Showing papers in "Materials Science Forum in 2001"
TL;DR: WinPLOTR is a graphic program for the analysis of powder diffraction patterns that has been developed for a Windows 9x/2k/NT environment and can be used as a Graphic User Interface for programs defined by the user.
Abstract: WinPLOTR is a graphic program for the analysis of powder diffraction patterns. It has been developed for a Windows 9x/2k/NT environment. It takes advantage of this graphical environment to offer a powerful and user-friendly powder diffraction tool. The program is able to display and analyse many different kinds of diffraction patterns as well as calculated and observed profiles coming from the Windows/DOS version of the program FullProf. It can also be used as a Graphic User Interface (GUI) for programs defined by the user.
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TL;DR: In this paper, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.
Abstract: Friction stir processing is a new thermo-mechanical processing technique that leads to a microstructure amenable for high strain rate superplasticity in commercial aluminum alloys. Friction stirring produces a combination of very fine grain size and high grain boundary misorientation angles. Preliminary results on a 7075 Al demonstrate high strain rate superplasticity in the temperature range of 430-510 °C. For example, an elongation of >1000 % was observed at 490 °C and 1 × 10 -2 s -1 . This demonstrates a new possibility to economically obtain a superplastic microstructure in commercial aluminum alloys. Based on these results, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.
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TL;DR: In this article, the thermal oxidation of metals by water vapour and by oxygen is compared and several ideas are advanced to explain the differences often observed, including the incorporation of hydroxyl ions in the oxide, but possible only if vacancy or interstitial oxide defects are present.
Abstract: The thermal oxidation of metals by water vapour and by oxygen is compared and several ideas are advanced to explain the differences often observed. The incorporation of hydroxyl ions in the oxide is shown to be of importance, but possible only if vacancy or interstitial oxide defects are present. The decomposition rate of H 2 O is another key-point and is shown to be correlated with the surface acidity of the oxide. Non-acidic p-type oxides could be very passive in water vapour whereas more acidic n-type oxides may oxidize at the same rate in both atmospheres or even more rapidly provided that rapid OH transport can take place within the formed scales.
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TL;DR: In this paper, the effect of gas velocity on the oxidation of three chromia-forming steels; X20 (11%Cr), 304L (18%Cr) and 310 (25%) in O 2 +40% H 2 O at 600°C is reported.
Abstract: The effect of gas velocity on the oxidation of three chromia-forming steels; X20 (11%Cr), 304L (18%Cr) and 310 (25%Cr) in O 2 +40% H 2 O at 600°C is reported. The samples acre investigated by a number of surface analytical techniques including grazing angle XRD, SEM/EDX, and SAM. At low gas velocities a protective chromium-rich oxide forms on X2() and 310 while 304L shows partly protective behavior. With increasing gas velocity, the oxide tends to becomes more iron-rich and, consequently, less protective. The influence of gas velocity in O 2 /H 2 O environment is explained by the loss of chromium from the oxide through evaporation. The volatile species is suggested to be CrO 2 (OH) 2 . The different behavior shown by the three materials is explained in terms of the supply of chromium from the substrate to the oxide. Chromium depletion results in an increase in corrosion rate with gas velocity for all three materials and to breakaway corrosion in the case of X20.
57 citations
TL;DR: In this paper, the influence of water vapor on the high temperature oxidation of several stabilized ferritic stainless steels with various Si and Cr contents has been studied by thermogravimetry at 900°C in pure water vapor or dry oxygen, followed by the characterization of the oxide scales formed.
Abstract: The influence of water vapor on the high temperature oxidation of several stabilized ferritic stainless steels with various Si and Cr contents has been studied by thermogravimetry at 900°C in pure water vapor or dry oxygen, followed by the characterization of the oxide scales formed. Oxidation of Fe-Cr steels in pure water vapor was characterized by a non-protective breakaway oxidation due to the formation of iron oxide nodules on the sample surface. Decreasing the H 2 O partial pressure or increasing the Si or Cr content enhanced the oxidation resistance of the alloy by delaying the breakaway appearance. This abnormal oxidation is thought to be initiated by the arrival of oxygen- and hydrogen-containing species on the chromium depleted metal/oxide interface. The mechanism of the effect of water vapor is discussed and some assumptions proposed.
TL;DR: In this article, the critical Y/S ratio required for adhesion was estimated to be on the order of 3-to-1 by weight (1to1 atomic), in agreement with values estimated from solubility products for yttrium sulfides.
Abstract: To improve scale adhesion, single crystal superalloys have been desulfurized to levels below 1 ppmw by hydrogen annealing. A transition to fully adherent behavior has been shown to occur at a sulfur level of about 0.2 ppmw, as demonstrated for PWA 1480, PWA 1484, and Rene N5 single crystal superalloys in 1100-1150 C cyclic oxidation tests up to 2000 h. Small additions of yttrium (15 ppmw) also have been effective in producing adhesion for sulfur contents of about 5 ppmw. Thus the critical Y/S ratio required for adhesion was on the order of 3-to-1 by weight (1-to-1 atomic), in agreement with values estimated from solubility products for yttrium sulfides. While hydrogen annealing greatly improved an undoped alloy, yielding <= 0.01 ppmw S, it also produced benefits for Y-doped alloys without measurably reducing the sulfur content.
