Effect of metallic phase content on mechanical properties of （85Cu- 15Ni）/（10NiO-NiFe2O4） cermet inert anode for aluminum electrolysis

（85Cu-15Ni）/（10NiO-NiFe2O4） cermets were prepared with Cu-Ni mixed powders as toughening metallic phase and 10NiO-NiFe2O4 as ceramic matrix. The phase composition, microstructure of composite and the effect of metallic phase content on bending strength, hardness, fracture toughness and thermal shock resistance were studied. X-ray diffraction analysis indicates the coexistence of （Cu-Ni）, NiO and NiFe2O4 phases in the cermets. Within the content range of metallic phase from 0% to 20% （mass fraction）, the maximal bending strength （176.4 MPa） and the minimal porosity （3.9%） of composite appear at the metallic phase content of 5%. The fracture toughness increases and Vickers＇ hardness decreases with increasing metal content. When the thermal shock temperature difference （At） is below 200 ℃, the loss rate of residual strength for 10NiO-NiFe2O4 ceramic is only 8%, but about 40% for （85Cu-15Ni）/（10NiO-NiFe2O4）cermets. As At is above 200 ℃, the residual strength sharply decreases for sample CN0 and falls slowly for samples CN5-CN20.     

Effect of sintering atmosphere on corrosion resistance of Ni/(NiFe2O4–10NiO) cermet inert anode for aluminum electrolysis

A comparative study on the corrosion resistance of 17Ni/(NiFe₂O₄–10NiO) cermet inert anode prepared in different sintering atmospheres was conducted in Na₃AlF₆–Al₂O₃ melt. The results indicate that the corrosion rates of NiFe₂O₄-based cermet anodes prepared in the vacuum and the atmosphere with oxygen content of 2×10⁻³ (volume fraction) are 6.46 and 2.71 cm/a, respectively. Though there is a transition layer with lots of holes or pores, a densified layer is formed on the surface of anode due to some reactions producing aluminates. For the anode prepared in the atmosphere with oxygen content of 2×10⁻³, the thickness of the densification layer (about 50 μm) is thicker than that (about 30 μm) formed on the surface of anode prepared in the vacuum. The contents of NiO and Fe(II) in NiFe_2xO_4–y–z increase with the decrease of oxygen content in sintering atmosphere, which reduces the corrosion resistance of the material.     

Tribological performances of Ti(C,N)-based cermets with different graphite contents in dry sliding condition

Ti(C,N)-based cermets with different graphite (Gr) contents were prepared to solve the problem that the cermets self-mated couples suffer severe wear in dry sliding condition. The microstructure and mechanical properties of cermets with varying graphite contents were observed and evaluated, and the effects of graphite content on the tribological performances of the cermets self-mated rubbing pairs were investigated using a block-on-ring apparatus at room temperature. The results show that the distribution of graphite phase in cermets transforms from segregated floc-like to uniform granular with the increase of graphite content. The hardness and fracture toughness of the cermets self-lubricating materials rise firstly and then fall, whereas the transverse rupture strength decreases gradually with graphite adding. Compared with the cermet without graphite, the tribological properties of the cermets containing graphite are improved significantly. When the graphite addition reaches 0.8 wt%, the cermet exhibits the optimal wear resistance, which is attributed to the formation of a stable lubricating film composed of graphite and metal oxides (TiO₂, MoO₃, NiO) on the tribo-surface. Additionally, results reveal that the wear mechanism of cermets in dry condition is a complicated regime containing abrasion, adhesion and oxidation. After adding graphite, the adhesive wear of cermets is inhibited effectively, however the abrasive wear will be aggravated as the graphite content increases continuously.     

Influence of Yb2O3 addition on microstructure and corrosion resistance of 10Cu/（10NiO-NiFe2O4） cermets

10Cu/(10NiO-NiFe₂O₄) cermets doped with Yb₂O₃ were prepared by conventional powder metallurgy technique. The effects of Yb₂O₃ content and sintering temperature on the relative density, phase composition, microstructure of the sintered cermets and the corrosion resistance to Na₃AlF₆-Al₂O₃ melts were investigated by sintered density test, XRD analysis and SEM. YbFeO₃ phase, which distributes in the ceramics grain boundary as particles or film, is produced by the reaction between Yb₂O₃ and ceramics. The addition of Yb₂O₃ accelerates the sintering process of ceramics matrix, eliminates pores in the boundary and results in coarsened crystalline grain. The relative density of the cermets with about 1% (mass fraction) Yb₂O₃ sintered at 1275 °C increases to above 95%. Addition of about 1.0% Yb₂O₃ can inhibit obviously the corrosion of NiFe₂O₄ grain boundary and Cu phase in Na₃AlF₆-Al₂O₃ melts.     

