Decomposition of supersaturated MnCu solid solution during cold rolling

ScriptaMetallurgicaetMaterialia,Vol. 32,No.6,pp.895-900,1995 Copyright0 1995ElsevierScience Ltd PrintedintheUSA.AUrightsreserved 0956-716X/Y5$9.5Ot.OO

DIECOMPOSITION OF SUPERSATURATED Mn-Cu SOLID SOLUTION DURING COLD ROLLING

S.A.Demin, V.S.Skorodzievskii, A.I.Ustinov Insti.tute of Metal Physics of Ukrainian Academy of Sciences, 36, Vernadskogo str., Kiev-142, 252680, Ukraine.

(Received February 8,1993) (Revised October 20,1994)

Manganese-copper combination of high damping alloys have the unusual capacity and good mechanical properties (l-5). High damping capacity is achieved for an f.c.t. lattice, formed after quenching or by isostructural decomposition of the supersaturated y-phase by means of low temperature aging (2-5). Damping is connected with twinning of the f.c.t. lattice being subjected to external stresses and it depends mainly on the degree of decomposition of the y-phase (26). However, it has been noted that the damping capacity can be changed twice after plastic deformation (3-5). It is assumed that this phenomenon is due to the influence of a new dislocation structure the on the twin's boundary mobility. This is the case, but we found (7) that the decomposition of the yphase and precipitation of the equilibrium a-phase during cold rolling of quenched Mn-Cu alloys also took place. Precipitation can also restrict the movement of the twin boundaries (2-4) and deteriorate damping capacity. This work deals with the study of decomposition of quenched Mn-Cu alloys during cold rolling. Some possible reasons for such transformations are discussed. ExDerimental

DrOCedure

Examinations were conducted by X-ray diffractometry, transmission electron microscopy small X-ray scattering (TEM) and angle techniques. A (SAXS) monochromatized Fe Kg: line and a one-dimensional proportional counter as well as point and slit collimation of the primary beam were used in the SAXS measurements. Point collimation better angle resolution of diffraction pattern ensured and slit collimation more high intensity secured. All SAXS curves were corrected for the multifactor t*exp(-put) where p is the linear absorption factor, which was calculated in view of the chemical composition of alloys, and t is the sample thickness determined by absorption of the primary beam by the samples. The SAXS for cold rolled Cu was subtracted from SAXS-curves for cold rolled Mn-Cu alloys by analogy with the procedure proposed by (8). Such a procedure allowed us to drop the possible double Bragg reflections that are significant for deformed metals. We believe this approach is justified since the grain sizes and texture of the deformed Cu and Mn-Cu alloys were similar (9). The two alloys were induction-melted in alumina crucibles and had nominal compositions of Mn-10 % Cu and Mn-30 % Cu. The 3 mm plates were cut from the ingots and upset to 2 mm by hot forging. Annealing was performed at about 100 K below the solidus for 10 h in an argon atmosphere and was followed by water quenching. The plates were then polished to 1.5 mm and cold rolled to a 0.15 mm thickness. Some parts of the strips were homogenized and quenched again as

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Mn-CuSOLIDSOLUTION

before. Samples for SAXS experiments were prepared 0.03 mm in a solution consisting of: 20 ml nitric and 50 ml water. Foils for electron microscopy were technique in an electrolyte consisting of: 500 ml trioxide and 15 ml water. Results

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by thinning of the strips to acid, 15 g chromium trioxide polished by means of the jet acetic acid, 100 g chromium

