摘要:

AbstractOxidation studies on intermetallic compounds carried out in Japan in the last ten years have been summarized and critically reviewed. The major effort was expended to improve the oxidation resistance of TiAl‐base materials. The methods tried include alloying additions, surface treatments and combinations of the both. Alloying additions to TiAl were more closely reviewed here. An excellent oxidation resistance was obtained owing to the formation of Al2O3scales by alloying additions of 0.2Zr, 0.2Hf or 500 ppm Cl, and by Nb‐ion implantation. Preoxidation under reduced oxygen pressures also resulted in the formation of protective Al2O3scales. Ceramic coatings and MCrAlY type coatings showed some improvement in the oxidation resistance. Studies on other aluminides of current interest arc relatively few. On the other hand, studies on suicides arc becoming more active than before, though still few in comparison with TiAl‐base materials. In the concluding remarks directions for future studies are

ISSN:0947-5117    

DOI:10.1002/maco.19970480102

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

ISSN:0947-5117    

DOI:10.1002/maco.19970480103

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe oxidation behaviour of titanium aluminides containing 36 wt.‐% Al (Ti36Al) and 35 wt.‐% Al plus 5 wt.‐% Nb (Ti35Al5Nb) has been investigated by electron microscopic methods with emphasis on transmission electron microscopy (TEM). The oxidation experiments were carried out at 800 to 1000°C in laboratory air for 0.5 h to 4 h. In addition thermogravimetric measurements were made. It has been shown that the shortterm oxidation of TiAl can be divided into two stages. In stage I the preferred formation of aluminium oxide leads to an aluminium depletion of the metal subsurface zone and the subsequent formation of titanium nitrides which enhances the oxidation rate. After consumption of the depletion layer a repeated cycle of aluminium oxide formation, subsequent local depletion of the metal subsurface zone in Al and consumption of the Ti‐rich metal phase by nitride formation is observed leading to linear oxidation behaviour (stage II). In the niobium containing alloy the dissolution of alumina in titania is decreased and thus the formation of aluminium oxide at the metal/oxide interface is favoured. By electron diffraction it has been found that the aluminium oxide formed at the metal/oxide interface most probably is an aluminium oxynitride Al27O39N. The aluminium depleted metal phase has been analyzed to consist of α2‐Ti3Al and a new cubic phase with a composition between of α2‐Ti

ISSN:0947-5117    

DOI:10.1002/maco.19970480104

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe microstructure and development of the scale of oxidized γ‐TiAl were studied in cross section by electron microscopy and metallographic techniques. Ti‐50(at.%)Al samples were oxidized at 900°C in air for various times and especially the evolution of the scale/metal interface was investigated. The formation of the aluminium deplete d subsurface layer could be observed after 1 h and develops from a single phase to a two phase region with longer exposure times The single phase region is visible up to 100 h and consists of a simple cubic phase with lattice parameter a = 0.692 nm. After an oxidation time of 140 h the formation of a two phase region consisting of and the cubic phase occurred. Both phases are depleted in aluminium compared to the base metal and contain some amount of oxygen whereby the cubic phase contains more Al and O than α2‐Ti3Al. The metallographic cross sections suggest that the α2‐Ti3Al phase forms at the ‐γ‐TiAl/cubic‐phase interface and grows in

ISSN:0947-5117    

DOI:10.1002/maco.19970480105

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe isothermal oxidation of binary and ternary, Nh‐containing titanium aluminides was studied at 900°C in air and Ar/O2. It was found that the presence of nitrogen in the atmosphere is in some cases beneficial, in others detrimental. For an interpretation of the effects of nitrogen it is necessary to consider the various oxidation stages in which the mechanisms of titanium aluminide oxidation fundamentally differ. For High‐Al alloys, which tend to form protective alumina based scales in the early oxidation stages, nitrogen always scenes to he detrimental because it promotes the formation of a more rapidly growing mixed alumina/titania scale. After this shift to mixed oxide scales, nitrogen can be detrimental as well as beneficial. It is proposed that the last mentioned case prevails if a near‐continuous Ti‐rich nitride layer is stable at the oxide/alloy interface, thereby preventing internal oxidation of aluminium. This beneficial effect, of nitrogen was found under all studied conditions for the Nb‐containing alloys but only in some cases for the bin

