Determination of the Nb–Cr–Si phase diagram using diffusion multiples

A high-efficiency diffusion-multiple approach was employed to determine the phase diagram of the Nb–Cr–Si ternary system which is critical for the design of niobium silicide-based in situ composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. The formation of the Nb(Cr,Si)₂ Laves phase is beneficial to the high oxidation resistance of the composites and the Nb–Cr–Si system serves as the base for understanding the Laves phase formation. The results clearly demonstrate the applicability of the diffusion-multiple approach in determining such complex phase diagrams as Nb–Cr–Si which contains 14 phases. Two isothermal sections at 1000 and 1150 °C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD). Three ternary compounds, CrNbSi, (Cr,Nb)₆Si₅ and (Cr,Nb)₁₁Si₈, were observed at both temperatures, and the C14 Laves phase of the Cr–Nb binary system was stabilized by Si to lower temperatures.     

Study on high-temperature oxidation behaviors of Cr–Si–N films

The high-temperature oxidation behavior of CrN and Cr–Si–N films was investigated. These films were deposited on STS 304 substrates by a hybrid deposition system with arc ion plating (AIP) and DC magnetron sputtering method using separate Cr (99.99%) and Si (99.99%) targets in a gaseous mixture of Ar and N₂. Good oxidation resistance of the CrN film was further improved by the incorporation of Si into the CrN film. The oxidation products of the Cr–Si–N film were Cr₂O₃ and amorphous SiO₂, which were gradually formed by the outward diffusion of Cr, Si, and N as well as the inward diffusion of oxygen. The oxidation kinetics of the specimen showed parabolic behavior, indicating that the diffusion process prevailed during oxidation. The oxidation activation energies for CrN, CrSi_0.10N, and CrSi_0.15N coatings are 303.8, 316.4, and 333.9 kJ/mol, respectively.     

Microstructure and dry sliding wear resistance of Moss-toughened Mo2Ni3Si metal silicide alloys

A wear resistant Mo₂Ni₃Si-based metal silicide alloy toughened by molybdenum-based solid solution (Mo_ss) was fabricated by the laser melting deposition (LMD) manufacturing process. Microstructure of the alloy is composed of Mo_ss primary dendrites and the matrix of the single phase Mo₂Ni₃Si. Wear resistance and friction coefficient of the alloys were evaluated under metallic dry sliding wear test conditions as a function of contact load. Results showed that the alloys have a low friction coefficient and outstanding wear resistance due to the high hardness of Mo₂Ni₃Si matrix and the high strength, ductility and toughness of Mo_ss dendrites. The wear rate and the friction coefficient of alloys are extremely insensitive to the contact load owing to the abnormal hardness–temperature relation of Mo₂Ni₃Si. The Mo_ss dendrite played the role of trapping micro-cracks and restraining brittle spalling of the Mo₂Ni₃Si matrix during wear process and improved the wear properties of Mo_ss-toughened Mo₂Ni₃Si alloy.     

Elevated temperature wear behaviors of a Co–Mo–Si ternary metal silicide alloy

Wear resistant Co–Mo–Si ternary metal silicide alloy consisting of Co₃Mo₂Si primary dendrite and the interdendritic ductile Co-base solid solution (Co_ss) was designed and fabricated by the laser melting process. The alloy exhibited strong abnormal load- and temperature-dependence of wear under elevated temperature metallic sliding wear test conditions.     

The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si, FeNb₄Si, and Fe₂Nb₃ and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)₃Si, (Nb,Ti)₃Sn and the Ti rich oxide. In NV8 the formation of Nb₅Si₃ was destabilised, the stability of Nb₃Si was enhanced and the eutectic between Nb₅Si₃ and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb₄Si and Fe₂Nb₃ phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)₃Sn, β(Ti,Nb)_ss, (Ti,Nb)₃Si and (Ti,Nb)₅Si₃ phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)_ss + (Ti,Nb)₅Si₃ → (Ti,Nb)₃Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb₃Sn, Ti₃Sn, and Nb₃Si, Ti₃Si, and Ti and Nb base 5-3 silicides are discussed.     

