Ion induced changes of physical-mechanical properties in Al alloys

Nuclear

930

Instruments

and Methods

in Physics

Research

B59/60

(I 991) 930-935 North-Holland

Ion induced changes of physical-mechanical

properties

in Al alloys

V.P. Goltsev, V.V. JChodasevich, G.H. Choi and V.V. Uglov Byelorussian

Stute Unirvrsrty, Department

of Solids Phwics,

Lenrnsky

Pr. 4. Minsk.

2X080,

USSR

The influence of nitrogen and boron ion implantation on the microhardness of deformed AI alloys AI-Mg is investigated. The implantation was carried out by N, + ions with energy of 100 keV and by B + ions with energy of 40 keV in the dose range of 10’h-1.5~10’7 ions/cm2. Characteristics of surface layers were investigated by means of X-ray diffraction (XRD). transmission electron microscopy (TEM) and scanning electron microscopy @EM). It is shown that in the deformed Al-Mg altoys at implantatjon AIN and AIB, are formed. The dependence of the microhardness on the dose and the depth was doses of -1017 ions/cm’ investigated. The important role of the size and the concentration of the second phases which exist in the deformed alloy was observed. The dynamics of their dependence on the implantation dose of nitrogen and boron ion implantation was investigated.

1. Introduction

Aluminium and its alloys exhibit important technical properties, but relatively low strength and wear resistance restrict their wide utilization in some areas of technology. One way of hardening aluminium is a modification of the structure and the phase composition of the surface layers by ion implantation. At present some work has been reported on the modification of mechanical properties of light metals and alloys [l-7]. Singh [I] investigated surface microhardness of aluminium and its alloy 2024-T351 after lithium ion implantation. The improvement of microhardness with the increase of dose and energy of the impinging beam was noted. The implantation of heavier ions (Cu+, Sb+) also leads to the improvement of microhardness with an increase in dose [2,3]. A positive influence of ion implantation on such mechanical characteristics of alu~nium as wear resistance, friction and microhardness was observed [46]. Ohira and Iwaki [6] showed an increase of relative hardness by more than six times in a layer of 1.5 ym thick during implantation of NC at an energy of 100 keV in Al. In the explanation of the obtained mechanical characteristics the main emphasis is laid on the possibility of formation of nitride and boride phases [4-61. It is important to note that, in all the above mentioned papers, investigations of the physical-mechanical characteristics were carried out mainly on samples of pure (99.99 wt.%) or commercial (99 wt.%) aluminium. At the same time there are practically no papers in which the processes are studied in real, structural light alloys, which require accounting for intrinsic or element and phase composition and also the structElsevier Science Pub&hers

B.V. (North-Holi~d)

ural state. The aim of the present article is to investigate changes in the atomic composition, crystal structure and microhardness of deformed Al alloys in the AI-Mg system that have been implanted by nitrogen and boron ions.

2. Experimental Al alloys of the Al-Mg system are among the deformed alloys which are not hardened by heat treatment, and represent a complex multicomponent system containing Al (92.82 at.%) Mg (6.1 at.%), and also Mn (0.57 at.%), Fe (0.27 at.%), Si (0.16 at.%), Cu (0.06 at.%). and Zn (0.02 at.%). The deformation degree was - 60%. Implantation conditions were chosen so that the rate of formation and the distribution of the defects for nitrogen and boron ions coincide with the maximum ion range R, (Rr Ni = R!’ = 107 nm, IpFA = 95 nm, RvG = 92 nm) [8]. The energy of B+ ions was 40 keV, and of N: ions 100 keV. Implantation doses (D) varied within the limits of 10’6-1.5 x lOi ions/cm2, and the density of the ion current did not exceed 10 @AA/cm’. Changes of the mechanical properties of the near-surface ion implanted layers were estimated from results of the Vickers hardness measurement H,. The load range (P) on the diamond indenter was 2-80 g. In this case the interval of the probed depths (h) was 2-5 pm. The crystallography investigations were carried out using a diffractometer with monochromatized Cu K n radiation (XRD). Analysis of the element composition was made using scanning electron microscopy (SEM) with an energy of the electron beam of 30 keV.

