Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 1992"

A comparative overview of molybdenum disilicide composites

Abstract: MoSi 2 -based composites possess significant potential to meet the demands of advanced high temperature structural applications in the range 1200–1600 °C, in oxidizing and aggressive environments. These materials constitute an important new class of “high temperature structural silicides”. The intermetallic compound MoSi 2 possesses properties which make it a very desirable matrix for high temperature composites, and these properties are described and compared with those of other high melting point silicides. The developmental history of composites based on MoSi 2 is traced from its beginnings to the present. Mechanical property improvements derived from SiC and ZrO 2 reinforcements, as well as matrix alloying, are described, and properties of current MoSi 2 -based composites compared with those of silicon-based structural ceramics. Finally, important research and development directions for the continued improvement of MoSi 2 -based composites and their use as high temperature structural components are discussed.

Creep behavior of discontinuous SiCAl composites

Abstract: A review of creep data of discontinuous SiCAl composites (whisker and particulate) shows that the creep behavior of these composites exhibits two main characteristics: (a) the stress dependence of the steady state (or minimum) creep rate, as described by the value of the stress exponent, is high and variable and (b) the temperature dependence of the steady state (or minimum) creep rate, which is measured by the creep activation energy, is much larger than that for self-diffusion in aluminum. These two characteristics are examined in the light of theoretical treatments describing the origin of high temperature strengthening in discontinuous metal matrix composites and dislocation models proposed for dispersion-strengthened alloys.

Oxidation of MoSi2 and comparison with other silicide materials

Abstract: The oxidation behavior of MoSi2 in three fabrication conditions has been studied in oxygen and in air The cast material was studied over the temperature range 500–1400°C, the hot isostatically pressed (HIP) material was studied at 500 and 1000°C and the single crystals were studied at 500°C The cast material exhibited three regimes of behavior Above 1000°C a continuous protective silica scale formed Between 600 and 1000°C a silica scale formed, but formation of silica within grain boundaries, which are believed to be cracked, was observed At temperatures near 500°C accelerated linear oxidation, involving the formation of both Mo and Si oxides was observed and the specimen fragmented into powder (“pested”) The HIP material exhibited two regimes At 1000°C a protective silica film formed At temperatures around 500°C the HIP material underwent accelerated oxidation but did not fragment The oxidation of the single crystal was qualitatively the same as that for the HIP material at 500°C It was concluded, therefore, that accelerated oxidation is a necessary, but not sufficient, condition for pesting to occur The pesting of the cast material was concluded to occur by oxidation along pre-existing microcracks in the MoSi2 Preoxidation at 1000°C was found to be only partially successful in limiting accelerated oxidation during subsequent exposure at 500°C The oxidation of TaSi2 was observed to be qualitatively the same as that for MoSi2 However, the high thermodynamic stability and low volatility of Ta2O5 result in much higher temperatures being required for the occurrence of selective oxidation of Si

Appraisal of other silicides as structural materials

Abstract: With the recognition of oxidation-resistant MoSi 2 as the ideal matrix material for high temperature structural composites, there has been a growing interest in “other” silicides. This paper is an attempt to place such interest in proper perspective with a comprehensive review of recent activities. The “other” silicides are appraised in terms of diverse strategies for the development of structural materials, which may be broadly grouped as monolithic, in situ composites and artificially reinforced composites. With alloying as an underlying common theme, it is argued that a broader and a more fundamental understanding of the silicides is necessary. The formation of silicides is reviewed with the focus on the disilicides, 5-3 silicides and monosilicides, as the three principal useful groups. For the refractory metal based disilicides, the relationship between the crystal structures C11 b , C40, C49 and C54 is examined in terms of stacking sequences and contrasted in relation to the structures of aluminides. The role of interstitial elements and the in situ composite approaches are emphasized for the complex 5-3 silicides and the monosilicides as being the most adjacent phases to refractory metal solid solutions. For most silicides with a non-cubic crystal structure, the effect of associated anisotropy of the coefficient of thermal expansion (CTE) on the mechanical integrity through processing and application of the material, is brought forth as a potentially critical issue. A qualitative model is proposed to rationalize the pronounced occurrence of “pest” disintegration in terms of the anisotropy of CTE, the nature of the grain structure and the ductile-brittle transition temperature of intermetallics.

Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Abstract: This paper is concerned with the intergranular crack tip oxidation mechanism in alloy 718 at elevated temperatures. The basic concept is based on the ability of the oxygen partial pressure to control the preferential formation of oxide layers at the crack tip. The time required to complete the build-up of the protective oxide type at the metal-oxide interface is considered a measure of the limits of the oxidation process. Identification by transmission electron microscopy of oxide scale formed along fracture surfaces during a low frequency fatigue crack process in alloy 718 at 650°C supports the proposed model concepts. An experimental program was carried out to investigate the role of passivation time in controlling the progressive process of crack tip oxidation. This was achieved by testing the influence of oxide buil-up during hold time at minimum load, as well as the effect of a minor high frequency cycle imposed on the hold time period. It was established that an increase in fatigue crack growth rate accompanies the increase in passivation time period. These results were interpreted on the basis of the oxidation formation concepts.

Dry sliding wear behaviour of squeeze cast aluminium alloy-silicon carbide composites

Abstract: Squeeze cast Al alloy (BSS: LM11) matrix composites, each containing 10 vol.% of SiC particles or fibres, have been investigated for their resistance to dry wear under varying applied pressures (1–3 MPa) at a sliding spped of 2.68 m s−1 against a rotating EN25 steel disc. Seizure pressure of the composites as well as the base alloy was determined using a pin-on-disc machine. The alloy containing SiC particles showed less wear rate than the one having SiC fibre dispersion. The base alloy showed maximum rate of wear. Dispersoid-matrix interfacial bonding and shape of the dispersoid were found to play an important role in governing the wear rate of the composites. Scanning electron microscopy examinations indicated relatively finer grooves on the wear surfaces prior to seizure, while seizure led to severely damaged surfaces. Similarly, wear debris generated during wear was thin and flaky prior to seizure, while bulky debris particles were observed during seizure. A few iron machining chips were also found in all the cases. The results obtained have been explained on the basis of wear-induced microstructural changes and deformation, leading to work hardening in the subsurface regions and wear debris.

An experimental evaluation of the phase relationships and solubilities in the nbcr system

Abstract: The phase relationships and solubilities have been determined experimentally in a revised evaluation of the NbCr alloy system The results from a series of microstructural investigations on arc-cast and equilibrated samples, together with X-ray diffraction and compositional analyses, provide a basis for modifying the invariant reaction phase compositions and phase boundaries In addition, new evidence based on X-ray diffraction measurements further establishes the existence of a high temperature C14 Laves polytype as well as an intermediate C36 structure for NbCr2 Moreover, lattice parameters have been determined as a function of composition for the bcc and C15 Laves phases Within the equilibrium solubility limits of the Nb and Cr terminal bcc solid solutions, the lattice parameters exhibit a slight positive deviation from Vegard's rule For the NbCr2 C15 Laves phase the solubility behavior and the distinct change in the composition dependence of the lattice parameters suggest a different defect structure on either side of the stoichiometric composition in the intermetallic phase field

Application of ternary phase diagrams to the development of MoSi2-based materials

Abstract: A review of the literature reveals ternary phase diagram data for a number of systems involving MoSi2. Although incomplete, this literature provides the initial basis for a rational approach to alloy design. For example, one can assess the high temperature stability of various artificially introduced reinforcements, such as niobium or SiC in an MoSi2 matrix and the possibilities for the development of stable two-phase microstructures in quasibinary alloys in the MoSi2TiSi2 and MoSi2TaSi2 systems. Revised phase diagrams for these latter systems are presented that indicate the absence of the C11b-to-C40 high temperature polymorphic transformation in pure MoSi2.

Mechanical behavior and interface design of MoSi2-based alloys and composites

Abstract: The mechanical behavior of hot pressed MoSi2-based composites containing M05Si3, SiO2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000-1300 °C. The effects of strain rate on the flow stress for M05Si~-, SiO2- and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi2-M05Si ~ composites are given. Of these four composite additions, M05Si 3 and CaO produce strengthening of MoSi 2 in the temperature range investigated. SiO 2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi 2 in a manner that suggests dislocation pumping into the MoSi 2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi2-MosSi 3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi2-based composites are also reported.