ISSN:0947-5117    

DOI:10.1002/maco.19970480106

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe influence of water vapour on the oxidation of Ti‐50 at.% Al was studied at 900°C. Thick, well‐adherent oxide scales were formed consisting of an outer TiO2layer and an inner heterogeneous mixture of TiO2and Al2O3. The interface between these layers is marked by large pores and Al2O3particles embedded in TiO2. No compact Al2O3barrier layer was observed. The oxidation leads to formation of a two phase, oxygen enriched subsurface zone, which is Al‐depleted by inner oxidation of Al to Al2O3.The oxidation kinetics were followed by continuous thermogravimetry. Exposure in moist oxygen leads to an accelerated attack compared with oxidation in dry oxygen. In moist oxygen the rate is linear after a short transition period. The oxidation rate is influenced by water partial pressure and oxygen partial pr

ISSN:0947-5117    

DOI:10.1002/maco.19970480107

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractTiAl‐based Intermetallic alloys are being considered as structural materials for high‐temperature applications due to their low density and substantial mechanical strength at high temperatures. The effect of various elements added by ion implantation on the oxidation behaviour of near‐γ Ti‐48Al‐2Cr and Ti‐48Al‐2Cr‐2Nb (compositions are given in at‐%) in air at 800°C has been studied. Ions implanted (1017Ions/Cm2) were Al, Ti, Cr, Mo, Y, Mn, Pt, Nb, and Si in case of Ti‐48Al‐2Cr and Nb in case of Ti‐48Al‐2Cr‐2Nb. Comparison was made with the oxidation behaviour of Ti‐48Al2Cr‐2Nb and Ti‐47Al‐2Cr‐0.2Si alloys in which quaternary elements were added by alloying. It is Concluded that ion implantation can serve as a research tool to study, in the frame of screening tests, the influence of various elements

ISSN:0947-5117    

DOI:10.1002/maco.19970480108

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe effectiveness of Fe+15%Cr+5%Al+0.3%Y coatings. produced by sputter ion plating, in improving the oxidation behaviour of γ‐2 Ti3Al at 800°C and γ‐TiAl at 900° and 1000°C. during cyclic exposures in air of up to 1000 hours duration, has been studied. The 45‐135 μm coatings were tested in both the as‐coated condition and following densification by peening. The kinetics of attack and of spallation were followed gravimetrically, while the reaction product compositions were examined by a range of surface analytical techniques.Oxidation protection was afforded by the formation on the coating surface of a γ‐Al2O3oxide scale but chemical reactions at the coating‐substrate interface with accompanying voidage development, eventually caused the coating to fail mechanically. To surmount this problem the potential of several ceramic interlayers produced by the same coating route, and interposed as diffusion barriers between the FeCrAlY overlayers and the Ti3Al substrate was examined in 1000 hour oxidat

ISSN:0947-5117    

DOI:10.1002/maco.19970480109

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThere is considerable interest in the use of γ‐TiAl within advanced gas turbines where they offer greater high temperature capability over conventional titanium at reduced weight. These factors would enable engines with greater thrust to weight ratio to be developed. Thus γ‐TiAl offers the potential of replacing nickel based alloys within the high pressure compressor and potentially the fourth stage turbine.Service in both of these locations would require that the γ‐TiAl be resistant, not only to oxidation, but hot salt corrosion. This paper presents a study of the hot salt corrosion resistance of γ‐TiAl over the temperature range 500–700°C.At 700°C, laboratory tests have shown that corrosion rates in a salt ladened environment are some 20X that of equivalent oxidation for exposures out to 100 h. The morphology of attack is consistent with a corrosion mechanism involving the vapour phase transport of aluminium from within the alloy and formation of a non protective oxide scale. A model for the corrosion mechanisms is presented, involving intermediate chloride phases. The mechanism is believed to be self sustaining, requires little chloride present, and leads to the observed accelerated

ISSN:0947-5117    

DOI:10.1002/maco.19970480110

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

ISSN:0947-5117    

DOI:10.1002/maco.19970480111

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY