Formation and microstructure of various thin film chromium silicide phases

The formation of the various chromium silicide phases, as predicted by the Cr-Si phase diagram for bulk materials, was studied when thin Cr/a-Si (where a-Si is amorphous silicon) bilayers were annealed in situ in a transmission electron microscope using a limited supply of a-Si. These results are compared with the silicide formed when unlimited a-Si or single-crystal silicon was used. The specimens were analysed using electron micrographs and diffraction patterns.The detailed studies of the bilayers were preceded by studies of single layers of chromium and a-Si. The chromium single layers proved to be continuous for films as thin as approximately 2 nm. The as-deposited films with thickness in excess of about 5 nm were crystalline with b.c.c. structure and comprised very small grains. A phase change occured at about 450°C from the b.c.c to a simple cubic lattice structure. The silicon single layers were completely amorphous in their as-deposited state and crystallized around 600°C. Very large grains formed.The self-supporting Cr/a-Si bilayers with a typical total thickness of about 50 nm, where the relative film thickness were adjusted to yield Cr:a-Si atomic ratios of 1:2, 1:1, 5:3 and 3:1, were prepared by sequential electron gun vacuum deposition of chromium and silicon onto photoresist-covered glass slides. In the early stages of phase formation, when both unreacted chromium and silicon were present, the CrSi₂ phase was formed at about 450°C for all of these specimens. This was the end phase for the 1:2 ratio specimen. For all the other specimens the metal- rich Cr₅Si₃ phase was next to grow at about 550°C. For the 5:3 ratio specimen this was the end phase.Upon further heating of specimens with a Cr:a-Si atomic ration of 3:1, a more chromium-rich phase of Cr₃Si was formed at about 650°C. In the 1:1 ratio specimen, however, the next and end phase observed was CrSi, also growing at about 650°C. The end phase was thus determined by the availability of chromium and silicon during the reactions and could be predicted from the phase diagram.When using an unlimited supply of a-Si (or of single-crystal silicon), instead of limiting it to the thickness necessary for the predetermined ratios mentioned above, the only phase that ever formed was CrSi₂.The grain sizes observed in the various final phase specimens were as follows: CrSi₂, 25 nm; Cr₃Si, 40 nm; Cr₅Si₃, 50 nm; CrSi, 100 nm.     

Formation of intermetallic phases during mechanical alloying and annealing of Cr + 20 wt % al mixtures

We have studied phase-formation processes in mixtures of Cr and Al (20 wt %) powders in the course of mechanical alloying (MA) and the phase transformations of the samples during subsequent annealing at temperatures of up to 800°C. The resultant x-ray amorphous intermetallic phases were identified by a differential dissolution method, which allows one to follow the formation of x-ray amorphous and partially crystallized phases. During MA of Cr + Al mixtures, the first to form is x-ray amorphous Cr₄Al, which then converts to partially crystallized Cr₂Al through reaction with aluminum. The peritectoid decomposition of Cr₄Al during heating of the MA samples is accompanied by heat release at 330–350°C. Heating to 420°C leads to the formation of Cr₅Al₈. At 800°C, Cr₅Al₈ reacts with Cr to form Cr₂Al.     

Microstructure and thermal stability of Al–Cr–X (X=Ni,Mo, Si) powders obtained by centrifugal atomisation

Abstract

The microstructure and the thermal stability of rapid solidified centrifugally atomised AI–3Cr–X (X=1Ni, 3Ni, 0·3Mo, 1Si, or 3Si, at.-%) powders were studied. Three main types of microstructure were observed in the powders: cellular, globular, and rosettelike. Some powders exhibited a mixture of these. In the atomised state the alloys usually had two phases, intermetallic Al₁₃Cr₂ and α-Al solid solution. Thermal stability was studied for a range of temperatures from 20 to 500°C. The phase Al₃Ni appeared in the nickel containing alloys and grew upon heat treatment. The molybdenum containing alloy did not show any noticeable change upon heat treatment. With respect to the silicon containing alloys, the intermetallic Al₁₃Cr₂ transformed into Al₁₃Cr₄Si₄ at high temperatures. On the basis of bibliographic information a nucleation map was calculated relating the prevalence of the intermetallic Al₁₃Cr₂ phase and the α-Al phase to the particle diameter and the chromium concentration of powder obtained by centrifugal atomisation.