TL;DR: In this article, the size and distribution of the oxide dispersions are of equal importance for obtaining optimum oxidation properties as the actual type and/or exact amount (0.1-1 wt.) of dispersion.
Abstract: In alumina and especially chromia forming alloys containing RE's in form of oxide dispersions the size and distribution of the RE is of equal importance for obtaining optimum oxidation properties as the actual type (Y 2 O 3 , La 2 O 3 , CeO 2 ) and/or exact amount (0.1-1 wt.-%) of the dispersion. In conventional cast or wrought alloys the effective distribution of the RE will strongly depend on the form in which it prevails in the alloy, e.g. dissolved or in form of an intermetallic compound. The mechanism of RE incorporation into the surface scale can be affected by presence of other reactive minor alloying elements, such as Ti. Common alloy impurities such as C and/or N, if present in sufficiently high amounts, lead to formation of RE-carbo/nitride precipitates which become embedded in the growing oxide scale. Oxidation of the embedded precipitates upon long term exposure leads to microcracking and subsequent increased oxidation rates.
TL;DR: Superplastic forming (SPF) is the process that can take a flat sheet of metal and with the application of heat and just 100 psi or so, shape it into a myriad of complex shapes using simple tools and equipment as discussed by the authors.
Abstract: What is the process that can take a flat sheet of metal and with the application of heat and just 100 psi or so, shape it into a myriad of complex shapes using simple tools and equipment? The answer is superplastic forming (SPF). Since the early 1970's SPF has been fulfilling its initial promise to create complex sheet metal parts for a wide range of end users; from the functional needs of the specialist automobile market to the creative world of architecture and sculpture. With all these beneficial attributes we might be prompted to ask this question, Why isn't SPF used everywhere and more often if it is that good? This paper examines current trends and the reasons for SPF's successes and what limits its wider application. Factors relating to available materials, their cost and performance, the process' techno-economics, R&D initiatives and the prospects of breakthrough developments are discussed.
TL;DR: In this article, a multi-year effort has examined various reactive element additions and shown that optimized Hf doping (0.05 at.% Hf) provides the best observed oxidation performance at 1100-1200°C.
Abstract: Iron aluminides (Fe 3 Al-based) are attractive corrosion-resistant, high temperature materials due to their excellent sulfidation resistance and large Al reservoir. However, at high temperatures (>1000°C) iron aluminides show excessive scale spallation compared to doped FeCrAl. This behavior is attributed to the inherently high coefficient of thermal expansion of these materials which results in higher cooling strains in the alumina scale. Although this problem cannot be corrected by minor alloying additions or impurity control, composition optimization can still be used to maximize performance. A multi-year effort has examined various reactive element additions and shown that optimized Hf doping (0.05 at.% Hf) provides the best observed oxidation performance at 1100-1200°C. Hafnium lowers the scale growth rate by an order of magnitude compared to undoped Fe 3 Al, but does not improve the resistance to scale spallation. Possible mechanisms for the role of Hf on the scale growth rate are discussed.
TL;DR: In this article, the influence of coating microstructure on the coating thermal conductivity and strain tolerance is discussed, and it is shown that by advanced processing it is possible to deposit EB-PVD coatings with thermal conductivities of 1.0W/mK, much lower than current commercial EB-PCV coatings at 1.5-1.9 W/mk and closely matching that for plasma sprayed ceramics.
Abstract: The continued drive for increased performance and higher engine efficiencies has seen the adoption of thermal barrier coating technologies as an integral part of modern engine design. Future engines will require even better performance from such thermal barrier coating system, towards the ultimate goal of designed-in TBC's. This paper reviews the advantages and disadvantages of various TBC systems, with the aim of custom designing a thermal barrier coating system to have good strain tolerance and low thermal conductivity. The influence of coating microstructure on the coating thermal conductivity and strain tolerance is discussed, and it is shown that by advanced processing it is possible to deposit EB-PVD coatings with thermal conductivities of 1.0W/mK, much lower than current commercial EB-PVD coatings at 1.5-1.9 W/mK and closely matching that for plasma sprayed ceramics (0.8-1.1 W/mK).
TL;DR: In this article, the influence of Co-, Cr-and Al-additions on phase composition and morphology of Ni(Co)CrAlY alloys and coatings is illustrated.
Abstract: Phase composition and distribution can strongly affect the transient oxide stages as well as the internal oxide morphology of Ni(Co)CrAlY alloys and coatings. In the present paper the influence of Co-, Cr- and Al-additions on phase composition and morphology is illustrated. Co, as well as Cr, destabilises the γ'-phase and promotes the formation of the σ-phase, Dilatometry is shown as an useful technique to determine phase transformations in Ni(Co)CrAlY alloys upon temperature changes. The phase distribution determines the morphology of the oxide formed during early stages of oxidation and the NiY-phase distribution. The distribution of the NiY-rich phase determines the final morphology of the internal oxidation. A coarse distribution of the NiY-phase on Ni(Co)CrAlY grain boundaries leads to a finger type internal oxidation which was found to have a detrimental effect on oxide scale adherence.
