World Journal Of Engineering
The effect of boundary layer on activated liquid phase sintering of W-bronze composites by Co and Fe
Additions.
1
M. S Kahtan, 2 R. Azmi
Universiti Malaysia Perlis
School of Material Engineering-Jejawi- Taman Muhibah 02600 Perlis-Malaysia
1
E mail: tompetorage954@yahoo.com
Abstract
W grains
on
the
verge
of
complete
dissolution
In this study, the effects of 1-5wt. % Fe and Co
additions on the densification of different
compositions of W-pre-alloy and pre-mixed bronze
compacts were examined. The sintering process was
conducted isothermally at temperatures ranging
from920 to1300ºC for 3h. Relative sintered densities
in the range of 70-90% were achieved. The results
showed that substantial improvement in hardness by
a factor of two folds and density by 5-15% was
achieved for the sintered compacts by the addition of
2–3 wt. % Fe and Co as activators.
Key words: Composite material; Microstructure;
sintering; grain boundaries.
Neck
W40 Bro. Fe3 1100ºC Mag. = 2.00 K X 5µm
Fig. 1: An image of W40wt. %- pre-alloy bronzeFe3wt. % sintered at 1100ºC.
Fig.2 SEM images and EDX line scan analysis show
the formation of Co-W intermetallics layer of
W40wt%-pre-alloy bronze sintered compacts of Co
3wt.% addition sintered at 1100ºC. Complete wetting
and the richness of this layer by Co element is evident.
Introduction
Tungsten metal and bronze alloy are almost completely
immiscible in both solid and liquid phases. This
limitation excludes the use of conventional alloying
techniques to yield W-bronze alloys. So far solid-state
(SS) and liquid phase sintering (LPS) techniques are
extensively used to sinter compacts of W-Cu
composite slugs according to their desired industrial
specifications [1]. Panichkina[2] and later by Johnson
and German[3] and Y. Boonyongmaneerat[4] have
demonstrated that addition of group VIII transition
metals including Ni, Co, Fe and Pd can beneficially
enhance the sintering kinetics of W powder and thus
lower its sintering temperature. It was found that
sintering additives which are insoluble in W can
segregate to the W interparticles zone, provide a high
diffusivity transport path for W atoms and
consequently lower the activation energy for bulk
transport of W [5].
O
C
Sn
Co
W
W40BRO. Co3 1100 ºC 15.0 kV x 5000 5µm
Fig.2 SEM images and EDX line scan analysis show
the formation of Co-W intermetallics layer. Peak of Co
element shoots up at the interboundary layer.
Experimental
Mixing of the as received powders is an essential step
in green compact preparation. Mixes of different
compositions varied from W40-80wt. %- pre-alloy
bronze were prepared from their elemental powders of
tungsten, pre-alloy and pre-mix bronze. The compacts
were held isothermally at sintering temperature ranged
from 920ºC to 1300ºC for 3 h. H2/N2 as protective gas
mixture of 20/80 wt% ratio was utilized.
Fig. 3: (a) Effects of Fe and Co addition on W50wt. %pre-alloy bronze compacts sintered at 920ºC and
1100ºC (b) W80 wt. %-pre-alloy bronze compacts with
Fe addition sintered at 1100, 1200 and 1300ºC. Figure
4 shows SEM fractograph of W40wt. %-pre-alloy
bronze compact of Fe3wt. % addition sintered at 965ºC
in H2/N2 as protective gas environment. It is apparent
that the Fe rich-W inter-boundary layer completely
Results
wets the W grain compared to the other adjacent non
wetted fractured one. It is apparent that the Fe rich-W
Fig. 1: An image of W40wt. %- pre-alloy bronze-Fe3wt. %
sintered at 1100ºC, dissociation of w grains in interboundary layer
inter-boundary layer completely wets the W grain
is evident
519
World Journal Of Engineering
sintering mechanisms of W were considered a mystery.