Microstructures and properties of high melting point intermetallic Ti5Si3 and TiSi2 compounds

Abstract: Physical and mechanical properties of high melting point Ti5Si3 and TiSi2 intermetallics with hexagonal D88 and orthorhombic C54 structure have been investigated. Young's moduli of about 160 GPa (Ti5Si3) and 250 GPa (TiSi2) were recorded at room temperature. At 1000°C the elastic moduli are 143 and 215 GPa respectively. Flow stresses of about 1050 MPa for Ti5Si3 and 230 MPa for TiSi2 at 1000°C were measured. With increasing temperature an exponential decrease of the flow stresses occurs. The low density of 4.3 g cm−3 and the pronounced creep resistance are important for high temperature applications of this material.

Effect of silicon and niobium on oxidation resistance of TiAl intermetallics

Abstract: The high temperature air oxidation of titanium aluminide intermetallics with small additions of silicon, niobium and a combination of silicon and niobium was studied in a temperature range between 973 and 1173 K. There was less weight gain when silicon and niobium were added individually. The combination of silicon and niobium resulted in much better oxidation resistance than the individual element additions. The effects of silicon and niobium on TiAl oxidation were different. For example, TiAlSi showed a parabolic weight gain whereas TiAlNb did not. The surface oxide scale was basically composed of three layers, i.e. TiO, Al 2 O 3 and TiO 2 + Al 2 O 3 from the surface inward. SiO 2 was detected in the TiO 2 + Al 2 O 3 layer. Niobium strengthened the tendency to form Al 2 O 3 in the early stage of oxidation, resulting in the formation of a continuous Al 2 O 3 layer and a dense TiO 2 + Al 2 O 3 layer.

Synthesis of molybdenum disilicide by mechanical alloying

Abstract: Considerable interest and effort are being directed towards developing molybdenum disilicide (MoSi2) alloys with low oxygen content. During alloy synthesis, oxygen combines with Si to form glassy SiO2 precipitates at the MoSi2 grain boundaries, resulting in a degradation of its mechanical properties. We have used mechanical alloying, a high-energy ball-milling process, to synthesize alloy powders of MoSi2, MoSi2-27 mol.% MoSi3, MoSi2-50 mol.% Mo5Si3 and MoSi2-50 mol.% WSi2 starting from elemental powders. The processing of the powders, as well as the loading of the powders in graphite dies, was performed under high-purity argon inside a glovebox. The finer grain and particle size of the mechanically alloyed powders enabled us to hot-press them at 1500 °C, which is 300 °C lower than the temperature currently used for hot-pressing commercial powders. We have been successful in reducing the oxygen content in our alloys to about 310 ppm by weight, as measured by nuclear (d,p) reactions. We report the formation of metastable phases in the mechanically alloyed powders and their characterization by X-ray diffraction and differential thermal analysis. We also report the characterization of the hot-pressed alloys by optical and transmission electron microscopy, and the measurement of high-temperature mechanical properties.

Behavior of iron aluminides in oxidizing and oxidizing/sulfidizing environments

Abstract: Fe3Al alloys have potential as structural materials and coatings for use in hostile environments because they form protective scales at very low oxygen partial pressures at elevated temperatures. Weight gains and parabolic rate constants were small for exposures to dry air at 800 and 900°C. Chromium additions to Fe3Al resulted in higher initial rates of oxidation in dry air at 800 and 900°C and substantially increased overall reaction rates in a very aggressive oxidizing/sulfidizing mixed gas when greater than 2 at.%. There was some evidence that an α-alumina layer formed during extended air exposure at 900°C. The ability to rapidly form an oxide layer appeared to limit damage caused by any loss of scale during thermal cycling. Preoxidation reduced the corrosion of Fe3Al5%Cr in the oxidizing/sulfidizing gas, but aluminizing this alloy to form an FeAl coating appeared to be the best way to assure long-term protection in sulfidizing environments.

On the measurement of local strain and temperature during the formation of adiabatic shear bands

Abstract: A series of experiments was performed to study the process of adiabatic shear band initiation and formation in steels. The steels include a low carbon cold-rolled steel and three martensitic steels (HY-100 and two tempers of AISI 4340 VAR steel of varying hardness). In each case the specimens are machined as thin-walled tubes that are deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar). Shear band initiation and formation are observed by ultrahigh-speed photography of a fine grid pattern deposited on the specimen's surface. It is shown that the critical strain for shear band initiation depends on the magnitude of a preexisting defect, in accordance with the predictions of Molinari and Clifton, J. Appl. Mech., 54 (1991) 806–812. Ultrahigh-speed photographs of the grid pattern show that local strains of 100–1000% may be attained and that the local strain rates reach 10 5 s −1 . In addition, the local temperature in the shear band is measured by employing an array of small high-speed infrared detectors that provide a plot of temperature as a function of time and position. Within the shear band region, temperatures of 600 °C have been measured.

