Influence of titanium addition on the microstructure of the novel ferrous-based stainless steel

The microstructural characteristics of the Fe-9Al-30Mn-1C-5Ti (wt.%) alloy were determined by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The microstructure of the alloy was essentially a mixture of (γ + TiC_x + (α + B2 + DO₃)) phases during solution treatment between 950 °C and 1150 °C. The TiC_x carbide had a face-center-cubic structure with a lattice parameter a of 0.432 nm. When the as-quenched alloy was subjected to aging treatment at temperatures of 450-850 °C, the following microstructural transformation occurred: (γ + TiC_x + κ + (α + DO₃)) → (γ + TiC_x + κ + (α + B2 + DO₃ + TiC_x)) → (γ + TiC_x + κ + κ′ + (α + B2 + DO₃)) → (γ + TiC_x + (α + B2 + DO₃)). Addition of Ti promotes the formation of the α phase at high temperatures.     

Supplementary measurements of the primary crystalline phases of the Co-Ni-Sb and the Co-Fe-Sb ternary systems

Some primary crystalline phases of the Co-Ni-Sb and the Co-Fe-Sb systems are studied on the basis of the crystalline structure analysis by X-ray diffraction (XRD), the microstructure observation by scanning electron microscopy (SEM), the composition determination by electron probe microanalysis (EPMA) and the phase equilibrium relations of the constituent binary systems. From the experimental measurements of the present work and the phase equilibrium relations of literature reports, the liquidus projections of the Co-Ni-Sb and the Co-Fe-Sb ternary systems are determined. There exist 6 primary crystalline phases in each of the ternary systems, which are γ(A1)-Fcc_A1, β(Co,Ni)₃Sb-D0₃, γ(Co,Ni)Sb-NiAs, ζ(Co,Ni)Sb₂-FeS₂, η(Co,Ni)Sb₃-CoAs₃ and Sb-Rhombo_A7 in the Co-Ni-Sb system and γ(A1)-Fcc_A1, α(A2)-Bcc_A2, γ(Co,Fe)Sb-NiAs, ζ(Co,Fe)Sb₂-FeS₂, η(Co,Fe)Sb₃-CoAs₃ and Sb-Rhombo_A7 in the Co-Fe-Sb system. Two of invariant reactions are proposed, which are the eutectic reaction L → γ(A1) + β(Co,Ni)₃Sb + γ(Co,Ni)Sb in the Co-Ni rich side of the Co-Ni-Sb system and the transition reaction L + γ(A1) → α(A2) + γ(Co,Fe)Sb in the Co-Fe rich side of the Co-Fe-Sb system.     

Microstructure and wear characteristics of hypereutectic Fe-Cr-C cladding with various carbon contents

The current study used flux core arc welding to produce a series of hypereutectic Fe-Cr-C claddings with various carbon content. Depending on the carbon content, this research produced hypereutectic microstructures of γ-Fe + (Cr,Fe)₇C₃ carbides. As the carbon content of a cladding increased from 3.73 to 4.85 wt.%, the surface fractions of carbides increased from 33.8% to 86.1%. The morphology of primary (Cr,Fe)₇C₃ carbides also transited from a blade-like to a rod-like shape. With regard to wear performance, the relationship between wear resistance and hardness (H) is non-linear. However, the mean free path (λ) of primary (Cr,Fe)₇C₃ carbides must be considered. Wear resistance is proportional to H/λ. The primary carbides can prevent the eutectic colonies from selective abrasion. The rod-like (Cr,Fe)₇C₃ carbides also provide much better wear resistance because rod-like carbides have a greater hardness. After an abrasive wear process, abrasive particles cause plastic plows when the cladding has lower surface fractions of carbides. The fracture of primary carbides leads into the craters where it occurs in the worn cladding surface with higher surface fractions of carbides.     

Phase evolution and dendrite growth in laser cladding of aluminium on zirconium

Aluminium was laser clad on a pure zirconium substrate using the blown powder method. The microstructure across the laser-clad coating was studied. Starting from the bottom to the top surface of the coating, a series of phase evolutions had occurred: (Zr) → (Zr) + AlZr₂ + AlZr₃ → Al₄Zr₅ + Al₃Zr₂ → Al₃Zr₂ + AlZr₂ → Al₂Zr → Al₂Zr + Al₃Zr. This resulted in an epitaxial columnar crystal growth at the re-melt substrate boundary, a band of backward growth Al₃Zr₂ dendrites towards the lower half of the coating, and a two-phase eutectic dendritic growth of Al₂Zr + Al₃Zr towards the top of the coating. The evolution of the various phases and microstructures is discussed in conjunction with the Al-Zr phase diagram, the criteria for planar interface instability, and the theory of eutectic growth under rapid solidification conditions (the TMK model).     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe₂Al₅ inter-metallic compound coating. The corrosion mechanism of the Fe₂Al₅ coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe₂Al₅ layer in molten zinc is as follows: Fe₂Al₅ → Fe₂Al₅Zn_x (η) → η + L(liquid phase) → L + η + δ(FeZn₇) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe₂Al₅ coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.     

