PREPARATION, STRUCTURE AND HIGH TEMPERATURE …€¦ · for composites of Ti and Al does not exceed 12001250°C. Layers hardening intermetallic compounds of Nb-3. Al and Nb. 2. Al

Journal of International Scientific Publications: Materials, Methods and Technologies

Volume 8, ISSN 1314-7269 (Online), Published at: http://www.scientific-publications.net

PREPARATION, STRUCTURE AND HIGH TEMPERATURE PROPERTIES

OF LAYERED Nb/Al- AND Ti/Al-COMPOSITES

Mikhail I. Karpov, Valery P. Korzhov, Dmitry V. Prokhorov, Irina S. Zheltyakova,

Tatiana S. Stroganov, Victor I. Vnukov

Institute of Solid State of Physics Russian Academy of Sciences, 142432 Moscow region,

Chernogolovka, Str. Academician Osip'yan, 2, Russia

Abstract

Multilayer composites were obtained by diffusion welding under pressure of packets collected from the thin foils of niobium or titanium and aluminum. Welding temperature for composites with Nb and Al was up to 1700°C, for composites of Ti and Al does not exceed 1200-1250°C. Layers hardening intermetallic compounds of Nb3Al and Nb2Al or Ti3Al and TiAl were formed by interdiffusion of niobium or titanium with aluminum. Mechanical properties at temperatures up to 1300-1350°C for Nb/Al- and to 850-900°C for Ti/Al-composites were determined by short-term tests for 3-point bending. Creep tests at the temperature were also. For example, a multilayer Ti/Al-composite showed that when stress σ, equal to 200 MPa its creep rate at 750°C is ∼6·10-4 h-1. When σ = 50 MPa, it was equal to 10 h-4, which corresponds to 1% deformation of the sample for 100 hours.

Key words: multilayer composite, high-resistant material, intermetallic compound, diffusion welding, layered structure, bending strength.

1. INTRODUCTION

Currently to alloys based on Nb-Si and, to a lesser extent, Nb-Al is a strong interest in connection with a real opportunity to use them as high-temperature materials. This is due to the fact that modern complex doped superalloys of Ni-Al, because of its relatively low (about 1400°C), melting temperature reached operating temperature limit (Светлов 2010, с. 29). Advantages of heat-resistant Nb-alloy are their lower density and higher melting temperature (Bewley 2003, p. 2043; Jackson 1996, p. 39). And strength values of ∼650 MPa at 1500°C (Bewley 1999, p. 32) achieved for alloys Nb-Mo-Ti-Si, obtained by directional solidification, give for researchers a real hope that products made of it can operate at temperatures significantly higher than similar products of Ni-alloys.

However, the use of the melting method of investment casting (Bewley 1999, p. 32) used to obtain products of Ni-superalloys difficult for alloys based on Nb-Si and Nb-Al in connection with the problem of inertia and deformation stability of ceramic production at foundries temperatures above 1700°C.

Methods of powder metallurgy (PM) are discussed. This – mechanochemical synthesis of nanostructure alloys (Dymek 1997, p. 507; Портной 2004, c. 79), mechanical alloying followed by hot pressing (Светлов 2007, с. 48), and so-called granular metallurgy of powders obtaining by cooling of the melt with a large (103-104 °C/s) speed (Береснев 2009, с. 24).

To heat-resistant alloys based system Ti-Al are alloys of Ti-Al with chromium, manganese, molybdenum, vanadium and certain other metals. They all have a high specific strength, good anticorrosive properties and heat resistance at temperatures up to ∼850°C. Their advantages over other systems are an insignificant specific weight and specific stresses in centrifugal conditions. The very same aluminum is widely distributed in nature, available and easy. Alloys of Ti-Al-Me are promising in the aviation and rocket engine – wheels, blades, and parts of gas turbines. However, low (∼1500°C) melting point limits their use by chamber of low pressure and static parts. (Ti-Al)-alloys and wares of them are produced mainly by smelting methods.

Common to the two families of alloys is the presence within their structures durable high-temperature intermetallic phases and solid solution based on titanium or niobium – relatively ductile phases – as spacers between structural elements of the intermetallic compounds. In alloys produced by melting methods and PM, such structures are formed naturally. In this study will be considered alloys based Nb-Al and Ti-Al. Their layered structure of the intermetallic compound and solid solution was produced by diffusion interaction of several tens of alternating thin foils of niobium or titanium with aluminum foils during the diffusion welding of pre-assembled multilayer Nb/Al- and Ti/Al-packages.

