Microstructures and Metallographic Characterization of Superalloys

International audienceSuperalloys are metallic alloys with specific microstructures and atoms in solid solution allowing good mechanical and chemical properties at high temperature, typically higher than 1000°C. They present many different types of {matrix-precipitates} combinations which can be characterized at each step of their elaboration, by specifying the chemical compositions and the crystalline network of the phases present. After test on a sample (laboratory scale), or after the deterioration in service of a component leading to its replacement by a new one (industrial scale), the superalloys constituting these pieces are often usefully examined. This aims to specify the changes occurred in their microstructures as well as in their surface or sub-surface, due to the endured thermal, mechanical and chemical solicitations. These characterizations can be simply done using basic low-magnification optical microscopic examinations. Other means such as electronic microscopy and other advanced apparatus may be usefully involved for further investigations. After a brief history about superalloys, an overview of the phenomenological aspects of their mechanical and chemical solicitations during their use at high temperature, the metallographic techniques which are usually employed to characterize them are presented and illustrated in the case of a chosen family of superalloys

Thermogravimetric Study of Oxide Spallation for Chromium-Rich Cast Cobalt-Based and Iron-Based Alloys Oxidized at High Temperature

International audienceDuring temperature cycles, metallic alloys for high temperature applications are usually oxidized with formation of an external protective oxide scale, but they loose it during the cooling. This problem of oxide spallation can be studied by specific tests of cyclic oxidation but first indications can be provided by analyzing the cooling parts of thermo-gravimetry curves. This possibility was studied in this work for simple {Co or Fe}-30 wt.% Cr alloys containing between 0 and 0.8 wt.% of carbon, and for two of these alloys after addition of about 5 wt.% of tantalum. These alloys, elaborated by foundry process, were exposed to oxidation at 1000, 1100 or 1200°C for 50 hours, with recording of their mass gain during heating, isothermal dwell and cooling. The analysis of the cooling part of the thermogravimetry files allowed specifying several data characterizing the spallation of the external oxide, such as the temperature at which the phenomenon begins and the rate with which the mass decreases because of the loss of oxide. Metallographic data about oxidation and spallation were obtained by examination of cross-sections of the oxidized samples. Oxide spallation, characterized by its beginning's temperature and its corresponding mass loss rate, was obviously more severe for the cobalt alloys than for the iron ones, for the carbon-free alloys than for carbides-containing ones and for tantalum-containing alloys than for the tantalum-free ones. Most of these effects were explained

High temperature properties of several chromium-containing Co-based alloys reinforced by different types of MC carbides (M=Ta, Nb, Hf and/or Zr)

International audienceFive cast cobalt alloys based on Co-8Ni-30Cr-0.4/0.45C and containing Ta, Nb, Hf and/or Zr were studied by metallography in the as-cast condition and after treatments at 1300°C. The obtained MC carbides were all interdendritic with a eutectic script-like morphology. For similar carbon contents, the HfC carbides are the most developed in the as-cast microstructure and the most stable at 1300°C. As-cast, the TaC carbides are less developed than the former and they tend to become more fractioned and less present in microstructure at 1300°C. The NbC carbides, which have initially the same morphology and the same fraction as TaC, rapidly dissolve at 1300°C. The cobalt alloys containing HfC or TaC are chromia-forming at 1300°C. The NbC-containing alloy catastrophically oxides after only few hours at 1300°C. The average hardness is the highest for the HfC-containing alloy and the lowest for the NbC-containing alloy

Oxidation Start Detection on Heating Parts of Thermogravimetry Curves for High Temperature Alloys Based on Nickel, Cobalt or Iron

International audienceThe heating parts of thermogravimetry curves performed for cast alloys in air at high temperatures were analysed, after correction from the effects of air buoyancy variation, to characterize the transient oxidation before reaching the targeted temperature for the isothermal stage usually applied thereafter. This was done here for three binary M-30Cr alloys, six ternary M-30Cr-0.4 and 0.8%C alloys, and three M-30Cr-0.4C-6Ta quaternary alloys, heated in synthetic air at 20 K min-1 until 1000, 1100 or 1200°C. The cobalt alloys begin to be oxidized with a sufficient mass gain to be detected by thermogravimetry, earlier than the nickel or iron alloys (i.e. at lower temperatures). The temperature of oxidation start is lowered by the presence of tantalum for the three families of alloys. When the carbon content (or the carbides density) increases in the alloy, the temperature of oxidation start decreases for the cobalt alloys and remains almost constant for the nickel or iron alloys, while the total mass gain due to oxidation during heating increases for the nickel and cobalt alloys and decreases for the iron alloys

