粉末冶金制备工艺对TiC增强高铬铸铁基复合材料性能的影响EI北大核心CSCD

采用高能球磨和真空烧结的方法制备TiC增强高铬铸铁(HCCI)基复合材料。利用SEM,DSC等方法对不同球磨时间的粉末进行分析,研究不同烧结温度对高铬铸铁基复合材料的显微组织、硬度及密度的影响,比较相同工艺下复合材料与高铬铸铁材料的耐磨性。结果表明:球磨12 h后的粉末颗粒大小趋于稳定,粉末活性提高,烧结性能改善,烧结试样中TiC均匀地分布在基体中。随着烧结温度的升高,复合材料内部晶粒逐渐长大,密度和硬度逐渐提高。在1280℃超固相线液相烧结的条件下烧结2 h后,致密度达94.17%,硬度和抗弯强度分别为49.2HRC和980 MPa。在销盘磨损实验中复合材料的耐磨性为单一高铬铸铁材料的1.52倍,磨损机制为磨粒磨损+轻微氧化磨损。     

Wear properties of high pressure torsion processed ultrafine grained Al–7%Si alloy

In this paper, Al–7 wt% Si alloy was processed via high pressure torsion (HPT) at an applied pressure 8 GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT samples were investigated and compared with those of the as-cast samples. The wear properties of as-cast and the HPT samples under dry sliding conditions using different sliding distances and loads were investigated by reciprocated sliding wear tests.The HPT process successfully resulted in nanostructure Al–7 wt% Si samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more homogeneous distribution of the Si particles than those in the as-cast samples.The wear mass loss and coefficient of friction values were decreased after the HPT process. The wear mechanism was observed to be adhesive, delamination, plastic deformation bands and oxidization in the case of the as-cast alloy. Then, the wear mechanism was transformed into a combination of abrasive and adhesive wear after the HPT process. The oxidization cannot be considered as a mechanism that contributes to wear in the case of HPT samples, because O₂ was not detected in all conditions.     

Microstructure and mechanical properties of liquid phase sintered Mo2FeB2 based cermets

Mo₂FeB₂ based cermets were fabricated by liquid phase sintering at different sintering temperatures and soaking time. Almost full densification was achieved at 1080 °C without soaking time. However, a lower sintering temperature resulted in the aggregation of binder phase and a higher contiguity of hard phase. An increase of sintering temperature or soaking time promoted an in situ growth of elongated Mo₂FeB₂ grains in the intermediate state of sintering. Abnormally large and faceted Mo₂FeB₂ grains appeared at 1320 °C for 20 min. The growth of the abnormally large grains was associated with grain coalescence. The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured, and attempts were made to relate them to the microstructural development.     

Effect of sintering temperature on properties of Al2O3 whisker reinforced 3 mol% Y2O3 stabilized tetragonal ZrO2 (TZ-3Y) nanocomposites

In present study, effect of sintering temperature on density and hardness of 3 mol% yttria-stabilized tetragonal zirconia (referred to as TZ-3Y) composite reinforced with alumina whiskers (5, 10, 15 and 20 wt.%) has been studied. Initially, Ammonium Aluminum Carbonate Hydroxide (AACH) whiskers were added in TZ-3Y composite and transformed into alumina during sintering performed at different temperatures i.e. 1400, 1500 and 1650 °C. Results revealed that for all sintering temperatures, with increase in whisker concentration, sintered density decreased and hardness increased conversely. Maximum hardness of 14.47 GPa was achieved with 10 wt.% whiskers addition when sintered at 1500 °C. However, with addition of CTAB (1 wt.%) as deflocculating agent the hardness was further improved to 15.11 GPa. While sintering at 1650 °C a decrease in hardness was observed. It was mainly due to high temperature morphological change of whiskers i.e. transformation of whiskers into alumina rich grains.     

