Intermetallic Fe-Al layers obtained by the powder cloth method

POWDER TECHNOLOGY ELSEVIER

Powder Technology 94 (1997) 25%263

Intermetallic Fe-A1 layers obtained by the powder cloth method F. Binczyk a,., S.T. Skrzypek b, A. Gierek a a Silesian Technical UniversiO; ul. Krasins4~iego 8, 40019 Katowiee, Poland h University of Mining and Metallurgy, ul. Mickiewicza 30, 30059 Cracou', Poland

Received 19 February 1996: revised 25 April 1997

Abstract Three-layer composites, i.e. substrate, diffusion and sinter layers, were obtained by the powder cloth method. The sinter layer consisted of self-disintegrated powders of Fe-AI-Me intermetallic phases with the addition of pure iron. Plain carbon steel was chosen as a substrate. High temperature pressing was applied, The quality of joints can be defined by the properties of the sinter and diffusion layers. The diffusion layer measurements were verified by the solid state and reactive diffusion laws, The best correspondence of the measured and calculated thicknesses of the diffusion layers was found for diffusion coefficients a little different than those given in the literature, i.e. DI' = 1.8 × 10-9 cm2/s at 1373KandD2'=l.5×10 ~cm2/satl173K. ©1997 Elsevier Science S.A. Keywords: Powder cloth method; Diffusion layer; Sintering; Pressing; Fe-AI intermetallic phases; Composites

1. Introduction One of the interesting phenomena occurring in high-aluminium alloys of the Fe-AI-Me type is their self-disintegration into the powder form. Self-disintegration can be applied as an easy powder production method [ 1,2]. The powder fractions, size and phase composition can be controlled by the size of the A14C 3 carbide and by some foundry parameters [3,4]. Because of the useful properties of high-aluminium alloys, the application of such self-disintegrated powders is desirable, and one of the solutions could be a powder metallurgy method, e.g. the powder cloth method. The conventional technology of pressing and sintering could be the best solution but in such an approach there are still some difficulties with porosity [5,6].

2. Methodological approach The alloy powders of Fe-A1-Me type made by the selfdisintegration method can contain 32--40 wt.% of A1 and other elements, such as Ni, Cr, Si, Cu, Mn and Co. These intermetallic phases appear in the form of the FesAl, FeAI and FeA12 superlattices [ 4,5 ]. The lattice parameters of these phases depend on the concentration of AI and other elements and they can appear as partially ordered and modulated [7]. * Corresponding author. 0032-5910/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved PI1S0032-5910(97)03324-X

The self-disintegrated powders were obtained from an alloy containing 36.5 wt.% A1, 1.2 wt.% C with Fe making up the balance. The powder contained slightly different amounts of the chemical elements because of chemical reactions occurring in the course of the self-disintegration [ 3-5 ]. Smaller fractions of powders contained more A1 and the concentration of C in these powders decreased to 0.1~0.2 wt.% [4,5]. Different cooling rates for solidification affected the morphology of the A14C3 carbide, which is a major factor affecting the grain size of the powder. In the case of the metal mould, we obtained 40/63/125 ixm powder fractions, and in the case of the sand mould the main fractions were 63/125/ 200 Ixm and constituted 90% of the total powder (Fig. 1). The brittle fracture of self-disintegration powders induced by residual microstresses cause the specific shape of the grains (Fig. 2). Stable Fe-AI-Me alloys have a good resistance to high temperature and a good abrasive resistance. Powder metallurgy methods are expected to yield high-aluminium alloy products of high performance. These experiments deal with the sintering of layers of the Fe-AI-Me type on a substrate of hypo-eutectoid steel. Because of the high diffusion rate of AI in c~-Fe, good adhesion of the layer-substrate joint is expected. Taking into account the poor results of the application of the conventional powder metallurgy method [ 5 ], we decided to investigate the advantages of high temperature pressing,

260

F. Binczyk et a l . / Powder Technology 94 (1997) 259-263

48"246.5

50

r'l Cu mould 1 [] Sand mould j

45 40

35,2

35 30 •

25 20

14.5

15 l0 5

6.4

[ ~

1.2 0.2

0 <20

20-40

40-63

63-125

fraction

125-200

>200

[P.ml

Fig. 1. Size composition of self-disintegrated Fe-AI powder (36.5 wt,% A1 ). Table 1 The two-level statistical experimental package Variable Basic level Variable range

