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Phase Composition and Mechanical Properties of Titanium Alloyed by Chromium and Molybdenum Atoms

under High-Current Electron Beams1

N.N. Cherenda, V.V. Uglov, V.I. Shymanski, N.N. Koval*, Yu.F. Ivanov*, and A.D. Teresov*

Belarusian State University, 4, Nezavisimosti ave., Minsk, 220030, Belarus Phone: (+375 17) 2095512, E-mail: uglov@bsu.by

*Institute of High Current Electronics, SB RAS, 2/3, Academicheskiy ave., Tomsk, 634055, Russia

1 The work was supported by BRFFR Grant (Project No. F10R-085).

Abstract – In the present work the influence of high-current low energy (20 keV) electron beam on the molybdenum/titanium and molybdenum-chromium/titanium systems is considered. Main regularities of the phase and element composition changes depending on the treatment parameters are found. The optimal parameters of β-Ti forma-tion are determined.

1. Introduction

The concentrated energy flows are widely used for modification of materials physical properties. The value of energy density is usually varied in the range of 10–40 J/cm2 to avoid the sufficient evaporation of the material and to attain an essential effect [1]. Sur-face properties changes took place due to high cooling rate (up to 107 K/s) that can provide refining and me-tastable structures formation. Phase composition of the modified layer can also be changed by alloying with additional elements. Coating or thin film deposi-tion on the modified material surface with the subse-quent treatment by ion, electron, laser or plasma beam is one of the possible alloying techniques [2]. Varia-tion of the treatment parameters allows to receive the required composition of the alloyed layer.

This treatment technique can be applied for the ti-tanium surface properties modification. One of the actual problems of the titanium alloys application is low hardness and low tribological properties. Me-chanical properties of titanium alloys can be varied by the change of the high-temperature phase (β-phase) concentration in alloy.

Investigation of the phase composition and me-chanical properties of titanium alloyed with molybde-num and chromium atoms by means of high-current low energy electron beam (HCEB) treatment [3–6]. Molybdenum and chromium were chosen as alloying elements due to their different solubility in the bcc β-phase of titanium. Molybdenum can form unlimited rank of solid solutions while chromium can form in-termetallides. Moreover, β-phase stabilized by molyb-denum or chromium atoms can be applied as biocom-patible material [7].

2. Experimental

The VT1-0 titanium alloy was chosen as an object of investigation because it contains small concentration of impurities.

The molybdenum coating was deposited on the ti-tanium surface by means of arc vacuum deposition wit the following parameters: the arc current is 180 A, bias voltage is 120 V, deposition time is 10 minutes. Another series of the samples was a binary system with simultaneous deposition of molybdenum and chromium atoms. The coating contains 88 at.% Mo and 12 at.% Cr. The thickness of coatings was 1–1.5 micrometers.

Afterwards the “coating/substrate” system was ir-radiated by high-current low energy electron beams at the “SOLO” equipment. The electron energy was equal to 20 keV. Three pulses with the frequency 0.3 Hz were used to provide more homogeneous dis-tribution of the alloying element on the surface. The absorbed energy density was changed from 20 to 35 J/cm2 at the constant time of pulse duration (50 μs).

The surface morphology and cross-section struc-ture of the samples were investigated by means of scanning electron microscope using LEO1455VP de-vice equipped with an energy-dispersive X-ray Röntec detector. Before investigations the samples cross-section was etched in HF + HNO3 (1:1) solution. The phase composition was determined by the X-ray dif-fraction analysis (XRD) in Bragg–Brentano geometry and CuKα radiation (λ = 0.154178 nm) using a DRON 4–13 diffractometer. The microhardness of the sam-ples was tested by means of a PMT-3 microhardmeter with a Vickers indenter under the load ranging from 0.5 to 2.0 N.

