Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.6, pp.559-568, 2010 Printed in the USA. All rights reserved
Preparation and Structural Characterization of Vanadium Doped Ni – B Binary
Hard Alloys
J. A. Ajao
Materials and Electronics Division, Centre for Energy Research and Development, Obafemi Awolowo
University, Ile-Ife, Osun State, Nigeria
E-mail: or
The microstructure of a series of binary Ni-B alloys containing various amounts of vanadium additions
were investigated by Differential Thermal Analysis (DTA), Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Analysis (EDXA). Due to the
vanadium addition in the alloys quenched from the liquidus, the formation of the Ni
B phase was
enhanced even when the nominal composition was hypoeutectic. The addition of vanadium led to the
formation of
) phase during the present investigation. In addition, the crystallographic
orientation relationship of this ternary phase with the nickel matrix was reported. The solid-state
eutectoid transformation of the Ni
B phase during cooling was also reported and discussed.
Keywords: Alloy, Solidification, Microstructure, Solid-state transformation.
One of the basic components of the Nickel-based hardfacing alloys are the Ni – B hard phases. These
hardfacing alloys are specially developed to solve the problems of wear and corrosion in
petrochemical, glass, automobile, nuclear and other related industries. They are employed as coatings
material deposited by various coating techniques [1-3]. The hard alloys usually consist of nickel as a
base metal while chromium, tungsten, molybdenum and vanadium are used as metallic additives and
boron, silicon and carbon as non-metallic additives [4]. Addition of chromium promotes the oxidation
and corrosion resistance property at elevated temperatures and enhances the hardness of the coating by
formation of hard phases. Boron depresses the melting temperature and plays active role in the
J. A. Ajao Vol.9, No.6
formation of hard phases. The wear resistance property is directly dependent on the relationship
between the nickel matrix and hard phases (boride, silicide, carbide) [5, 6]. It also depends on the
dimensions and the crystallographic orientation relationship between the phases. Given the various
complex hard phases involved in these alloys, the importance of detailed microstructural studies of the
alloys cannot be overemphasized. However, only few works report their microstructural features [7, 8].
Hence, this has aroused the interests in the present work to investigate the effect of vanadium additions
on the thermal and microstructural properties of the Ni-B hard alloys. The possible crystallographic
orientation relationships between the various phases present in the system were also explored and
2.1. Preparation of the Alloys
A series of hypo- and hyper-eutectic alloys were prepared in the region of the Ni-Ni
B eutectic
composition. These alloys were prepared from pure nickel, NiB (containing 15wt%B) and pure
vanadium (99.99% pure). The accurately weighed components of each alloy were arc-melted in an
argon atmosphere using a non-consumable tungsten electrode. This operation was repeated several
times to ensure the homogenization of the samples. Fig. 1a shows the equilibrium phase diagram for
the Ni-B-V system and the position of the alloys investigated.
2.2. Differential Thermal Analysis (DTA)
The melting behavior and phase formation were determined by differential thermal analysis (DTA)
model 404PC. This apparatus allows the automatic control of thermal programs and automatic analysis
of the results. The heating and cooling rate was 5
C min
. The samples were also quenched from the
liquidus with the cooling rate estimated as 10
[9]. The chemical compositions as well as transition
temperatures of the alloys are presented in Table 1.
Table 1: Composition (at %) and transition temperatures (
C) of the alloys
Ni B V Transition Temperatures
82.8 16.2 1 1138 954 825
81 17 2 1022 962
80 17 3 1076 1015 940
78 17 5 08 1038 968
Vol.9, No.6 Preparation and Structural Characterization 561
2.3. Characterization of the Alloys
Microstructural characterizations of the alloys were carried out using optical and scanning electron
microscopes. The samples for SEM observations were slightly etched with an etchant consisting of 5g
+ 10ml Conc. HCI dissolved in 50ml H
0. The crystallographic nature of the phases present in
these samples was examined by X-ray diffraction (XRD). Transmission electron microscopy (TEM)
and electron diffraction were performed on thin foils of the samples in a JEM-200CX microscope. Thin
foils were obtained by electropolish in an electrolyte containing 57% H
at room temperature under
3.1. Morphology of the Alloys
3.1.1. Samples Cooled at Rapid Rate
A certain number of alloys with varied compositions of vanadium l at % < V < 5 at % were prepared.
For alloy containing l at % V and quenched from the liquids, the microstructure observed in SEM
shows the primary phase of Ni
B (Fig. 1b) surrounded by the lamellar binary eutectic Ni (α) - Ni
It should be noted that for alloy of similar composition but without vanadium addition, the primary
phase observed was Ni (α). Hence, vanadium addition has probably caused the microstructure of the
alloy to shift from the hypoeutectic to hypereutectic region during rapid cooling. Detailed observations
in TEM of the Ni
B primary phase showed the presence of some precipitates in coherence with the
matrix. Fig. 2a shows the transmission electron micrograph on these precipitates while Figs. 2b and c
present the diffraction pattern and the key diagram on the precipitates respectively.
From the diffraction pattern obtained on the precipitates and the energy dispersive X-ray analysis
(EDXA) performed on them, a ternary τ (Ni
) phase could be proposed. The lattice parameter
of this ternary phase varies between 10.47Å and 10.49Å [10]. Similar ternary phase has been reported
in Ni-B-Si [11] and Ni-B-Ti [12, 13] systems.
