首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 156 毫秒
1.
A Ni-aluminide/Re-base diffusion-barrier-coating system and, for comparison, a sole Ni-aluminide coating were formed on the Ni–Mo based alloy Hastelloy-X, and the oxidation behavior of the coated alloys was investigated in air at 1,100 °C for up to 400 h. After 100 h of oxidation the diffusion-barrier-coating system consisted of a duplex structure comprised of an inner σ-(Re, Mo, Cr, Ni) and an outer β-NiAl layers and covered by a protective Al2O3 scale. By contrast, the Ni-aluminide coating had degraded, forming a reaction diffusion zone, γ-Ni(Cr, Mo, Fe, Al), with voids, and the external scale exfoliated extensively. After 400 h of oxidation of the diffusion-barrier-coating system, an intermediate reaction-diffusion zone was present. The Mo of the alloy substrate was enriched in the inner σ-layer, changing it from a Re-base alloy to a Mo-base alloy. The Mo-base alloy appeared to enhance both inward-Al diffusion and outward diffusion of alloying elements such as Mo, Fe, and Cr to form the intermediate reaction-diffusion zone. The outer layer consisted mainly of β-NiAl with channels of γ-Ni(Cr, Mo, Fe, Al). It was concluded that the Re-base alloy such as a σ-phase in the Ni–Cr–Re system acts as an effective diffusion-barrier layer, but that this is not the case for the Mo-base alloy layer.  相似文献   

2.
A diffusion-barrier-coating system having a duplex structure comprised of an inner Re(W)–Cr–Ni layer and an outer Ni-aluminide layer was formed on a fourth generation, single-crystal Ni-base superalloy by using electroplating of Re(Ni) and Ni(W) films, Al- and Cr- (high-Cr and low-Cr) pack cementations, and a combination of the two treatments. With the ReW-high-Cr coating, fine needle- or plate-like precipitates formed in the alloy substrate below the inner Re(W, Cr, Ni) layer, while there was little of this precipitate with the ReW-low-Cr pack-cementation coating. The inner, Re-base alloy layer in the ReW-high-Cr coating was identified to be a σ-(Re,Cr,W,Ni) phase, while the inner layer of the ReW-low-Cr was a mixture of σ-(Re,Cr,W,Ni) and δ-Re(Cr,W,Ni) phases. After heating the coated alloys at 1,150 °C for 100 h in air, the outer Al reservoir layer became β-NiAl containing (31–33)Al with α-Cr particles and fine precipitates of γ′-Ni3Al with both the ReW-high-Cr and ReW-low-Cr treatments. In the case of the ReW-high-Cr coating there were numerous light-colored, needle-like precipitates formed deep in the alloy substrate under the inner layer, while in the case of the ReW-low-Cr coating γ′ appeared near the inner layer. It was found that the inner, Re-base alloy layer acted as a diffusion barrier, and that its structure was maintained with little change in composition after 100 h of oxidation at 1,150 °C. K. Z. Thosin is from Indonesian Institute of Sciences, LIPI.  相似文献   

3.
Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni2Al3 and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram  相似文献   

4.
Matsumura  Y.  Fukumoto  M.  Hayashi  S.  Kasama  A.  Iwanaga  I.  Tanaka  R.  Narita  T. 《Oxidation of Metals》2004,61(1-2):105-124
A β-NiAl coating with or without a Re-base alloy layer was formed on a Nb–5Mo–15W alloy. The coated alloys were oxidized isothermally in air at 1373 and 1473 K. Electroplating of a high (more than 70at.%)-Re–Ni film, Cr-pack cementation, Ni plating, and then Al-pack cementation, in this sequence, formed a coating structure with Re-base alloy and β-NiAl layers. The Re-base alloy layers were comprised of an outer σ-phase in the Re–Cr(Ni) system and an inner χ-phase in the Re–Nb(Cr) system. It was found that reaction between the β-NiAl and the alloy substrate was significantly suppressed when the Re-base alloy layers were present. The Re-base σ and χ phases were found to be good candidates for a diffusion barrier against inward-Al diffusion because they have very low solubilities for Al.  相似文献   