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2. DIFFUSION WELDING UNDER PRESSURE

2.1. The assembly of multilayer packages

This paper investigates the composites consisting in the initial state of the deformable components – niobium or titanium and aluminum. Therefore, multilayered bags collected from thin metal foils. But by our method can also obtain a multilayer composite material reinforced with compounds layers of niobium and deformable silicon. Then the packet is going out of Nb-foils coated with Si (Коржов 2013, p. 120; Korzhov 2013, p. 343).

A building of packages was carried out on a specially designed scheme of individual elements

Fig. 1. Construction scheme of assembled multilayer Ti/Al-stack

of book form (Fig. 1). This scheme allows simply and quickly to build packages from dozens of refractory metal component elements connected to each other and alternating fusible foils (aluminum) component. After this assembly the package is a single design allowed with him to produce technological procedures. The described process also allows the use of metal foils with a coating.

2.2. Diffusion bonding of packages

The main idea realized in the work for the production of heat-resistant composite material, was to create a multi-layer structure. It is an alternation of layers of relatively ductile solid solution of aluminum in titanium or niobium and strong but brittle intermetallic layers with the aluminum of a type Me3Al and (or) MeAl (Me = Nb, Ti). Intermetallic layers provide the heat resistance to a composite, and the layers of the solid solution, inhibiting crack caused in a brittle layers – crack resistance. The formation of the layered structure was carried out in the process of diffusion welding (DW) of multilayer packages under pressure (Fig. 2) collected from dozens of alternating foils niobium or titanium 30-70 µm thick and Al-foils of 10-30 µm thickness. The thickness of the source packet – from 1.5 to 3-3.5 mm, but can be given a certain thickness. The structure described above was obtained as a result of the mutual diffusion between the layers of niobium or titanium and aluminum. DW regime was conducted in two stages. The temperature in the first stage does not exceed the melting point of aluminum. Aluminum layers fully or partially were transferred to an intermetallic compound with maximum aluminum content. These are NbAl3 and TiAl3. In the second stage of DW the temperature was determined by components of the package. For Ti/Al-package was sufficiently 1150-1250°C for Nb/Al-package temperature was significantly more – 1500-1700°C.

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Fig. 2. Mutual location of major plant components for the DW: 1 – camera body, cooled by water, 2 – system of graphite heat shields out of pressed wool, 3 – fixed punch, 4 – from high-strength graphite heater, 5 – tested

multilayer package, 6 – movable punch

In the studied composites the thickness of the diffusion layers is varied in the range of tens of microns. However, previous conducted experiments on reusable packet rolling are showed the possibility of obtaining of composites with layers of nanometer range.

3. Nb/Al-COMPOSITES

3.1. The multilayer structure of Nb/Al-composites

Multilayer composite materials researches continue the work of the results which are set out in (Korzhov 2013, p. 120; Korzhov 2013, p. 343).

Multilayer Nb/Al-composites were obtained in the form of flat plates with the thickness 2-3 mm and a size ∼30×60 mm2 using diffusion welding of packages consisting of Nb-foils with thickness of 45 µm in an amount of 56 units, alternating with 55 units of aluminum foils of 10 µm thick. Aluminum foil on the surface was artificially formed layer Al2O3. Opting for Al-oxide-coated foil was made with a view to strengthening intermetallic diffusion layers by oxide particles of Al2O3.

To prevent a leakage of aluminum DW of packet under pressure initially was conducted at a temperature below the melting point of aluminum of 500°C / 9 h / 0.78 MPa, and then proceed also an additional vacuum annealing of package at 615°C for 10 h. As a result, all of the aluminum was contacted in a chemical compound NbAl3. Only then the diffusion bonding at a high temperature 1700°C for 15 min under a pressure 13 MPa was undertaken.

Macro-and microstructure of the cross section of the package is shown in Fig. 3, the results of X-ray analysis – Fig. 4.

