Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
1
PECULIARITIES OF TENSILE DEFORMATION OF MOLYBDENUM
AND MOLYBDENUM-RHENIUM SINGLE CRYSTALS AT ROOM
TEMPERATURE
G.S. Burkhanov, V.M. Kirillova, A.R. Kadyrbaev, V.V. Sdobyrev, and
V.A. Dement’ev
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,
Leninskii pr. 49, Moscow, 119991 Russia
Abstract
In this work, we report data on the tensile deformation of molybdenum and molybdenum-
rhenium (by an example of the Mo-7 wt % Re alloy) single crystals at room temperature. The
rhenium alloying was shown to change both the low-temperature deformation regime, which
is typical of pure molybdenum, to the mediate-temperature regime of unidirectional slips and
dependence of properties on the crystallographic orientation. Molybdenum single crystals
having the [110] orientation are characterized by minimum strengthening and maximum
plasticity, whereas the maximum plasticity of molybdenum alloyed with rhenium is observed
for [100] single crystals. An experiment consisting in the noncontinuous tensile deformation
of the Mo-7 wt % Re single crystal is described for the first time. Each interruption and
subsequent continuation were found to cause an increase in the stress that is proportional to
the interruption time and disappears on continuation the tension.
The alloying-induced change in the deformation behavior of the single crystals agrees with
results obtained in studying the slipbands on the surface of deformed samples by an Opton
optical microscope using a Nomarskii interference-contrast method.
1. INTRODUCTION
Study of plastic deformation of refractory metals is of practical importance from the
viewpoint of their machiability and increase in the mechanical properties. In this connection,
some theoretical problems related to crystallographic and atomic mechanisms of plastic flow
of metals depending on their orientation, purity, and deformation conditions should be
clarified. Single-crystal metals are ideal subjects to solve the problems since they allow one to
avoid difficulties related to nonuniform deformation of grains in polycrystalline samples.
Most complete data on the plastic deformation and mechanical properties (determined in
tensile testing) of molybdenum single crystals produced by zone melting are given in
monography [1]. Substantial dependence of properties on the crystallographic direction is
observed, namely, the anisotropy of strength and plastic properties can reach 30-40 and 100%,
respectively. The tensile deformation exhibits a local character; in this case, the minimum
strengthening and maximum plasticity are observed for the deformation along the [110]
direction, whereas the maximum strengthening and minimum plasticity are observed in
testing along the [100] direction. The behavior of deformation curves for different
crystallographic orientations and the anisotropy of plasticity were explained assuming the
orientational dependence of intersecting slip for the bcc lattice and the probability of
formation of α<100> dislocations. To confirm the assumption, X-ray techniques were used.
The aim of the study is to study the effect of alloying on the tensile deformation of
molybdemun alloys single crystals and character of dependence of the mechanical properties
on the crystallographic directions. As the model alloy, we used the molybdenum alloy
containing 7 wt% Re. Such a choice is explained by the fact that molybdenum-rhenium alloys
occupy a highly important place owing to the rhenium effect on the physical and mechanical
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
2
properties of molybdenum. Rhenium increases both strength and plasticity of molybdenum
(so-called “rhenium effect”) [2]. Such an effect is explained by causes related to the
neutralization of unfavourable effect of interstitial impurities and realization of the additional
mechanism of twinning deformation.
2. EXPERIMENTAL
[100] pure molybdenum single crystals and [100] and [110] Mo-7 wt% Re single crystals
were grown by electron-beam zone melting [1] in a vacuum of ~5⋅10-5 mmHg that was
produced by roughing-down and diffusion oil-vapor pumps, nitrogen trap, and sorption trap
with anodized titanium sorbent.
As the starting material, we use the alloy produced by vacuum arc melting. Molybdenum-
rhenium alloy blanks were prepared from molybdenum and rhenium powders that were
mixed, annealed and wetted with an alcoholic solution of glycerol, and pressed to rods, which
were sintered in hydrogen atmosphere and degassed in vacuum to remove gas impurities.
Subsequently, the rods were arc melted.
The single crystals were grown using threefold run of zone; the third run was realized when
melting from the seed. After each melting, the single crystal was turned over. The speed of
two first runs is 4 mm/min; the speed of the third run is 2 mm/min. The diameter of single
crystals is 15 and 22 mm; their length is 200 mm.
The concentrations of impurities in the single crystals were determined by mass spectrometry
(see Table)
Table. Concentrations of impurities in the single crystals
Impurities C O H Fe W Cr P Mn Na Ca K Co Ta
pure Mo 1.8 1.2 1.0 13 180 5.5 1.8 0.5 0.7 95 56 0.3 0.4 Mo-7 wt% Re 1.2 2.2 1.0 33 320 3.9 1.5 0.2 1.0 60 25 0.1 0.4
The study of the microstructure of Mo and Mo-7 wt % single crystals shows that the alloying
with Re increases the angular misorientation of the alloy by 1.5-2 times. Moreover, the
alloying changes the structure of subgrains (Figs. 1a and 1b), namely, subgrains do not form
closed polygons (like in the case of pure molybdenum) that are diffuse at the end.
Blanks ~40 mm in length were spark-cut from the single crystals obtained. For mechanical
tests, cylindrical samples having a gage length of 15 mm and a diameter of from 2.9 to 3.8
mm (Figs. 1c and 1d) were prepared using a grinding machine.
