Ni Aluminide Layer Research Articles

Formation of Ni Aluminide Coating Containing Reactive Elements by Simultaneous Electrodeposition and Cyclic Oxidation Resistance

A Ni aluminide layer containing Zr or Hf was formed on a Ni specimen by the simultaneous electrodeposition of Al and Zr or Al and Hf using a molten-salt bath. When the simultaneous electrodeposition of Al and Zr was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%ZrF4, the electrodeposited layers were formed in the order of Ni2Al3, NiAl3 and Al from the Ni substrate side. The ZrAl3 particles were uniformly formed in the surface region of the NiAl3 and Al layers. On the other hand, when the simultaneous electrodeposition of Al and Hf was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%HfF4, the electrodeposited layer consisted of Ni2Al3 as the inner layer and NiAl3 as the outer layer were formed with HfAl3 particles uniformly formed in the surface region of the NiAl3 layer. For the sample treated with the simultaneous electrodeposition of Al and Zr, no significant change in the mass gain was observed during the cyclic-oxidation test at 1423 K, suggesting that the sample had a high cyclic-oxidation resistance. Similarly, the sample treated by the simultaneous electrodeposition of Al and Hf had a high cyclic-oxidation resistance. An adhesive scale, having localized inward penetrations consisting of Al2O3 containing ZrO2 or HfO2, was formed on the samples having the high cyclic-oxidation resistance.

Formation of Ni Aluminide Containing Hf by Simultaneous Electrodeposition of Al and Hf and Cyclic-Oxidation Resistance

A Ni aluminide layer containing Hf was formed on a Ni specimen by the simultaneous electrodeposition of Al and Hf using a molten-salt bath. Al and Hf were only slightly electrodeposited and an electrodeposited layer was not formed when the simultaneous electrodeposition of Al and Hf was tried using molten NaCl-KCl containing 3.5 mol%AlF3 and 3.5 mol%HfF4. On the other hand, an electrodeposited layer consisting of Ni2Al3 and NiAl3 layers, for which the Al3Hf particles were formed on the surface region, was formed, when the simultaneous electrodeposition of Al and Hf was carried out using the molten NaCl-KCl containing 3.5 mol% AlF3 and 0.05 mol% HfF4. The cyclic-oxidation resistance of the Ni specimens covered by the Ni aluminide layer containing the Al3Hf particles was evaluated in air at 1423 K. The sample covered with the Ni aluminide containing the Al3Hf particles, which was formed using the melt containing 3.5 mol%AlF3 and 0.05 mol%HfF4, showed a high cyclic-oxidation resistance. On this sample after the oxidation test, an adhesive scale having a spiked shape, which consisted of Al2O3 as the outer layer and HfO2 as the inner layer, was formed.

Al と Zr の同時電析法による Zr を含む Ni アルミナイド表面層の形成とその耐サイクル酸化性

A Ni aluminide layer containing Zr was formed on a Ni specimen by a synchronous electrodeposition of Al and Zr using molten-salt bath. Al and Zr were hardly electrodeposited and an electrodeposited layer was not formed when the simultaneous electrodeposition of Al and Zr was tried using molten NaCl-KCl containing 3.5 mol%AlF3 and 3.5 mol%ZrF4. On the other hand, the electrodeposited layer consisting of Ni2Al3 layer as an inner layer and NiAl3 layer as an outer layer, in which ZrAl3 particles were formed at the surface region, was formed when the simultaneous electrodeposition of Al and Zr was carried out using the molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%ZrF4. The cyclic oxidation resistance of the Ni covered by the Ni aluminide containing Zr was evaluated in air at 1423 K. The specimen covered with the Ni aluminide containing the ZrAl3 particles, which was formed using the melt containing 3.5 mol%AlF3 and 0.05 mol%ZrF4, showed a high cyclic-oxidation resistance. On this specimen after the oxidation test, an adhesive scale having a spiked shape, which consisted of Al2O3, was formed.