MST/3273     

Structure and properties of the diamond–WC–6Co composite doped by 1.5 wt % of CrSi<Subscript>2</Subscript>

The influence of chromium and silicon on the structure of the diamond–WC–6Co composite doped by chromium disilicide has been studied at the meso-, micro-, and submicron levels. It is established that under the conditions of the structure formation of the diamond–WC–Co + CrSi₂ chromium and silicon do not dissolve in particles of diamond and WC carbide, they form a Co(W,C,Cr,Si) solid solution decreasing the energy of the stacking fault, which contributes to the Co_(fcc) → Co_(hcp) polymorphic transformation. Chromium interacts with atoms of carbon in diamond and WC carbide, as a result of which the graphite layer in the diamond/Co phase contact zone disappears and the Co₃W₃C carbide forms in the volume of the WC–Co-matrix. Chromium and silicon contribute to a good retention of diamond particles by the WC–Co-matrix and increase the ultimate strength in compression of the diamond–WC–Co+CrSi₂ composite.     

Design of composite porous cermets synthesized by hydrothermal treatment of CrAl powder followed by calcination

The microstructure of the porous Cr–Al metal–oxide cermet was studied by means of XRD, SEM, EDX as well as IR and Raman spectroscopy. This cermet was synthesized by mechanical alloying of Cr–Al powders in an AGO-2 planetary ball mill followed by hydrothermal treatment in a special stainless steel die and calcination in air. As a result, a highly porous monolith comprised of metal-like particles randomly distributed in the oxide matrix (Cr₂O₃ and Al₂O₃) was formed. Two types of the composite cores were found in cermets. The first one consisted of chromium phase containing nanoparticles sized from 50 to 140 nm and Al-enriched phase at the interfaces. The second one consisted of new chromium oxide phases with hexagonal Cr₂N-like and fcc CrN-like structures probably with Cr₂O and CrO stoichiometry. These new phases were stabilized within aggregates of the nanocomposite particles containing inclusions of alumina. The relations between different preparation stages and the cermet microstructure are discussed.     

Effect of initial composition on phase selection in Ni–Si powder blends processed by mechanical alloying

The effect of initial powder blend composition on the synthesis and formation mechanism of nickel silicide phases was investigated by mechanical alloying in Ni-60 and Ni-66.7?at.% Si powder blends. It was noted that the equilibrium NiSi phase started to form in the early stages of milling and that the amount of the NiSi phase in the milled powder increased with increasing milling time. Even though, under equilibrium conditions, a mixture of both the NiSi and NiSi₂ phases was expected to be present in the Ni-60?at.% Si composition and the stoichiometric NiSi₂ phase in the Ni-66.7?at.% Si composition, the NiSi phase was present in both the compositions investigated. However, while only the NiSi phase was present homogeneously in the Ni-60?at.% Si powder blend, both the NiSi phase and a very small amount of unreacted Si were present in the powder blend of Ni-66.7?at.% Si composition. This unexpected phase constitution in the milled powders was attributed to a partial loss of Si during mechanical alloying of the powder blends, confirmed by energy dispersive X-ray spectrometer analyses, and explained on a thermodynamic basis.     

Formation of Composite Powders and Ceramics from β-SiC-Cr2O3-C Mixtures

The phase-formation process in the disperse -SiC + SiO₂ + Cr₂O₃ + C system treated in vacuum at 1273, 1473, 1673, and 1873 K was studied by X-ray phase analysis, IR spectroscopy, EPR, and other methods. It was found that at treatment temperatures of 1273, 1473, and 1673 K, the major process is carbothermal reduction of Cr₂O₃ and SiO₂ with the formation of chromium carbosilicide and chromium carbide (Cr₃C₂). At T = 1873 K, on the surface of the SiC particles, metastable chromium silicate forms on the base of SiO₂ and Cr₂O₃. The reduction of Cr₂SiO₄ by silicon carbide is accompanied by the formation of CrSi₂, Cr₅Si₃, and chromium carbosilicide. The phase composition of the products of interaction and their distribution over the volume of the composite powder and ceramic material are determined by the dispersion composition of the starting SiC powder and by the degree of development of reduction processes in local volumes of the disperse and compacted material.     