931

V. P. Golise~~et al. / ion induced changes in A I allays 3. Results and discussion

The dependences of the microhardness changes HP on the depth point of the diamond indenter for the unimplanted deformed alloy AI-Mg and the alloy implanted by nitrogen ions are presented in fig. 1. At doses of 10” and 3 X 10’” N:/cm-2, an increase of the microhardness in comparison with the initial deformed sample was observed. At doses higher than 3 x lOI NC/cm* a H,, decrease takes place in the whole area of penetrated depths of 2-7 urn. Thus at high doses of NT implantation (D > 3 X 10’” ions/cm”) a loss of strength is observed. An analogous situation is found after implantation with boron ions. In fig. 2 the dependences of the microhardness changes on the dose of B+ and NT implantation at the chosen print depths of the diamond indenter are presented. The experimental results show a nonlinear dependence of the H, change on dose and depth (figs. 1 and 2). The maximum H, increase was 60% at D = 1.5-4.5 x lOI B’/cm’ and h = 2 pm. The X-ray crystal and electron microscopic investigations show the presence of the following phase composition: Al, AIsMg,, MnAl,, FeMn,. and Mg,Si in the unimplanted deformed alloy Al-Mg IS]. The microstructural investigations showed (fig. 3a) that the average size of the second phase precipitates is 150 nm. and the Al grain size in the deformed Al-Mg alloy is 400 nm. After implantation by N: and B+ together with redistribution of the reflex intensities, which are

Fig. 1. Microhardness HP change dependences on the depth print of diamond indenter for the initial AI-Mg alloy (l), and the alloy implanted by NT ions at doses of 1Ol6 (2), 3 x 30” (3), 6 x lOI6 (4), and 10” (5) ions/cm2.

absent in the unimplanted deformed Al-Mg alloy of the second phases, the formation of nitride (AlN) and diboride (AIB,) takes place, respectively [7]. X-ray and electron diffraction patterns show additional reflexes, which are identifiable as Al nitride and diboride, hence AlN and AIB, are formed in the near-surface layers of the deformed Al-Mg alloy. AlN is formed at a nitrogen

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4,5 911 15 Bi DOSE(ions cmm2) x 1016 Fig. 2. Dependences of the microhardness change on the dose of the implantation by B+ (l-3) and N-j+ (4-6) at selected print depths of the diamond indenter: I,4 - h = 2 pm; 2.5 - h = 3 pm; 3.6 - h = 4 pm. VII. METALS / TRIBOLOGY

932

V.P. Goltsec~ et

ai. / ion induced changes in Al alloys

dose of 10” ions/cm’, and AlB, at a dose higher than 9.1 X 1016 ions/cm2. In our case a high level of residual stress, due to a high degree (- 60%) of deformation of the investigated alloy, and also an additional increase of the stress due to the induced radiation defects introduced during ion implantation, stimulate the formation and the growth of the nitride and diboride phases at an earlier stage of implantation and at a lower dose of implantation than are mentioned in refs (1,4-61. Experimental difficulties in determination (considering the selected reflex) of a real precipitate size of the particular

Fig. 3. TEM

phase by TEM and also a similarity of some diffraction characteristics of the second phases (AlsMg,, MnAl,, FeMn,, Mg,Si) do not allow exact identification of the observed microstructural components (fig. 3) i.e. to assign a phase composition to each observed precipitate. In this connection the analysis (size and concentration determination) of the second phase precipitate was carried out integrally throughout all precipitates regardless of their configuration and phase composition. The microstructure analysis (fig. 3), obtained from the electrochemically thinned samples by transmission electron

micrographs (a-c) and the corresponding TED patterns (d-f) of the deformed Al-Mg ions at different doses: (a,d) - unimplanted; (be) - 10’” Nj’/cm2; (e,f) - 1.5 x

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Fig. 4. Dose dependences of the change of the average linear size (1,2) and the average volume concentration (3.4) of second phases of A-Mg alloy, implanted by N: (1,3) and B” (2,4) ions, observed by TEM.

microscopy, revealed a behavior of the average linear size (fig. 4) of the second phase precipitates after AI-Mg alloy implantation by N: and B+ ions. From the above mentioned dependences it follows that the average linear size of the second phase precipitates decreases with

increasing dose of both B+ and N; . An average volume concentration of the second phase precipitate can be evaluated by TEM [9] by determining the foil thickness (- 200 nm) from the contours of thickness extinction. In fig. 4 the dose dependences of the average volume

Fig. 5. SEM micrographs of the characteristic X-ray FeK, during linear scanning along AI-Mg alloy, unimplant~d (a) and implanted at a dose of (bj 1016 Nz/cm2 and (c) 1.5 x 10”’ B+/cm’. VII. METALS / TRIBOLOGY

934

V. P. Golrset~ et al. / Ion induced changes in Al alloys

B+ DOSE (ions WI-~) ~10’~ Fig. 6. Dose dependences of the change of the relative sizes (1.2) and linear density (3,4) of the iron precipitates (1.2). of the deformed samples of AI-Mg alloy. implanted by nitrogen (1.3) and boron (2.4) ions observed by SEM.