3. Results and discussion

It was find that the value of the absorbed energy den-sity influences on the distribution of the alloyed ele-ments along the surface. SEM results (image in back-scattering electrons) show that there are zones in the surface where no-melted molybdenum is present after treatment at 20 J/cm2 (Fig. 1, a). Increase of the ab-sorbed energy density provides more homogeneous distribution of elements but the areas enriched with

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molybdenum are still present (Fig. 1, b). This effect can be connected with nonuniformity of electron beam in its cross-section.

a

b

Fig. 1. SEM-images of the titanium with molybdenum coa- ting after HCEB treatment at 20 J/cm2 (a) and 30 J/cm2 (b)

In the conditions of the high cooling rate the effect of the constitutional undercooling results in the forma-tion of the cellular structure in the areas containing both titanium and molybdenum atoms (Fig. 2). The average size of cells is about 300 nm and does not depend on the absorbed energy density.

Fig. 2. SEM-image of the cell-structure on the molybde- num/titanium system after HCEB treatment at 35 J/cm2

The molybdenum concentration in the melted layer (energy-dispersive X-ray microanalysis) has the ten-dency to decrease with the absorbed energy density growth (the Table). It can be explained both by inten-

sification of the molybdenum ablation and by the in-crease of the melted layer.

The molybdenum concentration in the modified layer with the thickness of ~ 1 µm at different absorbed energy density

Absorbed energy density, J/cm2 Mo concentration, at.% 20 28.9 25 5.5 30 8.5 35 3.4

One of the features of the internal structure of the

modified layer is non-uniform molybdenum distribu-tion along the depth of the sample (Figs. 3, 4). This effect can explain growth of molybdenum concentra-tion after treatment at 30 J/cm2 (the Table). A maxi-mum concentration is observed at the depth of 10–12 μm. The melting point of titanium is less than that of molybdenum. That is why nonmelted areas of mo-lybdenum coating can be transferred in to the depth by convection whirls under treatment regimes with lower energy density.

a

b

Fig. 3. SEM-image (a) of the cross-section and the charac-teristics X-ray radiation distribution along the depth AB (b) after HCEB treatment at 20 J/cm2

XRD investigations confirm presence of non-melted molybdenum in sample treated at 20 J/cm2 (Fig. 5). The presence of molybdenum atoms in the titanium with concentration greater than 5.8 at.% re-sults in stabilization of high-temperature β-phase at room temperature (Fig. 5). The relative content of β-phase increases with the growth of the absorbed

0 2 4 6 8 10 12 14 16 180

20

40

60

80

100

120Intensity, imp

Depth, μm

20 J/cm2

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energy density up to 25 J/cm2 that can be associated with the more uniform distribution of molybdenum atoms.

a

b

Fig. 4. SEM-image (a) of the cross-section and the charac-teristics X-ray radiation distribution along the depth AB (b) after HCEB treatment at 30 J/cm2

Fig. 5. XRD patterns of molybdenum/titanium system after HCEB treatment.

In this case the whole analyzed layer contains mainly β-phase. Further increase of absorbed energy density leads to the decrease of the relative β-phase that due to diminishing of molybdenum concentration. On the XRD patterns one can see shoulder on the less angle area of α–Ti diffraction peaks that could be at-tributed either to α′–Ti either to α″–Ti phase. It should be noted that diffraction peak of β-phase at

20 J/cm2 possesses a large width at a half magnitude. It proves the presence a high level internal stresses given by no-melted molybdenum.

The addition the chromium atoms in the binary (molybdenum-chromium)/titanium system also results in formation β-phase that could be stabilized both by molybdenum and chromium atoms. The maximum content of β-phase forms at absorbed energy density 25 J/cm2 as in the case of molybdenum/titanium sys-tem. At low absorbed energy density (20 J/cm2) for-mation of the solid solution Mo(Cr) is observed result-ing in shift of (110) Mo diffraction line to greater diffraction angle. Due to limited solubility of chro-mium and molybdenum in the α-Ti lattice some quan-tity of these elements can take part in the formation of metastable α′–Ti or α″–Ti phase.

