Analyses of the role of grain boundaries in mesoscale dynamic fracture resistance of SiC-Si3N4 intergranular nanocomposites

Silicon carbide (SiC)-silicon nitride (Si₃N₄) nanocomposites with SiC dispersions as well as Si₃N₄ matrix of mesoscale dimensions (∼1 μm) are considered to have exceptional strength attributed to interactions of SiC dispersions with Si₃N₄ grain boundaries (GBs). However, an account of GBs on the strength of these nanocomposites is not available. In order to analyze this issue, cohesive finite element method (CFEM) based mesoscale dynamic fracture analyses of SiC-Si₃N₄ nanocomposites with an explicit account of length scales associated with Si₃N₄ GBs, SiC particles, and Si₃N₄ grains are performed. Analyses indicate that primary mechanism of fracture in the nanocomposite microstructures is intergranular Si₃N₄ matrix cracking. GBs are responsible for crack deflection and accordingly damage is limited to a smaller geometric region in microstructures with GBs. On an average, a microstructure with GBs present is stronger than the corresponding microstructure with GBs removed. However, in cases where the second phase SiC particles are in the wake of microcracks the microstructure without GB becomes stronger against fracture in comparison to the corresponding one with GBs owing to the crack bridging effect caused by the second phase SiC particles.     

Effect of M7C3→M23C6 transformation on fracture behaviour of cast ferritic stainless steels

Abstract

The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M₇C₃ carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M₇C₃ to M₂₃ C₆ carbides. The M₂₃ C₆ carbides, unlike the coarse M₇C₃ carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M₇C₃ to M₂₃C₆ transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.

MST/1734     

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness.Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M₇C₃ types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M₂C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M₆C developing at the expense of the fine M₇C₃ carbides previously formed.     

Reactive deposition of tungsten and titanium carbides by induction plasma

A study is reported on the use of induction plasma technology for the preparation of dense free-standing deposits of tungsten carbide and titanium carbide from metallic powders and methane. Phase analysis by X-ray diffraction indicates that primary carburization of the particles takes place in-flight giving rise to the formation of W₂C and TiC_1–x . Secondary carburization occurs in the deposits resulting in the formation of tungsten and titanium carbides. Microstructures revealed by optical and scanning electron microscopy show uniform small grains of the carbides. The reactive plasma spray-formed tungsten carbide shows transgranular fracture, while pure tungsten deposits show intergranular fracture.     

M<Subscript>23</Subscript>C<Subscript>6</Subscript> precipitation behavior in a Ni-base superalloy M963

The M₂₃C₆ precipitation behavior in a cast Ni-base superalloy M963 was investigated after tensile creep testing at 800°C and strain-controlled low cycle fatigue testing at 700–950°C. During high temperature creep and low cycle fatigue, the primary MC decomposed into M₆C continuously, and a great amount of secondary carbide, chromium-rich M₂₃C₆, precipitated preferentially in the periphery of MC and γ +γ′ eutectic at grain boundaries. M₂₃C₆ was rarely present in grain interior, indicating that grain boundary promoted M₂₃C₆ carbide precipitation. The M₂₃C₆ precipitation was closely dependent on the stress state and testing temperature, seemed to be independent of the total strain amplitude. M₂₃C₆ is unstable during low cycle fatigue testing. The occurrence of M₂₃C₆ precipitation was sharply reduced during low cycle fatigue testing at 950°C. Crack was easily initiated at interface between MC and matrix, while fine M₂₃C₆ was effective to prevent grain boundary migration.     