The role of Sn and Ti additions in the microstructure of Nb–18Si base alloys

The effects of Sn and Ti on the microstructure and hardness of the as cast and heat treated Nb–18Si–5Sn (NV9) and Nb–24Ti–18Si–5Sn (NV6) alloys were studied. In both alloys the phases present in the as cast and heat treated microstructures were Nb_ss, Nb₃Sn and Nb₅Si_3. In NV9, Sn suppressed the formation of Nb₃Si, partitioned in Nb_ss stronger than in Nb₅Si₃ and did not affect significantly the solubility of Si in the Nb_ss. In NV6, the solubility of Ti in (Nb,Ti)_ss increased in the presence of Sn, the concentration of Ti in Nb₅Si₃ was sensitive to cooling rate and the solubility of Sn in Nb₅Si₃ decreased as the concentration of Ti increased. The Ti controlled the partitioning of Si between (Nb,Ti)_ss and Nb₃Sn and was considered responsible for the macrosegregation of Si in the as cast ingot. The transformation of β to Nb₅Si₃ was enhanced by the synergy of Sn and Ti. The addition of Ti did not destabilise the Nb₃Sn. Silicon increased the hardness of Nb₃Sn significantly, Sn did not affect the hardness of Nb₅Si₃ and Ti reduced the hardness of Nb₃Sn and Nb₅Si₃ significantly. The hardness of NV9 and NV6 decreased and increased, respectively, by heat treatment. The reduction of the hardness of NV6-AC compared to NV9-AC is attributed to the strong effect of Ti on the hardness of Nb₃Sn and Nb₅Si₃.     

Investigation of the interaction of the components in the Y–Si–Sb system at 670 K

Phase equilibria were established in the Y–Si–Sb ternary system at 670 K. The investigation of the phase relations was based on X-ray diffraction experiments made on arc-melted alloys, which were annealed up to 720 h. The 670 K isothermal section consists of 8 three-phase, 12 two-phase and 11 single-phase regions. The formation of a solid solution of Si in the binary YSb compound (8 at.% Si) has been observed. In the Y–Si–Sb system solid solutions between the isostructural binary compounds Y₅Si₃–Y₅Sb₃ form a continuous series. One ternary compound was observed: Y₅Si₂Sb₂ (Tm₅Si₂Sb₂ str. type, Cmca space group, a=1.4971(2), b=0.7855(2) and c=0.7820(2) nm).     

Effects of Ce and Mm additions on the glass forming ability of Al–Ni–Si metallic glass alloys

The effects of Ce and Mm contents on the glass forming ability (GFA) of melt-quenched Al_89−xNi₈Ce_xSi₃ and Al_89−xNi₈Mm_xSi₃ (x = 0, 1, 3, 5, 7 at.%) alloys have been systematically investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). According to the XRD and DSC results, both Ce and Mm elements can enhance the GFA and thermal stability of the Al–Ni–Si alloys. Moreover, only the x = 5 and x = 7 alloys are totally amorphous in both systems quenched at the wheel speed of 36.6 m/s. Compared with amorphous Al₈₄Ni₈Ce₅Si₃ alloy at different cooling rates, amorphous Al₈₄Ni₈Mm₅Si₃ alloy has higher GFA which is considered to have relation to the different atomic structure of the amorphous alloy.     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.     