and Discussion

X-ray phase analysis and TEM observations showed only a y-phase with a f.c.c. lattice for the Mn-30 'i;Cu alloy and a f.c.t. lattice for the Mn-10 9; Cu alloy present in the quenched samples. Weak additional maxima of a-Mn appeared on diffraction patterns from cold rolled samples (fig. 1). Formation of a cellular dislocation structure was found in the rolled strips by bright field TEM (fig. 2). Micro diffraction patterns from these strips had additional spots as compared to the y-phase picture corresponding to the a-phase lattice as well. A mottled contrast was seen on the dark field images, formed in the U-phase reflections. The nearly spherical spots of the mottling were mainly 4 - 6 nm in diameter and were spaced at distances from 2 to 5 diameters of particles. The mottling was so weak that photographs could not be reproduced. Figure 3 shows the variations in SAX.5 curves after cold rolling. A well separated diffraction maximum appeared on the curves from the rolled samples. The position and the magnitude of the maximum had no evident difference for lengthwise and cross rolling directions. A small difference in X-ray scattering amplitudes of the Mn and Cu atoms together with point collimation resulted in weak SAXS intensities. So, a similar study was carried out using slit collimation. The SAXS intensity was greatly enhanced, but smearing of the head part of the curves occurred simultaneously and the maximum was not visually observed (fig. 4). This maximum was separated by subtraction of the SAXS curve for the rolled pure Cu (fig. 4). The position of the reconstructed maximum was nearly the same as the one obtained directly. The results of the X-ray and TEM study allowed us to interpret the SAXS maxima as a result of the y-solid solution decomposition. Studies of the decomposition of the Mn-Cu alloys decomposition by low temperature aging using the small angle neutron scattering (SAWS) technique were carried out by Vintaikin et al (10). The time-depended maximum was observed on SAWS curves in these experiments as well. Our investigations of structural changes during aging also showed the appearance of a similar large maximum after aging at 700 K for 4 h (fig. 3). In the analysis of SAXS, we assumed that the diffraction maxima on SAXS curves could be caused either by the presence of inhomogeneities defined as enriched nuclei enveloped by depleted shells (the Guinier model) or by the interference of the scattered radiation associated with a correlation in the distribution of the precipitates. To verify the first assumption the SAXS spectra were simulated in the framework of (11). The position and half-width of the diffraction peak were controlled. We found that the variation of the model parameters, both the nucleus radius and the depleted shell thickness, failed to give reasonable matching of the measured curves. For instance, when these parameters were the most favorable for matching, the calculated half-width of the peak (in its experimentally determined position) was twice that of the observed one for the point collimation experiment. So, the concentration profile of the inhomogeneities did not correspond to the Guinier model and instead a two-phase system forms in the Mn-Cu alloys during cold rolling. The dimensions of the inhomogeneities were calculated in the framework of the two-phase model with drawing Guinier approximation for the diluted system of scattering particles (12) and also through the correlation function Y(r) (13) (fig. 5). Inhomogeneities of 4.5 and 7.0 nm diameter were correspondingly obtained proceeding the assumption of a spherical form for the precipitates. Taking into account that both methods were developed for the dilute system of particles, whereas we dealt with the high density system of inhomogeneities, we

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89-l

consider the obtained values to be rough estimates. Nevertheless, they are in good agreement with TEM data. Figure 5 shows a weak maxima on the y(r) function that shows some correlation in the occurrence of the precipitate distribution. The most probable distance between centers of the inhomogeneities is roughly 15 nm. The tail part of the SAKS curves showed significant deviation from Pored's law (14) both for point and for slit collimation. An intensity decreasing as I(sj=ks-4 (s-scattering vector) is assured in the case of the dilute system of uniform density spheres at the point collimation (14). In fact, a power of -2.8 was obtained in the experiment. This difference is sufficiently large, even for testifies that density of precipitations, and one the case of a high had no sharp rolling of Mn-Cu alloys cold inhomogeneities appeared at boundaries. Thus, the SAKS experiments show that cold rolling of Mn-Cu alloys results in decomposition of the supersaturated y-solid solution and that 4 - 6 nm in diameter inhomogeneities appear. It was :shown by (4,5,10) that decomposition of supersaturated Mn-Cu alloys by means of :Low temperature aging proceeds in two stages. In the first one, the isostructural decomposition into Mn-rich and Cu-rich areas takes place and this process can #continue for 2 - 10 h at aging temperatures of 650 - 700 K. Later, the a-phase precipitations appear mainly on the grain boundaries. Hence, cold rolling not only initiates decomposition of the supersaturated solid solution, kinetics. Cold rolling suppresses the stage of its but also changes accelerates isostructural decomposition as well as promotes and the precipitation of the stable a-phase. Freundertal and Weiner (15) estimated possible heating during cold rolling and showed that the overheating did not exceed 200 K. Clearly, aging for a few seconds during rolling at these temperatures not can result in any significant investigations of decomposition. The structural during changes plastic deformation of some aged Cu-Ti and Al-Cu alloys with similar multiphase transitions were described by Chuistov (16). He showed that the decomposition of solid solutions through homogeneous nucleation does not depend on the plastic deformation, whereas decomposition through heterogeneous nucleation is accelerated by plastic deformation. This phenomenon is connected with an increase the density of defects in the crystal lattice that makes the new phase nucleate easily. Springer and Schwink (17) studied the effect of the elongation on dynamic deformation aging of Cu-Mn alloys with Mn contents up to I4 0 and showed that additional pipe-diffusion along the dislocation lines occurs in deformed material. Thus, following the above cited examples, we can describe the decomposition of Mn-Cu alloy at cold rolling as follows. The main reason for the stability of supersaturated y-Mn-Cu alloys at low temperatures is a large misfit of embryo and parent lattices, provided heterogeneous nucleation of the a-phase is possible. Segregation of the alloys into two metastable isostructural phases on quenching or during usual low temperature aging results in a little additional decrease of '-he free energy of the alloy and "freezing1 of the formed metastable Accumulation of lattice state. defects by means of plastic deformation facilitates the heterogeneous nucleation of the energetically favorable aphase. Thus, it is possible to pass over the metastable state of isostructural decomposition during rolling. It should be noted that dark field TEM studies showed that size of the coherent precipitates (forming a single crystal like a diffraction pattern of the a-phase) did not exceed 4 - 6 nm. We estimated the critical size of the coherent roughly precipitates using the ratio D 5 1.5*b/a (where D is the diameter of the coherent precipitate, b is the Burgers vector and a is the misfit of lattice parameters of the y- and a-phases) derived by Rouhtburd (18). a-Mn is precipitated in the orientation relationship {lll}y//{lOl)a, CllO>y//a_ For the dcllO>y = 0.265 nm, dclll>a = 0,257 nm and Burgers vector b = dcllO>y the critical size of the coherent precipitates is approximately equal to 13 nm. This value is significantly decreased for another habit planes. Probably, particles experimentally observed by SAKS and TEM

