Synthesis of Al -TiC in-situ composites: Effect of processing temperature and Ti:C ratio

Al-TiC insitu composites are gaining increasing importance because of good wettability of TiC with Al melt and its favourable properties. However, incomplete reaction between the released Ti and C in Al melt leads to formation of undesirable phases like Al₃Ti which is detrimental to the properties. In this investigation, Al-TiC composites are processed at different temperatures from 700 °C to 1200 °C and with different Ti:C ratios to asses the effect of these two parameters on the formation of TiC particles. Microstructural features and X-ray diffraction results show that at temperatures below 1000 °C blocky type Al₃Ti forms. As the temperature is increased the blocky nature of Al₃Ti changes to needle like indicating release of Ti to a greater extent and as a result more and more TiC particles form. At 1200 °C there is no evidence of Al₃Ti formation after a reaction time of 30 minutes. Increasing the carbon content (Ti:C ratio) to two fold of the stoichimetric amount does not show any significant effect. Though a four fold increase in carbon content showed some promise, however, some amount of free carbon was found to remain.     

Isothermal oxidation behaviour of beta gamma powder metallurgy TiAl–4Nb–3Mn alloys

Abstract

A new TiAl–4Nb–3Mn beta gamma alloy was synthesised by a powder metallurgy process. HIPed and vacuum heat treated specimens were isothermally oxidised at 800 and 900°C in air up to 500 h. The TiAl–4Nb–3Mn alloys oxidised parabolically up to 500 h at both 800 and 900°C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of α₂ lamellae in the vicinity of the interface forms a diffusion zone (lamellar depleted zone) leading to the formation of Nb and Mn rich white layers just below the interface by outward diffusion of Nb and Mn which are released from breakdown of α₂ lamellae. As exposure time increases, Nb begins to diffuse earlier than Mn and diffuses more actively at higher temperature. The activation energy for oxidation of TiAl–4Nb–3Mn alloy was lower than that of Ti–48Al alloy and was higher than those of Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.

On a synthétisé un nouvel alliage TiAl–4Nb–3Mn de type bêta gamma par un procédé de métallurgie des poudres. On a oxydé en isotherme à 800 et à 900°C à l’air, jusqu’à une durée de 500 heures, les spécimens pressés par HIP et traités thermiquement sous vide. Les alliages de TiAl-4Nb-3Mn s’oxydaient paraboliquement jusqu’à 500 heures tant à 800°C qu’à 900°C. Les oxydes consistaient en une couche externe de TiO₂, en une couche intermédiaire d’Al₂O₃ et en une couche interne mixe riche en TiO₂ et les mécanismes d’oxydations de l’alliage étaient identiques aux deux températures. Lors de l’oxydation, la dégradation de lamelles d’α₂ dans le voisinage de l’interface forme une zone de diffusion (zone lamellaire appauvrie) menant à la formation de couches blanches riches en Nb et en Mn juste au-dessous de l’interface par diffusion vers l’extérieur du Nb et du Mn, qui sont relâchés par la dégradation des lamelles d’α₂. À mesure que la durée de l’exposition augmente, le Nb commence à se diffuser plus tôt que le Mn et se diffuse plus activement à une température plus élevée. L’énergie d’activation pour l’oxydation de l’alliage de TiAl–4Nb–3Mn était plus basse que celle de l’alliage de Ti–48Al et était plus élevée que celle des alliages de Ti–48Al–2Nb–2Cr et de Ti–48Al–2Nb–2Cr–W.     