Diamond and graphite coated with polyalloys by an immersion method

A method of coating diamond and graphite with polyalloys, such as Ti–Co–Cu, Cr–Ni–Cu, W–Cu–Co, etc., is presented in this paper. By adding a small amount of Ti powder, CoCl₂ and CuCl into NaCl–KCl molten salt system, and immersing diamond or graphite into it, the reactions which listed thereafter: Ti+CoCl₂→TiCl₂+Co, Ti+CuCl→TiCl₂+Cu, C+TiCl₂→TiCl₄+TiC, etc., occurred, and finally a Ti–Co–Cu coating was deposited on the diamond or graphite surface. Other polyalloy coatings, such as Cr–Ni–Cu, Cr–Co–Cu, and W–Cu–Co, etc., were also deposited in almost the same way. The coated materials were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with a wavelength dispersive spectrometer (WDS) line profile analysis. XRD analysis showed that the carbides of Ti, Cr, or W were formed during the coating process. It was also found that no vacuum or protective atmosphere was needed during the process.     

Effect of immersion Ag surface finish on interfacial reaction and mechanical reliability of Sn–3.5Ag–0.7Cu solder joint

A comparative study on the preparation of specific titanium silicides (including Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi, and TiSi₂) in the Ti–Si system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. The effect of initial sample stoichiometry was investigated on combustion characteristics and product composition. Experimental evidence shows a distinct combustion front traversing the entire reactant compact in a self-sustaining manner for all of the samples adopted in this study. However, as a result of the eutectic reaction, test samples composed of Ti:Si = 3:1 and 1:1 experienced excessive melting during the propagation of the reaction front. Moreover, the flame-front propagation rate and combustion temperature were found to vary significantly with starting stoichiometry of the powder compact. The flame-front velocity up to 50 mm/s was observed in the samples of Ti:Si = 5:4 and 5:3, in contrast to the values in the range of 2.6–3.5 mm/s for the compact of Ti:Si = 1:2. Reaction front temperatures between 1380 and 1460 °C were achieved by the powder compacts with Ti:Si = 5:4 and 5:3. For the samples of Ti:Si = 3:1 and 1:1, their flame-front temperatures were comparable and about 1330 °C. Consistent with its slow reaction front, the Ti + 2Si sample had the lowest combustion temperature close to 1150 °C. The XRD analysis confirms complete conversion from the reactant with Ti:Si = 5:3 to a single-phase silicide Ti₅Si₃. Powder compacts of Ti:Si = 5:4 and 1:1 yielded two silicide phases, Ti₅Si₄ and TiSi, in their end products. In spite of formation of a minor amount of TiSi, the disilicide TiSi₂ was identified as the dominant composition in the product obtained from the Ti + 2Si sample. Combustion products containing Ti₅Si₃ and a large amount of unreacted Ti were synthesized from the powder compacts of Ti:Si = 3:1, implying a poor degree of phase conversion.     

β-phase transformation and thermoelectric power in FeSi2 and Fe2Si5 based alloys containing small amounts of Cu

The effects of Cu addition on the β phase formation rate and the thermoelectric power in various FeSi₂ and Fe₂Si₅ based alloys was examined. The peritectoid reaction (a+→β) in FeSi₂ alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic ε was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi₂)_99−XMn₁Cu_X (X=0–1.O at.%), (FeSi₂)_99−X Co₁Cu_X (X=0–1.0 at.%) alloys were attained after the elimination of ε. In the case of eutectoid reaction (→β+Si). Differential thermal analysis, X-ray diffraction and microscopic observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The rate also depends on the annealing temperature and reached a maximum at about 1073 K for most alloys. The addition of only 0.1 at.% Cu was still very effective even in Mn or Co doped alloys. The thermoelectric power of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. The value of the 0.1 at.% Cu added alloy was higher than that of both the conventional p- and a-type FeSi₂ based alloys. These results suggest that the Fe₂Si₅ alloys with a small amount of Cu may be attractive as new thermoelectric materials.     

Phase diagram of the Nb–Al–Si ternary system

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb₅Si₃, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb₃Si₅Al₂ was observed. The solubility data of Al in Nb₅Si₃ and NbSi₂ as well as Si solubility in Nb₃Al, Nb₂Al and NbAl₃ were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.     