Aging behavior of Al2O3 short fiber reinforced Al-Cu alloy composites

Al2O3 short fiber reinforced AI-Cu composites containing 1%, 3%, 5% and 7% Cu were fabricated by a squeeze casting technique. The as-cast Al2O3/Al-Cu composites were solution treated at 535 ℃ and then aged at 170, 190 and 210 ℃, respectively. Age hardening behavior of the Al2O3/Al-Cu composites was analyzed by measuring the hardness of the samples at different aging temperatures and aging time. Microstructures of the composites were observed by transmission electron microscope（TEM）. The results indicate that the hardness of the Al2O3/Al-Cu composites containing 7% Cu is much higher than that containing 1%-5% Cu because of the large amount of CuAl2 precipitant in the Al2O3/Al-Cu composite. With the increase of Cu content from 1% to 7%, the time needed for the appearance of peak hardness shortened, indicating that the addition of Cu can accelerate the kinetic of CuAl2 precipitation in the Al2O3/Al-Cu composites. The Al2O3/Al-Cu composite containing 7% Cu shows the highest increment of hardness by aging treatment. Therefore, in order to get a higher peak hardness, the Al2O3/Al-Cu composites need more Cu addition as compared with the un-reinforced Al-Cu alloys.     

Effect of carbon content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with different carbon contents were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. The free carbon present in the green compact significantly decreased the grain size; however, a high carbon content resulted in the formation of graphite phase and Fe₃C phase. An increasing carbon content promoted the dissolution of Mo in the binder phase. In addition, the binder phase varied from ferrite to martensite with increasing carbon content. The highest hardness was found for the cermets with 0.5 wt.% carbon addition, whereas the cermets without carbon addition exhibited the maximum TRS and fracture toughness.     

Novel alumina titanium-carbonitride nickel composites

Alumina-containing titanium-carbonitride nickel composites (Al₂O₃-TiCN-Ni-Mo₂C) have been synthesized with up to 20 vol.% alumina particles and nickel contents of 10–15 vol.%. Alumina in the form of platelets as well as powders have been successfully incorporated into the composites. The mechanical properties of these composites show a marked increase in toughness while retaining a good hardness and a low density when compared to TiCN-Ni cemented carbides. The presence of alumina with its superior chemical and wear resistance makes these composites very attractive for all wear parts—particularly for pieces exposed to chemically aggressive environments and high-temperature applications. Moreover, the complementary properties of these new composites (e.g., light weight, hardness, toughness, and chemical stability) allow a wide range of applications to be envisaged.     

Effect of carbon content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with different carbon contents were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. The free carbon present in the green compact significantly decreased the grain size; however, a high carbon content resulted in the formation of graphite phase and Fe₃C phase. An increasing carbon content promoted the dissolution of Mo in the binder phase. In addition, the binder phase varied from ferrite to martensite with increasing carbon content. The highest hardness was found for the cermets with 0.5 wt.% carbon addition, whereas the cermets without carbon addition exhibited the maximum TRS and fracture toughness.     

NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>-based cermet inert anodes for aluminum electrolysis

Recent developments in the preparation, sintering process, mechanical properties, and thermal shock resistance of cermet inert anodes for aluminum electrolysis are reviewed in this paper. To obtain the desired technologies of low-temperature activated sintering of cermet inert anodes, the effects of material composition, sintering atmosphere, sintering temperature, and sintering aids on the densifi cation and microstructure of NiFe₂O₄-10NiO- based ceramics and cermets were studied. To obtain the toughening and strengthening technology of the cermet, the effects of material composition including ceramic and metallic phases are discussed. The cermet inert anodes with high density and mechanical properties were prepared through adjustment of material composition and sintering technology and selection of feasible sintering aids.     

Mechanical properties of polypropylene/CaCC3 composites

The effects of calcium carbonate (CaCO₃) content on the hardness, tensile and dynamic mechanical properties and impact strength of the filled polypropylene (PP) composites have been investigated in the present paper. The results show that the moduli and hardness of the composites increase with addition of CaCO₃ content, but the tensile strength and strain at break is opposite to this; there are extreme values of the tensile yield stress and Izod impact strength at 5% and 10% of CaCO₃ weight fraction, respectively. It indicates that the strength and toughness of PP can be improved to some extent when PP is filled with suitable concentration of inorganic rigid particles.     

Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on Electrical Breakdown Properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

Al₂O₃/Cu composites were prepared by external addition of Al₂O₃, and the effect of Al₂O₃ content on microstructure, density, hardness, electrical conductivity and vacuum electrical breakdown properties was studied. The results show that with increasing Al₂O₃ addition, the density of Al₂O₃/Cu composite significantly decreases, the hardness sharply increases and then slowly decreases, but the electrical conductivity invariably decreases. The vacuum breakdown test shows that with increasing Al₂O₃ addition, the breakdown strength first sharply increases and then decreases when the Al₂O₃ content exceeds 1.2 wt.%; the chopping current always exhibits a decreasing trend and the arc life first increases and then decreases. According to the morphology of arc erosion and analysis, the arc erosion resistance increases and then decreases sharply. In the range of experiments, the optimal arc erosion resistance of Al₂O₃/Cu composite can be obtained with the addition of 1.2 wt.% Al₂O₃.     

The Microstructures and Mechanical Properties of V2O5-Doped Al2O3/TiAl In-Situ Composites by Reactive Hot Pressing Process

Al₂O₃/TiAl composites are successfully fabricated by the in-situ hot pressing method from the elemental powders of Ti, Al, TiO₂, and V₂O₅. The effect of V₂O₅ addition on the microstructure and mechanical properties of the Al₂O₃/TiAl in-situ composites is investigated in detail. It is found that the as-synthesized composites mainly consist of V-dissolved γ-TiAl, α₂-Ti₃Al, and Al₂O₃ particles along with a small amount of V₃Al phase, and the in-situ-formed fine Al₂O₃ particles tend to disperse on the grain boundaries of TiAl matrix. With increasing V₂O₅ content, the density and Vickers hardness of the resulting composites gradually increase, whereas the fracture toughness and flexural strength first increase and then decrease with the increase of V₂O₅ content. The composite with 3.5 wt.% V₂O₅ has the maximum value of 9.35 MPa m^1/2 and 713.36 MPa for the fracture toughness and flexural strength, respectively. The toughening mechanism is also discussed in detail.     

Hot corrosion of a novel NiO/NiFe2O4 composite coating thermally converted from the electroplated Ni–Fe alloy

A novel NiO/NiFe₂O₄ composite coating thermally converted from an electroplated Ni–Fe alloy was successfully fabricated. The composite coating consisted of a NiO matrix and homogeneously distributed criss-cross intragranular and intergranular NiFe₂O₄ precipitates, with a very dense and flat structure. The composite, compared to bare Ni metal, exhibited increased hot corrosion resistance under an atmosphere of Na₃AlF₆–AlF₃–CaF molten salts and air at 960 °C, mainly because of the dense structure and well-adhered, homogeneously dispersed intragranular and intergranular NiFe₂O₄ precipitates.     

Effects of Cr3C2 addition on the corrosion behavior of Ti(C,N)-based cermets

The Ti(C, N)-based cermets with different Cr₃C₂ addition were prepared and the effects of Cr₃C₂ addition on the microstructure and properties of cermets were discussed. The corrosion behavior of the cermets with different Cr₃C₂ addition was investigated emphatically in 2 mol/L HNO₃ solution. The results indicate that there is no obvious effect of Cr₃C₂ addition on the densification of the cermets, and all cermets are almost fully densified during sintering. The thickness of rim phase is reduced and the core size is increased remarkably in the cermets with 1 wt.% and 3 wt.% Cr₃C₂ addition; the grains are refined significantly in the cermets with the increase of Cr₃C₂ addition to 5 wt.%. The hardness and transverse rapture strength of the cermets are improved with Cr₃C₂ added properly. In HNO₃ solution, the corrosion resistance of cermets is improved remarkably by Cr₃C₂ addition. The corrosion of binder phase is predominant in the cermets in which the Ni binder phase without Cr has lower corrosion resistance than the rim phase; whereas the corrosion resistance of binder phase with high Cr content is better compared to the rim phase, so that the degradation of rim phase is predominant and a reticulate binder phase forms. With the increase of Cr₃C₂ addition, the Mo content in rim increases, and it is bad for the corrosion resistance of rim phase. Additionally, the inner rim phase has lower corrosion resistance than the outer rim phase owing to the higher Mo content.     

Development and performance of new hard and wear-resistant engineering materials

Wear is a major problem in many industrial applications, and the development of wear-resistant materials is therefore both a technical and an economic advantage. Iron-base composites bring new possibilities into the production of wear-resistant materials because of their high hardness and sufficient fracture toughness. They are suitable replacements for the conventional WC/Co cermets owing to their lower fabrication cost, better machinability, weldability, and corrosion resistance. In this study, hot-work steel/Cr₃C₂ composites and reference wear-resistant materials were produced by hot isostatic pressing. It was found that the matrix powder size used during processing did not affect the resultant wear properties of the composite. On the other hand, the impact toughness increased when fine matrix powders were used. The increasing reinforcement volume fraction increased significantly the hardness and wear resistance of the composite; however, the impact resistance decreased. The newly proposed hot-work steel/30 vol% Cr₃C₂ composite demonstrated a better combination of properties than some of the most abrasion-resistant materials available today.     