Influence of the thermo-chemical conditions on the carbo-chromization process of α-Fe matrices obtained by powder sintering technique

We report on the superficial layer formation resulting from the carburization followed by chromization of α-Fe samples obtained by powder sintering technique. The carburization and chromization were carried out by thermal diffusion between 880-980 °C and 950-1050 °C in a solid powder mixture of charcoal/BaCO₃ and ferrochromium/alumina/NH₄Cl, respectively. The obtained layers were investigated using X-ray diffraction, optical microscopy, Vickers micro-hardness technique and scanning electron microscopy. The results show that the layers are of micrometric size and consist mostly of chromium carbides of different phases. These phases as well as the thickness of the layers are closely related to the treatment temperature used for carburization and to the temperature and Cr initial concentration in the mixture used for chromization. For highly reactive carbo-chromization conditions (high concentration of Cr, and high carburization and chromization temperatures) the superficial layer is constituted of two chromium carbide sub-layers (Cr₃C₂/Cr₇C₃) separated by a sharp interface. The thickness and hardness of the coating layer reached 45 μm and 2300 HV, respectively. Such coating could be used for tools that have to be abrasion and oxygen resistant at high temperatures.     

Microstructural changes of aluminized Alloy 617 during high-temperature aging

In this study, aluminized Alloy 617 was prepared by Al-pack cementation of high temperature high Al activity process. The microstructure evolution and microstructural changes of aluminide coating were investigated after Al-pack cementation and high-temperature aging. The aluminide coating was composed of Ni-aluminide layers, such as δ-Ni₂Al₃, β-NiAl, Cr₂Al, Al_3 + xMo_1 − x, and inter-diffusion zone by pack cementation. After high-temperature aging, the aluminide coating was transformed from the δ-Ni₂Al₃ to the β-NiAl because of outward Ni diffusion from substrate. The Cr₂Al and the Al_3 + xMo_1 − x were dissolved during aging. On the other hand, the α-(Cr, Mo) particles were precipitated during aging due to the low solubility of alloying elements in the β-NiAl. The β-NiAl newly formed by the outward Ni diffusion during aging and resulted in the formation of the inter-diffusion zone. The inter-diffusion zone consisted of β-NiAl, Ni₃(Al, Ti), Cr-rich M₂₃C₆ carbide, and sigma phases.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

Characterisation of HVOF sprayed Cr3C2-50(Ni20Cr) coating and the influence of binder properties on solid particle erosion behaviour

The coating Cr₃C₂ with 50 wt.% Ni20Cr deposited by high velocity oxy-fuel (HVOF) spray process was characterized in detail to investigate the effect of annealing on the solid particle erosion behaviour and understand the influence of the binder properties. Systematic characterization of the coating was carried out using electron microscopy (scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA)), X-ray diffraction (XRD), microindentation and nanoindentation techniques. The solid particle erosion tests were done on the as-sprayed coating and coatings annealed at 400 °C, 600 °C and 800 °C using silica erodent particles. The coefficient of restitution of the coated samples was also measured by WC ball impact tests to simulate dynamic impacts. The as-sprayed coating consisted of primary carbides and binder that was a mixture of amorphous and nanocrystalline phases. Annealing leads to recrystallisation of binder phase and precipitation of secondary carbides. The coating hardness and binder ductility change with annealing temperature. The erosion resistance improves with annealing up to 600 °C. In the as-sprayed coating, the amorphous phase, inter-splat boundaries and the elastic rebound characteristics affect the erosion response. While in the case of the coating annealed at 600 °C, the presence of ductile crystalline binder, fine carbide precipitates and embedment of erodent particles together improve solid particle erosion resistance.     