Cr-Cu-Si金属硅化物合金组织与耐磨性

利用激光熔炼材料制备技术，制得了由铜基固溶体增韧的Cr5Si3／CrSi金属硅化物新型耐磨合金，分析了合金的显微组织结构，测定了合金的显微硬度，考察了合金在室温干滑动磨损条件下的耐磨性能。研究结果表明：Cr-Cu-Si金属硅化物合金显微组织由Cr5Si3金属硅化物初生树枝晶、CrSi相的初生树枝晶及枝晶间铜基固溶体组成，由于金属硅化物Cr5Si3及CrSi的高硬度、强原子间结合力与铜基固溶体的优异导热性、摩擦相容性，上述激光熔炼Cr-Cu-Si金属硅化物合金材料在室温滑动干摩擦试验条件下表现出优异的耐磨性。     

Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys

By the methods of DTA, X-ray diffraction, metallography and microprobe analysis, phase equilibria in the Ti-corner (more than 50 at.% Ti) of the Ti–Si–Sn system were studied. The solidus projection and the melting diagram (solidus+liquidus) were constructed. A new ternary compound T of composition Ti₅Si_1.2–1.6Sn_1.8–1.4 was found to form with the crystal structure of W₅Si₃-type. The ternary eutectic equilibrium L↔β-Ti+Ti₅Si₃+Ti₃Sn was established to occur at 1460 °C with the composition of the invariant point E at 77Ti–9Si–14Sn. Microhardness measurements were carried out for the primary grains of the alloys with 5 at.% Si.     

硅含量对一种耐蚀合金组织与耐蚀性能的影响

设计并熔炼了不同硅含量的三种耐磨耐蚀合金30Ni-35Cr-6.5Mo-1Nb-0.25N-xSi(x=1、2、3)-Fe,分别命名为1Si、2Si、3Si合金,利用SEM、XRD、硬度测试以及电化学测试研究了三种合金的组织和性能。结果表明,三种合金均为双相结构,两相分别为γ相和σ-Cr₁₃Ni₅Si₂相,且随着硅含量的增加,σ-Cr₁₃Ni₅Si₂相所占的比例逐渐增加,合金的洛氏硬度也逐渐增大。动电位极化曲线以及电化学阻抗谱结果表明,3Si合金在10%H₂SO₄溶液中的耐蚀效果最优,而1Si合金的耐蚀性最差。合金硬度和耐蚀能力的提高主要得益于合金中σ相比例的增大。     

Small-angle neutron scattering and differential scanning calorimetry studies on the copper clustering stage of Fe–Si–B–Nb–Cu nanocrystalline alloys

The kinetics of copper clustering and primary crystallization of FINEMET type alloys with the compositions Fe_74.5−xSi_13.5B₉Nb₃Cu_x and Fe₇₇Si₁₁B₉Nb_3−xCu_x have been studied by small-angle neutron scattering (SANS) and high-sensitivity differential scanning calorimetry (DSC) in order to explain the different optimized Cu contents, x, for obtaining the highest permeability in these two alloys. SANS results have shown that the alloys with the optimized Cu contents have the finest nanocrystalline microstructures. Kinetic analyses of Cu clustering prior to primary crystallization have shown that the number density of Cu clusters becomes highest at the crystallization stage of -Fe primary crystals in the alloy containing an optimized amount of Cu.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

Activity measurements of Al and Cu in Si–Al–Cu melt at 1273 and 1373 K by the equilibration with molten Pb

For the effective control of Al introduction to solidified Si during the solidification refining of Si with the Si–Al-based melt for the solar cell material or the LPE Si film growth processes from the Si–Cu–Al solvent, thermodynamic properties of the Si–Al–Cu melt were investigated at 1273 and 1373 K. Activities of Al and Cu in the Si–Al–Cu melt were measured by the equilibration with molten Pb. Also, the excess Gibbs energy of the melt was studied by the ternary regular solution model.

The evaluated thermodynamic properties of the Si–Al–Cu melt indicated that Cu addition to the Si–Al melt brings the smaller activity coefficient of Al and is effective for reducing the Al content of solidified Si from the melt more effectively than its dilution effect for Al.