Effect of heat treatment on the tribological properties of Al–Cu–Mg/nanoSiC composites

In this study, the effect of heat treatment on the tribological properties of Al–Cu–Mg alloy reinforced with 4 wt.% SiC particles with 650 nm average particle size has been investigated. The age hardening process consists of solution treatment at 540 °C for 6 h, followed by water quenching and ageing at different temperatures of 175, 200 and 225 °C with soaking times of 3, 6 and 9 h. Hardness measurements were applied to monitor the precipitation effect and the aged samples were then subjected to wear tests under dry sliding conditions against steel and alumina counterfaces. The results showed that the reinforced material exhibits an enhanced ageing response compared to the unreinforced material in the same heat treatment conditions. The rate of ageing increases with increasing temperature; however, ageing at 200 and 225 °C for more than 6 h resulted in over-ageing. The best combinations for the enhanced tribological properties for the composite material were selected as 6 h ageing at 225 °C. The precipitation effect for this alloy can be enhanced by the small addition of SiC nanoparticles. Having a small amount of nanoSiC particles with fine precipitates inside the matrix further increases the hardness and wear properties.     

Improvement of mechanical properties of stainless maraging steel laser weldments by post-weld ageing treatments

The response of stainless maraging steel weldments to post-weld ageing treatment has been investigated. Post-weld ageing was performed at five different temperatures, viz., 420 °C, 460 °C, 500 °C, 540 °C, and 580 °C. Metallographic characterization of weldment revealed three zones, namely fusion zone, heat-affected zone (HAZ) and unaffected parent metal zone. Hardness and tensile properties were evaluated after ageing at different temperatures. Hardness in HAZ and fusion zone varied with ageing temperature differently from that of the parent metal; it became higher in HAZ and fusion zone than in parent metal zone above 420 °C. Among the applied ageing treatments, ageing at 460 °C achieved the highest tensile strength. A graph was constructed for determination of fracture location and post-weld heat treatment efficiency based on experimental results, using hardness ratio of HAZ to the treated parent material and hardness ratio of HAZ to the as-received parent material.     

The effect of silver on wear behaviour of zinc–aluminium-based ZA-12 alloy produced by gravity casting

The aim of this study was to investigate the effect of different weight concentrations of silver (0.1, 0.3, 0.5 and 0.7 wt.%) on the microstructure, hardness and wear properties of the gravity cast zinc–aluminium based alloy ZA-12. The alloys were manufactured under nitrogen protective atmosphere by a gravity casting process. Metallographic studies reveal that the addition of silver to the standard ZA-12 alloy changed the volume fraction and structure of the primary β-dendrites in the ZA-12 alloy. Also, it was observed that the addition of silver to ZA12 alloy enhanced the hardness and wear properties effectively. However, the corrosion resistance was decreased with increasing silver content. In addition to this, the highest hardness value among experimental alloys was obtained for the alloy containing 0.7 wt.% Ag with 116 HB. The wear rate for all applied loads is decreased with rising silver content. A similar trend was observed for the friction coefficient. The alloy containing 0.7 wt.% Ag exhibited the highest wear resistance at all loads.     

The effect of copper oxide on sintering, microstructure, mechanical properties and hydrothermal ageing of coated 2.5Y-TZP ceramics

Copper oxide dopants in amounts up to 1 wt% were added to 2.5 mol% yttria-coated zirconia powders in studies of sintering, microstructure, mechanical properties and hydrothermal ageing behaviour. High densities (>6 Mgm^–3), high tetragonal phase content (>95%), and phenomenal fracture toughness values (>17 MPam^1/2), were obtained for lower dopant levels. Grain sizes of 0.13 to 0.25 m were measured for all samples sintered at 1300°C. Rounded pores in some doped samples indicated that a liquid phase was involved during sintering. Copper oxide additions aid low temperature sintering and offer potential for property enhancement with a particularly high toughness being measured as well as improving resistance to structural degradation in 180°C hydrothermal ageing.     