Reciprocating wear response of Ti(C,N)–Ni3Al cermets

Abstract

Ti(C,N) based cermets have received significant attention due to their enhanced mechanical properties at elevated temperatures, low mass and increased chemical stability, when compared to WC–Co hardmetals. These properties have been found to be superior in many cases to traditional hardmetal replacements, such as TiC–Ni. The current study focuses on Ti(C,N)–Ni₃Al cermets formed through melt infiltration (with binder contents ranging from 20 to 40 vol.-%Ni₃Al). Comparison is made with TiC–Ni₃Al cermets using an identical binder alloy. The reciprocating ball on flat wear properties of the cermets were evaluated as a function of applied load (using a WC–Co counterface), together with the composite hardness and indentation fracture resistance. It is shown that nitrogen content negatively affects infiltration, resulting in non-infiltrated areas within low binder content cermets, which decreases the indentation fracture resistance and hardness. This problem can be largely mitigated by increasing binder content. When comparing fully infiltrated cermets, increasing nitrogen content decreases hardness and increases toughness, while all Ti(C,N) cermets outperform TiC (at 40 vol.-% binder). Reciprocating wear increased with increasing load, and typically was the most severe for the lowest binder contents. A combination of wear mechanisms were apparent, including both abrasive and adhesive wear, with the formation of an oxide tribolayer containing components from both the tested cermets and the WC–Co counterface material.

Les cermets à base de Ti(C,N) ont reçu une attention importante grâce à leurs propriétés mécaniques améliorées aux températures élevées, à leur faible masse et à leur stabilité chimique augmentée, lorsque comparés aux carbures métalliques de WC–Co. Dans plusieurs cas, on a trouvé que ces propriétés étaient supérieures à celles des remplacements traditionnels de carbures métalliques, comme le TiC–Ni. La présente étude se concentre sur les cermets de Ti(C,N) –Ni₃Al formés par infiltration du bain (la teneur du liant variant de 20 à 40% en volume de Ni₃Al). On les compare aux cermets de TiC–Ni₃Al en utilisant un alliage de liaison identique. On a évalué les propriétés d’usure par déplacement alternatif de bille sur disque des cermets en fonction de la charge appliquée (utilisant un antagoniste en WC–Co), ainsi que de la dureté du composite et de la résistance à la fracture d’indentation. On montre que la teneur en azote affecte négativement l’infiltration, ayant pour résultat des régions non infiltrées à l’intérieur des cermets à faible teneur en liant, ce qui diminue la résistance à la fracture d’indentation et la dureté. On peut largement atténuer ce problème en augmentant la teneur en liant. Lorsque l’on compare des cermets entièrement infiltrés, l’augmentation de la teneur en azote diminue la dureté et augmente la ténacité, alors que tous les cermets de Ti(C,N) surpassent le TiC (à 40% en volume de liant). L’usure par déplacement alternatif augmentait avec une augmentation de la charge, et était typiquement plus sévère pour les plus faibles teneurs en liant. Une combinaison de mécanismes d’usure était apparente, incluant tant l’usure par abrasion que l’usure d’adhérence, avec formation d’une couche tribologique oxyde contenant des composantes tant des cermets évalués que du matériau antagoniste en WC–Co.     

Reaction Pathways of Nanocomposite Synthesized in-situ from Mechanical Activated Al–C–TiO<Subscript>2</Subscript> Powder Mixture

In-situ Al matrix composite was synthesized from Al–TiO₂–C powder mixtures using mechanical alloying and heat treatment, subsequently. The effect of ball milling on reaction processes of the resulting nanocomposite was investigated. The evaluation of powder mixture without mechanical activation showed that at 900°C aluminum reduced TiO₂, forming Al₃Ti and Al₂O₃. After 20 h mechanical activation of powder mixture, Al₃Ti and Al₂O₃ were fabricated. After that, by increasing milling time up to 30 h, no new phases formed. The DTA analysis of 30 h milled powder indicated two peaks after aluminum melting at 730 and 900°C. The XRD results confirmed that at 730°C, molten Al reacted with TiO₂ and C, forming Al₃Ti, Al₂O₃ and Al₄C₃. After that, at 900°C, Al₃Ti reacted with Al₄C₃, causing TiC formation. This results proposed that the TiC formation is associated by a series of reactions between intermediate products, Al₃Ti and Al₄C₃ and the resultant nanocomposite was successfully synthesized after 30 h milling and heated by DTA analysis up to 1200°C.     

