Termomechanical processing of titanium alloys by high power pulsed ion beams

Materials Science and Engineering A243 (1998) 290 – 293

Termomechanical processing of titanium alloys by high power pulsed ion beams V.A. Shulov a,*, N.A. Nochovnaya b, G.E. Remnev c a

b

Moscow A6iation Institute, 4 Volokolamskoye shosse, 125871 Moscow, Russian Federation All-Russian Institute of A6iation Materials, 17 Radio Street, 107005 Moscow, Russian Federation c Nuclear Physics Institute, 2a, Lenin Street, 634050 Tomsk, Russian Federation

Abstract The effect of thermomechanical processing by high power pulsed ion beams (ions of C + and H + , energy, E= 300 keV, ion current density in a pulse, j=20–220 A · cm − 2, number of pulses, n=1 – 100, pulse duration, t=50 – 100 ns) on the chemical composition and structure of titanium alloy parts surface layers (VT8M, VT9, VT18U and VT25U) were studied. It is shown that ion beam treatment and a post-process annealing allow the modification of the surface layers of the titanium alloy parts with thicknesses up to 100 mm. As a result of this complex treatment, service properties of the titanium alloy details can be improved cardinally. The prospects of high power pulsed ion beam treatment application for aircraft engine building are discussed. © 1998 Elsevier Science S.A. Keywords: Ion beam; Titanium alloys; Service properties

1. Introduction High power pulsed ion beam (HPPIB) treatment is one of the most perspective methods for surface thermomechanical processing of metals and alloys [1–3]. It is conditioned by the possibility of carrying out highspeed heat treatment and mechanical processing of varied details using HPPIBs, simultaneously [4]. Furthermore, this method allows a record level of the operating characteristics to be achieved of some parts from titanium alloys, steels, superalloys, ceramics etc. [4,5]. The present paper reviews the latest experimental data on HPPIB treatment of the a +b (VT8M, VT9) and pseudo-a (VT18U, VT25U) titanium alloys and exhibits behavior of irradiated samples and parts (compressor blades) manufactured from these alloys.

2. Experimental The samples and compressor blades were manufac* Corresponding author. Tel.: +7 095 1584424; fax: + 7 095 1582977; e-mail: [email protected] 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 8 1 6 - 2

tured from rods of the following titanium alloys: VT8M (Ti–6.2Al–3.7Mo–0.2Fe–0.3Si), VT9 (Ti– 7.0Al–3.8Mo–0.2Fe–2.5Zr–0.3Si), VT18U (Ti–6.3Al– 3.4Mo–0.2Fe–4.5Zr–1.5Nb–0.25Si–3.0Sn) and VT25U (Ti–7.2Al–2.5Mo–1.5W–0.2Fe–2.5Zr–0.25Si). Irradiation of these targets have been carried out in TEMP accelerator [6,7] under the conditions: E =300 keV, j= 20–220 A · cm − 2, n=1–100 pulses, t=50 ns. After irradiation, the targets have been annealed at the operating temperatures of 500oC for VT8M and VT9, and 550oC for VT18U and VT25U. The duration of heat treatment have been varied from 10 min up to 2 h. The aim of a post-process annealing was stabilization of a phase-structural state in the surface layer of the targets. Surface conditions of the initial and treated samples have been studied by Auger-electron spectroscopy (AES), scanning electron microscopy (SEM), X-ray diffraction analysis, optical microscopy and exo-electron emission (EEE). Furthermore, roughness and microhardness of the target surface have been measured (Ra, Hm). The optimal regimes of irradiation have been determined after the completion of these investigations. The control parties of the compressor blades have been irradiated and annealed in

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Fig. 1. Element distributions on a depth of VT18U alloy samples (initial state).