Effect of the Electrodeposition Temperature on the Cyclic-Oxidation Resistance of Ni Aluminide Containing Zr Formed by Molten-Salt Electrodeposition

The effect of the Al electrodeposition temperature on the cyclic oxidation resistance of Ni aluminide containing Zr formed by molten salt electrodeposition was investigated. Zr and Al were deposited by molten salt electrolysis. For the sample treated with the Al deposition at 1073 K, a layer consisting of Ni2Al3 was uniformly formed. On the other hand, for the sample treated by Zr deposition, followed by Al deposition at 1073 K, a layer consisting of Ni2Al3 and a Ni aluminide layer containing Zr on the Ni2Al3 layer were formed. Furthermore, when the Al electrodeposition temperature was changed, the concentration of Zr in the Ni aluminide layer containing Zr changed. When the Al electrodeposition was carried out at 1153 and 1173 K, the Zr was scarcely observed in the surface region of the Ni aluminide layer. The cyclic oxidation test showed that for the sample treated with only the Al deposition and the sample treated with the Zr deposition, followed by Al deposition at 1073 K, a mass reduction due to scale exfoliation took place, whereas for the samples treated with the Zr deposition, followed by Al deposition at 1153 and 1173 K, no mass reduction was observed. For these samples, after the cyclic oxidation test, a scale consisting of � -Al2O3 adhering to the substrate was formed. Consequently, it was found that the cyclic oxidation resistance of Ni was improved by Zr deposition, followed by Al deposition at 1153 and 1173 K. [doi:10.2320/matertrans.MRA2008293]

Formation of Ni-aluminide Coating Containing Hf by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

The formation of a coating layer consisting of Ni-aluminide containing Hf on a Ni–10at.%Cr–8at.%Al alloy substrate was attempted by the electrodeposition of Hf, Ni, and Al. The cyclic-oxidation resistance for the alloy covered with this coating was then evaluated in air at 1,423 K. Ni was deposited by aqueous solution electrolysis. Hf and Al were deposited by molten-salt electrolysis. For the sample first treated with the Hf-deposition, subsequently treated with the Ni-deposition, followed by Al-deposition, a coating consisting of Ni2Al3 of about 40-μm thickness was uniformly formed on the alloy. At the center region of this coating, a Hf-concentration layer was formed. The cyclic-oxidation test showed that, for the untreated sample and the sample with only Ni and Al depositions, a mass reduction due to spallation of a scale took place during the initial oxidation period. On the contrary, for the sample with Hf, Ni, and Al depositions, a mass reduction due to spallation of a scale scarcely took place. The cross-sectional observation using SEM showed that, for the sample with Hf, Ni, and Al depositions after the oxidation test, an adhesive scale having a spiked shape was formed. This scale mainly consisted of α-Al2O3, and contained HfO2 particles. It was postulated that, for this sample, the Hf in the Hf-concentration layer diffused into the surface region of the Ni-aluminide layer, contributing to improvement in the exfoliation resistance of the scale.

電析法による Ni アルミナイド/Ni-Hf 合金コーティングとその耐サイクル酸化性

The formation of a bi-layer coating consisting of Ni aluminide/Ni-Hf alloy on a Ni substrate was attempted by the electrodeposition of Hf on the Ni substrate, followed by the electrodeposition of Ni and Al. Before the formation of this coating, it was confirmed by the diffusion couple test that the Ni-Hf alloy consisting of Ni7Hf2 acted as a diffusion barrier of aluminum. Upon forming this bi-layer coating, Hf and Al were deposited by molten salt electrolysis at 1023 K. The cyclic oxidation resistance for the Ni covered with this coating was then evaluated in air at 1373 K. The electrodeposition of Hf on the Ni substrate led to the formation of a layer consisting of the Ni-Hf alloy. The electrodeposition of Ni and Al on the Ni-Hf alloy layer led to the formation of a layer consisting of Ni2Al3 as a surface layer. The cyclic oxidation test showed that the mass gain due to oxidation for the sample covered with the bi-layer coating consisting of the Ni2Al3/Ni-Hf alloy was lower than that for the sample covered with a single-layer coating consisting of Ni2Al3. The analytical results of a cross-section of the sample covered with the bi-layer coating after the 5-cycle oxidation test showed that the chemical composition of the Ni-Hf alloy layer was uniform, and this layer consisted of Ni7Hf2. Furthermore, it was confirmed that the aluminum concentration in the Ni aluminide layer was constant at about 40 at%, and the Ni-Hf alloy layer depressed the diffusion of aluminum from the Ni aluminide layer to the Ni substrate. For the sample covered with the bi-layer coating after the 100-cycle oxidation test, the aluminum concentration in the Ni aluminide layer was constant at about 20 at%, and a protective alumina scale containing small amounts of hafnium oxide was formed. On the contrary, for the sample covered with the single-layer coating consisting of Ni2Al3, the aluminum concentration in the Ni aluminide layer was only about 10 at%, and an internal oxide was largely formed in the Ni aluminide layer.