High-temperature phase equilibria in the Al-rich corner of the Al-Ti-C system

A thermodynamic analysis of the Al-rich corner in the ternary Al-Ti-C diagram, providing phase relations and regions of phase stability, is presented. An invariant four-phase equilibrium between Al, Al₄C₃, Al₃Ti, and TiC _x takes place at 0.53 at. pct Ti, 7.10⁻⁶ at. pct C, and TiC_0.883 at 966 K. The carbon content of the TiC _x phase, which extends from x=0.48 to 0.98, exerts a significant effect on phase relationships in this ternary system. In particular, it is shown that stoichiometric TiC is not stable in the presence of liquid Al. For example, at 1300 K, a two-phase equilibrium between Al _L and TiC _x exists only in the 0.91<x<0.82 range. Thus, the interaction of Al _L with stoichiometric TiC leads to the formation of the Al₄C₃ aluminum carbide phase, whereas for x<0.82, only the intermetallic compound Al₃Ti can form at this temperature. The results of this analysis were confirmed by X-ray diffraction (XRD) measurements of relevant composites.     

Thermodynamic Properties of Dilute Aluminum in Liquid Zinc

Abstract

The thermodynamic properties of the liquid zinc-aluminum system in the range of 0 to 0.36 mole fraction of aluminum and over a temperature range of 430 to 530 °C were investigated by using an electromotive force (EMF) cell of the type

Al(s)|LiCl–KCl–AlCl₃|Zn–Al(l),Mo

The activity of aluminum was found to show a significant positive deviation from Raoultian behaviour. The activity coefficient of aluminum in the dilute solution range (γ°) and the free energy of solution of aluminum based on the one weight percent (1 wt %) standard state were determined as a function of temperature and found to follow the relationships:

logγ°=(1195/T)?0.874

ΔG_s=5457?11.4T (cal/mole)

Over the temperature range investigated, the self-interaction coefficient of aluminum in liquid zinc (e^Al_Al) was found to be approximately constant at ?0.01.

On a étudié les propriétés thermodynamiques du système liquide zinc-aluminium, entre 0 et 0.36 fraction molaire d'aluminium, à des températures de 430 à 530 °C, en utilisant une cellule à force électromotrice (FEM) du type:

Al(s)|LiCl–KCl–AlCl₃|Zn–Al(l),Mo

On a trouve que l'factivite de l'faluminium montrait une deviation positive importante du comportement Raoultien. On a determine le coefficient d'factivite, γ°, de l'faluminium pour une solution diluee ainsi que l'fenergie libre de solution de l'faluminium, base sur l'fetat standard de un pour-cent en poids (1%), en fonction de la temperature. On a trouve que ceux-ci suivaient les relations suivantes:

logγ°=(1195/T)?0.874

ΔG_s=5457?11.4T (cal/mole)

Dans le domaine de température étudié, on a trouvé que le coefficient d'auto-interaction de l'aluminium dans le zinc liquide (e_Al^Al) était approximativement constant à ?0.01.     

SULFIDATION OF Al2O3 WITH CS2 GAS FOR COMPACT ALUMINIUM PRODUCTION PROCESS

Abstract

An experimental study was conducted to investigate the sulfidation kinetics of Al₂O₃ with CS₂ gas. The experiments were carried out in a laboratory scale fluidized bed reactor at various temperatures. The objective was to investigate the feasibility of an alternative process for the production of primary aluminum using a sulfide intermediate. The results confirmed that the Al₂O₃ could be converted into Al2S3 to a limited degree within 4 to 5 hours. An optimum temperature for sulfidation of γ-Al₂O₃ was found to be about 850 °C. The alumina with a higher specific surface area results in a higher sulfidation ratio. The particle size of γ-Al₂O₃ has no effect on the reaction ratio under similar experimental conditions. The highest conversion ratio of about 40% was obtained under the present experimental conditions. According to a thermodynamic evaluation, a higher sulfidation ratio could be achieved in the next stage with improved reaction conditions.