Regular layered structure of composite was consisted of light layers of a solid solution of Al in Nb and diffusion layers of dark, overgrown by the diffusion of aluminum in niobium (Fig. 3a). At high magnification, and the results of the local X-ray analysis the diffusion zone was consisted of the intermetallic compound layer Nb2Al (Fig. 3b and 4) of ∼28 µm thick and two thinner (about 5.5 microns) intermetallic layers Nb3Al, containing respectively ∼33 and 20-23 at.% Al.

Inclusions of black discretely distributed predominantly middle Nb2Al-layer, were identified as oxide particles Al2O3. In Fig. 4b the range of 15 is corresponds to this case: 2.8 Nb, 36.4 Al and 60.7 at.% O. The ratio of Al/O is close to 2/3. At sufficiently high temperatures of final step of welding surface oxide layer of Al-foils was

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coagulated in A2O3-particles with sizes from a few to tens of microns, from what unlikely was expected of them reinforcing effect.

a b

Fig. 3. Macro- (a) and the microstructure of the Nb/Al-package after two stages of DW and an intermediate of vacuum annealing at 615°C (b)

a b

Fig. 4. Concentration dependences of niobium, aluminum, and oxygen (a) to points along the profile 1-14 indicated on the fragmentary pictures of microstructure (b)

Tests for 3-point bending at room temperature were showed 1100 MPa and at 1300°C – 330 MPa.

A subsequent vacuum annealing of this composite at 1300°C for 5 h was undertaken to see how the microstructure of it changes during high temperature tests. It turned out that after a 5-hour annealing phase

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structure of the diffusion layer is not changed (Fig. 5a). Compared with the previous structural picture only thickness of Nb3A-layers increased to 7-7.5 µm and thickness of Nb2Al-layer decreased to ∼19 µm.

Annealing at 1700°C for 1 h completely changed the structure of composite (Fig. 5b). If prior to annealing the sample had a multilayer structure, now the entire volume of it has turned into a solid solution of Al in niobium containing 8.9-9.3 at.% Al, with the lines of Al2O3-particles (40.2 at.% Al + 59.8 at.% Nb), spaced at a distance of 58-60 µm. And the size of the oxide particles in the surface layer could reach 35 or more microns. In the volume of the sample the size of Al2O3-particles did not exceed a few microns.

a b

Fig. 5. Microstructure of Nb/Al-composites after DW (Stage 1: 500°C / 9 h / 0.78 MPa) + annealing in vacuum (615°C / 10 h) + DW (Stage 2: 1700°C / 15 min /13 MPa) and then an annealing in vacuum: a – 1300°C, 5 h; b –

1700°C, 1 h

4. Ti/Al-COMPOSITES

4.1. Microstructure of Ti/Al-composites: scanning electron microscopy and X-ray analysis

a b

Fig. 6. Cross-sectional microstructure of the multilayer composite of Ti/Al after the DW regime: 550°C for 5 h + 1100°C for 30 min under a pressure of 16.7 MPa

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In the initial state, the package with size of ∼30×40 mm2 contained 42 Ti-layer of 50 µm thick and 41 layer of aluminum with thickness of 20 µm.

Structure of a Ti/Al-composite after DW with the final temperature of 1100°C for 30 min under a load of 2 T was shown in Fig. 6. Evident that in place of Al-layers diffusion layers were formed, each of which consisted of 2 layers of the intermetallic compounds of Ti3Al (22.5-26.8 at.% Al; 72.4-77.1 at.% Ti ), 2 layers of TiAl (∼53.5 at.% Al; ∼46.2 at.% Ti) and one inner TiAl2- layer (37.4-38.2 at.% Al; 61.6-61.7 at.% Ti). Ti-layers were turned into a solid solution of aluminum in titanium Ti(Al).

The structure of the Ti/Al-composite after DW and subsequent 10-hour annealing at 850°C was changed in favor of the TiAl-compound (Fig. 7): diffusion zone became consist of 2 layers of intermetallic Ti3Al (21.4-29.4 at.% Al; 70.2-78.1 at.% Ti) and one TiAl-layer (51.1-52.5 at.% Al; 41.5-47.5 at.% Ti). Inside TiAl-layer the presence of a significant amount of Al2O3-particles was noted. Ti-layers of a solid solution contains from 0.3 to 7.2 at.% Al.

a b

Fig. 7. Cross-sectional microstructure of the multilayer Ti/Al-composite after the CW regime: 550°C for 5 h + 1100°C for 30 min under a pressure of 16.7 MPa and annealing: 850°C, 10 h

4.2. Tests for 3-point bending

Tests on a 3-point bending at room temperature and temperatures up to 1350°C and creep tests were carried out using a testing machine of the brand «Instron» in pure argon atmosphere. Calculations were carried out according to known methods (Рудицин 1970). Load was applied to the sample midway between the supports.

