Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.10, pp.775-785, 2009 Printed in the USA. All rights reserved
Effect of Solidification Process Parameters on the Microstructure of Some
Meltspun Nickel-Based Hardfacing Alloys
J. A. Ajao
Materials and Electronics Division, Centre for Energy Research and Development, Obafemi
Awolowo University, Ile – Ife, Osun State, Nigeria.
The influence of solidification parameters on the morphology of a number of meltspun Nickel-
based hard alloys has been investigated by means of scanning electron microscopy (SEM),
conventional transmission electron microscopy and energ y dispersive X-ray analysis (EDXA). It
was established that the wheel velocity played a prominent role in the types of microstructure
observed. Very high wheel velocity could lead to the formation of amorphous structure as
observed in some of the alloys investigated. No appreciable effect of the ribbon thickness was
observed on the microhardness of the a lloys. It was also repo rted that the thermal transfer mod e
between the ideal and newtonian coolings was observed in all the alloys investigated.
Keywords: Alloy, Microstructure, Cooling rate, Ribbon, Wheel velocity
It is well known that the processing techniques (and also the processing parameters) can affect
the microstructure of coatings and that dramatic changes can be observed within coatings of the
same alloy processed differently [1]. It has also been shown that drastic solidification conditions
normally achieved good results in microstructural refinement and in the partial or total
dissolution of second phase reinforcement particles, in iron-based alloys [2], cobalt-based alloys
[3] and nickel-based alloys [4]. In general, these microstructural changes lead to modifications of
the mechanical properties of the material and, consequently, to modifications of its wear
behaviour. Therefore, particular attention must be given to the relations between processing
conditions, microstructure and the wear resistance of the material [5]. It has been reported that
rapid solidification of alloys may result in increasing solubility, decreasing grain size, change in
microsegregation and formation of metastable phases [6]. These microstructural properties
depend largely on the cooling rate and hence on the solidification parameters of the cooling
776  J. A. AjaoVol.8, No.10
techniques. For meltspinning technique, a change in the microstructure may be expected even
within the same ribbon considering the decrease in the solidification rate from the wheel contact
side to the free surface of the ribbon [7]. Rapid solidification by meltspinning technique has been
applied to a variety of new materials [8, 9]. Cooling rate measurements during this technique
have given some insight into the ribbon formation mechanisms and the development of
microstructure features [10, 11]. Nickel-based hard alloys have a unique combination of
properties that enables them to be used in a variety of special purpose applications especially in
the automobile, pharmaceutical, petrochemical etc industries to enhance resistance to wear and
corrosion which are some of the major problems facing these industries. The Ni–Cr–B–Si–C
colmonoy alloys provide adhesive wear and corrosion resistance at ambient and high
temperatures. The NiCrBSi coatings are widely employed to improve the quality of components
whose surface is subjected to severe tribological conditions such as coal-fired boilers, heat
exchangers, turbines, tools, extruders, plungers, rolls for rolling mills, agriculture machinery, etc.
[12]. They exhibit excellent resistance to abrasive wear because of the boride and carbide
dispersions within their microstructures. These alloys are usually applied in the form of thick
protective coatings which lead to improved tool life and performance. Hence in the present
study, the influence of some process parameters on the phase formations and transformations in
some of these nickel-based hard alloys employed as surface coatings were investigated after
rapid solidification by meltspinning.
2.1 Production of Meltspun Ribbon
The nominal composition of the alloys investigated is presented in Table 1.
Table 1: Nominal compositions of the alloys in weight percent.
Alloys B C Si Cr W Ni
NBT1 16.2 - - - - 83.8
NBT2 4.7 6.00 1.90 28.40 3.40 55.60
NBT3 4.9 4.75 5.85 27.90 0.03 56.57
The ribbons of the alloys were produced by the rapid solidification technique of meltspinning.
This technique consists of induction-melting the accurately weighed mixtures of the alloys in a
crucible with a nozzle of diameter 0.8mm through which the molten alloys is ejected onto a
rotating cooled copper wheel under an ambient pressure of about 200mbar helium. The
temperature of the liquid alloy before ejection onto the rotating copper wheel is kept at about
500C above the melting point to ensure complete homogenisation of the molten alloy. The wheel
surface velocity used in this work varied from 15 to 45m.s-1. The ribbons formed after cooling
were 1 to 2mm wide and 20 to 50µm thick. The cooling rate was estimated between 2 and
5x105K.s-1. The value of the heat transfer coefficient (h) between the melt and the copper
substrate of 105WK-1m-2 which corresponds to newtonian thermal transfer has been previously
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calculated for this apparatus [13]. The summary of the process parameters used for the
optimisation of the ribbon quality is shown in Table 2.
Table 2: Solidification process parameters for the meltspun ribbons.