Formation of a Rhenium-base Diffusion-barrier-coating System on Ni-base Single Crystal Superalloy and its Stability at 1,423 K

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.

電析法による Zr を含む Ni アルミナイド膜のコーティングとその耐サイクル酸化性

The formation of a coating layer consisting of Ni aluminide containing Zr on a Ni-10 at%Cr-8 at%Al alloy substrate was attempted by the electrodeposition of Zr, Ni and Al. The cyclic oxidation resistance for the alloys covered with this coating was then evaluated. Ni was deposited by aqueous solution electrolysis. Zr and Al were deposited by molten salt electrolysis. For the sample treated with the Ni deposition, followed by the Al deposition, a layer consisting of Ni2Al3 was uniformly formed. On the other hand, for the sample treated with the Zr deposition, followed by the Ni and Al depositions, a Ni-Zr alloy layer and a Ni2Al3 layer were formed in that order from the substrate alloy. The cyclic oxidation test showed that for the untreated sample and the sample only with Ni and Al depositions, a mass reduction due to exfoliation of a scale took place during the initial oxidation period. For the sample treated with the Zr deposition for 0.03 and 0.24 ks, followed by the Ni and Al depositions, a mass reduction was observed during the course of the oxidation test. However, for the sample treated with the Zr deposition for 0.18 ks, no mass reduction was observed. The result of a cross-sectional observation for this sample treated with the Zr deposition for 0.18 ks after the 5-cycle oxidation test showed that the Ni-Zr alloy layer disappeared and a wedge-shaped α-Al2O3 scale was formed on the Ni aluminide layer. Consequently, it was considered that for this sample, the Zr in the Ni-Zr alloy layer diffused into the Ni aluminide layer, contributing to an improvement in the exfoliation resistance of the Al2O3 scale.

溶融塩電析法による Zr を含む Ni アルミナイド表面層の形成とその耐サイクル酸化性

The formation of a Ni aluminide coating layer containing Zr on a Ni substrate was attempted by the electrodeposition of Zr and Al. The cyclic oxidation resistance of the Ni covered with the coating was then evaluated. Zr and Al were deposited by molten salt electrolysis. For the sample treated with the Al deposition, a layer consisting of Ni2Al3 was uniformly formed. On the other hand, for the sample treated with the Zr deposition, followed by the Al deposition, a layer consisting of Ni2Al3 and a Ni aluminide layer containing Zr on the Ni2Al3 layer were formed. Furthermore, when the Zr electrodeposition conditions were changed, the concentration of Zr in the Ni aluminide layer containing Zr, formed on the surface, changed. The cyclic oxidation test showed that for the sample with the Al deposition, the mass reduction due to scale exfoliation took place, whereas for the sample treated with a small deposit of Zr, followed by Al deposition, no mass reduction was observed. For this sample, after the cyclic oxidation test, a scale consisting of α-Al2O3 adhering to the substrate was formed. Consequently, it was found that the cyclic oxidation resistance of the Ni was improved by a small of Zr, followed by Al deposition.

Formation of Ni Aluminide Layer Containing La by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

In order to improve the cyclic-oxidation resistance of Ni and SUS304 steel, the coating of a Ni aluminide containing La on both samples were carried out by the electrodeposition of Al and La in a molten salt. The electrodepositions of Al and La were conducted using a potentiostatic polarization method at 1023 K in an equimolar NaCl-KCl melt containing 3.5 mol%AlF3, and that containing 4 mol%La2O3-24 mol%NH4Cl, or 3.5 mol%LaF3. Observation and analysis of the cross-section of the specimen after polarization showed that a deposited layer consisting of a thick inner Ni aluminide layer and a thin outer layer containing a large amount of La was formed on both samples. The cyclic-oxidation resistance of the Ni specimen covered with the deposit layer having the La layer as the outer layer was higher than those of the untreated Ni specimen and the Ni specimen coated with the deposit layer without La. The stainless steel covered with the Ni aluminide showed a high cyclic-oxidation resistance in the atmosphere containing water vapor at 1273 K in comparison with untreated stainless steel, without depending on the existence of La in the deposited layer.

