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Thermodynamic Modeling and Studying of Process of a Self-Extending High-Temperature Synthesis of Carbides and Borides Chrome, Tungsten and Molybdenum in Vacuum

V.M. Khaltanova, A.N. Chagdurov, T.B. Kim, T.B. Tsyrenov, B.B. Dorzhiev, and N.N. Smirnyagina

Department of Physical Problems of Buryat Scientific Center SD RAS, 8, Sakhyanovoy str., Ulan-Ude, 670047, Russia

E-mail: ionbeam@pres.bscnet.ru

Abstract – In the present report results of the thermodynamic calculations modeling interaction of transitive metal oxides with carbon are resulted. Thermodynamic research of phase equilibriums in Mo–C–O, W–C–O and Cr–C–O systems is carried with the purpose of optimization of conditions of carbides formation in vacuum. At a choice of con-ditions of carbides synthesis considered tempera-ture and the common pressure in system.

1. Introduction

At studies of nuclear reactions (dd, pd, etc.) in the surface layers saturation of metals and alloys by boron will carry out with the purpose of increase of their surface hardness, abrasion resistance and etc.

Multicomponent layers, containing borides of re-fractory metals, as a rule, form by methods of chemi-cal-thermal treatment in result of the boriding compo-nent interaction with refractory, or at the expense of boron saturation a refractory impurity of metal or alloy.

In work [1] was informed about hardening coating formation on basis refractory metal borides (TiB2, CrB2, VB2, W2B5) on carbon steels under influence of an electron beam in vacuum. As the boride layers formation came true on steel surface, at a choice of conditions of refractory metal borides synthesis it was necessary to take into account peculiarities metal basis melting.

As in the literature there are no full thermody-namic data about borides of chrome, molybdenum and tungsten to model phase balance in systems with their participation not probably. Earlier it has been estab-lished [2, 3], that borides in systems with participation oxides, boron and carbon it is formed intermediate carbides, therefore it was necessary to investigate phase formation in systems MeO–C.

In the work formation conditions for carbides MoC, Mo2C, WC, W2C, Cr3C2, Cr7C3, and Cr23C6 are studied, but we have presented in more detail results of calculations in system Cr–C.

Carbides layers are formed by electron beam sur-facing of self-extending high-temperature synthesis products. The carbides layers structures on carbona-ceous steel 20 was investigated and discussed.

2. Thermodynamic calculations

Thermodynamic modeling is executed under the pro-grams the ASTRA 4/рс and TERRA [4, 5].

Calculations are lead in a temperature interval 273–3873 K for the common pressure in system in a range 105 – 10–4 Pa.

3. Results and discussion 3.1. Chrome carbide

As is known [6], double system Cr–B is characterized by the most difficult phase parities in comparison with other transitive metals of IV period of the table of chemical elements of D.I. Mendeleeva. In this system formation of the greatest quantity borides is revealed: Cr2B, Cr5B3, CrB, Cr3B4, and CrB2 [6].

In double system Cr–C are formed 3 carbide: – Cr3C2, Cr7C3, Cr23C6 [7].

The interaction chrome oxide Cr2O3 with carbon is very sensitive to pressure of the gas environment which role at different stages of interaction essentially varies. In an initial stage of restoration a limiting stage is crystal reorganization of Cr2O3 in the metal or other oxides. Then, the regeneration stage of oxide carbon CO proceeds with rather small speed C. In the subse-quent, the role of gasification of carbon, therefore process of interaction increases becomes sensitive to pressure of the gas environment. By the end of in-teraction when the surface of a reactionary zone no-ticeably decreases, at presence of surplus of carbon again limiting there is crystal stage.

Carbide Cr3C2. Chrome carbide Cr3C2 is most thermally steady connection among the others car-bides [8]. In Fig. 1 influence of the common pressure of a gas phase on formation temperature of Cr3C2 in mixture 6Cr : 9O : 13C is presented. The formation temperature of Cr3C2 decreases with 1413 up to 713 K at pressure decrease from 105 up to 10–4 Pa.

Interaction of chrome oxide is rather interesting at pressure in a range from 10–4 up to 10 Pa. In this pressure area the formation of single-phase Cr3C2 which further decays is observed. Thus chrome passes in a gas phase, and a remaining rest of the condensed carbon appears in a gas phase at more heat (Fig. 1). Interaction of chrome oxide Cr2O3 with carbon is observed at temperature 733 K and accompanied

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Fig. 1. Formation temperature of carbide Cr3C2 in the mixture 6Cr : 9O : 13C

by thermal effect ∆Н = –1127 kJ/mol. It is necessary to note, that this thermal effect is total two processes – restoration Cr2O3 and gasification carbon C.

Change of phase structure in a mixture 6Cr : 9О : 13C confirms chemical interaction at tem-perature 733 K. Gasification of carbon (interaction with the oxide СО2) with formation of oxide CO pro-ceeds mainly in the field of temperatures from 453 K before chemical interaction, and further, up to tem-perature 873 K. The chrome oxide Cr2O3 in these con-ditions is partially restored up to chrome which passes in a gas phase. Single-phase carbide Cr3C2 it is fixed only in a temperature interval from 873 up to 1033 K, becomes soiled an impurity of carbon, further, with 1273 K starts to decay, and at the further rise in tem-perature with 1373 K completely is absent. Decompo-sition of carbide Cr3C2 happens at presence of the gas phase containing oxides CO and СО2. The contents of the last oxide in a gas phase decreases with rise in temperature, thus there are pairs of chrome oxide CrО. Chrome is ionized. It is necessary to note, that traces of a carbon impurity disappear in a temperature inter-val from 1653 up to 2153 K, due to evaporation, thus in a gas phase there are carbon pairs.