TL;DR: In this article, two new EPR spectra with trigonal symmetry and spin S = 1/2 were observed in irradiated p-type material, which are other configurations of the vacancy-related EI1 and EI3 centers.
Abstract: Electron paramagnetic resonance (EPR) has been used for investigation of intrinsic defects in 4H and 6H SiC. At W-band frequency (similar to 95 GHz), the detailed structures of most of the EPR spectra in 4H and 6H SiC irradiated with electrons have been observed. We report our observation of two new EPR spectra, labelled EI1 ' and EI3 ', which are other configurations of the vacancy-related EI1 and EI3 centers. The transformation from the EI1 and EI3 centers to the EI1 ' and EI3 ' configurations, respectively, can be realised by annealing. Two new EPR spectra, labelled EI5 and EI6, with trigonal symmetry and spin S=1/2 were observed in irradiated p-type material. The EI5 and EI6 centers can be identified as the C vacancy and Si antisite in the positive-charge state.
TL;DR: In this paper, the influence of Y 2 O 3, Pr 2 o 3, Nd 2 O3, Sm 2 o3 and Yb o 3 coatings on Fe-30Cr alloy oxidation behavior was investigated at 1000°C in air under atmospheric pressure.
Abstract: The influence of Y 2 O 3 , Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 and Yb 2 O 3 coatings on Fe-30Cr alloy oxidation behaviour was investigated at 1000°C in air under atmospheric pressure. Isothermal exposures indicated that the Y 2 O 3 coating was the most protective after 100 hours. Pr 2 O 3 , Nd 2 O 3 and Sm 2 O 3 coatings were less effective, but the less beneficial effect was observed when Yb 2 O 3 coating was applied onto the Fe-Cr alloy surface. Two-stage oxidation experiments in 16 O 2 and then 18 O 2 were performed to get information about the chromia growth phenomena with and without reactive elements. The 18 O-tracer distribution was determined by secondary ion mass spectrometry (SIMS) and sputtered neutral mass spectrometry (SNMS). The experiments performed on uncoated samples clearly demonstrated that chromia growth mechanism was controlled by chromium cationic diffusion, whereas on reactive element coated samples the external diffusion of chromium ions was not predominant.
TL;DR: In this article, the effects of bond-coats surface modifications prior to ceramic deposition is presented. But the results of the study were limited to a single crystal Ni-based superalloy.
Abstract: A systematic study of the effects of bond-coats surface modifications prior to ceramic deposition is presented. TBC systems have been made and tested with most combinations of the following parameters: 1/substrate: IN 100 or AM3 (single crystal) Ni-based superalloys; 2/ bond-coat: VPS NiCoCrAlYTa or CoNiCrAlY, (Ni,Pd)Al, (Ni,Pt)Al; 3/ bond-coat surface finish: as deposited, machined, grit blasted, preoxidized; 4/ top coat: APS or EB-PVD Y 2 O 3 stabilized ZrO 2 . The preoxidation treatments of bond-coats were determined from the results of a study (TGA-GIXRD-SEM-SIMS) of their short-term (6 hours) isothermal oxidation behavior as a function of temperature (900 to 1100°C), oxygen partial pressure and heating rate. Under these conditions all bond-coats are alumina formers, but depending on the oxidation conditions, transition aluminas may be formed. The characterization of complete TBC systems was also performed, including isothermal oxidation (TGA), cyclic oxidation at 1100°C, cyclic corrosion at 900°C and thermal shock (fast cooling from 1200°C).
TL;DR: The phase transformations and structural changes of Al 2 O 3 scales formed on an Al-deposited Fe-19Cr-3.2Al foil (totally 4.8 %Al, Al deposited foil) during oxidation in air were studied mainly at 1373 K as mentioned in this paper.
Abstract: The phase transformations and structural changes of Al 2 O 3 scales formed on an Al-deposited Fe-19Cr-3.2Al foil (totally 4.8 %Al, Al-deposited foil) during oxidation in air were studied mainly at 1373 K. A conventional Fe-20Cr-5Al foil (alloy foil) was similarly studied for comparison. The two kinds of foils had a thickness of 50 μm and similar amounts of rare earth elements. Extensive study using X-ray diffractometry, SEM and TEM revealed that scales formed on the Al-deposited foils transform from γ- to θ- to α-Al 2 O 3 with definite stages in a TTT type diagram. As the oxidation temperature rises, each transformation completes in a shorter period. Conversely, even 10 s oxidation resulted in scales consisting mainly of α-Al 2 O 3 on the alloy foil. The Al-deposited foil oxidises faster than the alloy foil at 1373 K during the initial period of about 9 ks, after which this relationship is inverted. These features are attributable to the fact that during the initial stage scales on the Al-deposited foils consist mainly of θ-Al 2 O 3 , while those on the alloy foils are α-Al 2 O 3 , and during the later stage equiaxial and columnar grains composing scales are larger for the Al-deposited foils.