Wheel material Cu - Cr
Nozzle 0.8mm
Melt temperature (K) 1343 – 1493K
Wheel surface speed (m.s-1) 15 - 45
Ejection pressure (mbar) 200
Nozzle – wheel – gap (mm) 2 - 5
Melt weight (g) 10 - 15
2.2 Characterization of the Ribbons
The solidification structure of the ribbons was investigated by scanning electron microscope
(SEM – JSM35) equipped with energy dispersive X-ray analysis system (EDX – TRACOR),
transmission electron microscope (TEM), and X – ray diffraction (XRD). The samples for SEM
observations were slightly etched with an etchant consisting of 5g FeCl3 +10ml HCl dissolved in
50ml H2O while thin foils for TEM were electrolytically thinned in an electrolyte containing 9:1
acetic – perchloric solution at room temperature under 15V. The samples for TEM were
examined in a direction vertical to the foil surface. Some thin foils were prepared by ion
bombardment to reduce the level of contamination of the samples.
3.1 Solidification structure and the cooling rate
A variety of microstructure was observed in the alloys investigated depending on the nominal
composition of the alloys and the cooling rates.
3.1.1 Cross – sectional view of the meltspun ribbon
The transversal cross – section of the ribbon of NBT1 alloy is shown in the scanning electron
micrograph of Fig. 1a. In this figure, three zones can be distinguished. On the wheel side of the
ribbon, a crystalline structure was observed followed by a columnar structure. The third zone at
the free wheel side of the ribbon is a dendrite structure. The columnar grains are inclined at an
angle of typically 5 to 150 towards the spin direction. The triplex microstructure that occurs in
these alloys is the result of the varying cooling rates through the thickness of the material as it
solidified from the bottom to the top.
778  J. A. AjaoVol.8, No.10
Figure 1: (a) The scanning electron micrograph of the transversal cross – section of the ribbon of
alloy NBT1. Transmission electron micrographs of the same alloy at different wheel velocities:
(b) Vr = 15m.s-1 (c) Vr= 28m.s-1 (d) Vr= 45m.s-1.
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3.1.2 Detailed morphological observations of the meltspun ribbons
A detailed observation of these alloys was performed on transmission electron microscope. NBT1 (Ni - Ni3B Eutectic) ribbon
The transmission electron micrographs of the eutectic composition meltspun under different
wheel surface speeds (15m.s-1 < Vr < 45m.s-1) are shown in Fig. 1. The effect of the wheel
surface speed (velocity) could be seen as the lamellae of Ni and Ni3B take different morphology
under different wheel velocities. In Fig. 1b, with the wheel velocity of 15m.s-1 the interlamellar
spacing d is about 90nm and the lamellae of Ni and Ni3B are approximately the same size. In
Fig. 1c with the wheel velocity of 28 m.s-1, the interlamellar spacing d reduced to about 75nm
with the Ni3B lamellae (~ 50nm) becoming twice as large as the Ni lamellae (~ 25nm). However
in the same alloy meltspun with the wheel velocity of about 45m.s-1, the eutectic structure
became coarse and globular as shown in Fig. 1d. For this globular structure, the interparticle
spacing d was about 65nm between the two phases Ni and Ni3B. Fig. 2 shows the variation of the
interlamellar spacing, d, with the wheel velocity, Vr. From the figure, it can be seen that the
interlamellar spacing decreases as the wheel velocity increases.
Figure 2: Interparticle spacing, d, as a function of wheel velocity.
780  J. A. AjaoVol.8, No.10 NBT2 and NBT3 Alloys
Unlike the NBT1 alloy, the microstructure of the ribbon of the other two alloys (NBT2 and
NBT3) containing various metallic and non metallic additions is not homogeneous. This is
probably partly due to the complex composition of the alloys and partly due to gas trapping
between the ribbon and the wheel. This made a direct correlation between the microstructure and
the cooling rate very difficult. It is understood that the diffusion-controlled processes proceeded
very rapidly; hence the final microstructure was not influenced by cooling rate alone. Fig. 3a
shows the transmission electron micrograph observed in alloy NBT2 after meltspinning with a
wheel velocity of 28m.s-1. The microstructure is complex. However, from the EDX analyses
performed on the alloy, three major phases could be identified namely:
(i) a phase (marked 1) with cubic structure (dark in bright field) rich in chromium. The EDX
analysis is shown in Fig. 3b. From the EDXA, the composition of this phase approaches that of
M23C6 metastable phase. The formation of cubic metastable phases has been observed in
different ternary systems such as Fe-Ni-B [14] or Ni-B-Si [15]. This carbide phase has been
reported to be a brittle phase [16].
(ii) a phase (marked 2) bright in bright field and particularly rich in nickel, chromium and
tungsten (Fig. 3c). The composition of this phase approaches that of M2W.