On a effectué une étude expérimentale pour investiguer la cinétique de sulfuration de l’Al₂O₃ avec du gaz CS₂. On a exécuté les expériences dans un réacteur à lit fluidisé à l’échelle de laboratoire, à des températures variées. L’objectif est d’investiguer la faisabilité d’un procédé de substitution pour la production de l’aluminium primaire par voie d’un intermédiaire de sulfure. Les résultats ont confirmé que l’Al₂O₃ pouvait être convertie en Al2S3 jusqu’à un certain point en moins de 4 à 5 heures. On a trouvé que la température optimale de sulfuration de γ-Al₂O₃ était d’environ 850 °C. L’alumine à surface spécifique plus élevée résulte en un rapport plus élevé de sulfuration. La taille de particule de γ-Al₂O₃ n’a pas d’effet sur le rapport de réaction sous des conditions expérimentales similaires. Le rapport le plus élevé de conversion d’environ 40% a été obtenu sous les présentes conditions expérimentales. D’après une évaluation thermodynamique, on pourrait obtenir un rapport plus élevé de sulfuration à l’étape suivante avec des conditions de réaction améliorées.     

Reaction products of Al/TiC composites fabricated by the pressureless infiltration technique

The interfacial reaction products of the Al-Mg/TiC _p composite fabricated by the pressureless infiltration method were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). During the fabrication of composites, reaction products with various morphologies and sizes were formed in the A1 matrix as well as in the vicinity of the TiC particles by the interfacial reaction between the Al alloy and the TiC particles. From the EDS and selected-area diffraction pattern (SADP) analysis, Al₄C₃, Al₁₈Ti₂Mg₃, Ti₂AlC, Al₃Ti, and TiAl could be identified to form as interfacial reaction products. Both the size and the amount of the reaction products were increased with increasing fabrication temperature as well as fabrication time. Coarse Al₃Ti was barely observed in water-quenched composites, while it was observed at all fabrication temperatures (700 °C to 1000 °C) in furnace-cooled conditions. An erratum to this article is available at .     

Interfacial segregation of Ti in the brazing of diamond grits onto a steel substrate using a Cu-Sn-Ti brazing alloy

Diamond grits were brazed onto a steel substrate using a prealloyed Cu-10Sn-15Ti (wt pct) brazing alloy at 925 °C and 1050 °C. Due to the relatively high concentration of Ti in the brazing alloy, the braze matrix exhibited a composite structure, composed of β-(Cu,Sn), a Cu-based solid solution, and various intermetallic compounds with different morphologies. The reaction of Ti with diamond yielded a continuous TiC layer on the surfaces of the diamond grits. On top of the TiC growth front, an intermetallic compound, composed of Sn and Ti, nucleated and grew into a randomly interwoven fine lacey structure. An interfacial structure developed as the interwoven fine lacey phase was semicoherently bonded to the TiC layer, with the Cu-based braze matrix filling its interstices. The thickness of such a composite layer was increased linearly with the square root of isothermal holding time at 925 °C, complying with the law of a diffusion-controlled process. However, at 1050 °C, the segregation behavior of Ti and Sn to the interfaces between the TiC layer and the braze matrix diminished, due to the increased solubility of Ti in the Cu-based liquid phase. The enhanced dissolution of Ti in the Cu-based liquid phase at 1050 °C also caused the precipitation of rod-like CuTi with an average diameter of about 0.2 μm during cooling. SnTi₃ was the predominant intermetallic compound and existed in three different forms in the braze matrix. It existed as interconnected grains of large size which either floated to the surface of the braze matrix or grew into faceted grains. It also exhibited a nail-like structure with a mean diameter of about 1 μm for the rod section and a lamellar structure arising from a eutectic reaction during cooling.     