As-Cast Microstructures of High Entropy Alloys Designed to Be TaC-Strengthened

In this work two new alloys were obtained by extrapolation from a well known high entropy alloy, the equimolar CoNiFeMnCr one. This was done by the addition of carbon and of tantalum, Ta being one of the strongest MC-former elements. They were produced by conventional casting under inert atmosphere. The obtained microstructures were characterized by X-ray diffraction, metallography, electron microscopy, and energy dispersion spectrometry. Their hardness was also measured by hardness indentation. In parallel, the original CoNiFeMnCr alloy was also synthesized and characterized for comparison. The reference HEA alloy is single-phased with an austenitic structure, while the two {Ta, C}-added alloys are double-phased, with an austenitic matrix and interdendritic script-like TaC carbides. The matrixes of these HEA/TaC alloy are equivalent toan equimolar CoNiFeMnCr alloy to which 2 wt.% Ta is present in solid solution. The presence of the TaC carbides caused a significant increase in hardness which suggests that the HEA/TaC alloys may be mechanically stronger than the HEA reference alloy at high temperature

Thermal and mechanical properties at high temperature of Co-based Superalloys strengthened by MC carbides with M=Ta or Nb

Polycrystalline Co-based superalloys strengthened by carbides can be used at high temperature in several domains (aeronautics, energy production, hot industrial processes…). Versions involving MC carbides often display particularly good mechanical properties at more than 1100°C. Most of them contain tantalum carbides but strengthening may be also achieved by other MC carbides in such alloy base. In this work, the effects of the presence of niobium carbides, and of their morphological modifications after aging, on the high temperature properties of cobalt-based superalloys were investigated. Comparison will be done with more classical TaC-strengthened Co-based superalloys. Alloys containing 25wt.%Cr, 0.5wt.%C and 7.5 wt.%Ta or 3.9 wt.% Nb were elaborated by casting. Samples were kept as-cast and others were heat-treated at 1200°C during 100h to simulate the aging of these alloys in service. In their two states the superalloys were first subjected to electron microscope observation for characterizing their microstructures, and to differential thermal analysis (DTA) for specifying their melting ranges. As-cast and aged samples were tested in creep resistance at 1100°C, 1150°C and 1200°C under 20MPa. Please click Additional Files below to see the full abstract

High temperature behaviour of chromium-nickel alloys with Ni varying from 50 to 0 Wt.%

Chromium is widely used in superalloys, as alloying element able to improve their high temperature mechanical behavior by forming strengthening carbides and to enhance their resistance against oxidation by gases and corrosion by aggressive molten substances. Thanks to its particularly high melting point, chromium can also be considered as base element for a next generation of high temperature alloys, and lead to Cr-based alloys able to be competitors for the best cobalt-based and nickel-based superalloys for particularly elevated temperatures. Unfortunately, despite its high melting point Cr is mechanically rather weak at high temperature and alloying is necessary to improve its mechanical properties as well as its tendency to brittleness at room temperature. In this work nickel was chosen as primary alloying element for chromium to try reinforcing it at high temperature. A series of Cr-xNi (x=50wt.% to) alloys were synthesized by foundry and exposed to 1200°C in air. Their as-cast microstructures (SEM/BSE) and room temperature hardness (Vickers indentation) were characterized and interpreted by regards to the Ni content. The evolution of their microstructures at high temperature as well as their oxidized surface states (SEM/SE and BSE, XRD), were also specified. The as-cast microstructures were double-phased for most alloys and the hardness evolution versus the Ni content followed a bell-shape. The microstructures significantly evolved during the high temperature stage while oxidation induced microstructure change in the sub-surfaces. Chormia formed exclusively on surface but was lost by spallation during cooling in most cases. Nitridation also occurred for some of the alloys. Please click Additional Files below to see the full abstract

