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FIM-ATOM PROBE STUDIES OF THE
DECOMPOSITION IN THE METALLIC GLASS Ni45 Pd35 P20
M. Oehring, P. Haasen
To cite this version:
M. Oehring, P. Haasen. FIM-ATOM PROBE STUDIES OF THE DECOMPOSITION IN THE
METALLIC GLASS Ni45 Pd35 P20. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-275-C7-
280. �10.1051/jphyscol:1986747�. �jpa-00225941�
JOURNAL DE PHYSIQUE
Colloque C7, supplement au n o 11, Tome 47, Novembre 1986
FIM-ATOM PROBE STUDIES OF THE DECOMPOSITION IN THE METALLIC GLASS N i ~ 5 Pd3 5 ' 2 0
M. OEHRING and P. HAASEN
Institut fiir Metallphysik der Universitat Gbttingen, 0-3400 Gdttingen, F.R.G.
and Sonderforschungsbereich 126, 0-3392 Clausthal-Zellerfeld, (Gottingen), F.R.G.
Abstract
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A Field Ion Microscope (FIM) combined with an Atom Probe CAP) was used to study decomposition in the metallic glass N45Pd35P20. As-quenched specimens are found to be homogeneous whereas isothermally annealed specimens show phosphorous enrichment of small concentration amplitudes and diameters.Two glass temperatures are observed in annealed specimens. The temper em- brittlement in this glass must be caused by crystallisation, not by decom- position.
I
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INTRODUCTIONDue to a nucleation barrier of crystalline phases, it is possible that metallic glasses which are metastable undercooled liquids decompose into amorphous phases as it is known from silicate and borate glasses. Such a decomposition can affect strongly the macroscopic properties of a metallic glass. Various experimental methods as Transmission Electron Microscopy (TEM), Small-Angle X-Ray Scattering (SAXS), FIM-AP, Differential Scanning Calorimetry (DSC) and electrical resistance measurements have been employed to detect phase separation in metallic glasses, partly with controversial results 111. On continuous heating two glass tempera- tures T can be found by DSC in phase separated metallic glasses /2/,/3/.
~chluckegier and Predel investigated systematically the decomposition of NiPdP glasses of several compositions by DSC. As-quenched specimens (e.g. Ni41Pd35P18) showed on first heating in the DSC-scan one glass temperature and an exothermic peak which is identified with the enthalpy of a phase separation in the amorphous state. If heating is stopped prior to crystallisation and the specimens are rapidly cooled down to room temperature two glass temperatures and no more ex- othermic peak are found in the second DSC-run providing clear evidence for decom- position into two amorphous phases. Additionally, Jing et al. 141 found structural changes by MoDbauer spectroscopy on annealing of Ni45Pd35P20 above the glass temperature. To get direct information about the two-phase microstructure in annealed Ni,, Pd3, P2 we have applied FIM-AP spectroscopy.
Piller and Haasen 151 have shown by FIM-AP studies of a-Fe40Ni40B,o that a) phase separation can lead to the well-known temper embrittlement of a metallic glass on annealing prior to crystallisation. It has been further demonstrated by Gerling and Wagner 161 that b) annealing out of free volume or both effects a) and b) to- gether occurring at different temperatures during isochronal annealing can cause temper embrittlement. By investigating the decomposition of NiPdP glasses further information is expected about the possible correlation between temper embrittlement and phase separation.
I1
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EXPERIMENTALMelt spun ribbons of Ni4,Pd35P20
,
1 mm wide and (32f7) pm thick were kindly Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986747C7-276 J O U R N A L DE PHYSIQUE
provided by J. Jing and U. Gonser, Universitat Saarbriicken. The true alloy com- position was determined by microprobe analysis which yielded 45.4 at% Ni, 34.9 at%
Pd and 19.7 at% P.
For isothermal annealing the specimens were encapsulated in evacuated quartz tubes mbar) which were subsequently filled with 99.996 % Ar. The heat treatment was stopped by quenching the quartz tubes into water.
