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Title: M 5Si 3(M=Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications

Abstract

Transition metal silicides are being considered for future engine turbine components at temperatures up to 1600 C. Although significant improvement in high temperature strength, room temperature fracture toughness has been realized in the past decade, further improvement in oxidation resistance is needed. Oxidation mechanism of Ti 5Si 3-based alloys was investigated. Oxidation behavior of Ti 5Si 3-based alloy strongly depends on the atmosphere. Presence of Nitrogen alters the oxidation behavior of Ti 5Si 3 by nucleation and growth of nitride subscale. Ti 5Si 3.2and Ti 5Si 3C 0.5 alloys exhibited an excellent oxidation resistance in nitrogen bearing atmosphere due to limited dissolution of nitrogen and increased Si/Ti activity ratio. MoSi 2 coating developed by pack cementation to protect Mo-based Mo-Si-B composites was found to be effective up to 1500 C. Shifting coating composition to T1+T2+Mo 3Si region showed the possibility to extend the coating lifetime above 1500 C by more than ten times via formation of slow growing Mo 3Si or T2 interlayer without sacrificing the oxidation resistance of the coating. The phase equilibria in the Nb-rich portion of Nb-B system has been evaluated experimentally using metallographic analysis and differential thermal analyzer (DTA). It was shown that Nb ss (solidmore » solution) and NbB are the only two primary phases in the 0-40 at.% B composition range, and the eutectic reaction L {leftrightarrow} Nb SS + NbB was determined to occur at 2104 ± 5 C by DTA.« less

Authors:
 [1]
  1. Iowa State Univ., Ames, IA (United States)
Publication Date:
Research Org.:
Ames Lab., Ames, IA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
933031
Report Number(s):
IS-T 2491
TRN: US200814%%818
DOE Contract Number:
AC02-07CH11358
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; BEARINGS; COATINGS; DISSOLUTION; ENGINES; EUTECTICS; FRACTURE PROPERTIES; LIFETIME; NITRIDES; NITROGEN; NUCLEATION; OXIDATION; SILICIDES; TRANSITION ELEMENTS; TURBINES