Very recently, numerous experiments were devoted to
elucidate the long-standing mystery of subsolidus
activated sintering mechanisms in solid-state sintering
of W-Pd, W-Ni, W-Co, W-Fe materials. Specifically,
that part related to the initiation of activated sintering,
the wetting-prewetting transition and the formation of
quasi-liquid phase at subsolidus eutectic/peretectic
temperatures. In the classical models of Brophy et al.,
[7] and Toth and Lockington, [8], the solid-state
activator is presumed to be the secondary crystalline
Ni-rich phase which completely ‘‘wets’’ the tungsten
primary phase and penetrates along the grain
boundaries (GBs). The work of Luo and Shi, [9] and
Luo, [10] and Gupta et al.[11] proposed a mechanism
in which the stabilized nanometer-thick, quasi-liquid
GB layers in Ni-doped W well below the bulk eutectic
temperature is considered fully responsible for the
densification enhancement in solid-state subsolidus
activated sintering. The outcome of this study came on
the contrary and provides compelling evidences on
different conclusions. It seems to follow the
mechanism proposed by and Toth and Lockington, [8].
compared to the other adjacent non wetted fractured
one.
a
Conclusion
The sintering activation mechanisms seemed to follow
the model proposed earlier by Toth and Lockington
rather than the recently proposed mechanism by Gupta
and coworkers.
b
Fig.3 depicts the effects of Fe and Co addition on W50
and 80wt. %-pre-alloy bronze compacts.
References
b
Element
Wt%
At%
OK
01.38
10.27
FeK
17.39
37.10
WL
81.23
52.63
[1] L. J. Kecskes, M. D. Trexler, B. R. Klotz, K. C.
Cho, R. J. Dowding, Metall. Mater. Trans. A 32A
(2001) 2885-2893.
[2] V. V. Panichkina, Article translated from
Poroshkovaya Metallurgiya 2 (50) (1966) 1-5.
[3] J. L. Johnson, R. M. German, Metall. Mater. Trans.
B 27B (1996) 901-909.
[4] Y. Boonyongmaneerat, J. Mater. Processing
Technol. (2008) 1-4.
[5] R. M. German, Z. A. Munir, Metall. Mater. Trans.
A7 (1976) 1873-1877.
[6] IceNine Phase diagram, ESM Software, (1997)
version 1.0, http://www.esmsoftware. com.
[7] J.H. Brophy, H.W. Hayden, A.L. Prill, J. Wulff,
Prepared under U.S. navy, bureau of naval weapons,
contract no. w 61-0326-d1. (1961).
Fractured W grain
W-Fe rich layer completely wets
the W grain
Fig.4 shows SEM of W40wt. %-pre-alloy bronze
compact fractograph of Fe3wt. % addition
Discussion
As the sintering process commences, the activator
elements of Co and Fe start to segregate at W grain
boundary and owing to high solubility for W, they
form stable intermetallic compounds of W6Co7 and
W6Fe7 with W. Consulting the relevant Fe-W and CoW binary phase diagrams in [6], reveals that these
intermetallic compounds melt at 1538ºC and 1471ºC
respectively, while W melts at 3430ºC. This means that
the diffusion energies for those two intermetallic
compounds are substantially lower than the solid-state
diffusion energy of W element. Based on that, those
two intermetallic compounds could be responsible for
sintering enhancement. However, clarification of the
process is not simple. For decades, the activated
[8] J.I. Toth, N.A. Lockington, J. Less-common Me12
(1967) 353-36.
[9] J. Luo, X. Shi, App. Phys. Let. 92 (2009) 101901.
[10] J. Houze, S. Kim, S.J. Park, R.M. German, M.F.
Horstemeyer, S.G. Kim, J. App. Phys. 103(10) (2009)
106103.
[11] V.K. Gupta, D.H. Yoon, H.M. Meyer III, J. Luo,
Act. Mat. 55 (2007) 3131-3142.
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