Si Mo Alloys Research Articles

Influence of Cooling Rate and Alloying by “Cr, V, and Ni” on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

AbstractSiMo ductile irons, typical heat-resistant materials, are subjected to varied wear environments during operation in high-temperature applications. SiMo ductile iron castings of different thicknesses were cast in investment and greensand molds, achieving a wide range of cooling rates. The present work aims to investigate the effect of the cooling rate and alloying elements (Cr, V, and Ni) on the microstructure and the abrasive wear behavior of these grades of SiMo ductile iron at high-temperature 700 °C under different loads. Thermodynamic calculations were used to propose the phase diagrams, critical transformation temperatures, and phase volume fractions in all SiMo alloys by using the Thermo-Calc software then verified by and Differential Scanning Calorimetry (DSC). The microstructure of unalloyed SiMo ductile cast iron consists of graphite nodules and carbides embedded in the precipitates at the grain boundary regions in a ferrite matrix. The alloyed SiMo microstructure contains nodular graphite and the carbides promoted by the alloying elements (Cr and V). The alloyed SiMo alloys exhibit higher wear resistance than unalloyed ones. These wear results support that the microstructure plays a chief role in wear loss. The combination of M6C, VC, and M7C3 carbides embedded in a ferrite-pearlite matrix (alloyed SiMo) seems to be more resistant to wear than the ferritic matrix with lamellar pearlite and eutectic M6C carbides (unalloyed SiMo).

Effect of Molybdenum Contents on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

High-silicon and molybdenum (SiMo) ductile iron is a common heat-resistant alloy that may be exposed to high-temperature wear during service in many of its applications. The wear behavior of four SiMo ductile iron alloys was evaluated at different temperatures up to 750 °C. This research focuses on the influence of various Mo contents on the microstructure, structural stability, and hence, the wear performance of such alloys. Thermodynamic calculations proposed the phase diagrams, critical transformation temperatures, and phase volume fractions in all samples by means of Thermo-Calc software. The dilatometry measurements were carried for confirming the theoretical results of Thermo-Calc thermodynamic calculations. The results revealed that the microstructure of SiMo ductile cast iron consists of nodular graphite and a ferrite matrix with carbides embedded in the fine precipitates at the grain boundary regions. The type of carbides and the nature of these fine precipitates are discussed according to EDX and SEM results. Adding molybdenum enhanced the wear performance of SiMo by decreasing the weight loss by about 40–70% compared to a Mo-free alloy. This is due to the increased molybdenum carbides, which increase hardness and improve wear resistance in SiMo alloys. The high temperatures have a negative effect on reducing the wear resistance at 250 °C. On the other hand, the wear resistance unexpectedly started to increase at higher temperatures of 500 °C and 750 °C because of the contribution of oxidative wear with abrasive wear by forming a protective oxide layer. Furthermore, the obtained results supported the idea that adding molybdenum improves wear resistance at high temperatures. Hence, SiMo has the potential to be wear-resistant material in wider applications requiring high-temperature wear resistance.

Effect of Ti addition on the kinetics and mechanism of non-isothermal and isothermal oxidation of Nb–Si–Mo alloys at 900 °C–1200 °C

The effect of 20 at% Ti addition and annealing (1500 °C/100 h) on non-isothermal and isothermal oxidation behavior of as-cast hypoeutectic Nb–12Si–5Mo or hypereutectic Nb–19Si–5Mo alloys has been investigated. On heating to 1300 °C in air, the mass gain begins at lower temperature, with a steeper rise in the ternary alloys, compared to the Ti-containing alloys. On isothermal exposure for 24 h, the ternary alloys have exhibited lower mass gain at 900 °C than the quaternary alloys, but the trend has reversed at 1100 °C and 1200 °C, with the hypereutectic Ti-containing alloy exhibiting the least mass gain. The products of oxidation are found as Nb2O5 and SiO2, with TiO2, TiNb2O7 and Ti2Nb10O29 being formed additionally in the oxide scales of the Nb–12Si–5Mo–20Ti and Nb–19Si–5Mo–20Ti alloys. Analysis of the oxidation kinetics suggest an initial phase of interface-reaction controlled rapid increase of mass, followed by stage of parabolic behavior, where diffusion of O2− or Ti4+ through the oxide scale could be the rate-controlling in ternary or Ti-containing quaternary alloys, respectively. Coarser microstructure obtained by annealing has caused higher mass gain than that observed for the corresponding as cast sample. The as cast or annealed Nb–19Si–5Mo–20Ti alloy has exhibited lower values of parabolic rate constant than other alloys at ≥1100 °C, due to the formation of a defect-free oxide scale. Reduced oxygen solubility and diffusivity due to the presence of Mo and Ti in Nb-solid solution, along with formation of SiO2-rich oxide on hypereutectic quaternary alloy at 1100 °C and 1200 °C appear to be responsible for their superior oxidation resistance.