柱塞表面激光熔覆铁基涂层的强韧化机理

文中在柱塞表面激光熔覆制备高硬度铁基涂层,采用SEM,XRD,EPMA和TEM等手段研究熔覆层组织特征及耐磨性,阐述其强韧化机理.结果表明,激光熔覆铁基合金涂层成形良好,无裂纹及气孔等缺陷,熔覆层与基体呈冶金结合,组织由(Ni,Fe)固溶体、(Cr,Fe)₂₃C₆碳化物和少量孪晶马氏体组成.铁基熔覆层的强化机制主要有细晶强化、固溶强化、弥散强化以及马氏体强化;熔覆层内(Ni,Fe)固溶体及细晶强化的综合作用,保证了高硬度铁基涂层的韧性.铁基熔覆层显微硬度较45钢提高4倍,最大值H_HV0.2=850 GPa;熔覆层耐磨性明显高于45钢,45钢表面出现大面积疲劳剥落,铁基熔覆层磨损面平整,磨痕很浅且少,磨损机制为轻微的磨粒磨损.     

Effect of laser irradiation on failure mechanism of TiCp reinforced titanium composite coating produced by laser cladding

Laser cladding is an effective technique to coat a metallic substrate with a layer of a different nature. It has been widely reported that the most important combined parameters controlling the quality of the coating are the specific energy (E) and the powder density (Ψ). In the present work, clad deposits of Ti6Al4V + 60 wt.% TiC were prepared on a Ti6Al4V substrate using an optimum combination of E_c = 24 J/mm² and ψ_c = 3 mg/mm². These experiments were performed using a laser power of 400 and 600 W, in order to study the effect of laser power on the properties of the clad. The microstructure, phase composition and nanohardness of the coatings were investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. During laser processing, TiC can be partially converted to TiC_X (X = 0.5) due mainly to the TiC dissolution into the laser-generated melting pool and subsequent precipitation during cooling. It was observed that the lower laser power limit reduces primary TiC dissolution but it also promotes secondary carbide alignment at the interface. On the other hand, the damage mechanism induced by high laser power is dominated by primary TiC particle cracking by the high stress concentration at the particle–matrix interface followed by ductile failure of the matrix. It is also remarkable that irradiance affects the TiC/TiC_x ratio despite E_c and ψ_c are fixed and it determines hardness distribution inside the coating.     

Multilayered decorative a-C:H/CrC coating on stainless steel

A nano-structured multilayer coating of a combination of a soft layer (a-C:H) produced by PECVD together with a hard layer (Cr/Cr_3 − xC_x) produced by PVD allows to tailor the coating properties by choosing the thickness of the individual layers. The resulting coating is well adherent, may have the elastic properties compatible with the substrate and is much harder than the substrate. The best coating properties were found for about 7-8 nm thick layer thickness for both CrC/Cr and a-C:H. The adhesion (L_C1 > 30 N) and abrasion resistance (Rösler test > 1 h) indicate a coating resistant to normal handling of a handheld consumer product. The colour of the coating may be chosen by adjusting the thickness of the top Cr layer between black (5 nm thick Cr layer) and metallic Cr like (> 30 nm thick Cr layer). The colour and the adhesion (as verified by tape test) did not change during a 6 d humidity cycle tests or 3 month exposure to normal atmospheric conditions.     

Experimental investigation of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

The microstructure and the phase composition of a heat-resistant Fe–Cr–Ni alloy (0. 45C–25Cr–35Ni) has been investigated in the cast state and after annealing at 1150°C for 2–100 h. After a 2-h high-temperature annealing, the fragmentation of the crystal structure of the eutectic M ₇C₃ carbides into domains of ~500 nm in size with a partial transition into M ₂₃C₆ carbides is observed. After a 100-h holding, the complete transition of the hexagonal M ₇C₃ carbides into M ₂₃C₆ with a face-centered cubic structure occurs. The carbide transition M ₇C₃ →M ₂₃ can be considered to be an in situ transformation.     

Solidification and microstructural characterisation of iron-chromium based hardfaced coatings deposited by SMAW and electric arc spraying