Achieving Al Melt/Carbon and Al-Ti Melts/Carbon Interfaces Wetting via Ultrasonic Couple Processing

Good wetting among Al, Al-Ti melts and C (carbon) solid was achieved by ultrasonic couple processing (UCP). Due to the action of interfacial tension of wetting, Al melt and Al-Ti melts climb along the side wall of C solid. The wetting angle at the triple junction of gaseous, liquid, and solid phases is all lower than 15 deg. In the meantime, good wetting between Al melt and C powder was also achieved by the ultrasonic couple processing. Al melt infiltrated into the interior of C powder through the capillarity. When the ultrasonic couple processing was applied in the system of wetting between Al-Ti melts and C powder, owing to the superimposed effect of incident acoustic wave and reflected acoustic wave at Al-Ti melts/C interface, local high temperature occurred in the Al-Ti melts near C interface, and the superimposed effect can effectively obstruct a direct reaction of Al melt and C, inhibit the formation of undesirable Al₄C₃, and promote dissolved Ti to react with C and dissociated C to form TiC particle phase.     

Titanium deoxidation reactions in liquid iron

Abstract

An experimental study has been made of the equilibrium between titanium and oxygen in liquid iron at 1625° and 1700°C. Equilibrium was established between liquid Fe-Ti-O alloys held in Ti₃O₅ or Al₂O₃ crucibles and water vapor-hydrogen gas mixtures which were used to control the oxygen potential of the melt. The gas mixture was bubbled through the melt to give good gas-metal contact and to avoid thermal diffusion effects. In the reaction

Ti₃O₅(s) + 5H₂(g) ? 3 Ti + 5H₂O(g)

equilibrium was attained both by reduction of the solid oxide crucible and by oxidation of titanium in the alloy. Quenched samples obtained by suction into SiO₂ tubes were analyzed chemically for titanium and by vacuum fusion for oxygen. X-ray diffraction techniques were used to identify the products of deoxidation. Thermodynamic data for Fe-Ti-O alloys are presented and the use of SiO₂ sampling tubes at low oxygen potentials is discussed. Results are compared with published data and it is suggested that titanium decreases appreciably the activity coefficient of oxygen in liquid iron and that the deoxidizing power of titanium is closer to that of aluminum than that of silicon or vanadium.

Résumé

Les auteurs ont étudié l'équilibre entre le tatane et l'oxygène dans le fer liquide à 1625 et 1700°C. Des mélanges vaporeux d'eau-hydrogène ont servi à contrô1er le potentiel d'oxygène dans les alliages liquides Fe-Ti-O dans des creusets de Ti₃O₅ ou de Al₂O₃. Le mélange gazeux circulait à travers le bain liquide pour favoriser les échanges gaz-métal et éviter des effets de diffusion thermique.

Le point d'équilibre de la réaction:

Ti₃O₅(s) + 5H₂(g) ? 3 Ti + 5H₂O(g)

a été atteint par réduction du creuset d'oxyde solide et par oxydation du titane de l'alliage. Le titane a été dosé par voie chimique et l'oxygéne par fusion sous vide. Les échantillons pour le dosage ont été prélevés par aspiration de l'alliage dans des tubes de silice. Les produits de désoxydation ont été identifiés par diffraction des rayons-X. Les auteurs présentent des données thermodynamiques et discutent de l'utilisation des tubes de silice pour l'echantillonnage.

Ils comparent leurs résultats aux données de la littérature: ils rapportent que le titane abaisse le coefficient d'activité de l'oxygéne dans le fer liquide et que le pouvoir désoxydant du titane se rapproche plus de celui de l'aluminiumque de celui du silicium ou du vanadium.     

Al-TiC Composites Fabricated by a Thermally Activated Reaction Process in an Al Melt Using Al-Ti-C-CuO Powder Mixtures. Part I: Microstructural Evolution and Reaction Mechanism

Al matrix composites reinforced with TiC particles are fabricated by a thermally activated reaction of Al-Ti-C powder mixtures in an Al melt. In the presence of CuO, reactant mixtures in the form of a pellet added to molten Al at temperatures higher than 1093 K (820 °C) instantly reach the peak temperature over 1785 K (1512 °C), followed by combustion wave propagation with in situ synthesizing TiC with a size of approximately 1 μm. Incomplete reaction products such as unreacted C, Al₃Ti, and TiC aggregates are also observed. The pellet microstructure evolution upon the combustion reaction indicates that preheating temperature, i.e., the initial melt temperature, affects both the thermodynamic and kinetic characteristics of the reaction, and thereby influences the final microstructure of the Al/TiC composites. Based on the experimental and theoretical results, a sequence of the reaction leading upto the in situ synthesis of TiC is illustrated and the corresponding mechanism for the present process is proposed.     