Temperature (K) (x,) 1273 100

Fe (wt.%) (xO 40 10

Sintering time (h) (x3) 1,0 0.5

1 2 3 4 5 6 7 8

+ + + + -

+ +

50 50

+ -

-

30

+

1.5

-

30

-

+ +

50 50

+ -

0.5 1.5 0.5

1373 1373 1373 1373 1173 1173 1173 1173

1.5 0.5

-

30

+

1.5

-

30

-

0.5

IP

Fig. 2. Micrograph of Fe-A1 powder (in sand mould, grain size 63-125 ~m).

changing such factors like sintering temperature, pressure and time of sintering which affect the properties of sinters, i.e. unit density, porosity and microhardness. Introductory experiments made it possible to reduce the number and ranges of the sintering parameters.

induction ~nter

3. Experimental procedure and results The so-called full two-level 23 package for statistical experiments [ 8 ] was applied to establish the influence of the technological factors (as input parameters, xi) on the properties of joints, i.e. the sinter and the intermediate diffusion layer (as output variables, Yj) (Table 1). As already mentioned, the input parameters were the pressing temperature (x~), the wt.% of pure iron powder (x2) and the time of sintering (x3). Self-disintegrated Fe-AI powder (36.5 wt.% AI) of grain size 40-63 Ixm and Fe powder of grain size 20-40 ~zm used. The mixture of both powder fractions was poured onto a 3 mm plate of substrate made of ferritic-perlitic non-alloyed steel (Fig. 3).

plate of

~r'a0t~ematn'~ h ~rmt:~/o PtR~4-1 Fig. 3. Schema of high temperature pressing.

This composite was placed in a graphite mould under a vacuum of 1.33 Pa; then an induction heating and pressing system of 10 MPa was turned on, After the necessary time it

F. Binczyk et al. / P o w d e r Technology 94 (1997) 259-263

261

Table 2 Resulting output values for high temperature pressing Exp. no.

txHV2o (MPa)

Average thickness (l,zm)

Real density ( g / c m 3)

Theoretical density ( g / c m ~)

Porosity (%)

AI in sinters, C~ (wt,%)

1 2 3 4 5 6 7 8

3260 3260 3030 2920 6530 6090 6240 6040

81 62 96 93 8 4.5 10.0 5.5

6,07 6.03 5.31 5.19 5.18 4.77 4.73 4.68

6,42 6,42 6.03 6.03 6.42 6.42 6.03 6.03

5.5 6.1 12.0 14.0 19.0 25.7 21.6 22.4

18.25 18.25 25.55 25.55 18.25 18.25 25.55 25,55

Table 3 Estimation of the probability p of the impact of experimental variable x~ on the output properties Y, at the significance level of c~=0.1 Input variable

x~ x2 x3

Probability p Y~

Y2

Y~

Y4

0.006 0.025 0.367

0.004 0.371 0.333

0.001 0.057 0.095

0.001 0.081 0.394

The above relations hold true when p _
was cooled to 100°C under vacuum. The output variables of the 23 experimental package, i.e. real density (Y~), porosity (I/2), microhardness (IxHV2o) of the sintered layer (Y3) and thickness of the diffusion layer (Y4) were then measured (Table 2). The porosity was calculated from

P=lOO(l-pr/pO

(%)

(1)

where p, is the theoretical density (from the chemical composition and the lattice parameters) and Pr is the real density. A multiple regression analysis of the input parameters (&) on the output parameters ( ~ ) was performed. The value of the probability p of the zero hypothesis is used as a criterion of the estimation. It is a measure of the influence of the input characteristics on the output values. When the p value is less than the level of significance o~, the zero hypothesis is rejected. If the p value is greater than the level of significance then the defined random variable has an influence on the value of the result. The results of this calculation are shown in Table 3. Most of the results were obtained by metallographical investigations, i.e. the microstructure of the sinter layer and the intermediate diffusion layer (Figs. 4 and 5).

4. Discussion For the assumed significance level, i.e. o~= 0.1, the sintering temperature was a prominent factor in sintering and in the creation of the diffusion layer. With the increase of the

sintering temperature the porosity decreased and the thickness of the diffusion layer increased (Table 3). A higher concentration of Fe yields a higher density, lower porosity and greater microhardness of the sinter matrix and a thinner diffusion layer. The contact points between the Fe and FeAI grains lead to better diffusion in the range of grain size used here, giving lower porosity. Since a higher concentration of Fe results in a lower gradient of aluminium concentration between the substrate and the sinter layer, it produces a thinner diffusion layer (Tables 2 and 4). The sintering time has no effect on the output features. This may be the result of too long a time taken at the basic level (Table 1). The calculations of the significance tests using probability coefficient p of the input-output relations versus the technological variables showed that the most influential factor was the sintering temperature (Table 3), which is the most influential factor in diffusion. Diffusion layers are subject to diffusion laws, and in this case two approaches were taken, namely, reactive diffusion and solid state diffusion. The diffusion layer thickness for reactive diffusion was calculated according to the formula

x2=Dt

(2)

where x is the thickness of the diffusion layer, D is the diffusion coefficient [9] and t is the sintering time. For solid state diffusion, the following formula was used:

C~-Co

1/2

(3)

where C, is the concentration of AI at the lowest distinguishable level (1 wt,%) for the measurement of the diffusion layer thickness, C~ is the potential maximum concentration of A1 in the diffusion layer (Table 2) and Co is the background concentration of A1 in the substrate. The calculated results are presented in Table 4. The thicknesses of the diffusion layers were calculated by means of the diffusion coefficient D for A1 in pure Fe [9]. The application of the solid state diffusion procedure according to Eq. (3) gives a better fit. For the same diffusion coefficients the ratios of the calculated/measured thicknesses are closer to 1 than the ratios calculated for the reactive diffusion

F. Binczyk et a l . / Powder Technology 94 (1997) 259-263

262

Exp 2 tt

f

170 ~aml~.

") #

-~

liPl[

Exp3~

~

:~

s

•

•

-

I

b -

:*'11" -

t

70tim ~' L

Fig. 4. Microstructure of the sinters and intermediate layers.

Fig. 5. Microstructure of the sinter and diffusion layers from experiment no. 3 - - line scan indicating relative aluminium concentration.

using Eq. (2). Further calculations indicate that quite a good fit of the measured and calculated thicknesses is achieved for diffusion coefficients of D ~ ' = 1.8 × 10 - 9 c m 2 / s at 1373 K and Dz' = 1.5 × 10- ~ cm2/s at 1173 K. These diffusion coefficients are obtained taking into account the nearest oscilla-

tion of the ratio m/c to I. The opposite diffusion flow, i.e. the Fe atoms in the Fe-AI sinter layer, was not taken into account. At lower temperatures D2' D~ (Table 4). The applied pressure of 10 MPa reveals a new contact area between the grains themselves and between the grains and the substrate, which should intensify diffusion. This conclusion is fulfilled only for higher temperatures. Probably, the applied stress was able to break the oxide films around the grains because the yield point of the intermetallic FeA1 at 1373 K had decreased sufficiently. The results of the reactive diffusion show a considerable discrepancy between the measured and calculated thicknesses in spite of the possibility of the occurrence of such a mechanism in Fe-A1 alloys being reported in the literature [ 10] and confirmed by exothermic effects due to the intermetallic phase formation.

5. Summary and conclusions It was experimentally proved that a sinter FeAI intermetallic layer can be achieved on ordinary substrate materials.

F. Binczyk et aL / Powder Technology 94 (1997) 259--263

263

Table 4 A comparison of measured and calculated thicknesses of diffusion layers due to the reactive diffusion and solid state diffusion procedures Exp. no.

Reactive diffusion according to Eq. (2) D (cm2/s)

1 2 3 4 5 6 7 8

1 × 10-'~

4 × 1 0 -It

Solid state diffusion according to Eq. (3)

Measured thickness m (Ixm)

Calculated thickness a (~m)

m/a

D

ratio

(cm2/s)