A Perkin Elmer DSC-2 instrument was used for thermal analyses. DSC-runs of a.q.
material were recorded with heating rates 0 of 5, 20 and 40 K/min. Additionally, DSC-scans were taken at a heatin8 rate of 20 K/min from specimens, which had been isothermallg annealed at 330 C for 2 h and 4 h. The glass temperature T =(319+2) C and the crystallisation temperature T =(388+2) OC are in fair agree- mgnt with those obtained from Schluckebier and ~reselts DSC-plot for Ni4,Pd3,P,, (T =310 OC, T =390 OC) 131, who however found in contrast to us two crystallisa- tign peaks.
fie
following table summarizes our DSC-results:Table
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Iheat treatment 0 [Klmin] T [OC] Tg2 c°C] Tc [OC] AHc [kJ/mol]
g 1
Fig. 1 shows the DSC-scans for an a.q. and two at 330 OC isothermally annealed specimens with two glass temperatures indicating that the glass has decomposed into two amorphous phases.
As a measure of embrittlement the relative strain at fracture XF was determined from bending tests ($=d/(Zr-dl; d: thickness of the ribbon, r: radius of cur- vature of the semi-circular loop).
X-ray diffraction of both surfaces of a 6h at 330 OC isothermally annealed specimen showed several sharp peaks. It is concluded that a large volume fraction of the specimen has crystallised at least at the surfaces. As Drehman and Greer point out /7/ heterogeneous nucleation plays an important role in the crystallisation of the glass Ni40Pd40P20. Judging by the full ductility of the 4h annealed specimens and their only slightly decreased enthalpy of crystallisation they are however amor- phous.
I11
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FIM-ATOM PROBE STUDIESFIM-tips were prepared by mechanically grinding both narrow sides of metallic glass ribbons in order to obtain wedge-shaped specimens which were electropolished between 12 V and 6 V (DC) in a solution of 10 % perchloric acid in acetic acid.
The anode DC voltage was pulsed, using a 2 Hz frequency and pulse lengths down to 5 nsec for the last stage of electropolishing.
The FIM and AP studies were performed on the Gottingen instrument described by Piller /8/ and Wagner 191. Measurement conditions were a background pressure of 2*10-9 mbar, a tip temperature of about 80 K and usually 5*10-' mbar Ne pressure.
Field ion micrographs of both a.q. and annealed specimens (fig. 2a,b) showed uni- formly distributed bright spots without ring structures or any contrast between phases. Therefore we suppose that we have hit an amorphous region in the partially crystallised 6h annealed s ecimen. In the AP mass spectra P'
P ,
P+, Ni2+, Ni+, pd2+ and ~ d + were present. Most of the Ni and.Pd atoms were field evaporated as NiZ+ and ~ d + , respectively. Due to overlapping of 6'~iz+, 6'~i2f and 6 4 ~ i 2 f with $'P+ and (3'~~)+ 5.4 % of Ni atoms are taken as P atoms. For N i 4 ~ P d ~ s PZO all AP investigations give a total composi- tion of 47.5 at% Ni, 27.4 at% Pd and 25.1 at% P. If these values are corrected for the peak overlapping 50.0 at% Ni, 27.4 at% Pd and 22.6 at% P are obtained in- dicating preferential field evaporation of Pd atoms at the chosen pulse ratio of0.17. This could be confirmed by decreasing the pulse ratio to 0.085 resulting in an additional loss of 17 % of the Pd atoms. Further, preferential field evapora- tion is increased by removing the imaging gas which leads to a decrease of 14 % in the detected Pd atoms. For our AP studies we chose 5.10~ mbar Ne and a pulse ratio of 0.17 which allowed to control atomprobing in the FIM image.
Because there is no depth scale available for atomprobing of metallic glasses we defined one atomic layer after catching 25 atoms. In the typical voltage range of 6-7 kV 30-40 atoms are detected from a crystalline Ni but such a comparison cannot be very accurate because e.g. the different field evaporations end forms of crys- talline and amorphous tips.