Oxide Spallation During Post-isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys Highly Alloyed with Hf

International audienceCobalt, nickel and iron-based alloys containing 25wt.%Cr and strengthened either by chromium carbides or by HfC were oxidized at 1000, 1100 and 1200°C to obtain external oxide scales. The spallation of these oxides during the post-oxidation cooling was studied by exploiting the cooling part of the thermogravimetry curves. The best resistant alloys against scale spallation were the iron-based alloys which did not lose oxide. The nickel-based alloys well behaved too. The worst alloys were the cobalt alloys, principally due to the complex composition of the oxide scales. Beside the base element, the thickness of the oxide was also identified as a major parameter. The presence of Hf with contents much higher than usual was also beneficial for the resistance against spallation

Electrochemical Behavior of Alloys for Fixed Partial Denture

International audienceThe electrochemical behaviors of eight parent alloys and four post-solder alloys used in dental prostheses were specified in a (pH=7.4; [NaCl]=9g/L)-aqueous solution at 37°C. For each alloy, the free potential was followed during two hours, and a cyclic polarization between the cathodic domain and the solvent's oxidation was performed. The gal-vanic corrosion between each parent alloy and its usual post-solder was also studied. The free potentials stay in the immunity domain of the alloy's base-elements (precious metals) or in their passivation domain (nickel, chromium). A relatively strong oxidation of the alloys is possible only for high values of applied potential. Corrosion of the noblest alloys is almost inexistent. It occurs for the nickel-base alloys but it is very slow. Most of the elements present can be oxidized, but only for high potentials which cannot be really achieved by dissolved oxygen. Galvanic corrosion can occur between the parent alloy and the post-solder alloy in some cases

OXYDATION A HAUTE TEMPERATURE D’ALLIAGES BASE COBALT DE FONDERIE RENFORCES PAR DIFFERENTS TYPES DE CARBURES MC (M = Ta, Nb, Hf ou Zr)

International audienceThe aim of this work is to study if NbC, HfC and ZrC carbides can be obtained in a cobalt-based alloy, and what can be the high temperature behaviours of alloys containing such MC carbides in both stability and oxidation. Several alloys, all based on Co-8Ni-30Cr-0,2 or 0,4C (weight contents), and containing quantities of Nb, Hf or Zr which allow obtaining the same carbides densities, were synthesized by high frequency induction melting, and compared to a similar TaC-containing alloy. The as-cast alloys display NbC, HfC or ZrC carbides forming an interdendritic eutectic with matrix. After several tens hours at 1200°C, all carbides are affected by a decrease in volume fraction and by a fragmentation phenomenon. This effect does not influence significantly the room temperature hardness of the alloys. Oxidation of the (Nb, Hf or Zr)C-containing alloys at 1200°C is much faster than for a TaC-containing alloy. After thermogravimetry runs, external oxides formed on the (Nb, Hf or Zr)C-containing samples are mainly spinels while a TaC-containing alloy is still covered by chromia.La morphologie des carbures NbC, HfC et ZrC dans des alliages de fonderie à base de cobalt, ainsi que leur stabilité et leur comportement en oxydation à haute température ont été étudiés. Des alliages de type Co-8Ni-30Cr-0,2 ou 0,4C (%massique) additionnés de niobium, d’hafnium ou de zirconium, avec des teneurs permettant d’obtenir exclusivement des carbures MC de densités similaires, ont été élaborés par fonderie par induction à haute fréquence et comparés à un alliage classique contenant des carbures TaC de même densité. A l’état brut de coulée, les carbures NbC, HfC et ZrC ont la même morphologie que les TaC et ils forment aussi un eutectique interdendritique avec la matrice. De façon similaire aux carbures TaC, lors d’une exposition prolongée à 1200°C leur fraction volumique tend à diminuer et leur forme devient fragmentée, mais sans diminution systématique de la dureté des alliages. En oxydation à 1200°C, les alliages contenant les carbures NbC, HfC ou ZrC présentent des prises de masse beaucoup plus rapides que l’alliage équivalent avec carbures TaC, et en fin d’essai, leur surface est majoritairement recouverte d’oxydes de type spinelle au lieu de la chromine dans le cas de l’alliage avec carbures TaC