Fig. 2. Element distributions on a depth of VT18U alloy samples treated by HPPIB ( j = 150 A · cm − 2, n = 100 pulses, t =50 ns, E =300 keV).

the optimal regimes. Finally, the following properties of the treated blades were determined and compared with the serial details: fatigue strength (s − 1, MPa; loading frequency, f= 1800 – 3200 Hz; temperature, T= 450– 500°C; air; the base of tests, 2 · 107 cycles), oxidation resistance (h0, thickness of an oxidized layer; T= 500– 550°C; duration of exposure, t= 100 – 500 h), salt corrosion resistance (Dm/S, specific mass loss; thermocycling: heating in air up to 550°C and cooling in sea water down to 20°C; number of cycles, N=250) [4,5].

3. Results It is well known [1–3] the main technological parameter of the HPPIB irradiation process is ion current density ( j ) in a pulse because a value of j determines density of absorbed energy in the surface layer, if other parameters (energy, content of beam and number of pulses) are not varied in the experiments. Therefore, the following phenomena proceeds in a near-surface zone of titanium alloys during HPPIB irradiation with a rise of ion current density [4,5]:

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Table 1 The influence of HPPIB irradiation on microhardness Hm, roughness Ra, intensity of exo-electron emission IEEE, crystal lattice parameter c, and half width of X-ray line W1/2 of compressor blades manufactured from titanium alloys Alloy

j (A · cm−2)

n (number of pulses)

Hm (HV) p= 0.5 N (loading)

Ra (mm9 0.02)

IEEE (pulses s−1)

c (nm 90.0005)

W1/2 (grad9 0.05)

VT18U VT18Ua VT18U VT18U VT9 VT9a VT9 VT9 VT9 VT9 VT9 VT9 VT9

— — 120 150 — — 100 100 100 100 120 150 180

— — 3 3 — — 1 3 5 7 1 1 1

330–340 410–450 330–340 330-340 330–360 510–570 370–390 270-290 230–250 210–230 380–400 400–420 410–430

0.24 0.25 0.15 0.10 0.10 0.11 0.13 0.13 0.11 0.11 0.08 0.05 0.05

75 9 10 50 910 100 910 120 910 45 95 30 95 55 95 80 910 50 95 40 95 80 910 95 95 100 95

0.4691 0.4706 0.4687 0.4696 0.4679 0.4697 0.4675 0.4651 0.4657 0.4668 0.4670 0.4665 0.4661

0.93 0.65 1.16 1.26 0.78 0.58 0.99 0.92 0.77 0.71 1.06 1.11 1.19

a

Indicates the results for blades subjected to annealing at 750°C for 2 h.

Fig. 3. Auger-spectrum of a surface of VT9 alloy sample treated by HPPIB ( j =60 A · cm − 2, n =15 pulses, t =50 ns, E=300 keV).

evaporation of organic impurities ( j = 20 – 40 A · cm − 2); melting of a surface material (thickness of a melting layer is equal to 0.4–2 mm if j= 40 – 80 A · cm − 2); crater and crack formation, evaporation and element sublimation ( j= 100–180 A · cm − 2); total evaporation and sublimation, plasma formation, development of a compressive pulse during intensive removal of a substance, formation of a shock wave, spreading of the shock wave in a depth (a maximum amplitude of this wave achieves 200 MPa). These phenomena determine the experimental results fixed after irradiation (Figs. 1 and 2, Table 1). It is shown that HPPIB treatment leads to redistribution of alloy components in the surface layer (elements having low coefficients of distribution, ki B1, are driven back by the crystallization front at a surface —Al, Zr,

Mo, C; the other way if ki \ 1, then elements are removed from a depth—O, N. This redistribution of elements can be observed only at the higher number of pulses (n\10–20 pulses) and the low values of ion current density ( j: 60 A · cm − 2) when appreciable removal of the material is absent. As a result of melting, a surface is smooth ( j= 40–80 A · cm − 2) and residual tensile stresses are formed under the remelting zone. After irradiation the surface layer with a thickness near 4–7 mm contains only a and a% phases. There are many craters and even microcracks on a surface treated by HPPIBs with j\ 80 A · cm − 2. A post-process annealing brings about some important alterations in the chemical composition and structure of the surface layer material. For example, distributions of elements from a depth become more homogeneous and formation of

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surface layer. Thus, the results allow the conclusion that there are the following regimes of HPPIB thermomechanical processing of the titanium alloys: (i) a+ b titanium alloys, j= 609 10 A · cm − 2, n]15 pulses; (ii) pseudo-a titanium alloys, j=150910 A · cm − 2, n]3 pulses. The test results of the initial and irradiated samples are presented in Table 2. One can see that HPPIB treatment allows the improvement of service properties of the titanium alloy parts considerably.