The mechanical properties of modified layer are mainly determined by its structure and phase composi-tion. In particular, the alloyed layer possesses en-hanced microhardness (Figs. 8, 9).

Fig. 7. XRD patterns of (molybdenum-chromium)/titanium system after HCEB treatment

Fig. 8. Dependence of microhardness of modified layer on the indentation depth for molybdenum/titanium system after HCEB treatment

After HCEB treatment the microhardness in mo-lybdenum/titanium system increases up to 4 GPa (the microhardness of the initial titanium is equal 2 GPa).

0 2 4 6 8 10 12 14 16 180

20

40

60

80 Intensity, imp

Depth, μm

30 J/cm 2

37 38 39 40 41 42

30 J/cm2

25 J/cm2

Intensity

Diffraction angle (2θ), degree

20 J/cm2

β–Ti(Mo) (110)

α–Ti(002)

α–T

i(101

)

Mo(

110)

38 39 40 41 42

β–Ti (110)

Mo(

Cr)

(110

)

α–T

i (10

1)30 J/cm2

25 J/cm2

Intensity

Diffraction angle (2θ), degree

20 J/cm2

α–T

i (00

2)

2 3 4 5

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8HV, GPa

Indentation depth, μm

20 J/cm2 25 J/cm2 30 J/cm2

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The maximum hardening in all range of loads takes place after HCEB treatment at 25 J/cm2 when the quantity of β-phase is the largest (Fig. 8).

Fig. 9. Dependence of microhardness of modified layer on the indentation depth for binary (molybdenum-chromium)/ titanium system after HCEB treatment

In the case of the (molybdenum-chromium)/tita- nium system the microhardness growth up to 5 GPa at the depth 2 μm is observed after HCEB treatment at 20 J/cm2 due to presence of β-Ti(Mo,Cr) and Mo(Cr) solid solutions (Fig. 9). Thus solid solution hardening is the main mechanism of hardness growth in molyb-denum/titanium and (molybdenum-chromium)/tita- nium systems after HCEB treatment.

4. Conclusions The treatment of molybdenum/titanium and (molyb-denum-chromium)/titanium systems by high-current low energy electron beams allows to produce a modi-fied titanium layer with different concentration of al-loying elements. The presence of molybdenum atoms provides the formation of β–Ti (Mo) solid solution The maximum content of β-phase is observed at ab-sorbed energy density of 25 J/cm2. Chromium atoms take part in Mo(Cr) solid solution formation at ab-sorbed energy density of 20 J/cm2. Solid solution hardening is the main mechanism providing micro-hardness increase up to 4 GPa in case of molybde-num/titanium system and 5 GPa in case of (molybde-num-chromium)/titanium system.

References [1] R.J. Adler, R.J. Richter-Sand, and J. Abercrombie,

Surface and Coatings Technology 136, 40 (2001). [2] N.N. Cherenda, V.V. Uglov, V.M. Anishchik et

al., Vacuum 78, 483 (2005). [3] Y. Qin, J. Zou, Ch. Dong et al., Nuclear Instru-

ments and Methods in Physics Research B. 225, 544 (2004).

[4] V.P. Rotshtein, Yu.F. Ivanov, D.I. Proskurovsky et al., Surface and Coatings Technology 180–181, 382 (2004).

[5] V.P. Rotshtein, D.I. Proskurovsky, G.E. Ozur et al., Surface and Coatings Technology 180–181, 377 (2004).

[6] J. An, X.X. Shen, Y. Lu et al., Surface and Coat-ings Technology 200, 5590 (2006).

[7] M. Rahman, I. Reid, P. Duggan et al., Surface and Coatings Technology 210, 4865 (2007).

2 3 4 5 62.0

2.5

3.0

3.5

4.0

4.5

5.0

Indentation depth, μm

20 J/cm2

25 J/cm2

30 J/cm2

HV, GPa