Thermodynamic calculation of the ternary TiAlNb system

Abstract: Phase equilibria of the ternary TiAlNb system are dominated by the large range of homogeneity of (β-Ti,Nb), the binary intermetallic compounds of the NbAl and TiAl systems and the formation of two ternary compounds. The available ternary experimental data, together with a thermodynamic extrapolation of the ternary system from the binary systems, have been used to calculate the ternary phase diagram. The model descriptions of the Gibbs energies of most of these compounds are given by the existing calculations of the binary systems. In order to model a phase which is present in only one binary system, but has a ternary homogeneity range, a hypothetical phase with the same structure was analytically described for each binary system. Such a phase would, of course, be metastable in the other binary systems. Constraints on the Gibbs energies of formation were derived from the crystal structures of the corresponding ordered compounds. These same constraints were employed for the corresponding phases in the ternary system. In a final optimization step, ternary parameters were introduced and adjusted to the available experimental data. The as-derived description of the ternary TiAlNb system can be used to estimate single or multiphase fields and thermodynamic quantities where no experimental data are yet available. It is also useful as an indicator of problem areas for which additional experimental data are required.

Depth of intergranular oxygen diffusion during environment-dependent fatigue crack growth in alloy 718

Abstract: The elevated-temperture fatigue crack growth behavior in alloy 718, when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed as fully environment dependent. In this process, the crack growth increment per loading cycle is assumed to be equal to the intergranular oxygen diffusion depth at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimens made of alloy 718 at 650 °C in which fatigue crack growth measurements were made for cyclic load conditions with and without hold time periods at minimum load level. This work resulted in establishing a relationship correlating the intergranular oxygen diffusion depth and the value of the stress intensity factor range ΔK. This relationship, when integrated over the cycle effective oxidation time, results in a closed-form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison shows good agreement between the two sets of results. Furthermore, by combining the parabolic rate law of diffusion and the equation for the intergranular oxygen diffusion depth, an explicit expression for the oxygen diffusivity of grain boundaries is derived. It is found that this diffusivity is both a ΔK- and a frequency-dependent parameter.

Oxidation of MoSi2-based composites

Abstract: The oxidation behavior of MoSi2-based composites reinforced with 30 vol% TiB2, ZrB2, HfB2 or SiC were tested under isothermal and cyclic conditions at temperatures from 500 to 1500°C The effects of thermal cycling on subsequent pesting behavior were studied A second series of cyclic oxidation and pesting tests was then performed on MoSi2 composites containing from 15 to 45 vol% TiB2 or SiC, in order to determine the effects of reinforcement volume fraction and porosity on oxidation behavior Oxidation kinetics of the composites under static pesting (500°C) conditions and cyclic (1200°C peak temperature) conditions were studied Scanning electron microscopy analyses were performed to determine the structures and compositions of the oxide layers formed under various test conditions

Combustion-assisted synthesis of TiTiBTiC composite via the casting route

Abstract: A processing method combining conventional melting and combustion synthesis reaction was used to produce TiTiBTiC composite. Simple thermochemical calculations on the reaction system gave the operating “excess moles-initial temperature” regime for making such composites. Room temperature tension tests were performed on the as-cast and the rolled composites. The increase in strength of the rolled composite compared with the as-cast composite was attributed to the absence of dendritic structure. Fractographic examinations were done to observe the fracture behaviour of matrix and precipitated TiBTiC phase.

Solid state amorphization in binary Ti-Ni, Ti-Cu and ternary Ti-Ni-Cu system by mechanical alloying

Abstract: An amorphous phase has been synthesized by mechanical alloying in a planetary mill over a nickel content range of 10–70 at.% in the TiNi system and a copper content range of 10–50 at.% in the TiCu system. In the case of ternary TiNiCu alloys the glass-forming composition range has been found to be given by x = 10–20 for Ti60Ni40 − xCux, x = 10 – 30 for Ti50Ni50 − xCux and x = 10 – 40 for Ti40Ni60 − xCux alloys. The difficulty in the amorphization of copper-rich compositions is explained in the light of enthalpy composition diagrams calculated for the ternary solid solution and the amorphous phase.