Simulation and analysis of the solidification characteristics of a Si-Mo ductile iron

High silicon and molybdenum ductile cast irons (Si-Mo alloys) are commonly used as exhaust manifold materials suffering from high temperature-oxidation and thermal-mechanical fatigue. The structural integrity of cast Si-Mo alloys under these service conditions is attributed to their microstructure consisting of spheroidal graphite and Mo-rich carbide embedded in a ferritic matrix. However, the cast structure includes also pearlite structure having a detrimental effect on the mechanical properties, therefore the cast matrix needs to be heat treated. In this study, the solidification of a Si-Mo ductile iron was investigated using (i) thermodynamic and kinetic calculations by Thermo-Calc and DICTRA software and (ii) thermal analysis in order to reveal out the sequence of phase formation and the phase transformations during solidification and (iii) microanalysis by energy dispersive spectrometer in order to determine elemental segregation and compare with the calculated values. The solidified structure was also characterized and all microstructural features were specified.

Synthesis, Lithium Insertion and Thermal Stability of Si–Mo Alloys

Li insertion was investigated in SixMo100−x (90 ≥ x ≥ 70, Δx = 10) alloys prepared by mechanical ball milling. X-ray diffraction (XRD) and quantitative phase analysis were used to analyze phase compositions of these Si–Mo alloys, and how these phase compositions changed with milling times. The results of quantitative phase analysis showed that cr-Si converted into a-Si within 1 h during milling, and the Si–Mo reactions were nearly complete after only 4-h milling. During electrochemical cycling, the Si–Mo samples with high initial Mo contents and long milling times displayed good crystalline Li15Si4 suppression and stable cycling performance. In addition, thermal stability of some selected Si–Mo alloys was studied. The Si80Mo20 16 h alloy combines good thermal stability and a high volumetric capacity of about 1300–1400 Ah L−1 after heat treatment at 600 °C or 800 °C, which may allow the alloy to be further improved by carbon coating at high temperature.

Matrix design of a novel ductile cast iron modified by W and Al: A comparison between thermodynamic modeling and experimental data

In high-temperature applications of ferrous materials, as in the case of exhaust manifolds, high thermal and mechanical stability are required. Stainless steels and Ni-resist alloys having austenitic matrices are good candidates to meet these requirements at elevated temperatures; however, they are expensive materials and present difficulties in casting. Ferritic ductile cast irons, like the commercial SiMo alloy, are comparatively cheaper materials with better castability but they cannot be used above approximately 800 °C. Thus, to meet the requirements with low-cost materials having improved high-temperature properties, new alloys must be developed by ferrite forming elements having the potential to increase equilibrium temperature. In this study, initially, a novel ductile cast iron matrix was designed using 1 W and 0-4 Al wt.-% and their phases stable at room temperature, transformation temperatures, solidification sequences and thermal expansivity values were determined using thermodynamic calculations with Thermo-Calc software. Computational studies revealed that (i) designed alloy matrices had graphite and M6C type carbides embedded in a ferritic matrix at room temperature as expected, (ii) A1 temperature increased as aluminum content increased. The obtained values were all above that of commercial SiMo alloy, (iii) the detrimental effect of increased aluminum addition on graphite content, and thermal expansivity was observed. Secondly, microstructural and thermal characterizations of cast alloys were performed for validation – the obtained data were in good agreement with the thermodynamic calculations.

Microstructure and ablation behavior of SiMo/graphite composites with excellent short-time ablation resistance

Si-Mo alloy was infiltrated into porous graphite to develop a short-term low-ablative SiMo/graphite composite material. 19.4 vol.% residual Si was retained in the composites and significantly improved the short-term (less than 60 s) ablation resistance without sacrificing long-term (∼180 s) ablation resistance. The excellent short-term low-ablative performance is ascribed to the melt and evaporation of Si, which reduces the rate of temperature rise and supplies the consumption of SiO2 in the ablation surface. With the progress of ablation time, the ablation surface forms a SiO2-MoSi2 double layer, which acts as a barrier between the oxyacetylene flame and the ablation surface.

Tailoring the Mechanical Properties of Laser Cladding-Deposited Ferrous Alloys with a Mixture of 410L Alloy and Fe⁻Cr⁻B⁻Si⁻Mo Alloy Powders.