Nonequilibrium microstructures and their evolution in a Fe–Cr–W–Ni–C laser clad coating

The laser-solidified microstructural and compositional characterization and phase evolution during tempering at 963 K were investigated using an analytical transmission electron microscope with energy dispersive X-ray analysis. The cladded alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weight ratio of 10:5:1:1:1, was processed with a 3 kW continuous wave CO₂ laser. The processing parameters were 16 mm/s beam scanning speed, 3 mm beam diameter, 2 kW laser power, and 0.3 g/s feed rate. The coating was metallurgically bonded to the substrate, with a maximum thickness of 730 μm, a microhardness of about 860 Hv and a volumetric dilution ratio of about 6%. Microanalyses revealed that the cladded coating possessed the hypoeutectic microstructure comprising the primary dendritic γ-austenite and interdendritic eutectic consisted of γ-austenite and M₇C₃ carbide. The γ-austenite was a non-equilibrium phase with extended solid solution of alloying elements and a great deal of defect structures, i.e. a high density of dislocations, twins, and stacking faults existed in γ phase. During high temperature aging, in situ carbide transformation occurred of M₇C₃ to M₂₃C₆ and M₆C. The precipitation of M₂₃C₆, MC and M₂C carbides from austenite was also observed.     

Review of Microstructure–Mechanical Property Relationships in Cast and Wrought Ni-Based Superalloys with Boron,Carbon, and Zirconium Microalloying Additions

Cast and wrought Ni-based superalloys are materials of choice for harsh high-temperature environments of aircraft engines and gas turbines. Their compositional complexity requires sophisticated thermo-mechanical processing. A typical microstructure consists of a polycrystalline γ-matrix, strengthening Ni₃(Al,Ti) γ′ precipitates, carbides (MC, M₆C, and M₂₃C₆), borides (M₂B, M₃B₂, and M₅B₃), and other inclusions. Microalloying additions of B, C, and Zr commonly improve high-temperature strength and creep resistance, although excessive additions are detrimental. Grain boundary (GB) segregation may improve cohesion and displace embrittling impurities. Finely dispersed carbides and borides are desired to control grain size via GB pinning. However, excessive decoration of GBs may lead to failure during processing and in-service. Hence, a systematic review on the roles of B, C, and Zr in cast and wrought Ni-based superalloys is required. The current state of knowledge on GB segregation and precipitation is reviewed. Experimental and modeling results are compared across various processing steps. The impact of GB precipitation on mechanical properties is most well researched. Co-precipitation in proximity to GBs interacting with local microstructure evolution and mechanical properties remains less explored. Addressing these gaps in knowledge allows a more complete understanding of processing–microstructure–properties relationships in advanced cast and wrought Ni-based superalloys.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

The morphology and microtexture of M7C3 carbides in Fe-Cr-C and Fe-Cr-C-Si alloys of near eutectic composition

The microtexture of M₇C₃ carbides in undercooled 40 g samples of hyper- and hypo-eutectic Fe-Cr-C alloys was determined by electron back scatter diffraction. In the hyper-eutectic alloy the carbides were monocrystalline, while those in the hypo-eutectic alloy were polycrystalline. While in the former the preferred growth direction of the M₇C₃ carbides was [0001], in the hypo-eutectic alloy there was a relatively weak texture near [10¯11]. There was no evidence for the presence of growth twins in either the M₇C₃ carbide rods or in the branching mechanism in the joint between the carbide rods of the hypo-eutectic sample. The morphologies of the M₇C₃ carbides resulting from undercooling were used to explain the microstructure of hardfacing Fe-Cr-C weld deposits applied by the manual metal arc process. The effect of silicon additions on the morphology of M₇C₃ carbides in Fe-Cr-C-Si alloys is explained in terms of the effect of silicon on undercooling.     

Failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting. Part I: Material and fractographic characterization

The failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting and used in cast glass rolling was carried out by means of traditional characterization techniques (optical metallography, SEM, EDX microanalysis, tensile testing and XRD). The roll was in the as-cast condition and its microstructure featured large proportion of δ ferrite (between 20% and 27%) in a martensitic (α′) matrix, with the δ/α′ interfaces presenting an intergranular network of M₂₃C₆ carbides. The crack propagation began in the internal surface of the roll, with δ/α′ intergranular and transgranular cleavage in the “equiaxed region” of the casting, progressing to δ/α′ intergranular ductile fracture in the “columnar” and “chilled regions”. Tensile thermal stresses in the internal surface of the roll associated with microstructural embrittlement (network of interfacial carbide and microporosities) are thought to be the main causes for the premature failure of the roll. Finally, materials selection was performed to replace the CA-15M stainless steel with another class of stainless steel for centrifugal casting.