Permeability for flow parallel to primary dendrite arms

Abstract: The permeability for the flow of interdendritic liquid parallel to primary dendrite arms in columnar structures was calculated using the boundary element method for fully developed flow. The permeability was calculated because when the volume fraction of liquid (gL)exceeds approximately 0.60 –0.65, experiments fail. The calculated results, based on the microstructures of directionally solidified alloys, agreed with analytical results for flow parallel to circular cylinders arranged in square and triangular arrays. It appears that there is a transition in the behavior of the permeability at gL ≈ 0.65.

TiNi shape memory alloys prepared by normal sintering

Abstract: This paper covers the use of conventional press and sinter of titanium and nickel equiatomic blends, involving transient liquid phases. A notable development has been the fabrication of homogeneous compacts from the elemental powders. Results indicate that employment of a double-stage sintering cycle is effective to sustain the geometrical integrity of the sintered sample. The TiNi alloy prepared in this manner exhibits a recovery strain of 4.6% by generating a restoring stress of 230 MPa, with a martensitic transformation point Ms of 34 °C.

Microstructural evolution in rolled aluminium

Abstract: The evolution of the microstructure of cold-rolled pure aluminium (99.996%) was studied by transmission electron microscopy (TEM), extending a previous study that was limited to 30% strain ( e = 0.36). The present work concentrates on rolling strains from 50% ( e = 0.69) to 90% ( e = 2.3). The observations are explained through the governing principle that, in the course of polyslip, grains break up into volume elements within each of which fewer slip systems operate simultaneously than required by the Taylor model. The boundaries between the volume elements accommodate the lattice misorientations arising from the correspondingly different glide. They are therefore geometrically necessary boundaries and in TEM they appear variously as dense dislocation walls, different types of microbands and subgrain boundaries. The morphology and the spatial arrangement of the geometrically necessary boundaries were characterized and it is discussed how these boundaries form with increasing strain as an integral part of the microstructural evolution.

The oxidation behavior of intermetallic compounds

Abstract: The selective oxidation of intermetallic compounds is described. It is shown that the fundamental concepts developed for the oxidation of conventional alloys ( e . g . iron- and nickel-base alloys) pertain also to the oxidation of intermetallics with slight modification. The effects of temperature on the development of protective external scales and on the occurrence of “pesting” are considered. The effects of third element additions on the oxidation mechanisms and influence of gas composition on oxidation rates and morphologies and on the formation of volatile reaction products are described. Finally, the effects of interstitial embrittlement of intermetallic compounds are briefly considered. Selected data for compounds of primary interest as high temperature materials (aluminides, silicides) are used to illustrate the above-mentioned fundamentals.

The effect of orientation and lamellar structure on the plastic behavior of TiAl crystals

Abstract: Crystals of binary TiAl (48.1–51.6 at.% Al) containing a single set of lamellae were grown using the floating zone method, and the spacings between the lamellae were changed by altering the crystal growth rate and the alloy composition. The plastic behavior of TiAl was found to depend strongly on the lamellar spacing and the angle between the loading axis and the lamellar planes. When the stress is applied parallel or perpendicular to the lamellar planes, shear deformation proceeds on {111} planes which cross the lamellae, the specimens are very strong and they exhibit low ductility. However, when the stress is applied at intermediate angles, slip occurs on {111} planes parallel to the lamellae at a low applied stress and good ductility is observed. The mode of deformation in the α2 phase, i.e. {11 2 1}〈 11 26〉 or 〈10 1 0〉〈 2 10〉 slip, strongly affects the plastic behavior of lamellar TiAl compounds. A fine and uniform distribution of lamellae is necessary to obtain high strength and good ductility.

Diffusional reactions in the combustion synthesis of MoSi2

Abstract: An attempt has been made to understand the nature of the reactions occurring in the combustion wave during the combustion synthesis of MoSi 2 from powders of molybdenum and silicon. The study is also extended to determine the effect of heating rate on the product formation by heating the compacts of molybdenum and silicon at various heating rates. In combustion synthesis, MoSi 2 is formed in a single step suggesting the occurrence of liquid silicon-solid molybdenum diffusional reactions during the propagation of the combustion wave. In contrast, at low heating rates, Mo 5 Si 3 is formed first, which is then enriched with liquid silicon by the diffusional reaction of liquid silicon through the product layer of Mo 5 Si 3 , forming MoSi 2 . Evidence for these reactions has been obtained by correlating the exothermic effects observed from the differential thermal analysis curves with the X-ray identification of the products of the reaction.