The effects of different ratios of 410L alloy and Fe–Cr–B–Si–Mo alloy powders on the microstructure and mechanical properties of laser cladding-deposited ferrous alloys were investigated. The experimental results revealed that the 410L alloy had good strength and excellent ductility due to its microstructure consisting of large elongated ferrite dendrites surrounded by a small number of martensite grains, while the Fe–Cr–B–Si–Mo alloy had high strength and poor ductility because of its eutectic microstructure composed of ferrite and Fe2B/Cr2B. As the concentration of Fe–Cr–B–Si–Mo alloy powder added to the 410L alloy powder increased, the ferrite grains became finer and the volume fraction of the eutectic increased, which eventually improved the strength and reduced the plasticity. Then, 410L + 12.5% Fe–Cr–B–Si–Mo alloy powder was successfully deposited onto AISI 1060 steel substrate via laser cladding deposition, and the mechanical properties met those of the substrate, which verified that tailoring the mechanical properties of the laser cladding-deposited alloys with a mixture of 410L and Fe–Cr–B–Si–Mo alloy powders for steel repairing applications is a feasible solution.

Microstructure and Hot Oxidation Resistance of SiMo Ductile Cast Irons Containing Si-Mo-Al

SiMo ductile cast irons are used as high-temperature materials in automotive components, because they are microstructurally stable at high operating temperatures. The effect of different amounts of Si and Mo as well as the addition of 3 wt pct Al on the microstructure, high-temperature oxidation, and mechanical properties of SiMo ductile cast iron was studied. Dilatometric measurements of SiMo ductile iron exhibited obvious differences in the transformation temperature A 1 due to presence of Al and the increase of Si. The microstructure of the SiMo alloys without Al addition showed outstanding nodularity and uniform nodule distribution. However, by adding 3 wt pct Al to low Si-SiMo ductile iron, some compacted graphite was observed. The results of oxidation experiments indicated that high Si-SiMo ductile iron containing 4 and 4.9 wt pct Si had superior resistance to lower Si-SiMo and SiMo ductile iron containing 3 wt pct Al. The results showed also that with increasing Si up to 4.9 wt pct or by replacing a part of Si with 3 wt pct Al, tensile strength increased while elongation and impact toughness decreased.

In-situ synthesis of MoSi2 coating on molybdenum substrate by electro-deoxidation of a SiO2 layer in molten salt

MoSi2 coating was fabricated in situ by electro-deoxidation of SiO2 layer pre-fabricated on molybdenum substrate in molten calcium chloride at 900°C. The characteristics of the composition and microstructure of the coating under different conditions were investigated. And the cyclic voltammograms of the silica layer in molten calcium chloride were also analyzed, aiming to achieve a better understanding of the electro-deoxidation and MoSi2 formation processes. It was found that the reduction rate increased with the cell voltage and temperature of molten salt increasing. MoSi2 coating with compact structure was obtained after the electro-deoxidation of the silica layer at 3.1V for 3h. The preliminary study on the mechanism of MoSi2 coating formation suggested that SiO2, CaSiO3 and Ca3Si2O7 were reduced directly on Mo substrate to form Si–Mo alloys. As the process continued, the ratio of Si in Si–Mo alloys increased, and finally the compact MoSi2 coating with a thickness of about 10μm was obtained. The electro-reduction of SiO2 layer was highly irreversible and controlled mainly by the slow rate of electron transfer.

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

Isothermal oxidation behavior of a refractory high-entropy NbCrMo0.5Ta0.5TiZr alloy was studied during heating at 1273 K for 100 h in flowing air. Continuous weight gain occurred during oxidation, and the time dependence of the weight gain per unit surface area was described by a parabolic dependence with the time exponent n = 0.6. X-ray diffraction and scanning electron microscopy accompanied by energy-dispersive X-ray spectroscopy showed that the continuous oxide scale was made of complex oxides and only local (on the submicron levels) redistribution of the alloying elements occurred during oxidation. The alloy has a better combination of mechanical properties and oxidation resistance than commercial Nb alloys and earlier reported developmental Nb–Si–Al–Ti and Nb–Si–Mo alloys.