For alloys containing 3 to 5 at %V and quenched from the liquidus, SEM observations only revealed
the binary eutectic structure between Ni (α) and Ni
B. Unlike boron, vanadium is more soluble in
nickel. Hence the domain of existence of the ternary phase τ (Fig. 1a) is much more reduced.
Furthermore, Fig. 3 shows the TEM observations on the binary eutectic Ni (α) – Ni
B obtained in
alloys containing 3 – 5 at %V quenched from the liquidus. The binary eutectic is lamellar with the
lamellae of Ni
B (1500Å) about twice the size of the Ni (α) lamellae (800Å) as shown in the
transmission electron micrograph of Fig. 3a.
J. A. Ajao Vol.9, No.6
Fig. 1: (a) Equilibrium phase diagram for the Ni-B-V system and the position of the alloys investigated
(b) Scanning electron micrograph of alloy containing 1 at %V showing the primary phase of Ni
surrounded by the Ni (α) – Ni
B eutectic
Vol.9, No.6 Preparation and Structural Characterization 563
Fig. 2: (a) TEM on the τ (Ni
) ternary phase (b and c) Diffraction pattern and the key diagram
on the τ precipitates
J. A. Ajao Vol.9, No.6
Fig. 3: (a) TEM observations on Ni (α) – Ni
B eutectic quenched from the liquidus (b and c)
Diffraction pattern and the key diagram on the Ni (α) – Ni
B eutectic.
3.1.2 Differential Thermal Analysis (DTA) Samples
For alloys containing 2 at % V, the crystallization of the alloys started with the primary formation of
the Ni (α) phase at 1022
C followed by a solid state precipitation of the Ni
B phase at 962
C (Fig. 4b).
This appears as slabs in the figure. It would be recalled that for similar alloys quenched from the
liquidus, the primary phase formed was Ni
B followed by the binary eutectic Ni (α) – Ni
Vol.9, No.6 Preparation and Structural Characterization 565
Fig. 4: (a) Solid-state precipitation of the Ni
B phase in alloy containing 2 at % V during cooling in
DTA (b) SEM observations on alloys containing 3 – 5 at % V slowly cooled in DTA (c) Haloes of Ni
(α) in alloys containing 5 at % V with interdendritic precipitations
In the same vein, for alloys containing 5 at % V, three transitions temperatures were observed in DTA
experiments. This alloys started its crystallization by the formation of the primary phase of Ni (α) at
C followed by the crystallization of the binary eutectic Ni (α) – Ni
B at 1038
C as shown in Fig.
4a. As the temperature continued to decrease, this Ni
B phase underwent a solid-state transformation at
J. A. Ajao Vol.9, No.6
C as evidenced in the interdendritic grooves in the micrograph in Fig. 4c. Similar observations
have been reported during slow cooling of Ni-B alloys doped with titanium [13]. In fact, at 968
C, the
non-stoichiometric Ni
B was saturated with the atoms of nickel and vanadium at the same time. The
rejection of these atoms of nickel and vanadium to the interface gave rise to the solid-state
transformation (eutectoid-like structure) as follows:
B (Ni, V) Ni
B + Ni (V) [H= 20kJ kg
The relation between the thermal effect due to the change of state and the surface area of the peak in
DTA experiment is given by
mH = kS
m = mass of the melt
H = enthalpy of transformation
S = surface area of the peak in DTA
k = heat transfer coefficient
For the alloy undergoing solid-state transformation (with m = 0.002kg), the enthalpy of transformation
has been estimated as 20 x 10
[9]. From the DTA trace, the surface area of the peak during
transition at 954
C was calculated as 55 x 10
. Hence, from the above relation and data, the heat
transfer coefficient (k) between the crucible and the melt has been estimated as 8 x10
W m
. This
is in good agreement with the value obtained by Casanova et al. [15]. It is also important to report that
the hardness measurements carried out on these samples showed a higher value for alloys with
vanadium additions due probably to the formation of some pockets of VB hard phases which might be
present in the alloys.
3.2 Crystallographic Orientation Relationship between Ni (α) and Ni
The superposed diffraction pattern of the two phases is shown in Fig. 3b while the key diagram is
presented in Fig. 3c. From the superposed electron diffraction pattern of the two phases, the following
relative crystallographic orientation relationship between the two phases could be deduced:
Ni (α)
// (221)
// [110]
This orientation relationship is quite different from those obtained from pure Ni (α) - Ni
B eutectic
quenched from the liquidus [9] showing the pronounced influence of vanadium addition. However, this
mutual crystallographic orientation between the two phases is comparable to the orientation
relationship observed between them in meltspun Ni-B alloys [14]. From the above crystallographic
orientation relationship, the lattice mismatch could be estimated as 33% for the [013]
// [110]
direction. This value is far greater than 10% which could have made the planes special. These
Vol.9, No.6 Preparation and Structural Characterization 567
observations show the apparent effect of cooling conditions on the orientation and the growth of the
A series of Ni-B alloys containing various amount of vanadium additions were studied using various
characterization techniques. Precipitates of a ternary phase identified as the τ- phase and coherent with
the matrix were observed in the alloys during quench from the liquidus. The mutual crystallographic
orientation relationship of this ternary phase with the matrix was also reported. In addition, solid-state
transformation of the Ni
B phase during slow cooling was reported and an explanation attempted.
The author is thankful to Dr. D. A. Pelemo for helpful discussions on electron microscopy. The Centre
for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria is appreciated
for granting the author leave of absence during the preparation of this work.
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