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part III. Tension-compression anisotropy

Single crystal superalloy specimens with various crystallographic orientations were tested in compression at room temperature, 650, 760, 870, and 980 °C. These results are compared with the tensile behavior studied previously. The alloy, René* N4, was developed for gas turbine engine blades and has the nominal composition 3.7 Al, 4.2 Ti, 4 Ta, 0.5 Nb, 6 W, 1.5 Mo, 9 Cr, 7.5 Co, balance Ni, in weight percent. Slip trace analysis showed that primary cube slip had occurred even at room temperature for the [111] specimens. With increasing test temperature more orientations exhibited primary cube slip, until at 870 °C only the orientations near [001] and [011] exhibited normal octahedral slip. The yield strength for octahedral slip was numerically analyzed using a model proposed by Lall, Chin, and Pope to explain deviations from Schmid’s law in the yielding behavior of a single phase γ’ alloy, Ni₃(Al,Nb). The Schmid’s law deviations in René N4 were found to be largely due to a tension-compression anisotropy. This is one of the sources of the Schmid’s law violations observed in Ni₃(Al, Nb) which are rationalized by the model. A second effect, which increases the strength of orientations away from [001], was found to be small in René N4. Analysis of recently published data on the single crystal superalloy PWA 1480 yielded the same result. Formerly Co-op Student, University of Cincinnati     

Microstructure,excess solid solubility,and elevated-temperature mechanical behavior of spray-atomized and codeposited Al-Ti-SiCP

In the present study, the microstructure, thermal stability, and elevated temperature mechanical behavior of Al-Ti-SiC_P metal matrix composites (MMCs) processed by spray atomization and codeposition were investigated. The evolution of the microstructure of the spray-deposited material before and after thermal annealing was studied using X-ray diffractometry, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy. The thermal stability of the spray-deposited materials was determined by monitoring the changes in hardness after isochronal thermal anneals at various temperatures. The results of X-ray and microanalysis studies revealed the presence of a supersaturated solid solution of Ti in α Al in the spray-atomized and codeposited material, with Ti concentrations in the 0.8 to 1.1 wt pet range. The formation of an extended solid solution was discussed in light of the cooling rates present during atomization and, subsequently, during deposition. Regarding mechanical behavior, the present results suggest that the as-spray deposited and hot extruded Al-Ti matrix is thermally stable up to a temperature of 400 °C and that the excess solid solubility of Ti in a Al, resulting from the rapid quench during processing, is maintained up to a temperature of 300 °C. The elevated-temperature mechanical properties of the hot extruded spray-deposited materials were studied following a 100-hour exposure at 250 °C, 350 °C, and 450 °C; the roomtemperature mechanical properties were also determined. Results show that the elevated-temperature yield strength of the spray-deposited and extruded materials compared favorably to those of an equivalent alloy made by powder metallurgical materials, were superior to those of the ingot material, but were inferior to those of mechanically alloyed Al-Ti materials. In addition, TEM studies showed no evidence of interfacial reactions at the Al-Ti/SiC_P interface. This article is based on a presentation made in the symposium “Spray Processing Fundamentals: Coating and Deposition” presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee.     

Synthesis of TiC–TiB–TiB2 composite powders by solid reaction of powders

In the present work, TiC–TiB–TiB₂ diffusion-layer-coated B₄C composite powders were synthesised via a powder immersion method using Ti and B₄C powders as reactants. The phase compositions and microstructure of the treated powders were characterised by employing X-ray diffraction and scanning electron microscopy. No significant reaction between B₄C and Ti could be detected at 800°C. After treatment at 900°C, the products generated were composed of TiC and TiB. After treatment at 1000°C, the products generated were primarily composed of TiC and TiB, with a small amount of TiB₂. The composition and proportions of the produced phases varied with process temperatures and the composition of the initial powders used. Powder mixtures with a Ti/B₄C molar ratio of 3.5:1 and treated at 1000°C for 14?h were more suitable for synthesis of TiC–TiB–TiB₂-coated B₄C composite powders.     