For the load-displacement curves were characterized by the presence of several peaks, each of which corresponded to the formation and inhibition of single crack. And often happened that the second peak significantly surpassed the first (Fig. 8). In addition to the load-displacement curves, usually after the temperature tests, could be isolated PMAX (or σMAX), corresponding to the maximum of the curve, and the PPR (or σPR). At this point, the experimental curve deviated from the tangent, signaling the beginning of the inelastic deformation of the sample. Values of maximum strain σMAX and strain of proportionality at bending σPR were calculated by formula:

𝜎 = 𝑀𝑊

, where 𝑀 = 𝑃×𝑙4

and 𝑊 = 𝑏×ℎ2

6 (l – distance between supports, b and h – width and height of the cross

section of the sample, respectively, Р – appropriate load).

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Tests for 3-point bending performed after additional annealing at 850°C for 10 h at room temperature σPR = 300-450 and 1050-1300 MPa (Fig. 9), when the applied load P is perpendicular (⊥) and parallel (||) a surface of layers (ab). At 750°C and P ⊥ (ab) σPR was increased to 700 MPa. At 850°C and P ⊥ (ab) σPR and σMAX had considerable variation – from 400 to 850 MPa.

4.3. Creep test

The sample was loaded given load and kept at it given time. Moving of punches was recorded depending on time. In the initial part of the test a selection of clearances in range of supports and a

Fig. 8. Experimental load-displacement curve

Fig. 9. σMAX and σPR in dependence on the temperature of the test: − σPR, P ⊥ (ab); − σMAX, P ⊥ (ab); − σPR, P || (ab); − σMAX, P || (ab); − σPR, P ⊥ (ab), after creep test at 1300°C

Fig. 10. Creep test results: 3-point bending at 750°C under Ar-atmosphere.

Dependence of the deflection of the test time

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Fig. 11. The creep strain rate in tensile tests depending on stress. Mathematical processing of the experimental

data obtained from testing the 3-point bending (Fig. 10)

snap was occurred. Over time, this process came to an end, and the movement of the punch was reflected only the deflection of the sample (linear plots in Fig. 10). Samples were tested at intervals. Before the break the sample was unloaded and filmed heating. Before the next test sample is first heated again, and it was applied already higher load. Such tests in conjunction with the special scheme calculation and data processing (Mileiko 2002, p. 195; Милейко 2004, с. 523) were afforded in relatively short period of time to estimate creep characteristics of the material. Mathematical processing of the test results was made on the basis of power-law of tensile creep (Rabotnov 1979), i.e., bending test was simulated behavior of the material in tensile tests.

𝜀 = 𝜂𝑛 �𝜎𝜎𝑛�𝑛

, where: ε – creep rate, and ηn, σn and n – constants. The first of them is chosen arbitrarily. We assume ηn = 10-4 h-1. This means that σn – the strain causing deformation of 1% for 100 hours. Following the scheme of solving the problem of bending of the rod under steady-state creep, we obtain an expression that relates the magnitude of the applied load P [MPa] and the deflection rate F [µm/h]. Determined from the ratio F1/F2 = (P1/P2) n value of index n, solve one of the equations Fi = = f (Pi) (i = 1 and 2) with respect to the remaining unknown parameter σn. Knowing σn and n, we can construct the desired power dependence ε = const∙σn (Fig. 11).

The creep tests at 750 ° C and P ⊥ (ab) was showed that strains equated to 50 and 100 MPa (Fig. 11) were caused the creep rate at the tensile equal to 10-4 and 2.5∙10-4 h-1 respectively. Then for 100 hours, a sample will be extended to 2.5% (2.5∙10-4 × 100 h = 0.025), i.e. 0.025 = ∆l/l or ∆l = 0.025×l.

