Phase and structural transformations in the Ni65Mo 20Cr15 alloy at changing the temperature of heat treatment

Article abstract of DOI:10.1016/j.jallcom.2013.11.100

The ternary Ni₆₅Mo₂₀Cr₁₅ alloy was studied using TEM after heat treatment at different temperatures. It has been shown that the tendency to phase separation which exists in the Ni/Mo diffusion couple at high temperatures leads to precipitation of crystalline particles of Mo atoms in the liquid solution. At lowering the heat treatment temperature to 1200 C, a phase transition ordering - phase separation occurs in the Ni/Mo diffusion couple, which results in dissolution of the particles of Mo atoms. At 650 C, there takes place precipitation of Ni₂Mo phase particles, which can initiate intercrystalline embrittlement of Hastelloy-type alloys.

Full text of DOI:10.1016/j.jallcom.2013.11.100

Journal of Alloys and Compounds 588 (2014) 470–473
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Phase and structural transformations in the Ni65Mo20Cr15 alloy
at changing the temperature of heat treatment
⇑
Y. Ustinovshikov
Physical-Technical Institute, 132 Kirov St., Izhevsk 426000, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The ternary Ni65Mo20Cr15 alloy was studied using TEM after heat treatment at different temperatures. It
has been shown that the tendency to phase separation which exists in the Ni/Mo diffusion couple at high
temperatures leads to precipitation of crystalline particles of Mo atoms in the liquid solution. At lowering
the heat treatment temperature to 1200 °C, a phase transition ordering – phase separation occurs in the
Ni/Mo diffusion couple, which results in dissolution of the particles of Mo atoms. At 650 °C, there takes
place precipitation of Ni2Mo phase particles, which can initiate intercrystalline embrittlement of
Hastelloy-type alloys.
Received 20 September 2013
Received in revised form 2 November 2013
Accepted 15 November 2013
Available online 25 November 2013
Keywords:
Alloys
Transmission electron microscopy
Microstructure
Ó 2013 Published by Elsevier B.V.
Phase transitions
1. Introduction
treatment of Hastelloy alloys, Cieslak et.al. [3] supposed that
perhaps they were equilibrium structures in the Ni–Cr–Mo ternary
The group of nickel-based alloys having high corrosion resis-
tance and containing in its composition molybdenum and chro-
mium has long been known under the common name of
‘‘Hastelloy’’. These alloys have different compositions but almost
always contain Mo and Cr as main alloying elements. The main
advantage of these alloys is their effective work in the condition
of contact with aggressive media at high pressures and tempera-
tures. All properties of these alloys have been sufficiently well
studied, but the microstructural aspects of the formation of these
properties are still not fully cleared up. It is believed that particles
of the ordered L12 phase (c0-phase) formed in the temperature
range of 600–700 °C, are the primary strengthening phase in these
alloys and the chemical composition of this phase corresponds to
Ni3 (Mo, Cr) [1]. In other works, for example in Ref. [2] it is sup-
posed that the main strengthening effect in these alloys occurs
when the ordered Ni2 (Mo, Cr) phase of the Pt2Mo type, with aver-
age particle size of about 10 nm, precipitates in these alloys.
In the study of the microstructure of welds of Hastelloy alloys
C-4, C-22 and C-276, sufficiently large particles of topologically
close-packed intermetallic compounds have been found, which
turned out to be the cause of hot cracking in the weld [3]. Those
system.
The purpose of this paper was to find out, through the case
study of the Ni65Mo20Cr15 alloy, what phases are formed in
Hastelloy type alloys in the entire temperature range of heat treat-
ment, taking into account the fact that the energy of the chemical
interaction between the atoms of Ni and Cr as well Ni and Mo
changes sign with temperature [4,5].
2. Experimental
The investigated alloy was melted from pure components in an induction fur-
nace. Samples (discs 3 mm diameter with 0.2 mm thickness) were cut from pieces
for research by the method of TEM. Samples for TEM, placed in evacuated quartz
ampoules were heat treated at 1300, 800 and 650 °C for different periods of time
and quenched in water to fix the structure existing at these temperatures. Quench-
ing from the liquid state (1600 °C) was performed simply by pouring the alloy from
the crucible into water. Thin foils were prepared by electropolishing the discs in a
10% solution of perchloric acid in ethanol. The foils were examined in an EM-125
electron microscope at an accelerating voltage of 100 kV.
3. Results and discussion
were
was almost the same, but their crystal structures were different:
tetragonal ( ), orthorhombic (P) and hexagonal ( ) [3]. Although
r, P and l phases. The chemical composition of these phases
The diffusion processes in the studied alloy were considered as
occurring in three diffusion couples-Ni/Mo, Ni/Cr and Mo/Cr. It was
believed that phase transitions taking place in these diffusion cou-
ples occur similarly to phase transitions in corresponding binary
alloys of the Ni–Mo, Ni–Cr and Mo–Cr systems. It is obvious that
the order and completeness of these phase transitions depends
on the temperature of the heat treatment, as well as the ratio
r
l
previously, these phases had not been detected during usual heat
⇑
Tel.: +7 3412 216633; fax: +7 3412 722529.
E-mail address: ust@ftiudm.ru
0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jallcom.2013.11.100