Citation Formats

Tang, Zhihong. M5Si3(M=Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications. United States: N. p., 2007. Web. doi:10.2172/933031.
Tang, Zhihong. M5Si3(M=Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications. United States. doi:10.2172/933031.
Tang, Zhihong. Mon . "M5Si3(M=Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications". United States. doi:10.2172/933031. https://www.osti.gov/servlets/purl/933031.
@article{osti_933031,
title = {M5Si3(M=Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications},
author = {Tang, Zhihong},
abstractNote = {Transition metal silicides are being considered for future engine turbine components at temperatures up to 1600 C. Although significant improvement in high temperature strength, room temperature fracture toughness has been realized in the past decade, further improvement in oxidation resistance is needed. Oxidation mechanism of Ti5Si3-based alloys was investigated. Oxidation behavior of Ti5Si3-based alloy strongly depends on the atmosphere. Presence of Nitrogen alters the oxidation behavior of Ti5Si3 by nucleation and growth of nitride subscale. Ti5Si3.2and Ti5Si3C0.5 alloys exhibited an excellent oxidation resistance in nitrogen bearing atmosphere due to limited dissolution of nitrogen and increased Si/Ti activity ratio. MoSi2 coating developed by pack cementation to protect Mo-based Mo-Si-B composites was found to be effective up to 1500 C. Shifting coating composition to T1+T2+Mo3Si region showed the possibility to extend the coating lifetime above 1500 C by more than ten times via formation of slow growing Mo3Si or T2 interlayer without sacrificing the oxidation resistance of the coating. The phase equilibria in the Nb-rich portion of Nb-B system has been evaluated experimentally using metallographic analysis and differential thermal analyzer (DTA). It was shown that Nbss (solid solution) and NbB are the only two primary phases in the 0-40 at.% B composition range, and the eutectic reaction L {leftrightarrow} NbSS + NbB was determined to occur at 2104 ± 5 C by DTA.},
doi = {10.2172/933031},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Thesis/Dissertation:
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  • In the process of exploring and understanding the influence of crystal structure on the system of compounds with the composition Gd 5(Si xGe 1-x) 4 several new compounds were synthesized with different crystal structures, but similar structural features. In Gd 5(Si xGe 1-x) 4, the main feature of interest is the magnetocaloric effect (MCE), which allows the material to be useful in magnetic refrigeration applications. The MCE is based on the magnetic interactions of the Gd atoms in the crystal structure, which varies with x (the amount of Si in the compound). The crystal structure of Gd 5(Si xGe 1-x)more » 4 can be thought of as being formed from two 3 2434 nets of Gd atoms, with additional Gd atoms in the cubic voids and Si/Ge atoms in the trigonal prismatic voids. Attempts were made to substitute nonmagnetic atoms for magnetic Gd using In, Mg and Al. Gd 2MgGe 2 and Gd 2InGe 2 both possess the same 3 2434 nets of Gd atoms as Gd 5(Si xGe 1-x) 4, but these nets are connected differently, forming the Mo 2FeB 2 crystal structure. A search of the literature revealed that compounds with the composition R 2XM 2 (R=Sc, Y, Ti, Zr, Hf, rare earth; X=main group element; M=transition metal, Si, Ge) crystallize in one of four crystal structures: the Mo 2FeB 2, Zr 3Al 2, Mn 2AlB 2 and W 2CoB 2 crystal structures. These crystal structures are described, and the relationships between them are highlighted. Gd 2AlGe 2 forms an entirely new crystal structure, and the details of its synthesis and characterization are given. Electronic structure calculations are performed to understand the nature of bonding in this compound and how electrons can be accounted for. A series of electronic structure calculations were performed on models with the U 3Si 2 and Zr 3Al 2 structures, using Zr and A1 as the building blocks. The starting point for these models was the U 3Si 2 structure, and models were created to simulate the transition from the idealized U 3Si 2 structure to the distorted Zr 3Al 2 structure. Analysis of the band structures of the models has shown that the transition from the U 3Si 2 structure to the Zr 3Al 2 structure lifts degeneracies along the Λ → Z direction, indicating a Peierls-type mechanism for the displacement occurring in the positions of the Zr atoms.« less
  • This study was motivated by the fact that previous research on the structure and properties of Ti 5Si 3 showed unacceptably inconsistent results. The primary reason for these inconsistencies was interstitial contamination of Ti 5Si 3 by carbon, nitrogen and oxygen. Thus, this study measured the effects that these interstitial atoms have on some of the previously reported properties. These properties include crystalline structure, thermal expansion anisotropy, electronic structure and bonding, and high temperature oxidation resistance. In Chapter 2 of this study, the lattice parameters and atomic positions of Ti 5Si 3 as a function of carbon, nitrogen or oxygenmore » content were measured via x-ray and neutron diffraction. Comparing these lattice parameters to those reported in other studies on supposedly pure Ti 5Si 3 confirmed that the majority of the previous studies had samples with a considerable amount of interstitial impurities. In fact, the lattice parameter trends given in Chapter 2 can be used to estimate the types and level of impurities in these studies. Furthermore, Chapter 2 discusses how atomic positions change as interstitial atoms are incorporated into