Mechanochemical synthesis of chromium-based alloys

This paper presents a detailed study of the formation of chromium-based alloys, Cr–Ta–W + plasticizing additives (Nb and Zr) and Cr–Ta–Si, during milling of powder mixtures in a Fritsch (P-7) planetary mill under an Ar atmosphere. It is shown that, after milling for 18 h, all the components of the starting mixtures convert into a Cr-based BCC solid solution. The powders of chromium alloys obtained in this study are readily compacted by hot isostatic pressing (HIP) under conditions typical of the processing of powders of high-temperature nickel alloys. Heating of the powders and compacts leads to the decomposition of the supersaturated solid solution and the formation of two forms of the Cr₂M Laves phase with cubic crystal lattices. The formation of a mixed-phase fine microstructure in the chromium alloys after HIP suggests that the materials studied here are potentially attractive as a base of next-generation chromium-based high-temperature alloys.     

Improved surface properties of D2 steel by laser surface alloying

The wear and the high-temperature oxidation resistance of the D2 steel (Fe-1.5 C-12 Cr-0.95 Mo-0.9 V-0.3 Mn) were increased by laser surface alloying after coating the surface with SiC or Cr₃C₂ powder. The surface alloys exhibit two microstructures: hypoeutectic and hypereutectic, respectively, all containing iron solid solutions and iron-chromium carbides, (Fe,Cr)₇C₃. The oxidation resistance of these alloys was measured in isothermal and cyclic conditions, and was shown to increase with silicon or chromium additions, particularly due to the formation of a chromia scale with excellent behaviour during thermal shoks. The surface alloy obtained with Cr₃C₂ also has shown a better resistance to wear due to its hypereutectic microstructure.     

Combustion synthesis of (Mo1 − xCrx)Si2 (x = 0.00–0.30) alloys in SHS mode

Combustion synthesis was adopted to successfully synthesize molybdenum–silicon–chromium (Mo?Si?Cr) alloys by the mode of self-propagating high-temperature synthesis (SHS). The experimental study of combustion synthesis of Mo?Si?Cr alloys was conducted on elemental powder compacts. Powder compacts with nominal compositions including MoSi₂, (Mo_0.95Cr_0.05)Si₂, (Mo_0.90Cr_0.10)Si₂, (Mo_0.85Cr_0.15)Si₂, (Mo_0.80Cr_0.20)Si₂, (Mo_0.75Cr_0.25)Si₂ and (Mo_0.70Cr_0.30)Si₂ were employed in combustion synthesis experiments. The combustion mode, combustion temperature, flame-front propagation velocity and product structure were investigated. The results showed that Mo?Si?Cr alloys were synthesized by an unsteady state combustion mode with a spiral-trajectory reaction front. The peak combustion temperature reduced with the addition of Cr to Mo–Si system. The flame-front propagation velocity decreased with an increase in Cr content of the powder compact. The X-ray diffraction (XRD) results showed that the crystal structure of the combustion product changed from Cll_b-type structure (Mo_0.90Cr_0.10)Si₂ to C40-type structure (Mo_0.85Cr_0.15)Si₂ with increase in Cr content of Mo–Cr–Si alloys. The intensities of diffraction peaks of the C40-type phase gradually increased with increase in Cr content.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Thermoelectric properties of chromium disilicide prepared by mechanical alloying

CrSi and Cr_1?x Fe _x Si particles embedded in a CrSi₂ matrix have been prepared by hot pressing from CrSi_1.9, CrSi₂, and CrSi_2.1 powders produced by ball milling using either WC or stainless steel milling media. The samples were characterized by powder X-ray diffraction, scanning, and transmission electron microscopy and electron microprobe analysis. The final crystallite size of CrSi₂ obtained from the XRD patterns is about 40 and 80 nm for SS- and WC-milled powders, respectively, whereas the size of the second phase inclusions in the hot pressed samples is about 1–5 μm. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, and figure of merit (ZT) were analyzed in the temperature range from 300 to 800 K. While the ball-milling process results in a lower electrical resistivity and thermal conductivity due to the presence of the inclusions and the refinement of the matrix microstructure, respectively, the Seebeck coefficient is negatively affected by the formation of the inclusions which leads to a modest improvement of ZT.     