CONCLUSIONS

1. A simple and reliable way to build multi-layer packets.

2. The change of the structure Nb/Al-composites assembled from Al-foils with artificially formed on the surface of the oxide layer was investigated. If annealing at 1300°C only slightly changed the ratio of the diffusion layers of Nb3Al and Nb2Al, after 1700°C for 1 h, the composite structure is a matrix of solid solution of aluminum in niobium with the lines of particles Al2O3, evenly spaced.

3. Structure of Ti/Al-composite after diffusion welding under pressure was consisted of layers of a solid solution of aluminum in titanium, with alternating diffusion bands of two layers of intermetallic Ti3Al and TiAl-one middle layer. Bending tests have shown that at 750-850°C strain of the proportionality σPR is varied from 400 to 850 MPa. A series of creep tests were conducted.

REFERENCES

Береснев А.Г., Разумовский И.М., Логунов Ф.В., Логачёва А.И. Порошковые и гранульные материалы. // Технология металлов, 2009, №12, с. 24-37.

Коржов В.П., Карпов М.И., Прохоров Д.В. Многослойные жаропрочные материалы на основе интерметаллических соединений ниобия и титана: получение, структура и механические свойства. Multilayer heat-resistant materials based on intermetallic compounds of niobium and titanium: preparation, structure and mechanical properties. // Scientific Proceedings (of the Scientific-technical Union of Mechanical Engineering), 2013, v. 10/147, Year XXI, p. 120-123.

Милейко С.Т., Кийко В.М. Высокотемпературная ползучесть волокнистых композитов с металлической матрицей при переменных напряжениях. // Механика композитных материалов, 2004, т. 40, с. 523-534.

Портной В.К., Третьяков К.В., Логачёва А.И., Логунов Ф.В., Разумовский И.М. Метод механохимического синтеза для создания нанокристаллических Nb-Al-сплавов. // Физика металлов и металловедение, 2004, т. 97, №2, с. 79-84.

Работнов Ю.Н. Механика деформируемого твердого тела. // М.: «Наука», 1979, с. 744.

Рудицин М.Н., Артёмов П.Я., Любошиц М.И. Справочное пособие по сопротивлению материалов. // Минск: «Вышэйшая Школа», 1970.

Светлов И.Л. Высокотемпературные Nb-Si-композиты. // Материаловедение, 2010, №9, с. 29-38.

184



Светлов И.Л., Абузин Ю.А., Бабич Б.Н., Власенко С.Я., Ефимочкин И.Ю., Тимофеева О.Б. Высокотемпературные ниобиевые композиты, упрочнённые силицидами ниобия. // Журнал функциональных материалов, 2007, т.1, №2, с. 48-53.

Bewley B.P., Jackson M.A., Zhao J.C., Subramanian P.A. A review of very-high-temperature Nb-silicide-based composites. // Metallurgical and Materials Transactions, 2003, v. 34A, N10, p. 2043-2052.

Bewley B.P., Jackson M.A., Subramanian P.A. Processing high temperature refractory metal-silicide in situ composites. // Journal of Metals, 1999, v. 51, N1, p. 32-36.

Dymek S., Dollar M., Leonard K. Synthesis and characterization of mechanically alloyed Nb3Al-based alloys. // Material Science & Engineering, 1997, v. A239-240, p. 507.

Jackson M.A., Bewley B.P., Rowe R.G., Skelly D.W., Lipsitt H.A. High temperature refractory metal-intermetallic composites. // Journal of Metals, 1996, v. 48, N1, p. 39-44.

Korzhov Valeriy P., Karpov Michael I., Prokhorov Dmitriy V. Structure evolution of multilayer materials of heat-resistant intermetallic compounds under the influence of temperature in the process of diffusion welding under pressure and their mechanical properties. // Journal of International Scientific Publications: Materials, Methods & Technologies, 2013, v. 7, p. 1, p. 343-361.

Mileiko S.T. Oxide-fibre/Ni-based matrix composites – III: a creep model and analysis of experimental data. // Composites Science and Technology, 2002, v.62, p. 195-204.

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PREPARATION, STRUCTURE AND HIGH TEMPERATURE …€¦ · for composites of Ti and Al does not exceed 12001250°C. Layers hardening intermetallic compounds of Nb-3. Al and Nb. 2. Al