(iii) a third phase (marked 3) very rich in nickel is the Ni (α) solid solution as depicted by Fig.
3d. The chromium-to-nickel ratio obtained by EDXA of around 0.48 is in good agreement with
the global composition of the alloy (chromium-to-nickel ratio, 0.50).
In the case of NBT3 alloy with nominal composition similar to that of alloy NBT2, two distinct
zones were observed – an amorphous zone (marked A) and a microcrystalline zone (marked C)
(Fig. 4a). The EDXA performed on the microcrystalline zone showed the presence of aggregates
of crystals of about 100Å in size consisting of nickel, chromium and tungsten (Fig. 4b). The
diffraction pattern (Fig. 4c) obtained on the amorphous zone (marked A) showed effectively the
existence of this zone. It should be noted from our observations that the high silicon content
(5.85wt%) in this alloy NBT3 compared to 1.90wt% in NBT2 alloy seemed to play an important
role in the amorphization of this alloy. High silicon content promotes glass formation in these
complex alloys. This is in agreement with reported work on the Ni-B-Si system where silicon
additions increased the composition range where glasses can be formed [17].
Furthermore, the microstructure observations of the above alloy meltspun at lower wheel
velocity (Vr ~ 15m.s-1) showed that the whole alloy became microcrystallized. No amorphous
zone was observed.
Vol.8, No.10 Effect of Solidification Process Parameters 781
Figure 3: (a) Transmission electron micrograph observed in the ribbon of alloyNBT2 at wheel
velocity of 28m.s-1. EDX analysis on (b) M23C6 metastable phase (c) M2W phase (d) Ni (α) solid
A general relationship has been proposed between the cooling rate, T, and the wheel velocity,
Vr, by Cantor [11]:
T 1.2x104 Vr
From the above relationship, the cooling rate in our experiments could be estimated between 2
and 5x105K.s-1 for wheel velocities of 15m.s-1 < Vr < 45m.s-1.
782  J. A. AjaoVol.8, No.10
Figure 4: (a) Transmission electron micrograph observed in alloy NBT3 after meltspinning with
a wheel velocity of 28m.s-1 depicting two zones – amorphous (A) and microcrystalline (C) (b)
EDX analysis on the microcrystalline zone (c) diffraction pattern on the amorphous zone.
3.2 Wheel velocity and the ribbon thickness
A number of relations [18, 19] have been proposed in order to calculate and relate the
solidification parameters such as (i) cooling rate T and (ii) the propagation rate of the solid –
Vol.8, No.10 Effect of Solidification Process Parameters 783
liquid interface, V, from accessible experimental factors such as the wheel velocity, Vr, and the
ribbon thickness, e. Hillmann and Hilzinger [18] in their work on the Fe40Ni40P14B6 alloy
proposed the following relationship between the ribbon thickness, e and the wheel velocity Vr:
e Vr-α
where α ~ 0.75 and 1.75 for ideal and newtonian coolings respectively. Experimentally, these
authors have reported α ~ 0.80 indicating a mixed regime. In the same vein, Kavesh [19] and
Lieberman and Graham [20] reported a slightly higher value (α ~ 0.83). A similar calculation in
this present work gives a value of approximately 1.03 for α. This value of α indicates a thermal
transfer mode intermediate between the ideal and newtonian coolings.
3.3 Micro-Hardness
The ribbons were subjected to Vicker’s hardness tests. Fig. 5 shows the microhardness of the
meltspun ribbons as a function of the ribbon thickness.
Figure 5: Variation of microhardness as a function of the ribbon thickness
From the figure, the hardness of the ribbon decreased as the ribbon thickness increased. This
strange behaviour can be explained by the localized segregations of the hard phases such as
M23C6, M2W etc. As reported earlier, different zones were observed in these alloys thereby
giving rise to non uniform hardness of the ribbons. But Vogt and Frommeyer [21] reported a
microhardness weakly influenced by the ribbon thickness in the Fe – 3.2wt%C alloys.
The influence of solidification parameters on some meltspun Nickel hard alloys has been
investigated with regard to their effects on the ribbon castability as well as on ribbon
microstructure. It has been established that the microstructure varied substantially with the wheel
velocity. In complex alloys (NBT2 and NBT3), depending on the wheel velocity, amorphous or
784  J. A. AjaoVol.8, No.10
microcrystalline structure was observed with the formation in microcrystallized samples of
metastable phases such as M23C6. At intermediate wheel velocity, a mixture of both amorphous
and microcrystalline phases was reported. Interestingly, it was also established that the
microhardness decreased as the ribbon thickness increased.
The author is indebted to Dr. M. Kemell for assistance in electron microscopy and to Centre for
Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria for granting
him leave of absence during the preparation of this work.
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