5.
The oxidation behavior of the nickel-base single-crystal superalloy TMS-82+ coated with a duplex Re(W)–Cr–Ni/Ni(Cr)–Al layer was investigated in air at 1,150 °C for up to 100 h. The coating layer was formed by electroplating Re(Ni) and Ni(W) films on the alloy, followed by Cr-pack cementation at 1,300 °C, and as a result, forming a continuous Re(W)–Cr–Ni diffusion-barrier layer. A Ni film containing fine Zr particles was then electroplated on the duplex layer, followed by Al pack cementation at 1,000 °C for 1 and 5 h to form an Al reservoir layer with a duplex Ni2Al3/γ-Ni layer, which changed quickly to γ-Ni phase containing (10∼13)at.% Al for the 1 h Al-pack coat and a mixture of γ′-Ni3Al and β-NiAl phases for the 5 h Al-pack coat during high-temperature oxidation. A protective α-Al2O3 scale formed during oxidation at 1,150 °C in air, and parabolic rate constants of 7.4 × 10−11 and 6.6 × 10−10 kg2 m−4 s−1 were obtained for the 1 h- and 5 h-Al pack-coatings, respectively. There was little change in the structures of the superalloy substrate after oxidation at 1,150 °C in air for up to 100 h. It was found that the Re(W)–Cr–Ni layer remained stable, acting as a diffusion barrier between the alloy substrate and Al reservoir layers.  相似文献   

6.
The oxidation of three Ni–xCr–10Al alloys with a constant Al content of 10 at% and containing 3, 5, and 10 at% Cr was investigated at 900–1000°C in 1 atm of pure oxygen and compared to the behavior of Ni–10Al. At both temperatures, an external NiO scale overlying a zone of internal-oxide precipitates formed on Ni–10Al and Ni–3Cr–10Al: in addition, a discontinuous Al2O3 layer formed at the front of the internal oxidation for Ni–3Cr–10Al. An exclusive external scale of Al2O3 formed at most places on Ni–5Cr–10Al at 900°C, while, at some sites, the same alloy formed an outer NiO layer overlying an internal oxidation zone. The scales formed on Ni–5Cr–10Al at 1000°C were complex, but eventually a protective Al2O3 layer developed either at the alloy surface or beneath a region containing a mixture of different oxides. Finally, an exclusive external Al2O3 layer formed on Ni–10Cr–10Al at both temperatures. Thus, the addition of sufficient chromium to Ni–10Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum under a constant Al content. A possible mechanism for the effect of chromium on the oxidation of Ni–10Al is discussed on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most-reactive component in binary alloys.  相似文献   

7.
《Intermetallics》2007,15(10):1285-1290
The phase transformation of alumina formed during oxidation of β-NiAl coating prepared on M38G alloy by pack cementation was investigated. Oxidation experiments were conducted at 950 °C for various times from 2 to 180 min. The phase composition and microstructure of the oxide scales were investigated by using glancing angle XRD, AFM and SEM. The results showed that at the initial oxidation stage needle-like θ-Al2O3 was formed and then it covered the sample surface rapidly. The formation of α-Al2O3 grains beneath the θ-Al2O3 layer was favored by depletion of Al in the β-NiAl coating during oxidation. α-Al2O3 preferred growing on the top (ridge) of β-NiAl grains, which resulted in the formation of net-like α-Al2O3 inner layer. With increasing time, θ-Al2O3 transformed to α-Al2O3 gradually. After 180 min oxidation, most of θ-Al2O3 grains transformed into α-Al2O3. A mechanism of excessive voids' formation at the oxide/coating interface was also proposed in this paper.  相似文献   

8.
γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al2O3 scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr2O3 layer, and an α-Al2O3 layer at the oxide/alloy interface. The evolution of such an NiO/Cr2O3/Al2O3 layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr2O3 outer layer and a discontinuous inner layer of Al2O3 initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al2O3, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr2O3 and Al2O3 to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al2O3 layer.  相似文献   

9.
《Intermetallics》2007,15(8):989-998
The early stages of Ti–46.5Al–5Nb (at%) oxidation at 900 °C have been investigated combined with TEM and STEM. The results reveal that a layer composed of polycrystalline TiO2 and amorphous Al2O3 phase formed firstly after 5-min oxidation. The base alloy connected with the oxide scale has some deformation compared with the inner full lamellar TiAl structure. After 30-min oxidation, the phases of γ-Al2O3, κ-Al2O3, titanium nitrides and Ti5Al3O2 were formed in the area from the nitride layer to base alloy. After 50 h of oxidation, Ti5Al3O2 vanishes at the interface of oxide scale/base alloy in Ti–46.5Al–5Nb, contrary to the continuous formation of Ti5Al3O2 in γ-TiAl at the interface of oxide scale/base alloy, Al3Nb phase formed in this zone, which hinders the continuous formation of Ti5Al3O2.  相似文献   

10.
The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O2. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al2O3 layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al2O3 layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al2O3–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al2O3 and Cr2O3.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号