The effect of salt bath cementation on mechanical behavior of hot-rolled and cold-drawn SAE 8620 and 16MnCr5 steels

In this study, the effect of salt bath cementation on mechanical behavior of SAE 8620 and 16MnCr5 cementation steels, which are widely used in industry, was investigated. The experiments were carried out with hot rolled and cold rolled specimens. The cementation processes were performed in NaCN salt bath at 920 °C temperature for 1, 2, 3 and 4 h. Abrasive wear tests of specimens were conducted with Wolfram Carbide (WC) ball for 1 h. After cementation processes, a martensite phase on the surface of specimens was confirmed by X-ray diffraction analysis. After cementation processes carried out with different times, a different surface hardness and effective cementation depth values were obtained. Experimental results showed that an effective cementation depth increased with increasing the cementation time. Wear tests showed that the wear resistance of specimens increased by the cementation processes. Experimental results revealed that the surface hardness of specimen affects the wear resistance of specimens.     

Investigating the effect of quench environment and deep cryogenic treatment on the wear behavior of AZ91

Deep cryogenic heat treatment is a conventional supplementary treatment for steels to improve their wear resistance and hardness. Despite a variety of researches about steels, lack of investigation for the other alloys and materials is obvious. In this study, the effect of the deep cryogenic heat treatment and different quenching environments on the magnesium alloy (AZ91) was investigated via the optical microscope (OM), scanning electron microscope (SEM), hardness evaluation and wear test. For this purpose, the samples were solutionized at 420 °C for 24 h followed by quenching in different environments of water, air and liquid nitrogen. After that the samples were deep cryogenically treated in liquid nitrogen, followed by aging. Results show that deep cryogenic heat treatment improves the hardness and wear resistance. This behavior is a consequence of aluminum atoms jumping to the nearby defects, including dislocations. Moreover, it was clarified that increasing the cooling rate of quench environment improves the hardness after aging and that the predominant wear mechanism is abrasive.     

Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel

The plasma nitrocarburizing of nanocrystallized 18Ni maraging steel was performed at 460 °C for 4 h. The surface phase composition, cross-sectional microstructure and hardness profile of the nitrocarburized layer were investigated by the X-ray diffractometer (XRD), optical microscope (OM) and microhardness tester. Plasticity of the surface layer of original and nitrocarburized samples was analyzed by Taylor factor obtained by electron backscattering diffraction (EBSD) data and nanoindentation tests. The nitrocarburized surface is composed of α-Fe, Fe₄N and a small fraction of low nitrogen compound FeN_0.049. The surface and core hardness of nitrocarburized samples are 200% and 130% of that of the original one, respectively. The Taylor factors for different slip systems of α-Fe grains are all decreased after nitrocarburizing and Taylor factors for Fe₄N grains are lower than those of basal slip system of α-Fe grains. Plasticity factor η_p, i.e. the ratio of plastic deformation work to total deformation work dissipated during loading-unloading process, of the surface layer is reduced about 20% after nitrocarburizing. This suggests that plasticity and wear resistance of the surface layer could be decreased and improved after nitrocarburizing, respectively. The surface layer of the nitrocarburized sample also possesses certain plasticity because its plasticity factor η_p is more than 60%.     

Tool wear and its effect on microstructure and properties of friction stir processed Ti–6Al–4V

Ti–6Al–4V alloy was subjected to friction stir processing at rotation rates of 400, 800 and 1200 rpm using a polycrystalline cubic boron nitride (pcBN) tool and tool wear at different travel distances was investigated. At high rotation rates of 800 and 1200 rpm, the greatest tool wear, including mechanical and chemical wear, occurred at the initial tool plunge point. Detailed microstructural examinations on the tool plunge point at 1200 rpm by transmission electron microscopy indicated that the “onion ring” structure in the stir zone was caused by a variation in the distribution of TiB particles. Two similar but not identical spatial phase sequences around BN particles, BN–TiB₂–TiB–α-Ti (N) and BN–TiB₂–TiB–transformed β-Ti (N), as well as Ti2N phase were identified. The reaction mechanism between the tool and the Ti matrix was discussed. Moreover, when the tool wear reached a steady-state condition, the effect of tool wear on the microstructure and mechanical properties of the stir zone was evaluated. A fully transformed β with a Widmanstatten structure was observed at all rotation rates and the average size of prior β grains increased with the rotation rate. The tool wear led to an increment in hardness and tensile strength but a loss of ductility of the stir zone.     