Experimental study on the as-cast solidification of the Si-rich alloys of the Nb–Si–Mo ternary system

This paper describes the as-cast solidification of the Nb–Si–Mo ternary alloys in the Si-rich region, which have attracted great attention for the use of the Nb–Si and the MoSi 2 based alloys as ultra-high temperature structural materials. The microstructural observation and phase identification for the as-cast Nb–Si-Mo alloys in Si-rich region were studied using backscattered electron (BSE) imaging, electron beam microprobe analysis (EPMA) and X-ray diffraction (XRD) analysis. Based on the present experimental study along with the literature reported data, the liquidus surface projection of the Nb–Si–Mo system is determined. Four primary solidification regions have been involved in the Si-rich region of the system, which are Diamond-(Si), β(Nb,Mo) 5Si 3, Nb(Mo)Si 2 and αMo(Nb)Si 2. Two invariant reactions exist in the Si-rich side, which are L + αMo(Nb)Si 2 = Nb(Mo)Si 2 + β(Nb,Mo) 5Si 3 and L + αMo(Nb)Si 2 = Nb(Mo)Si 2 + Diamond-(Si). According to the as-cast microstructural morphologies as well as their chemical compositions and crystalline structures, the solidification paths for different kinds of alloys are analyzed.

High-Temperature Low-Cycle Fatigue Property of Heat-Resistant Ductile-Cast Irons

This study examined the high-temperature degradation behavior of two types of heat-resistant Si-Mo ductile cast iron (Fe-3.4C-3.7Si-0.4Mo and Fe-3.1C-4.5Si-1.0Mo) with particular attention paid to the mechanical properties and overall oxidation resistance. Tension and low-cycle fatigue properties were examined at 600 °C and 800 °C. The mechanical tests and metallographic and fractographic analyses showed that cast iron containing higher Si and Mo contents had a higher tensile strength and longer fatigue life at both temperatures than cast iron with lower levels due to the phase transformations of pearlite and carbide. The Coffin–Manson type equation was used to assess the fatigue mechanism suggesting that the higher Si-Mo alloy was stronger but less ductile than the lower Si-Mo alloy at 600 °C. However, similar properties for both alloys were observed at 800 °C because of softening and oxidation effects. Analysis of the isothermal oxidation behavior at those temperatures showed that mixed Fe2SiO4 layers were formed and the resulting scaling kinetics was much faster for low Si-Mo containing iron. With increasing temperature, subsurface degradation such as decarburization, voids, and cracks played a significant role in the overall oxidation resistance.

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The eutectic morphology of ternary Nb–Si–Mo alloys with hypereutectic (Nb–19Si–5Mo, and Nb–18Si–26Mo) and hypoeutectic (Nb–13Si–4Mo, and Nb–12Si–15Mo) compositions has been examined. The eutectic in these materials has been found to consist of a mixture of Nb solid solution and Nb5Si3 with lamellar or non-lamellar morphology. For similar concentration of Si, the volume fraction of non-lamellar eutectic in the hypoeutectic alloys increases with Mo content. However, the concentration of Mo has little effect on the non-lamellar eutectic content of hypereutectic alloys. Hypereutectic compositions, having higher concentration of Si, show a much larger volume fraction of non-lamellar eutectic compared to that found in the hypoeutectic compositions. The lamellar or non-lamellar character of the eutectic is attributed to the composition-dependent under-cooling behaviour of the ternary Nb–Si–Mo alloys during solidification and their volume fractions in these alloys assist to explain some of the structure–property relations.

Microstructure of Reaction-Bonded Silicon Carbide including Molybdenum Disiliside

Residual silicon in the reaction-bonded silicon carbide (RBSiC) can be replaced by refractory MoSi2 by using Si-Mo alloy as infiltrants instead of pure Si. Transmission electron microscopy of these materials clarified the following results. The MoSi2 existed in the SiC grain boundaries and many fine grains of SiC were found in the MoSi2 phase. The carbon particles were thought to be dissolved into the Si melt including Mo and reprecipitated as SiC particles in the solution-reprecipitation mechanism. The Mo concentration in the Si-Mo melt increased with the reprecipitation of SiC particles, and the MoSi2 was precipitated in the grain boundaries. The presence of the fine SiC particles in the MoSi2 phase suggests that the carbon have a fairly strong interaction with silicon in the melt and diffuse not as isolated atoms, but as C-Si pairs.