Effect of Zirconium on Cyclic-Oxidation of Ni-40Cr-3Re Alloys with α-Cr / β-NiAl Coatings

A coating with duplex structure of a outer β-NiAl and an inner α-Cr layer, was formed on a Ni-40Cr-3Re (in at%) alloy with or without Zr addition, and the coated alloys were oxidized under thermal cycling in air for up to 2300ks. The coated alloys containing Zr showed a two-step parabolic oxidation, the kp1st in the early stage of oxidation was 2.6~5.4×10-11 kg2 m-4 s-1 for four alloys tested, and kp 2nd for the longer oxidation increased with increasing Zr content from 2.6~5.4×10-11 for the alloy with 0.1at%Zr to 9.6×10-11 kg2 m-4 s-1 for the alloy with 1.0at%Zr. The rapid oxidation for the alloy with 1.0at%Zr is due to the formation of ZrO2 as an internal oxide. The oxide scale in the 1st stage consisted of both α- and θ- Al2O3 with whiskers, and with further oxidation the α-Al2O3 became the major product in the 2nd stage. After the oxidation for 2300ks the as-prepared, outer β-NiAl was changed into a mixture of β-NiAl and γ’-Ni3Al for the Ni-40Cr-3Re alloy containing Zr, while in the coated Ni-40Cr-3Re alloy the outer layer became a mixture of γ’-Ni3Al and γ-Ni(Al,Cr). It was concluded that the addition of Zr into the coated Ni-40Cr-3Re alloy helps maintain high Al contents in the outer Ni-aluminide layer by forming a protective Al2O3 layer.

溶融塩電析法によるＬａを含むＮｉアルミナイド表面層の形成とその耐サイクル酸化性

In order to improve the cyclic-oxidation resistance of Ni, the coating of a Ni aluminide containing La on the Ni was carried out by the electrodeposition of Al and La in a molten salt. The electrodepositions of Al and La were conducted using a potentiostatic polarization method at 1023 K in an equimolar NaCl-KCl melt containing 3.5 mol%AlF3 and that containing 4 mol%La2O3 and 24 mol%NH4Cl, respectively. Observation and analysis of the cross-section of the specimen after polarization showed that a deposit layer consisting of a thick inner Ni aluminide layer and a thin outer layer containing a large amount of La was formed on the Ni specimen. The cyclic-oxidation resistance of the specimen coated with the deposit layer having the layer containing La as an outer layer was higher than those of the untreated Ni specimen and the specimen coated with deposit layer without La. The scale formed on the specimen coated with the deposit layer without La after the oxidation test was thick and consisted of NiO, whereas the scale formed on the specimen coated with the deposit layer having the layer containing La was extremely thin, and consisted of α-Al2O3.

電析法による金属Ｎｂ上へのＮｉ‐Ａｌ金属間化合物のコーティングとその耐酸化性

The coating of a Ni aluminide layer on a Nb substrate was carried out by the electrodeposition of Ni and Al. The improvement of the high-temperature oxidation resistance for Nb was then evaluated. Ni and Al were deposited by aqueous solution and molten salt electrolysis, respectively. The thickness of the deposited layer increased with time for the nickel electrodeposition. When the nickel electrodeposition time is short, NiAl3 and Ni2Al3 are formed as the outer and inner layers, respectively. When the nickel electrodeposition time becomes longer, on the other hand, Ni2Al3 and Ni are formed as the outer and inner layers, respectively. The oxidation resistance of Nb was improved by coating of the Ni-aluminum intermetallic compound. Especially, a Nb specimen covered with a coating layer consisting of Ni2Al3 and Ni as the outer and inner layers, respectively, showed an improved oxidation resistance during the prolonged oxidation test.

溶融塩電析によるNiアルミナイド表面層の作製とその耐高温酸化性

A Ni aluminide layer on Ni substrate was formed by electrodepositing Al and alloying it with Ni in molten salt. Electrolysis of Al was conducted using potentiostatic polarization method at constant potentials in an equimolar NaCl-KCl melt containing 1∼5 mol%AlF3 at 1023 K. The mass of electrodeposited material increased with an increase in polarization potential. Deposits formed at −1.2∼−1.4 V(vs. Ag/Ag+(0.1)) built up a homogeneous layer. These deposits consisted of Ni aluminide such as Ni2Al3 and NiAl. Al content in deposit layers slightly decreased with increasing depth from the surface to the deposit/substrate interface. Nickel covered by the electrodeposit layer was more resistant than bare nickel to high temperature oxidation.