81 62 96 93 8 4.5 10.0 5.5

23 13 23 !3 4.65 2.68 4.65 2.68

3.486 4.62 6.93 1.72 1.68 2.15 4.66 4.66

1 × 10 --9

4×10 -t~

T h e e x p e r i m e n t s s h o w e d t h a t the quality o f the w h o l e j o i n t c a n b e d e t e r m i n e d b y the quality o f the sinter a n d the interm e d i a t e d i f f u s i o n layers. T h i s a p p r o a c h m a y b e u s e d in m a t e rials e n g i n e e r i n g to o b t a i n v a r i o u s k i n d s o f sinter layers o n d i f f e r e n t s u b s t r a t e s w i t h i n t e r m e d i a t e d i f f u s i o n layers, i.e. three-layer composites. The following conclusions were reached. T h e r e c o m m e n d e d s i n t e r i n g t e m p e r a t u r e o f self-disinteg r a t e d p o w d e r s o f i n t e r m e t a l l i c FeA1 o n f e r r i t i c - p e r l i t i c steel is 1373 K. T h e s i n t e r i n g t i m e c a n b e s h o r t e r then 30 rain. S i n t e r layers m a d e o f s e l f - d i s i n t e g r a t e d p o w d e r s o f the F e A1 t y p e h a v e g o o d a d h e s i o n to f e r r i t i c - p e r l i t i c steel o w i n g to the c r e a t i o n o f the d i f f u s i o n layer. T h e F e - A 1 p o w d e r s c a n b e a p p l i e d to the t h e r m o - d i f f u s i o n t r e a t m e n t o f alloys; the d i f f u s i o n layers are s u b j e c t to the solid state d i f f u s i o n law.

Acknowledgements T h i s p a p e r w a s s p o n s o r e d b y the C o m m i t t e e o f Scientific R e s e a r c h ( K B N ) u n d e r C o n t r a c t Nos. 7 $201 033 0 6 a n d 7 $ 2 0 2 0 1 9 06.

Calculated thickness b (Ixm)

m/b

D'

ratio

(crn2/s)

64.4 37.2 69.0 40.0 13.0 7.4 13.8 8.0

1.258 1.667 1.391 2.334 0.621 0.605 0.725 0.690

1.8 X 10 -"

1.5×10 t~

Calculated thickness c (/xrn)

m/c

86.4 49.9 92.6 53.5 7.9 4.6 8.5 4.9

0.94 1.24 1.04 1.74 1.01 0.99 1.18 1.13

ratio

References [ 11 F. Binczyk, A. Gierek, G. Pucka and T. Mikuszewski, Otrzymywanie, wtasnogci i zastosowanie rnetalowych proszk6w samorozpadowych, Ekspl. Maszyn, (2/3) (1988) 21-23. I2] A. Gierek and F. Binczyk, Sarnorozpadowe proszki metalowe, technologia otrzyrnywania, wtasnogei i zastosowanie, lnz. Mater., (2) (1989) 51-57. [ 3 ] F. Binczyk, The factors which determine the structure of the Fe-A1-C alloys with high content of alurniniurn and the analysis of the phenomena leading to their self-decomposition, Hab. Dr Thesis, v. 40, Silesian Technical University, Katowice, Poland, 1991 (in Polish). [ 4] A. Wojtysiak and S.T. Skrzypek, Zrniana energii zrnagazynowanej w sieci krystalicznej nadstruktury FeA1 podczas sarnorzutnego rozpadu stopu Fe-A[-C, Arch. Nauki Mater., l I ( 1) (1990) 45-58. [5] S.T. Skrzypek, J. Karwan-Baczewska, E. Durowa and J. Kazior, Conventional pressing and sintering of the high alurniniurn powders type Fe-A1-C-Me, Metal. Proszkow, (2) (1994) 3-12 (in Polish). [6] S.T. Skrzypek, F. Binczyk and A. Gierek, Microstructure and mechanical properties of sinters of self-disintegrated powders with Cu addition, Proc. 4th Int. Conf. Advanced Materials and Processes, EUROMAT '95, Padua/Venice, Italy, 1995, pp. 541-544. [7] W. K6ster and T. Godecke, Physikalische Messungen an EisenAlurninium-Legierungen rnit 10 bis 50 Al, Z. Metallkde., (2-8) (1982). 18l W. Volk, Applied Statistics for Engineers (Polish edn.), WNT, Warsaw, 1973. [91 J. Fridberg, L.E. Tornahl and M. Hillert, Diffusion of iron, Jernkonterets Ann., 153 (1969) 263-276. I1o] J.S. Sheasby, Int. J. Powder Metall. Powder Technol., 15 (4) (1979) 301.