IV
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RESULTS AND DISCUSSIONWe took AP concentration profiles (typically 500 layers in length) from a.q. and annealed specimens (2h, 4h and 6h at 330 OC). The latter showed small concentra- tion variations in amplitude and diameter (see fig. 3). More information can be derived from autocorrelograms R (k) of P concentration profiles smoothed by a mov- ing average over 2 atomic layers. As can be seen from fig. P 4 Rp(k)"O, if k>O, for the a.q. specimen which means that the specimen is homogeneous whereas a posi- tive correlation is found in the autocorrelograms of annealed specimens which reveal an increase in k with larger annealing time. In autocorrelograms oflyhite noise 95 X of all val$es of R(k) for k>O lie between *2U with ~zl/(n-k) (n:
profile length) /lo/. Taking into account that the profiles w smoothed by the moving average over 2 atomic layers which gives a=11(0.5n-k)~~ we can conclude from ~(1)>20 that significant deviations from pure noise are present in our concen- tration profiles. With d =k d (d10.26 nm: atomic layer thickness) a precipitate diameter of 2.1 nm, 3.1 a t a n 8 3.9 nm is obtained for an annealing time of 2h, 4h and 6h at T =330 OC, respectively. From the concentration distribution a precipi- tate concenqration of c =24 at% P and a matrix concentration of cm=18 at% P are obtained which gives aP%lume fraction f=0.33 and for d =3.1 nm a precipitate number density N ~ = Z
*Ips
m-3 and a mean precipitate d i s t a n g s =3.7 nm.In order to examine our evaluation of concentration profilegP by autocorrelation analysis we simulated a concentration profile with the help of a computer. The program we used is described in 6111. We assumed the decomposition parameters which we derived for the 4h at 330 C annealed specimen (see fig. 5). The auotcor- relogram of this concentration profile which was smoothed by a moving average over 2 atomic layers, too, is displayed in fig. 5. From the low value of ~ ( 1 ) and the small ko=7 (ko=12 was assumed) we conclude that our obtained concentration ar,pli- tudes and precipitate diameters are minimum values.
It must be pointed out that c and c cannot be accurately determined due to the small precipitate diameters. 'qt woulz be difficult to distinguish between the numerous crystalline phosphides which are present in the binary phase diagrams con- stituting the ternary Ni-Pd-P diagram (Ni3 P ( tetragonal), Nil P2
,
Nil P, Pds P(cementite structure), Pds P2,.
.
.) 1121. Summing up the decomposed microstructure can be described according to Gaskellts model /I31 of Teo&o metallic glasses (T:transition metal, M: metalloid) which proposes TsM clusters (cementite structure) of about 1.5 nm in diameter in a M depleted matrix. AP-studies have verified this model for Fe40 Ni40 B2o /5/ and Fees & 5 / 141. In electrodeposited Nia I PI 9 Sonnberger et al. found a similar microstructure by SAXS with spherical precipitates 2 nm in diameter, but they got no information about the concentration of either precipi- tate or matrix /IS/.
From fig. 6 showing the relative strain at fracture hF plotted against the an- nealing time tA on isothermal annealing at 330 OC one can see that the embrittle- ment sets in very sharply between t =4h and t =6h. Up to 4h annealing time decomposed specimens of Ni,,5Pd35P20 are ductile. A ~ h a t A means that the temper em- brittlement is not affected by decomposition but is a consequence of partial crys- tallisation as confirmed by XRD. That phase separation can occur without temper embrittlement was also observed by Chou and Spaepen in the metallic glass Pd,,AySi,, /16/. In contrast to T~,Be,,oZrlo 1171 which embrittles in a first step by annealing out free volume as concluded from the parallel increase of the relative density (fig. 6, lower
art)
enough free volume remains in NL,, P d 3 ~ P20 toC7-278 JOURNAL DE PHYSIQUE
enable plastic deformation to occur. Annealing out of free volume may also be the cause of the more continuous temper embrittlement in Mos,Rb, &, and NI,,Nb,, (fig. 6).