Y. Ustinovshikov / Journal of Alloys and Compounds 588 (2014) 470–473
471
between the values of chemical affinity between the elements in
these or those diffusion couples at each of the studied
temperatures.
Let us first discuss experimental results obtained previously for
alloys of binary systems corresponding to the above-mentioned
diffusion couples.
transition leads to the fact that, gradually, the whole solid solution
transforms into the Ni3Mo (D022) phase. This happens not as a re-
sult of a direct reaction A1 ? D022 (which, as shown in Ref. [8] by
the example of the Ni3V alloy, is crystallographically impossible),
but through formation of intermediate metastable Ni2Mo (Pt2Mo)
and Ni4Mo (D1a) phases [4].
What diffusion processes occur, for example, in the diffusion
couple of Cr–Mo, can be judged upon by the results, on the basis
of which the Cr–Mo phase diagram has been constructed (Fig. 1).
This diagram shows a continuous series of solid solutions at tem-
peratures above 890 °C, and a region of phase separation below
this temperature [6]. However, a thermodynamic study of the al-
loys of this system at high temperatures showed that a tendency
to phase separation also appears at temperatures significantly
above 890 °C. For example, positive deviations from Raoult’s law
were detected in the alloys of this system, in the liquid (1600 °C)
state, by measuring the partial vapor pressure above the sample
surface [7]. A comparison of the data obtained in the study of alloys
of this system leads to the conclusion that in this system, at all
temperatures, there is a tendency to phase separation. This means,
that at any temperature, atoms of Mo and Cr tend to diffuse not to-
wards each other to form a chemical compound, but on the con-
trary, from each other, i.e., these atoms, in principle, cannot
participate in joint formation of either the Ni2 (Mo, Cr) phase of
the Pt2Mo type, or the Ni3 (Mo, Cr) phase of the L12 type (c0-phase),
as claimed in Refs. [1,2].
As is known, in binary alloys of the Ni–Mo system, the phase
transition ordering–phase separation occurs at temperatures close
to 1200 °C [4]. Above these temperatures, in the alloys there is a
tendency to phase separation, which leads to precipitation of solid
particles consisting of Mo atoms in the liquid solution (these par-
ticles are found already after quenching from 1600 °C). Below the
phase transition line in the Ni–Mo system, a tendency to ordering
takes place, in the result of which particles consisting of Mo atoms,
which were formed at high temperatures, dissolve in the solid
solution. A longer exposure at temperatures below the phase
A somewhat different situation is observed in binary alloys of
the Ni–Cr system. In the part of the system where concentration
of chromium atoms (e.g., an alloy Ni40Cr60) prevails in the alloys,
the tendency to ordering remains at all temperatures, including
the liquid state [5]. As a result of this tendency, particles of the Cr2-
Ni chemical compound precipitate in alloys of this composition
both in the liquid state and at aging temperatures [5]. In the part
of the system of Ni–Cr, where concentration of nickel atoms pre-
vails in alloys (for instance, the Ni68Cr32 alloy), heat treatment at
a temperature of 1000 °C and above, leads to formation of clusters
of chromium atoms (after quenching from the liquid state, as well)
[5]. This means that, at these temperatures, a tendency to phase
separation takes place in these alloys. However, heat treatment
at lower temperatures, for example at 700 °C and below, does
not lead to formation of either clusters or any particles of new
phases in the microstructure [5]. Such experimental data usually
serve as the basis for the conclusion that a solid solution micro-
structure is formed in the alloys. Thus, microstructural data does
not allow judging which tendency appears in the Ni68Cr32 alloy
at aging temperatures. However, it is quite clear that at high tem-
peratures in the nickel–chromium system, the change of the sign of
chemical interaction between component atoms occurs at a change
in alloy concentration [5]. Thus, the Ni–Cr system is the first of the
systems, in which the possibility of a change in the sign of chemical
interaction, depending on a change of alloy concentration was
experimentally confirmed [5].
The microstructure of the studied alloy after quenching from
the liquid state (1600 °C) is represented in Fig. 2. Rounded precip-
itates are observed, which, in size and shape are very similar to
those observed in the Ni75Mo20Al5 alloy after quenching from the
Fig. 1. Cr–Mo phase diagram [6].