Observable behaviors features of the condensed phases of carbide Cr3C2 and carbon are shown at pre- sence of the gas phase which contain oxides CO and СО2. The carbide Cr3C2 is unstable in a range of pres-sure from 10–4 up to 10 Pa, decays owing to dissocia-tion as a result of which intensive evaporation of chrome is observed.

It is possible to present total sequence of chemical transformations:

Сr2O3 + CO → Cr + CO2 C + CO2 → 2CO

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Cr2O3 + C = Cr + CO Cr + C = Cr3C2.

Carbide Cr7C3 is formed through a stage of for-mation Cr3C2. Carbide Cr7C3 So, at Р = 105 Pa interac-tion begins with formation Cr3C2 at temperature 1393 K, and at pressure 10–3 Pa decrease in temperature to 763 K. Formation of carbide Cr7C3 is fixed is possible to observe at 1603 To (105 Pa), and at pressure 10–3 Pa – 843 K, accordingly. The thermal effect of formation reaction Cr7C3 makes ∆Н = –320.4 kJ/mol.

Thus, thermodynamic calculations have shown that formation of single-phase chrome carbides Cr23C6, Cr7C3, and Cr3C2 is possible at lower temperatures in vacuum, than at atmospheric pressure.

3.2. Boride CrB2-layers

The structure of boride layers CrB2 (Fig. 2) is the most interesting. Layers are homogeneous, without greater areas of inclusions as for a case with or without pro-tection of boron oxide В2О3. There are small oval grey inclusions of dendrite type which settle down in the certain order and the maintenance of chrome in them does not exceed 0.9% Cr (Fig. 2, a). Besides it is pos-sible to observe eutectic, and also separate black im-pregnations at which there are atoms Cr and C (B). Research of a microstructure and a chemical com-position characterization of layer СrB2 with protection from boron oxide В2О3 have revealed its complex structure (Fig. 2, c).

273

773

1273

1773

2273

2773

3273

1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05Pressure, Pa

Tem

pera

ture

, K

Cr3C2,C Cr3C2 Cr23C6,C C vapor

1

3

5

4

2

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a b c

Fig. 2. Microstructure of layers CrB2 (×250) (а), CrB2 + B2O3 (×300) (b)

Features of a structure of layer CrB2 + B2O3 are shown in the ordered arrangement of light, light grey oval inclusions in dark grey eutectic field of a layer. Inclusions are non-uniform on all volume and contain different quantities of chrome. A metal basis (Fig. 2, points 1, 2, and 3) and boride layer are separated from each other by a transitive thin zone in which there are atoms Fe and Cr (point 4). In light (points 7 and 10) and light grey inclusions (points 5, 8, 11, and 13) pre- sence 2.3–2.7 mass.% Cr. In light inclusions presence of atoms Mn is not fixed.

Special interest represents dark grey eutectic in which atoms B are found out, and interface area (Fig. 2, c). In interface layer zone a inclusions contain different quantities of chrome atoms from 3.05–3.35, up to 4.35–4.60 and 5.35–5.45 mass.%. Besides it is possi-ble to observe separate black impregnations at which there are atoms Cr and C (B). Boride CrB2 there is in reflowing areas of a layer, as him density below, than a density of liquid Fe–Cr–C/B. Measurement of microhardness also confirms non-uniform distribu-tion on structural components and a role of a metal sample reflowing zone the in formation of a layers. Phase composition of boride chrome layers to de-fine difficultly. Presence of following phases is identi-

fied: chrome ferrite Сr0.03Fe0.97 (PDF 03-065-4607, Sp.gr. Im3m, with the cubic cell a = 0.286920 nm), CrFeB (PDF 00-051-1410, Sp.gr. Fddd, with a rhom-bic cell a = 1.45349, b = 0.7303, c = 0.42149 nm).

References [1] N.N. Smirnyagina, I.G. Sizov, and A.P. Semenov,

Inoganic Mater. 38, 1, 48 (2002) [in Russian]. [2] N.N. Smirnyagina, Izv. Vuzov, Fizika 10, 450

(2006) [in Russian]. [3] N.N. Smirnyagina and A.S. Milonov, Izv. Vuzov,

Fizika 8, 273 (2006) [in Russian]. [4] N.A. Vatolin, G.K. Moiseyev, and B.G. Trusov,

Thermodynamic Modeling in High Temperature Inorganic Systems, Moscow, Metallurgy, 1994, 352 p.

[5] G.B. Sinyarev, N.A. Vatolin, B.G. Trusov, and G.K. Moiseyev, Application of the COMPUTER for Thermodynamic Calculations of Metallurgical Processes, Moscow, Science, 1982, 264 p.

[6] T.I. Serebpyakova, V.A. Neronov, and P.D. Peshev, High Temperature Borides, Moscow, Metallurgy, Chelyabinck branch, 1991, pp. 368.

[7] G.V. Samsonov and I.M. Vinistkiy, The Refractory Compounds, Moscow, Metallurgy, 1976, 560 p.