresses, but no significant variation on the grain refinement. Consequently, EPP-treated specimens exhibited higher tensile strength but lower ductility and toughness for the martensitic formation due to the rapid heating and quenching effects. EPP was also applied on PWHT samples, but which did not reveal any substantial effect on the tensile properties after PWHT at 650 °C. Finally the microstructures and fracture morphology are analyzed using scanning electron microscopy (SEM) and optical microscope to study the evolution of microstructures.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

On selective laser melting of ultra high carbon steel: Effect of scan speed and post heat treatment

Selective laser melting is a laser‐based additive manufacturing process applying layer manufacturing technology and is used to produce dense parts from metallic powders. The application of selective laser melting on carbon steels is still limited due to difficulties arising from carbon content. This experimental investigation aims at gaining an understanding of the application of the process on ultra high carbon steel, which is a special alloy with remarkable structural properties suitable for different industrial applications. The feedstock ultra high carbon steel (2.1% C) powder, 20 μm to 106 μm was prepared by water atomizing technique. This powder was used for the selective laser melting to build specimens 10×10×40 mm in dimensions. To decrease the thermal stresses during layer by layer building, laser scanning was done through 5×5 mm random island patterns while layer thickness was 30 μm. Laser beam diameter, maximum power output, layer thickness and scan speed range were 0.2 mm, 100 W, 30 μm and 50–200 mm/s respectively. The process was done inside high purity nitrogen environment, with less than 0.5% oxygen content. The results illustrate the influence of scan speed from 50 to 200 mm/s on product geometry and dimensions, surface roughness, internal porosity and cracks, microstructure and surface hardness. The effect of post heat treatment by heating and holding for one hour (annealing) at different temperatures of 700°C, 750°C, 950°C is studied. The results indicate that selective laser melting is able to produce near to 95% density of ultra high carbon steel parts with acceptable geometry and surface quality. Porosity cracks, and microstructure formed during the process could be controlled through proper selection of process parameters and post heat treatment. Industrial ultra high carbon steel products can be rapidly fabricated by selective laser melting.     

Effects of heat treatment processes on microstructure and creep properties of a high nitrogen 15Cr-15Ni austenitic heat resistant stainless steel

Conventional thermo-mechanical treatment (CTMT) and modified thermo-mechanical treatment (MTMT) process were applied for manufacturing a high nitrogen niobium-stabilized 15Cr-15Ni austenitic alloy. CTMT process consists of 5 h of solution treatment at 1270 °C followed by water quenching and subsequent aging at 820 °C for 50 h. MTMT process differs from CTMT process in hot plastic deformation performed immediately after the solution treatment at 1270 °C and longer aging time. Microstructure and creep properties of the steel obtained by both processing routes were investigated. Creep rupture tests at 750 °C showed double increase in rupture time brought about by MTMT process. Examination of crept microstructure by transmission electron microscopy revealed that the improved creep properties in MTMT process were mainly due to improved distribution uniformity of fine nano-sized carbonitride precipitates in the austenitic matrix and that MTMT process has no effects on the number density and distribution of copper precipitates present in the steel. However, the creep ductility in MTMT process drastically reduced comparing to CTMT process. The higher density of grain boundaries due to finer grain recrystallized microstructures and the formation of higher volume fraction of coarser M₂₃C₆ precipitates at the boundaries are believed to be the main reason for the lower creep ductility in MTMT process.     

The heat treatment effect on the structure and mechanical properties of electrodeposited nano grain size Ni-W alloy coatings

The Ni-W alloy coatings with tungsten content from 32.5 to 61.2 wt.% were prepared in this study by electro-deposition. Experimental results show that the grain size of Ni-W coatings evaluated by XRD decreased with increasing tungsten content in coatings, however, the micro-hardness increased with increasing tungsten content. As-deposited Ni-61.2 wt.%W coating has amorphous-like structure and the grain size is around 1.5 nm, after annealing at 500 °C, the hardness of the coating is promoted to 1293 Hv owing to formation of Ni₄W and NW precipitates. In addition, the heat-treated Ni-W coatings show a better wear resistance than the as-plated Ni-W coatings.     

Effect of heat treatment on the photoluminescence of CdS nanocrystallites in cadmium-rich organic Langmuir Blodgett matrix

CdS nanocrystallites were grown within an organic layered matrix by partial sulphidation of precursor cadmium arachidate LB multilayers. The cadmium arachidate–arachidic acid composite multilayers containing CdS nanocrystallites were enriched by intercalation with Cd²⁺ ions in aqueous solution of CdCl₂ and subsequently heat treated at different temperatures up to 300 °C, in air and in vacuum. CdS nanocrystallites within the composite multilayer have been found to exhibit treatment process dependent characteristic changes in optical absorption and luminescence. The optical data obtained at different stages/conditions of processing have been analyzed by considering changes in excitonic absorption and emission as well as contributions from surface and bulk defects related emission and a suitable energy level diagram has been proposed. The composition, microstructure and surface morphology of the composite multilayers were also studied at all stages of processing to develop a comprehensive understanding of the interaction of the organic matrix with CdS nanocrystallites and the consequent influence on the optical behavior of the nanocrystallites. These studies have shown that the organic moieties encapsulating the CdS nanocrystallites tend to restrict their growth and aggregation, while the presence of cadmium in the organic matrix is responsible for the passivation of surface defects as well as the reduction of bulk defects, and these factors have significant influence on the photoluminescence of CdS nanocrystallites.     