The microstructure and solidification of shielded metal arc welding (SMAW) hardfaced Fe-Cr-C deposits used in the sugar industry as well as electric arc-sprayed Fe-Cr-B coating have been determined using a combination of optical microscopy, image analysis, SEM and XRD. The aim of this study was to examine the morphology, microstructure and chemical composition of the coating. The weld microstructures consisted primarily of (Fe,Cr,Mn)₇C₃ carbides, austenite (γ) and ferrite (α), while the arc-sprayed coating was composed of two metallic phases, α (Fe,Cr) and Fe_1.1Cr_0.9B_0.9, which were intermingled with oxides of iron and chromium. The highest average hardness (850 kgf/mm²) occurred in weld coating A80, compared to the 730 kgf/mm² measured in the arc-sprayed coating. The results of the study also showed that different welding electrodes as well as weld procedure variation produced significant differences in the morphology of the carbides, structure of the deposit and microhardness. Although the microhardness of the welded deposits was higher than the arc-sprayed coating, the arc-sprayed coating exhibited a more consistent hardness value. Porosity and oxide inclusions were more evident in the arc-sprayed coating: 1% and 3% in the weld coatings S80 and A80, respectively, and 6.5% in the arc-sprayed coating. The implications of the result with respect to solidification and microstructure are discussed.     

Microstructural evolution of a Ni-based superalloy (617B) at 700 °C studied by electron microscopy and atom probe tomography

We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M₂₃C₆-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M₂₃C₆ carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M₂₃C₆ and γ′/M₂₃C₆ interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M₂₃C₆ precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography.     

Effects of nitrogen pressure and pulse bias voltage on the properties of Cr-N coatings deposited by arc ion plating

Cr-N coatings were deposited on 1Cr18Ni9Ti stainless steel in the pure N₂ atmosphere by arc ion plating (AIP). The relationships between deposition parameters and coating properties were investigated. X-ray diffraction showed a phase transformation from CrN + Cr₂N + Cr → CrN + Cr → CrN and the CrN preferred orientation changed from (200) to (220) as N₂ pressure increased. Increasing bias voltage led to CrN preferred orientation changed from (200) to (220) and the formation of Cr₂N. XPS results indicated that chemical composition of the coatings changed as N₂ pressure increased but it changed little with bias voltage. The lower melting point of chromium nitride formed on target surface induced the increase of macroparticles and deposition rate with increasing N₂ pressure; and bias voltage had an obvious effect on reducing macroparticles of the Cr-N coatings. Residual stresses were measured by substrate curvature technique, and the changing tendency coincided with the microhardness of the coatings.     

Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Laser cladding of SiC/Si composite coating on Si-SiC ceramic substrates

Pieces of silicon infiltrated silicon carbide (Si-SiC) were coated with a SiC particles reinforced Si matrix composite (SiC/Si) obtained from mixtures of SiC + SiO₂ and SiC + Si by laser cladding. A Nd:YAG pulsed laser delivering an average power of 920 W was used to apply such coatings using the powder blowing technique. The results demonstrate that the use of the SiC + SiO₂ powder mixture produces a severe damage on the base material, whereas the use of the SiC + Si mixture leads to the formation of sound coatings without substrate damage. XRD and nanoindentation measurements corroborate the production of silicon carbides surrounded by a metallic silicon matrix. This method could be used for repairing surface defects of silicon infiltrated silicon carbide ceramics (Si-SiC).     

Influence of alloy chemistry on microstructure and properties in NiCrBSi overlay coatings deposited by plasma transferred arc welding (PTAW)

The microstructures and performance of two NiCrBSi alloy overlays deposited by plasma transferred arc welding are studied. The coatings consist of a γ-Ni primary dendritic phase with harder Ni + Ni₃B or Ni + Ni₃Si eutectics and Cr-based particles (CrB, Cr₃C₂, and Cr₇C₃) situated at the interdendritic regions. It was found that the volume fraction of the soft primary dendritic phase drastically decreased and the proportion of chromium borides and carbides increased with an increase of C, B, Si, and Cr content. Microhardness testing revealed that the primary Ni dendrite, interdendritic, and Cr-particle phases had average hardness values of 405, 860, and 1200 HV respectively. An increase in the volume fraction of hard eutectics and Cr-particles lead to a substantial increase in hardness and wear resistance.     

Influence of metal ions on the formation of artificial zinc rusts

The artificial rust particles were prepared from ZnCl₂ solutions dissolving Al(III), Fe(III), Fe(II), Ni(II), Co(II) and Mg(II) at different atomic ratios from 0 to 0.3 in metal/Zn. With increasing metal/Zn the crystal phases of the products turned following as ZnO → a mixture of ZnO and Zn₅(OH)₈Cl₂ · H₂O (ZHC) → ZHC. Al(III) most facilitated the formation of ZHC but Mg(II) and Fe(III) produced no ZHC. The morphology of the formed particles varied following as agglomerate → fine → rod → sheet → irregular with the increase of metal/Zn. The sheet and irregularly shaped particles were identified as ZHC and the other particles as ZnO.