Mesure du coefficient de diffusion chimique dans le protoxyde de cobalt

Abstract

The chemical diffusion coefficient for cobalt protoxide at 1000°C has been measured by means of the concentration gradient relaxation method. The already predicted vacancy mobility variation is experimentally confirmed. Such a variation may be explained by a modified configuration for the V_M^′ defect within its existence range or, at least, by a modified activation energy for the diffusion jump process. The effective valency measured for the diffusing ion is close to two while, in this case, it should be equal to one according to Wagner's theory. The electrical measurements made close to an oxygen partial pressure of one atmosphere do not reveal the existence of the neutral vacancies V_M^x

Résumé

Le coefficient de diffusion chimique dans le protoxyde de cobalt à 1000°C a été mesuré par la méthode de relaxation du gradient de concentration. La variation de la mobilité des lacunes, prédite par ailleurs, se trouve confirmée expérimentalement et pourraitétre expliquée par une modification de la configuration du défaut V_M^′ aacute; l'intérieur de son domaine d'existence ou, du moins, par une modification de l'énergie d'activation des sauts de diffusion. La valence effective mesurée pour l'ion qui diffuse, est trés voisine de deux tandis que la theorie de Wagner praecute;voit une valence égale a un, dans ce cas. Les mesures de conductibilité electrique au voisinage d'une pression d'oxygene égale à un atm, ne confirment pas l'apparition de lacunes neutres V_M^x.     

Electrochemical Transport in Molten Lead Silicates

Abstract

Ionic transport in the molten system PbO-SiO₂ is discussed based upon available data on lead and silicon tracer diffusivities, inter diffusion coefficients, electrical conductivity and on measured values of lead and silicon transference numbers. The interrelationship of these parameters is illustrated through a theoretical treatment of the phenomenological transport equations. The Onsager transport coefficients for the independent set of equations are calculated for melt compositions between 0.3 < x _SiO2 < 0.6 at 850°C. The coincidence of large changes in the transport parameters between the orthosilicate and metasilicate compositions and the large changes in the silicate structure is discussed.

Résumé

Les phénomènes de transport ionique dans le systeme PbO-SiO₂ à l'état fondu sont analysés et discutés a partir des valeurs déjà publiées concernant la diffusion des isotopes traceurs de Pb et Si, les coefficients d'interdiffusion de PbO et SiO₂, les conductivityés ioniques et les mesures des nombres de transport pour ces éléments. Un traitement théorique des équations phénoménologiques de transport illustre les différentes relations entre ces paramètres. Les coefficients de transport d'Onsager sont calculés, à l'aide d'un système indépendant d'équations, pour les compositions comprises entre 30 et 60 % de SiO₂ en mole, a 850°C. La possibilité de coincidence des fortes variations des propriétés de transport avec les changements de structure ionique des bains silicates est discutée, pour des compositions correspondant à des structures d'ortho- ou de méta-silicates.     

Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis and improvement of the hardness and wear resistance of Ti-6Al-4V surface-alloyed materials fabricated by a high-energy electron beam. The mixtures of TiC, TiN, or TiC + TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then the electron beam was irradiated on these mixtures. In the specimens processed with a flux addition, the surface-alloyed layers of 1 mm in thickness were homogeneously formed without defects and contained a large amount (over 30 vol pct) of precipitates such as TiC, TiN, (Ti _x Al_1−x )N, and Ti(C _x N_1−x ) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification, including the formation of hard precipitates and hardened matrices in the surface-alloyed layers, improved the hardness and wear resistance. Particularly in the surface-alloyed material fabricated by the deposition of TiN powders, the wear resistance was greatly enhanced to a level 10 times higher than that of the Ti alloy substrate. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-based surface-alloyed materials with improved hardness and wear resistance.