Effect of copper addition and heat treatment on the depth dependence of the nanoindentation creep of aluminum at 300 K

Constant-load indentation creep tests were performed on pure aluminum and aluminum 4 wt% copper at 300 K to assess the influence of indentation depth, copper addition, and heat treatment upon the indentation creep rate. The stress dependence of the average indentation creep rate could be expressed for all the samples tested in terms of a mechanism of obstacle-limited dislocation glide. The calculated activation energy showed the same dependence upon indentation stress for all the conditions investigated.We therefore conclude that the indentation creep rate is limited by dislocation/dislocation interactions regardless of indentation depth, copper addition, or heat treatment. The presence of 4 wt% copper and heat treatment, however changes, the dislocation density, and hence the spacing of the dislocation–dislocation interactions.     

Effect of heat treatment on microstructures and mechanical properties of Al/Fe bimetal

The Al/Fe bimetallic castings were prepared by the compound casting combined with hot-dip galvanising and aluminising, and the effect of the heat treatment on microstructures and mechanical properties of the Al/Fe bimetallic castings was systematically studied in the present work. The thickness of the reaction layer of the Al/Fe bimetallic castings continuously increased with increasing solution temperature or prolonging solution time. However, the excessive solution temperature and solution time promoted cracks of the reaction layer. Regardless of as-cast or heat treatment, there were respectively the Fe₂Al_5, Al₉Fe₄Si_3, FeAl_3, Al₈Fe₂Si and Al_4.5FeSi phases in the reaction layers. The nano-hardnesses of the reaction layer after heat treatment exhibited an increase compared to that of the as-cast condition.     

Effect of heat treatment on the structure and stability of multiwalled carbon nanotubes produced by catalytic chemical vapor deposition technique

The multiwalled carbon nanotubes (MWCNTs) produced by catalytic chemical vapor deposition (CCVD) route were heat treated to 2500 °C to improve the structure, morphology and purity level. The process has lead to substantial reduction in the catalytic impurity along with an improved thermal stability and degree of graphitization of these tubes that can possibly lead to its better utilization in various applications. The structural changes following heat treatment have been correlated using various characterization techniques such as Raman spectroscopy, X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, thermo gravimetric analysis and electron paramagnetic resonance spectroscopy. The electrical and mechanical properties of the polymer composites prepared with heat treated MWCNT show improved properties over the one prepared by as produced MWCNT.     

石墨化处理对双层热解炭基2DC/C复合材料微观结构的影响

综合采用偏光显微镜(PLM),X射线衍射仪(XRD),扫描电子显微镜(SEM),高分辨透射电子显微镜(HRTEM)等分析方法,逐层研究了2500℃高温石墨化(HTT)处理前后双层热解炭基2D C/C复合材料的不同织构以及纤维-基体界面的变化.结果表明:HTT处理后,d002值减小,石墨化度显著提高;内层低织构热解炭的断口微观形貌与晶格条纹几乎没有变化,而外层高织构热解炭晶格条纹更加平直,尤其是发现层间裂纹密度明显增大,使得处理后的高织构热解炭在受力时,裂纹易于在层间扩展和偏转,因此可有效提高材料的韧性;同时,纤维-基体界面发生弱化也是提高材料韧性的另一机制.HTT处理前后试样的三点弯曲力学性能试验结果证实了以上机制.     

高温热处理对C/C-SiC复合材料制备与力学性能的影响

以针刺整体炭毡为坯体,采用树脂浸渍和化学气相沉积混合法制备C/C多孔体,然后熔硅浸渗制得C/C-S iC复合材料;研究了C/C多孔体的高温热处理对C/C-S iC复合材料密度、孔隙度、力学性能及断裂方式的影响。结果表明:炭涂层进行高温热处理可改变复合材料的弯曲断裂方式,使其具有一定的“假塑性”,弯曲强度下降约16%,压缩强度提高约20%,硬度增加;C/C多孔体的最终高温热处理可打开孔隙,有利于液S i的渗入,制备出密度较高(>2.0 g.cm-3)、开孔率较小(<4%)的复合材料,但导致其力学性能下降,基本上不影响其断裂方式。     

Effect of equal channel angular pressing and heat treatment on the microstructure of Cu–Al–Be–B shape memory alloy

The combination of equal channel angular pressing (ECAP) and heat treatment was carried out to modify the microstructure of a Cu–Al–Be–B shape memory alloy. Microstructures of the alloy after ECAP and subsequent quenching were investigated by optical microscopy and X-ray diffraction (XRD). The alloy with 8 passes of ECAP at 743 K is characterized with ultra-fine grains (~ 2 μm), but with smaller fraction of martensites which implies the lower shape memory effect (SME). After reheated at 873 K and oil-quenched to room temperature, the grains become coarsen (~ 50 μm) but still finer than that of as-received (100–300 μm), and the fraction and order of martensites were increased simultaneously.