The fabrication of multi-core structure cermets based on (Ti,W,Ta)CN and TiCN solid-solution powders

A novel multi-core structure cermets consisted of both black-core/rim structure and grey-core/rim structure were obtained by partially replacing TiCN powder with (Ti,20W,15Ta)CN powder via low-pressure sintering process. The toughness and strength of TiCN-based cermets were optimized and its feature of high hardness was maintained simultaneously. Systematically, it was investigated that the influences of various weight ratios of both (Ti,20W,15Ta)CN/TiCN and Co/Ni on the microstructure and mechanical properties of the multi-core cermets. The results showed that the addition of (Ti,20W,15Ta)CN powder could cause the refinement of the core size and the occurrence of the secondary phase (W,Mo,Ti)_3 + x(Co,Ni)_3 − xC (0 < x ≤ 1), both of which are responsible for the significant improvement of the mechanical properties. The appearance of the secondary phase was found under two circumstances, one was when the weight ratio of (Ti,20W,15Ta)CN/TiCN was 6:4 while that of Co/Ni was 5:5(cermet M60) and the other was when that of (Ti,20W,15Ta)CN/TiCN was 5:5 with pure Co binder (cermet C50). And there is a monotonous escalation of the fracture toughness (K_IC) of the cermets while increasing the (Ti,20W,15Ta)CN content. The optimal comprehensive mechanical performance was found in cermet M60 with transverse rupture strength (TRS) of 1903.32 MPa, Vickers hardness (HV₃₀) of 16.33 GPa and fracture toughness of 12.19 MPa·m^1/2.     

镍钴对超细Ti(C,N)基金属陶瓷性能的影响

研究了Co部分和全部代替Ni对超细Ti(C,N)基金属陶瓷性能的影响。采用X射线衍射仪对物相研究发现:金属相中w(Co)=10%和w(Co)=15%时,金属陶瓷中出现金属间化合物,并且有两种Co的同素异构体并存;力学性能测试表明:5%的Co取代5%的Ni可以明显提高金属陶瓷的抗弯强度、硬度和断裂韧性。金属相为10%Ni-5%Co时,材料的综合力学性能最好。     

Fabrication and properties of hot-pressing sintered WC-Al2O3 composites reinforced by graphene platelets

To enhance the toughness of the WC-Al₂O₃ composite, graphene platelets (GPLs) were incorporated to this ceramic composite by using ball milling and hot pressing sintering. The influences of graphene on microstructure and mechanical properties of the WC-Al₂O₃ composites were investigated. Results of the experiments showed that the grain size of the composite first diminished and then increased gradually along with the increase of graphene content. Both the Vickers hardness and toughness increased first and then diminished along with the increase of graphene content. The optimized composite exhibited the highest Vickers hardness (18.78 GPa) and the highest fracture toughness (11.09 MPa·m^1/2) at the indentation load of 294 N (30 kg) when incorporated with 0.3 wt% GPLs, which are about 18.7% and 40.8% higher than that of WC-Al₂O₃ without GPLs, respectively. However, the relative density of the WC-Al₂O₃ composites decreased stably along with the increase of graphene content. Agglomeration of GPLs and porosity were observed in the composites with high content, which weakened the properties. The toughening mechanisms are proposed to be crack deflection, crack bridging, graphene pull out and grain refinement.     

Welding of AA6061-(Al2O3 )p composite: effect of weld process variables and post-welding heat treatment on microstructure and mechanical properties

Abstract

Metal–matrix composites reinforced with Al₂O₃ particles combine the properties of the matrix (ductility and toughness) with the ceramic properties of the reinforcements (high strength). However, their wide application as structural materials requires a proper development of their joint process. The present work describes the results obtained from microstructural (optical and scanning electron microscopy) and mechanical evaluation (hardness and tensile tests) of the welded aluminium–matrix composite (AA6061) reinforced with 10% and 20% volume fraction Al₂O₃ particles (W6A10 and W6A20, respectively) using the MIG (metal inert gas) welding process and ER5356 (AlMg5) as filler material. A characteristic of the welds carried out in composites is that the size of the melt pool is wider than in the unreinforced materials, for the same welding conditions. This is caused by the lower thermal conductivity of the composites. Furthermore, composites act as an insulator reducing the cooling rate of the bath. The thermal effect of welding on different types of joints results in a loss of the mechanical properties in the heat affected zones (HAZ). These properties can be recovered with post-welding heat treatment.