concentration of the second phases after implantation by NT and B’ ions are presented. It follows from these dependences that the maximum values of the average volume concentration of the second phase in the nearsurface layer of - 200 nm thick occur for doses of 1 x 10lh and 4.5 x 10lh ions/cm2 for NT and B’ ions, respectively. It was of interest to study the behavior of the second phase precipitates in the layer to 7 pm thick, i.e. in the depth range where the main changes of H, of the alloy Al-Mg microhardness take place after implantation. The precipitate parameters (concentration and average size) in the layers of more than 200 nm thick were studied by SEM, using an X-ray microanalyzer. The image analysis of the intensity dist~bution of the characteristic X-rays FeK, (fig. 5), MnK, and SiK,, while linear scanning along the initial alloy surface, shows that Fe. Mn and Si unlike Mg in the generation area of - 4 nm are distributed nonhomogeneously. The average size of the precipitates and their concentration (per unit length) were determined from concentration profiles of the intensities of the characteristic X-ray while scanning along the alloy surface. In fig. 6 the dose dependences of the change of the average linear concentration and the relative precipitate size of Fe:, obtained by SEM from the deformed samples of Al-Mg alloy. implanted by nitrogen and horon ions, are presented. It follows from these dependences that the average linear density of Fe precipitates increases by more than two times after implantation by Nf ions at a dose of 10lh ions/cm’ (implanted dose of 2 x 10” NT/cm’ at 50 keV) and by B + ions at a dose of 4.5 X 10” ions/cm’. It is necessary to note that the maximum changes of the average size of Fe precipitates after implantation of Al-big alloy by NT and B* also

occurs within the doses of 10” and 1.5 x 10” ions/cm’. respectively. At these doses the average size of Fe precipitate decreases by more than two times relative to analogous ones in the initial deformed alloy. Thus from the above mentioned investigations it follows that the formation of new AlN and AlB, phases and also the change and the formation of the optimal parameters (concentration and size) of the second phase precipitates (Al,Mg,. FeMn,. Mg,Si) are the main reason of the microhardness change with target depth (figs. 1 and 2). This hypothesis is confirmed by well known experimental data on the alloy implantation of the Al by Sb’ and Cu+ ions j3.21. In these papers it was shown that a maximum improvement in mechanical properties (wear resistance, hardness) occurs for a dose of 10” ions/cm’. The authors noted that at this dose of implantation the optimal size of the second phase precipitates (AISb, FeAl,) is formed, which is equal to 1.2 nm as determined from SEM data. Further increase of the implantation dose leads to an increase of the precipitate size (3.5 pm). which induces a decrease of the wear resistance. In our case this value obtained by TEM analysis is -0.15 and - 0.25 urn (figs. 3 and 4) after Al-Mg alloy implantation by NT and Bi ions, respectively.

4. Conclusion The investigations of N; and Bt ion implantation in the deformed aluminium Al-Mg alloy showed the possibility for formation of the nitride (AIN) and the diboride (.AlB,) phases at doses of 10” ions/cm’. Here their formation takes place at smaller doses than ob-

935

V.P. Goltseo et al. / Ion induced chunges in A I alloys served after implantation of nondeformed polycrystal and monocrystal samples of pure aluminium j5.61. The main parameters which are necessary to take into account during the surface modification of the deformed aluminium alloy of Al-Mg type are size and concentration of the second phases.

References

[l] A. Singh. Adv. Surface Treat. Technol.

Appl. Eff. 3 (1986) 155. 121 P.B. Madakson and G. Dearnaley. Proc. Int. Ion Engineering Congress (ISIAT 83 and 1PAT 83) Kyoto. Japan (1983) p. 1805. [3] P.B. Madakson, J. Appl. Phys. 55 (1984) 3308.

and A.A. Smith. Nucl. Instr. and Meth. [41 P.B. Madakson 209/210 (1983) 983. 151S. Ghira, M. Iwaki and K. Hiei, Nucl. Instr. and Meth. 832 (1988) 66.

161S. Ohira and M. Iwaki. Mater. Sci. Eng. 90 (1987) 143. H. Choi Gim and V.V. 171 V.P. Goltsev, V.V. Khodasevich. Uglov, Proc. All-Union Conf. Property Modification of Construction Materials by Charged Particle Beams (IPI, Tomsk. 1988) p. 126. ISI A.Ph. Burenkov. Ph.Ph. Komarov, M.A. Kumakhov and M.M. Tyomkin. Space Distribution of Energy Injected in the Atomic Impingement Cascade in Solids (Energoatomizdat. 1985) p. 248. L.M. Malashenko and P.L. Tophienetz. 191 L.A. Vasiljeva. Electron Microscopy in Metallography of Nonferrous Metals (Nauka and Technika, Minsk, 1989) p. 208.

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