Comparison of microstructural evolution in Ti-Mo-Zr-Fe and Ti-15Mo biocompatible alloys

The microstructural evolution and attendant strengthening mechanisms in two biocompatible alloy systems, the binary Ti-15Mo and the quaternary Ti-13Mo-7Zr-3Fe (TMZF), have been compared and contrasted in this paper. In the homogenized condition, while the Ti-15Mo alloy exhibited a single phase microstructure consisting of large β grains, the TMZF alloy exhibited a microstructure consisting primarily of a β matrix with grain boundary α precipitates and a low volume fraction of intra-granular α precipitates. On ageing the homogenized alloys at 600 ^∘C for 4 h, both alloys exhibited the precipitation of refined scale secondary α precipitates homogeneously in the β matrix. However, while the hardness of the TMZF alloy marginally increased, that of the Ti-15Mo alloy decreased substantially as a result of the ageing treatment. In order to understand this difference in the mechanical properties after ageing, TEM studies have been carried out on both alloys in the homogenized and homogenized plus aged conditions. The results indicate that the ω precipitates dissolve on ageing in case of the Ti-15Mo alloy, consequently leading to a substantial decrease in the hardness. In contrast, the ω precipitates do not dissolve on ageing in the TMZF alloy and the precipitation of the fine scale secondary α leads to increased hardness.     

Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.

     

Abnormal grain growth enhanced densification of liquid phase-sintered WC–Co in support of the pore filling theory

This study examines the effect of grain growth on densification during liquid phase sintering of compacts with faceted grains. Two kinds of WC powders with different sizes were used to produce WC–Co alloys. Large pores of ~5 μm size were generated in 95WC–5Co (wt%) using spherical Co particles of the same size. The overall sintering behavior was observed by measuring grain growth and densification as a function of sintering time at a sintering temperature of 1350 °C. When the WC powder was fine (0.4 μm), large pores disappeared upon filling of pores by liquid with the formation of abnormal grains. On the contrary, when the WC powder was large (4.2 μm), grain growth is not observed, and large pores remained intact even after a long period of sintering (24 h). These observations confirm that densification during final stage liquid phase sintering occurs via filling of pores by liquid as a result of grain growth. This finding is consistent with the model of densification predicted by the pore filling theory.     

Influence of synthesis routes on the performance from weakly agglomerated yttria-stabilized zirconia nanomaterials

Weakly agglomerated nanocrystalline YSZ (yttria-stabilized zirconia) was synthesized by three different routes, namely homogeneous precipitation, ammonium hydroxide-based directly hydrothermal and urea-based homogeneously hydrothermal methods. The XRD and TEM results showed that well-crystallized nanocrystalline YSZ with homogenous and weakly agglomerated assembly was obtained from these processes. The Raman scattering results of the three samples suggested that cubic zirconia was formed as major phase, and consisted of the tetragonal zirconia as minor one. The sintering behavior of the two hydrothermal samples indicated a significant shrinkage at T < 570 °C. However, the shrinkage rates of the three samples showed similar at T > 570 °C. Impedance spectroscopy was used to determine the ac ionic conductivity. The total and grain boundary conductivities of the sample obtained from ammonium hydroxide-based hydrothermal method were higher than those of the samples obtained from homogeneous precipitation and urea-based homogeneously hydrothermal methods, whereas the total and grain boundary activation energies of the former being slightly lower.     

Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel

The plasma nitrocarburizing of nanocrystallized 18Ni maraging steel was performed at 460 °C for 4 h. The surface phase composition, cross-sectional microstructure and hardness profile of the nitrocarburized layer were investigated by the X-ray diffractometer (XRD), optical microscope (OM) and microhardness tester. Plasticity of the surface layer of original and nitrocarburized samples was analyzed by Taylor factor obtained by electron backscattering diffraction (EBSD) data and nanoindentation tests. The nitrocarburized surface is composed of α-Fe, Fe₄N and a small fraction of low nitrogen compound FeN_0.049. The surface and core hardness of nitrocarburized samples are 200% and 130% of that of the original one, respectively. The Taylor factors for different slip systems of α-Fe grains are all decreased after nitrocarburizing and Taylor factors for Fe₄N grains are lower than those of basal slip system of α-Fe grains. Plasticity factor η_p, i.e. the ratio of plastic deformation work to total deformation work dissipated during loading-unloading process, of the surface layer is reduced about 20% after nitrocarburizing. This suggests that plasticity and wear resistance of the surface layer could be decreased and improved after nitrocarburizing, respectively. The surface layer of the nitrocarburized sample also possesses certain plasticity because its plasticity factor η_p is more than 60%.     

Nanomechanical properties of hydroxyapatite (HAP) with DAB dendrimers (poly-propylene imine) coatings onto titanium surfaces

Coatings of hydroxyapatite (HAP) nanorods onto titanium surfaces were synthesized with the aim to improve coatings’ mechanical properties and adhesion to the substrate. The coatings are consisting of HAP nanorods synthesized in the presence of a cationic fourth generation diaminobutane poly(propylene imine) dendrimer (DAB) bearing 32 amine end groups employing varying calcium: dendrimer ratios and varying hydrothermal treatments. The quality, surface morphology and structure of the coatings were characterized with X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and energy dispersive microanalysis. Wear resistance and adhesion properties of the coatings onto titanium substrates were studied through nanoindentation analysis. The experimental conditions, namely the calcium: dendrimer molar ratio and the hydrothermal treatment temperature were carefully selected; thus, it was possible to produce coatings of high hardness and elastic modulus values (ranging between 1–4.5 GPa and 40–150 GPa, respectively) and/or high wear resistance and plastic deformation values.     

Microstructural and mechanical characteristics of Cu–Cu2O composites compacted with pulsed electric current sintering and hot isostatic pressing

This paper reports the influence of applied sintering process – pulsed electric current sintering (PECS) and hot isostatic pressing (HIP) – on the microstructure and mechanical properties of Cu–Cu₂O composites. In PECS fine-grained structure was obtained while in HIPing the grain growth was more noticeable, mostly due to the longer process time. The studies also showed that Cu₂O-phase distributed in Cu-matrix increased microhardness; at a fixed grains size Cu–Cu₂O structure had higher hardness than Cu so that 20% higher microhardness was obtained when Cu₂O was doubled from 19.1 to 37.2 vol%. At best, 99.1% density with 690 nm grain size and 1.35 GPa hardness were achieved by PECS whereas by HIP the same density with 1860 nm grain size gave 1.02 GPa hardness. The grain growth and the effect of second phase clustering on the grain growth were evaluated experimentally.     

Mechanical alloying and spark plasma sintering of CoCrFeNiMnAl high-entropy alloy

An equiatomic CoCrFeNiMnAl high-entropy alloy was synthesized by mechanical alloying, and alloying behaviors, microstructure and annealing behaviors were investigated. It was found that a solid solution with refined microstructure of 20 nm in grain size could be obtained after 30 h milling. As-milled powder transformed into a face-centered cubic phase above 500 °C. The as-milled powder was subsequently consolidated by spark plasma sintering at 800 °C, BCC phase and FCC phase coexisted in the consolidated HEA, which had excellent properties in Vickers hardness of 662 HV and compressive strength of 2142 MPa.