Effect of Mo and Si on morphology and volume fraction of eutectic in Nb–Si–Mo alloys

The eutectic morphology of ternary Nb–Si–Mo alloys with hypereutectic (Nb–19Si–5Mo, and Nb–18Si–26Mo) and hypoeutectic (Nb–13Si–4Mo, and Nb–12Si–15Mo) compositions has been examined. The eutectic in these materials has been found to consist of a mixture of Nb solid solution and Nb 5Si 3 with lamellar or non-lamellar morphology. For similar concentration of Si, the volume fraction of non-lamellar eutectic in the hypoeutectic alloys increases with Mo content. However, the concentration of Mo has little effect on the non-lamellar eutectic content of hypereutectic alloys. Hypereutectic compositions, having higher concentration of Si, show a much larger volume fraction of non-lamellar eutectic compared to that found in the hypoeutectic compositions. The lamellar or non-lamellar character of the eutectic is attributed to the composition-dependent under-cooling behaviour of the ternary Nb–Si–Mo alloys during solidification and their volume fractions in these alloys assist to explain some of the structure–property relations.

High-Temperature Corrosion of Cast Irons and Cast Steels in Dry Air

The oxidation in dry air of four cast alloys intended for exhaust gas systems has been examined. Particular interest was directed to how the oxide growth was related to the microstructures. The examined alloys were two cast ductile irons, a SiMo alloy (Fe3,86Si0,6Mo3C) and a Ni-Resist alloy (Fe32Ni5,3Si2,1C), and two cast stainless steels, one ferritic (Fe18Cr2,1Mn0,32C) and cane austenitic (Fe20Cr9Ni0,47C). Coupons were oxidised for 50 h at temperatures between 650°C and 1050°C. The samples were characterised by using XRD, SEM/EDX and AES. As expected, the overall oxide thickness increased with temperature and partial spallation occurred at the highest temperatures for all alloys. Porous Fe oxide nodules nucleate at the graphite nodules on the ductile irons. These Fe-oxide nodules formed above a continuous layer of Fe-Si-oxide for the SiMo and mixed Fe-Ni-Si oxides for the Ni-Resist. The total oxide thickness is about (60μm). Thick oxides at the interdendritic regions in the cast steels were attributed to non-Cr-carbides. Segregation of Cr directed the formation of iron oxide nodules to the centre of the dendrites in the austenitic alloy.

Microstructure and room temperature deformation of Nb ss/Nb 5Si 3 in situ composites alloyed with Mo

Room temperature deformation behavior of Nb 5Si 3 based intermetallics in the ternary Nb–Si–Mo alloy system is investigated by compression testing in relation to microstructure and compositional effect. The partial ternary phase diagram of the Nb-Si-Mo alloy system containing phase equilibrium information at 1973 K is determined by metallography, X-ray diffraction and scanning electron microscopy (SEM) equipped with wavelength-dispersive X-ray fluorescence spectroscopy (WDS). A pseudo-binary compound of Nb 5Si 3–Mo 5Si 3 is confirmed to form at Nb 5Si 3-rich compositions by Mo addition, without having a solubility range of Si. The phase transformation from α-Nb 5Si 3 to β-Nb 5Si 3 is found to occur at about 5 at.% of Mo content on the Nb 5Si 3–Mo 5Si 3 pseudo-binary line. The observed yield stresses are largely dependent on not only the volume fraction and morphology of constituent phases but also the strength of Nb 5Si 3 phase equilibrating with Nb solid solution. The in situ composites consisting of α-Nb 5Si 3 phase and bcc solid solution exhibit higher yield stress than those consisting of β-Nb 5Si 3 phase and bcc solid solution when yield stress is compared at the same volume fraction of bcc phase. It is suggested that room temperature deformability of β-Nb 5Si 3 is superior to that of α-Nb 5Si 3 phase. Room temperature deformation behavior of the present composites will be discussed in relation to microstructural evolution.

Magnetic properties of fine crystalline Fe–P–C–Cu–X alloys

The crystallization process and magnetic properties of Fe–P–C–Cu–Ge–Si amorphous alloys were investigated. After annealing above the crystallization temperature, the bcc Fe particles, with nanoscale grain size, precipitated in the amorphous alloy for a wide P concentration range. The volume ratio of the bcc Fe phase to amorphous was about 30%. The mixed phase of these alloys shows a drastic decrease of coercive force (Hc) and particle diameter (d) as P concentration increases. The lowest Hc was obtained for Fe78P16C2Cu0.5Ge3Si0.5, and Hc and d were 1.8 A/m and 16 nm, respectively. Observation by in situ Lorentz scanning electron microscopy (SEM) revealed that the width of magnetic domains were 0.01 or 0.1 mm and the domain walls with smaller bcc Fe particles moved in a weaker magnetic field than those with larger ones. So the low Hc is attributed to the decrease of magnetocrystalline anisotropy caused by fine structures. The core loss of fine crystalline Fe–P–C–Cu–Si–Mo alloy, W14/50, was 0.22 W/kg after annealing under a magnetic field of 2.4 kA/m.