472
Y. Ustinovshikov / Journal of Alloys and Compounds 588 (2014) 470–473
These inhomogeneities are observed at lower temperatures, as
well, for example at 800 °C, but now in the form of dark rounded
spots with diffuse edges, i.e., in the form of conventional clusters
(Fig. 4, clusters are visible due to absorption contrast). A similar
structure of clusters of chromium atoms was observed in the elec-
tron microscopic image of the Ni68Cr32 binary alloy after quenching
the alloy from 1200 and from 1000 °C [5] however, as it was no-
ticed, at lowering the temperature of heat treatment, the number
of clusters decreased and their contours became more blurred
[5]. As is known from reference books, the optimum combination
of corrosion resistance and ductility of most Hastelloy-type alloys
is observed after a heat treatment at 1150–1175 °C (cooling in
water or air). As it follows from our structural results, it is at these
temperatures that in the solid solution occurs complete dissolution
of particles of Mo atoms, formed when the alloy was in the liquid
state. And it is precisely in the range of these temperatures that the
phase separation microstructure forming in the Ni–Cr diffusion
couple has the highest dispersity (see in Figs. 3 and 4). Hence, it
can be concluded that the high level of corrosion resistance, form-
ing in Hastelloy-type alloys in the result of their heat treatment at
1150–1175 °C, owes its origin to the highly-dispersed phase sepa-
ration microstructure forming in the diffusion couple of Ni/Cr in
range of these temperatures.
At lowering the heat treatment temperature to 650 °C, the mor-
phology of the studied alloy changes dramatically. Fig. 5 shows the
bright-field image of the microstructure and the electron diffrac-
tion pattern, as well as the dark-field image, obtained from an
additional reflection indicated by an arrow in the electron diffrac-
tion pattern. Obviously, the precipitate particles in Fig. 5a are
chemical compounds of Ni2Mo with a structure of the Pt2Mo type
(Fig. 5b). Since such a compound in the Ni-20 at.% Mo binary alloy
is not a stable phase, the authors [1,2] supposed that in the Ni–Mo–
Cr alloys, the compound has the chemical composition of Ni2 (Mo,
Cr), in which atoms of chromium may replace some of the atoms of
molybdenum, to ‘‘strengthen the stability of this compound’’. How-
ever, this supposition is in conflict with the results [6,7], from
which it becomes clear that in alloys of the Cr–Mo system, phase
separation of the solid solution occurs at all temperatures. This
means that between the atoms of Cr and Mo, there takes place a
repulsive interaction but not attraction. This type of interaction ex-
cludes the possibility of the Ni2 (Mo, Cr) phase formation in the
investigated alloy. Actually, the attempt of the authors [10] to elec-
trolytically extract particles of Ni2 (Mo, Cr) from the solid solution
of the Hastelloy C-22HS alloy, and then to determine their chemi-
cal composition did not lead to the desired results: the amount of
chromium in the anodic deposit was much lower than expected.
Fig. 2. Microstructure after quenching from the liquid state.
liquid state [9]. They are particles of Mo atoms, which are formed
in the liquid solution because of the existence of a tendency to
phase separation in the Ni–Mo diffusion couple at high tempera-
tures. These particles are crystalline formations due to the high
solidification temperature of molybdenum.
After exposure of the alloy at 1300 °C for 1 h (with subsequent
quenching in water), particles of Mo atoms are still observed in the
structure (Fig. 3a,b) but their dimensions are considerably reduced.
Along with particles of Mo atoms, in Fig. 3, one can observe peri-
odic waves of contrast, which can be regarded as concentration
inhomogeneities in the distribution of chromium in the solid solu-
tion, formed in the Ni–Cr diffusion couple. It is obvious that such
inhomogeneities are formed as a result of the existence of a ten-
dency to phase separation in the Ni–Cr diffusion couple at high
temperatures (at that concentration of component atoms, when
atoms of Ni prevail) [5].
Fig. 3. Microstructure after quenching from 1300 °C in the different points of the
foil (a, b).
Fig. 4. Microstructure after quenching from 800 °C.

Y. Ustinovshikov / Journal of Alloys and Compounds 588 (2014) 470–473
473
Fig. 6. Microstructure after aging at 650 °C: particles of the Ni2Mo phase in
subboundaries.
4. Conclusion
1. In the investigated alloy, at high temperatures, a tendency
to phase separation takes place in all the three diffusion
couples. In the Ni/Mo and Cr/Mo diffusion couples, this
tendency leads to formation of crystalline particles of Mo
atoms in the liquid solution.
2. In the Ni/Mo diffusion couple, at a temperature of 1200 °C
the tendency to phase separation changes to a tendency
to ordering, which leads to dissolution of particles of Mo
atoms in the solid solution. At this temperature, the phase
separation microstructure, which was formed in the diffu-
sion couple of Ni/Cr, remains being highly dispersed. It is
this microstructure that leads to an optimum combination
of corrosion resistance (highly dispersed clusters of Cr
atoms) and ductility (dissolution of particles of Mo atoms
in the solid solution) after heat treatment at temperatures
of 1150–1175 °C.
3. In the diffusion couple of Ni/Mo, at aging temperatures,
particles of the Ni2Mo chemical compound precipitate as
a result of the tendency to ordering. Precipitation of such
particles along the boundaries leads to the appearance of
intercrystalline brittleness of Hastelloy-type alloys during
prolonged heat treatment at 600–650 °C.
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As it is known [3], intercrystalline corrosion develops in alloys
of the Hastelloy type during prolonged heat treatment in the tem-
perature range of 600–650 °C. As described above, the Ni2Mo phase
precipitates in the solid solution at these temperatures (Fig. 5). In
addition to that, along some grain and sub-grain boundaries, pre-
cipitates of this phase can be seen as a continuous layer (Fig. 6).
It can be supposed that the formation of this precisely phase at
the boundaries leads to intergranular embrittlement, which
usually occurs in Hastelloy-type alloys at aging temperatures.