Acknowledgements
We are grateful to J. Jing and U. Gonser (~aarbrucken) for the provision of the material. DSC-measurements were carried out at the Inst. f. Werkstoffkunde und Werkstofftechnik der TU Clausthal-Zellerfeld. We thank Mrs. A. Kolb-Telieps for her experimental assistance. Thanks are due to R. Wagner, L. v. Alvensleben, R. Grune and A. Hutten for many fruitful discussions.
References
/I/ R.W. Cahn, in: Physical Metallurgy, 3rd ed., Vol. 2, ed.
R.W. Cahn, P. Haasen, North Holland, Amsterdam 1983, p. 1779
/ 2 / A.R. Pelton, L.E. Tanner, Proc. 5th Int. Conf. Rapidly Quenched Me- tals, ed. S. Steeb, H. Warlimont, North Holland, Amsterdam 1985 /3/ G. Schluckebier, B. Predel, Z. Metallkde. 74(1983)569
141
J. Jing, H.-G. Wagner, U. Gonser, presented at DGM-Hauptversammlung, Stuttgart, 1985151 J. Piller, P. Haasen, Acta metall. 30(1982)1 /6/ R. Gerling, R. Wagner, Scripta metall. 17(1983)1129 /7/ A.J. Drehman, A.L. Greer, Acta metall. 32(1984)323 /8/ J. Piller, Diploma thesis, University of Gottingen, 1977
/9/ R. Wagner, Field Ion Microscopy in Materials Science, Crystals, Vol. 6, Springer Verlag, Berlin 1982
1101 G.E.P. Box, G.M. Jenkins, Time Series Analysis, Forecasting and Con- trol, Holden Day, San Francisco 1970
/11/ L. v. Alvensleben, R. Griine, A. Hutten, M. Oehring, this volume 1121 E. Wachtel, H. Haggag, T. Godecke, B. Predel, Z. Metallkde.
76(1985)121
1131 P.H. Gaskell, J. Non-Cryst. Solids 32(1979)207 1141 A. Menand, Ph.D. thesis, University of Rouen, 1984
1151 R. Sonnberger, H. Bestgen, G. Dietz, 2. Phys. ~56(1984)289 1161 C.P. Chou, F. Spaepen, Acta metall. 23(1975)609
1171 R. Grune, M. Oehring, R. Wagner, P. Haasen, Proc. 5th Int. Conf.
Rapidly Quenched Metals, ed. S. Steeb, H. Warlimont, North Holland, Amsterdam 1985
Fig. 1
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DSC-scans (dH/dt versus T) of an a.q. and two annealed specimens.Fig. 2
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Field ion micrographs ofa) an a.q. Ni,,bP&sP20 specimen ( ~ e , 6.8 k ~ )
b) a 6h at 330 C isothermally annealed specimen of NL,,P&,Pz0 ( ~ e , 8.5 k ~ )
JOURNAL DE PHYSIQUE
Correlation Length k
Fig. 3
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Autocorrelograms of P concentration profiles smoothed by a moving average over 2 atomic layers.Correlation Length k
Fig. 5
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Autocorrelogram of a com- puter simulated concentration pro- file which was smoothed by a moving average over 2 atomic layers. As- sumed parameters: c =20 at %, c =24 at %, c0=18 at %, dPPt-3.1 nmkl .O nm, r =7.04 nmPP t- aP
Number of Oesorbed Layers
Fig. 4
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Composition profile of a 6h at 330 OC annealed specimen smoothed by a moving average over 2 atomic layers.10 100 1000
Annealing time t,[minl
Fig. 6
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Relative strain at frac- ture XF of several metallic glasses and relative density of Ti,, Be,, Zr,, (values taken from1 1 7 1 ) . 0 : T b o B e c o Z r ~ o ;