the lattice. These changes in atomic separations suggest that strong bonds form between the interstitial atoms and the surrounding titanium atoms. This is in full agreement with the electronic structure calculations given in Chapter 4. These calculations show that bonding does occur between titanium d-states and interstitial atom p-states at the expense of bonding between some of the titanium and silicon atoms. In addition, carbon seems to be the most strongly bonded interstitial atom. Knowledge of the exact interstitial content and its effect on bonding is important because Chapters 3 and 5 have shown that interstitial atoms have a marked effect on the thermal expansion and oxidation resistance. As discussed in Chapter 3, all interstitial atoms lower the thermal expansion anisotropy of Ti 5Si 3 due to the formation of bonds between the interstitial atom and the surrounding titanium atoms. Although interstitial atoms do have an effect on the thermal expansion of Ti 5Si 3, these effects were not strong enough to explain all the scatter of previous studies. These studies most likely suffered from systematic errors as a result of poor experimental design. The experimental procedure used in this study was designed to significantly reduce these systematic errors.« less
  • A systematic study of the superconducting and normal state properties of some ternary rare earth transition metal silicides and germanides of the Sc/sub 5/Co/sub 4/Si/sub 10/0-type is reported. Low temperature heat capacity measurements indicate the presence of a complicated phonon density of states in these structurally complex compounds. A better description of the phonon spectrum of the high T/sub c/ materials Sc/sub 5/Rh/sub 4/Si/sub 10/, Sc/sub 5/Ir/sub 4/Si/sub 10/, and Y/sub 5/Os/sub 4/Ge/sub 10/, given by a model proposed by Junod et al., is presented and discussed. The large values of ..delta..C/..gamma../sub n/T/sub c/ and the electron-phonon coupling constant formore » these high T/sub c/ compounds indicate that they are strong-coupled superconductors. Relative to other ternary superconductors, many of these materials have large Debye temperatures. DC electrical resistivity measurements on these compounds show resistivity behavior deviating from those exhibited by simple metals. The rho(T) data for Y/sub 5/Ir/sub 4/Si/sub 10/, Lu/sub 5/Ir/sub 4/Si/sub 10/, and Y/sub 5/Os/sub 4/Ge/sub 10/, indicate the presence of anomalies. Static molar magnetic susceptibility measurements performed on these compounds indicate (1) a small effective magnetic moment of 0.26..mu../sub B/ on the Co atom and (2) anomalous behaviors in the Lu/sub 5/Rh/sub 4/Si/sub 10/, Lu/sub 5/Ir/sub 4/Si/sub 10/, Y/sub 5/Ir/sub 4/Si/sub 10/, Lu/sub 5/Ir/sub 4/Ge/sub 10/, and Y/sub 5/Rh/sub 4/Ge/sub 10/ data. Lastly, upper critical magnetic field measurements were performed on Sc/sub 5/Co/sub 4/Si/sub 10/, Sc/sub 5/Rh/sub 4/Si/sub 10/, Sc/sub 5/Ir/sub 4/Si/sub 10/, Lu/sub 5/Rh/sub 4/Si/sub 10/, Lu/sub 5/Ir/sub 4/Si/sub 10/, and Y/sub 5/Os/sub 4/Ge/sub 10/.« less
  • The ultrasonic attenuation of 600-700 MHz surface acoustic waves by two high T/sub c/, cubic crystal structure, superconducting thin films has been investigated. The films studied were two, 0.5 ..mu.. thin, Nb/sub 3/Sn samples, electron-beam codeposited on LiNbO/sub 3/ and Quartz, and eleven NbN samples from 3 x 10/sup 3/ angstrom to less than or equal to 200 angstrom thin, sputter deposited on LiNbO/sub 3/. The Nb/sub 3/Sn (A15 structure) film on Quartz was difficult to measure due to defects in the Quartz caused by the high deposition temperature (approx. =700/sup 0/C) used to make the high T/sub c/ formmore » of the compound. The Nb/sub 3/Sn film on LiNbO/sub 3/, however, provided information about the transition temperature and energy gap at T = 0 K when the attenuation was measured as a function of temperature in zero magnetic field. A theory is developed to predict the electron-phonon produced normal state attenuation of surface acoustic waves by a thin, loss producing film on a nonattenuating substrate. Using a viscous drag model for the attentuation, the predictions of the theory are compared to the measured normal state attenuation to find the electron mean-free-path for the Nb/sub 3/Sn film on LiNbO/sub 3/.« less
  • An iron-rich low-cost superalloy has been developed in conjunction with United Technologies Research Center under the NASA program, Conservation of Strategic Aerospace Materials. The alloy, when processed by conventional chill casting, has physical and mechanical properties that compare favorably with existing nickel - and cobalt-based superalloys while containing significantly lower amounts of strategic elements. The composition of the alloy is Cr(15)-Mn(15)-Mo(2)-C(1.5)-Si(1.0)-Nb(1.0)-Fe(bal.), and it can be produced with chromite ore deposits located within the United States. Studies were also made on the properties of Cr(20)-Mn(10)-C(3.4)-Fe(bal.), a eutectic alloy processed by chill casting and directional solidification (D.S.) which produced an aligned microstructuremore » consisting of M/sub 7/C/sub 3/ fibers in an ..gamma..-Fe matrix. This good alignment vanishes when molybdenum or aluminum is added in higher concentrations. Thermal expansion of the M/sub 7/C/sub 3/ (M = Fe, Cr, Mn) carbide lattice was measured up to 800/sup 0/C and found to be highly anisotropic, with the a-axis being the predominant mode of expansion. Repetitive impact-sliding wear experiments performed with the Fe-rich eutectic alloy showed that the directionally solidified microstructure greatly improved the alloy's wear resistance as compared to the chill-cast microstructure and conventional nickel-base superalloys.« less