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites VII:improved analyses for bonded interfaces

Abstract: The previous analytical solution, which was presented in part I of this series of papers, for the transfer of elastic stress from a bonded fiber to the matrix is improved in the present study. The present analysis resolves the following limitations, which existed in the previous analysis. (1) The radial dimension of the matrix is much greater than that of the fiber. (2) The radial dependence of the axial stress in the fiber is ignored. (3) The approximate shear stress distribution, which is used to derive the radial dependence of the axial stress in the matrix, satisfies the free surface condition only in an approximate sense. (4) A plane strain condition is assumed in the equilibrium equation in order to relate the tangential stress to the radial stress at the interface. The most significant improvement of the present analysis is that it is applicable for any radial dimension of matrix.

Engineering limitations of MoSi2 coatings

Abstract: An analysis is presented of MoSi2 for use as a protective coating against high temperature oxidation. Environmental issues such as low temperature behavior, passive-active oxidation, cyclic oxidation, and hot corrosion are reviewed. Thermal-mechanical issues such as thermal stresses, interdiffusion, and coating deformation are also presented. The ability of MoSi2 to form a protective silica film over a broad range of temperatures and the demonstrated ability to engineer coatings of MoSi2 suggest that MoSi2 may represent a significant opportunity for advancement in high temperature coating technology relative to current fused slurry silicide technology.

Creep of molybdenum disilicide composites

Abstract: The creep deformation behavior of MoSi 2 , MoSi 2 , with SiC and WSi 2 alloy, and the alloy with SiC reinforcement was determined under compression in the temperature range 1100–1450 °C. The effects of reinforcement and alloying were evaluated. Existing theories of composite strengthening were examined in relation to the present results. Models based on creeping matrix with elastically deforming fibers appear to predict the observed behavior better than any other models. Activation energies and volumes were evaluated to identify the creep mechanisms. Based on the stress exponents and the energies, it is concluded that creep progress with increasing temperature and stress from newtonian viscous flow involving diffusion processes to power-law creep involving dislocation climb.

Effects of chemistry, density and size distribution of inclusions on the nucleation of acicular ferrite of CMn steel shielded-metal-arc-welding weldments

Abstract: The influence of inclusions on the nucleation of acicular ferrite of six as-deposited CMnTi welds prepared by the shielded-metal-arc-welding process was studied. The microstructure of the welds, the inclusion size distribution, the density and the average chemical composition were characterized. It was found that a titanium-rich phase which partially covers the inclusions is responsible for the nucleation of acicular ferrite. It is estimated that a titanium content of about 39 wt.ppm in the weld metal is necessary for the formation of this titanium-rich phase. However, even if the presence of the titanium-rich phase in the inclusions is necessary, it is not the only factor affecting the nucleation of acicular ferrite (AF). After correcting for the effect of austenite grain size, it was found that the efficiency of the inclusions for the nucleation of AF is related to the total external surface area of the titanium-rich phase covering the inclusions per unit area of weld metal. This is in turn related to the density, the size distribution and the titanium level of the inclusions.

ZrO2 and ZrO2SiC particle reinforced MoSi2 matrix composites

Abstract: ZrO 2 MoSi 2 and (ZrO 2 SiC)MoSi 2 composites were fabricated by hot pressing-hot isostatic pressing at 1700 °C. No reactions between ZrO 2 , SiC and MoSi 2 were observed. An amorphous silica glassy phase was present in all composites. Composites with unstabilized ZrO 2 particles exhibited the highest room temperature fracture toughness, reaching a level three times that of pure MoSi 2 . Both the room temperature toughness and 1200 °C strength of (ZrO 2 SiC)MoSi 2 composites were higher than those of ZrO 2 MoSi 2 composites, indicating beneficial effects of combined reinforcement phases. Low strength levels were observed at 1400 °C as a result of the presence of the silica glassy phase. Elimination of glassy phases and refinements in microstructural homogeneity are processing routes important to the optimization of the mechanical properties of these types of composites.