Interdiffusion and growth of chromium silicide at the interface of Cr/Si(As) system during rapid thermal annealing

In this work, the solid-state reaction between a thin film of chromium and silicon has been studied using Rutherford backscattering spectroscopy, X-ray diffraction and the sheet resistance measurements. The thickness of 100 nm chromium layer has been deposited by electronic bombardment on Si (100) substrates, part of them had previously been implanted with arsenic ions of 10¹⁵ at/cm² doses and an energy of 100 keV. The samples were heat treated under rapid thermal annealing at 500 °C for time intervals ranging from 15 to 60 s. The rapid thermal annealing leads to a reaction at the interface Cr/Si inducing the formation and the growth of the unique silicide CrSi₂, but no other phase can be detected. For samples implanted with arsenic, the saturation value of the sheet resistance is approximately 1.5 times higher than for the non-implanted case.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.     

Effect of co-substitution of Mn and Al on thermoelectric properties of chromium disilicide

CrSi₂ was earlier reported to be an interesting thermoelectric material for high temperature applications because of its high oxidation resistance and good mechanical properties. In order to enhance its figure of merit, Mn at Cr site and Al at Si site were substituted into CrSi₂. Our results indicate that Cr_1?x Mn _x Si_2?x Al _x solid solutions exhibit significantly lower thermal conductivity and a higher figure of merit than CrSi₂.     

Synthesis and characterisation of TiC reinforced CoCrFeNi composite by hot-pressing sintering

ABSTRACT

Ti, Co, Cr, Fe, Ni and graphite powders were used to fabricate TiC reinforced CoCrFeNi composite by mechanical alloying and consequently hot pressing sintering at 1200°C for 1?h. Results indicated that Co, Cr, Fe and Ni powders were deformed, cold welded and crushed repeatedly during milling and an face-centred cubic-structured solid solution was obtained after milled for more than 10?h. Nano-sized TiC and micron-sized Cr₇C₃ type carbides were formed and embedded in the CoCrFeNi matrix dispersedly after sintering. The hardness and compressive fracture strength of the sintered composite reached 501 HV and 2.55?GPa, respectively, which could be ascribed to the presence of large amount of in-situ formed TiC and Cr₇C₃ type carbides in the composite.     

Phase formation sequence of Cr2AlC ceramics starting from Cr–Al–C powders

The reaction process of Cr₂AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr₂AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr₅Al₈, Cr₂Al and Cr₇C₃ were the intermediate phases appearing in turn in the heating process. The amount of Cr₂AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.     

Synthesis and characterization of nanostructured Cr3C2NiCr

Pre-alloyed Cr₃C₂-25 (Ni20Cr) powder was synthesized by mechanical ball milling in Hexane [H₃(CH₂)₄CH₃]and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average powder size drastically decreased with time during the first four hours of milling; then decreased slightly as milling continued up to 20 hours. For milling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subjected to heavier deformation than the chromium carbide powder. During the initial stages of milling, the brittle chromium carbide powders are fractured into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for times up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and composite powders. Milling times of up to 20 hours led to the formation of a poly crystal nanocomposite powder system in which chromium carbides, with average size of 15 nm, were uniformly distributed in NiCr matrix.     

Phase Reactions and Diffusion Path of the SiC/Cr System

Solid-state bonding at pressureless-sintered SiC has been carried out using 25-m Cr foil at temperatures from 1373 to 1773 K for 1.8 ks in vacuum. The formation of reaction phases and microstructures at the interface between SiC and Cr was investigated by X-ray diffraction and microprobe analysis. At the bonding temperature of 1373 K the cubic Cr₂₃C₆phase formed next to Cr, and the hexagonal Cr₇C₃ phase formed next to SiC. At 1473 K the cubic phase Cr₃SiC_x appeared additionally on the SiC side. At 1573 K the complete diffusion path was established. Upon increasing the joining temperature beyond 1573 K all the chromium was consumed, and Cr₂₃C₆ and Cr₃SiC _x dissolved. A layered structure consisting of SiC/Cr₅Si₃ C _x /Cr₇C₃/Cr₅Si₃ C _x /SiC occurred.     

The microstructure and its properties of Ni3Al based composite

 Composites of B-doped ductile Ni₃Al alloy matrix with no-oxide WC ceramic powders were produced by mechanical alloying, half-sintering and build-up welding. WC powders form non-continuous hardening phases, which are distributed in Ni₃Al matrix, wetting well with the matrix. The hardness and the structure stability are retained to temperatures of at least 850°C. After build-up welding, most of the NiAl phase left after sintering was changed into other phases and some graphite was precipitated in the matrix. The sand-laden water wear test showed expected results. Received: 12 October 1998/Accepted: 2 November 1998