4. Conclusion

Fig. 4. SEM micrograph of the VT25U alloy sample surface treated by HPPIB and annealed at 550°C for 2 h ( j= 150 A · cm − 2, n =3 pulses, t = 50 ns, E = 300 keV).

fine-disperged carbides takes place in the 0.2 mm-thick layer. The latter conclusion has a half quantitative character and follows from an analysis of 272 eV, peak C from Fig. 3. Further, almost the total decay of strengthening b-phase takes place in the surface layer of the samples from a + b titanium alloys already after a short exposure at 450°C, if irradiation is carried out at high values of ion current density ( j\ 80 A · cm − 2). Obviously, the b-phase decay will result in a decrease of the operating characteristics of the irradiated parts. Unlike a + b-titanium alloys, high current irradiation of the pseudo-a-titanium alloy samples and their annealing (j = 150 A · cm − 2, T =550°C, t =2 h) cause low temperature recrystallization forming the optimal size of grains (L:40 mm) in the near surface zone. The latter is accompanied by a decrease of the intersurface distance from 0.4691 to 0.4665 nm, formation of secondary a plate structure into an inch first a plate (Fig. 4), and a diminution of the defect concentration in the

It is shown that HPPIB treatment allows the increase of the following operating characteristics of the titanium alloy parts: fatigue strength, up to 180%; oxidation resistance, up to 220%; and salt corrosion resistance, in 6 times and more. The record level of service properties, high productivity, and ecological purity of treatment form a good prospect for introduction of HPPIB processing in aircraft engine building.

References [1] A.D. Pogrebnyak, Phys. State Solids 117 (17) (1990) 17–51. [2] J. Naragawa, T. Ariychi, H. Hanjo, Nucl. Instruments Methods Phys. Res. B39 (1989) 603 – 606. [3] K. Yatsui, Laser Particle Beams 7 (4) (1989) 733 – 741. [4] G.E. Remnev, V.A. Shulov, Laser Particle Beams 11 (4) (1993) 707 – 731. [5] D.J. Rej, H.A. Davis, J.C. Olson et al., Materials processing with intense pulsed ion beams, 43rd Natl. Symp. American Vacuum Society, Philadelphia, PA, 1996, p. 18. [6] V.A. Shulov, N.A. Nochovnaya, G.E. Remnev, The application of high power ion beams in aircraft engine building for reconstruction of refractory alloy parts, Proc. Beams 1996 Conf., vol. 2, Prague, 1996, pp. 878 – 882. [7] V.A. Shulov, N.A. Nochovnaya, G.E. Remnev et al., High power ion beam treatment of titanium alloy parts, Proc. 8th Int. Conf. on Titanium, vol. 3, UK, 1996, pp. 2126 – 2132.

Table 2 The influence of HPPIB irradiation on titanium alloy behavior Alloy

VT9 VT9 VT8M VT8M VT18U VT18U VT25U VT25U

Regime of irradiation

Service properties

j (A · cm−2)

n (number of pulses)

s−1 (MPa)

h0 (mm; t =200 h)

Dm/S (mg · mm−2; n =250 cycles)

— 60 — 60 — 150 — 150

— 15 — 20 — 3 — 3

420 9 10 495 910 160 910 440 910 360 910 420 9 10 4309 10 520 910

30 95 15 9 4 35 9 3 16 93 25 9 4 12 94 23 94 11 9 3

0.51 9 0.05 0.09 90.01 0.62 9 0.05 0.07 9 0.01 0.48 9 0.05 0.11 9 0.02 0.47 9 0.05 0.12 9 0.02