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techniques had critical size for the coherent precipitates. Thus, even with the heterogeneous nucleation coherent or semicoherent boundaries of the embryos restrict growth of the a-phase precipitates is restricted to not more than 4 6 nm in diameter. However, it is clear that such small scattering volumes cannot form the additional sharply defined diffraction maxima observed in the X-ray phase analysis (fig. 1). Thus, large particles of the a-phase at the cold rolling are formed too. Chuistov (16) showed that plastic deformation results in the loss of the precipitation coherence. Apparently in this case repeat rolling first results in the loss of the interface coherency embryo and matrix and second increases the density of dislocations. The former vanishes restricting influence of the parent phase lattice on the embryos and the latter increases diffusion mobility of Mn atoms through its pipe-diffusion along the dislocation lines. Combination of these factors promotes intensive growth of the embryos and formation of the large-scale particles of the a-phase. This work was financially supported by the State Technology of Ukraine (Project 7.02.01/006-92).

Committee

on Science

References L.M.Schetky and J.Perkins, Machine Design, 50, 202 (1978). 1. 2. D.Birchon, D.E.Bromley, and D.Healey, Mater. Sci. J., 2, 41 (1968). 3. R.J.Goodwin, Mater. Sci. J., 2, 121 (1968). 4. Yu.K.Favstov, Yu.N.Shulga and A.G.Rahshtadt, Metallovedenie vysokodempfiruyuschih splavov, p-66, Metallurgia, Moscow (1980) (in Russian). 5. K.Ito and M.Tsukishima, Trans. Jap. Inst. Metals, 26, 319 (1985). 91, 101 (1985) 6. S.A.Demin, A.I.Ustinov and K.V.Chuistov, Phys.Stat.Sol.(a), S.A.Demin and A.I.Ustinov, Metallofizika E.V.Barmashina, N.I.Glavatskaya, 15, N 12, 42 (1993). 8. C.Servant, G.Haeder and G.Cizeron, Met. Trans. A, 6, 981 (1975) 9. S.A.Demin, Crystal Res. Technol., 28, 1121 (1993). 10 E.Z.Vintaikin, V.B.Dmitriev and V.A.Udovenko, Fizika Metallov and Metallovedenie, 46, 790 (1978). 11 .B.Belbeock and A.Guinier, Acta Met., 3, 370 (1953). 12 .A.Guinier, Small angle scattering of X-rays, Gordon and Breach, London-New-York, (1955). 13 P.Debye and A.M.J.Bueche, J. Appl. Cryst., 20, 518 (1949). 14 :G.Porod, Kolloid. Z., 124, 83 (1951). 15 .A.M.Freudental and J.H.Weiner, J. Appl. Phys., 27, 44 (1956). 16 .K.V.Chuistov. Starenie Metallicheskih Splavov, p. 157, Naukova Dumka, Kiev (1985) (in Russian). 17 F.Springer and Ch.Schwink, Scripta Metall., 25, 2739 (1991). i Fiziki Metallov, 36, 235, 18 :A.L.Rouhtburd, Problemy Materialovedenia Metallurgia, Moskow (1964) (in Russian).

and

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i

50

52

54

Diffraction FIG.l. X-ray diffraction

pattern

56

angle, 20

for cold rolled Mn-10

%

Cu

alloy.

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Mn-CuSOLJDSOLUTION

1 - quenched

P

1

1

2 -rolled

3 .? . -2 *

3 - aged i

Mn cu

CL

3 -

I(1) -

I(2

2

3

1

Scattering

1 2 -

:\

* ; t

0.00

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0.08

4 0.16

vector,

l/A

-

5

10

Distance,

!&

il.00

Scattering

Fig.3. SAXS spectra for cold rolled Mn-10 % Cu alloy (point collimation).

0

oc

0.08

0.16

vector,

1/A

Fig.4. SAXS spectra for cold rolled Mn-30 % Cu alloy (slit collimation)

15

;o

nm

Fig.5. Correlation function for Mn-10 % Cu alloy, calculated from SAXS (point collimation).