Before loading, to remove deformed layer and obtain adequate surface (that is necessary to
study slipbands with an optical microscope) the surface of samples was electropolyshed at
room temperature at a voltage of 10 V and a current of 1.2 A using an electrolyte containing
86% ethanol and 14% sulphuric acid.
The tensile tests of the [110] Mo and [110] Mo-7 wt% Re single crystals were realized at
room temperature at a rate of 1.11⋅10-3 s
-1 using an Instron TT machine; the [100] Mo-7 wt%
Re single crystal was tested at a rate of 9.3⋅10-3 s
-1.
The deformed surface of the samples was studied in an Opton optical microscope using the
Nomarskii interference-contrast technique that allows the surface relief characterized by slight
difference in the hill heights to be revealed.
Moreover, a sample oriented so that an unidirectional slip is realized was cut from the Mo
single crystal. The sample was subjected to tensile test at a rate of 1.1⋅10-3 s
-1. The aim of the
experiment is to study changes in the surface relief as compared to that of the aforementioned
samples.
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
3
a b
c d
Fig. 1. Microstructure (x100) of (a) Mo and (b) Mo-7 wt% Re single crystals.
(c) Orientation of deformation axis. (d) Appearance of a tested sample
Two samples cut from the [110] Mo-7 wt% Re single crystal were subjected to tensile tests;
one of them was loaded noncontinuously and the other was tested continuously at a rate of
1.1⋅10-3 s
-1. The deformation curves and changes in the surface relief of samples were
analyzed comparatively.
3. RESULTS AND DISCUSSION
Figure 2 shows the deformation curves of the [100] Mo single crystals and [100] and [110]
Mo-7 wt% Re single crystals.
150
200
250
300
350
400
450
500
550
600
0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4
Elongation, mm
Tensile stress, MPa
Mo [100]
Mo - 7 wt% Re [110]
Mo - 7 wt% Re [100]
Fig. 2. Tensile deformation curves for the [100] Mo single crystal and [100] and [110]
Mo-7 wt% Re single crystals.
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
4
The [100] Mo single crystal loaded at room temperature exhibits a parabolic curve of
strengthening characterized by so-called low-temperature regime of deformation, at which
several equivalent slip systems are realized primarily. The ultimate strength is sufficiently
high; it is σb = 555 MPa.
This value agrees with data obtained earlier for a molybdenum single crystal produced by
electron-beam melting [3].
Figure 3 shows the micrograph of the surface of the [100] Mo single crystal subjected to
tension. A fine slightly distinguishable relief exhibiting several slip systems (Fig. 3a) is
observed; this is typical of the deformation of pure molybdenum [3].
a b`
c
Fig. 3. Relief (x400) of deformed surface of Mo single crystals subjected to tensile
deformation: (a) [100] orientation, several slip systems are observed; (b) orientation of
unidirectional slip: slipbands are more pronounced; primary slip system is dominant
(secondary slip systems are observed locally); and (c) relief at the sample neck.
Samples of Mo single crystals, which were especially oriented for the unidirectional slip,
exhibit more clear slipbands at the surface (Fig. 3b). The (101) [111] primary slip system is
mainly observed. Traces of secondary slipbands can be observed locally. The surface relief of
the sample neck that is characterized by substantial local plastic deformation is more
pronounced (Fig. 3c).
The alloying of molybdenum with rhenium changes the deformation character of the single
crystals (Fig. 2). The tension curves of the Mo-7 wt% Re single crystals with the [110] and
[100] orientations differ from not only the curves for the Mo single crystal but also from one
another.
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
5
(i) The deformation curve of the [110] single crystal is virtually parabolic, whereas the
deformation curve of the [100] single crystal exhibits the presence of three stages that
correspond to the “mediate-temperature” deformation regime with the unidirectional slip.
(ii) The elongation of the [110] single crystal is less than that of the [100] single crystal.
(iii) The maximum dσ/dε value for the [110] Mo-Re single crystal is less than that for the
[100] Mo-Re single crystal.
(iv) The temperature dependence of the critical stress for the [110] single crystal lies above
that for the [100] single crystal, whereas its strength is lower.
The surface relief of alloyed samples is more clear as compared to that of the Mo single
crystal. Figures 4a-4d show the surface of different sections of the [100] Mo-7 wt% Re single
crystal. As is seen, the relief character depends substantially on the section under study, i.e.,
on the surface orientation with respect to the operating slip systems. The slipbands are
observed the most clearly in Fig. 4d, which was obtained in rotating the sample by 45° with
respect to the plane shown in Fig. 4. The slipbands are clearly pronounced. The unidirectional
slip is dominant (traces of the secondary slip are observed). The height of some bands is
substantial.
a b
c d
Fig. 4. Relief (x400) of the deformed surface of the Mo-7 wt% Re single crystal (σb =
387 MPa): (a) section of the side surface with slightly distinguishable slipbands
(Burgers vector of the primary system is in-plain); (b) section with irregular surface
(the rotation of sample by 90° about an axis); (c) section of the surface with clear
slipbands (the rotation of sample by 45° about an axis from the plane shown in Fig.
4a; and (d) deformed surface of the same single crystal after tensile deformation, when
one of the slip systems is dominant.
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
6
Figure 5 shows the deformation curves for [110] Mo- 7 wt% Re single crystal subjected to
noncontinuous and continuous tensile tests. The subsequent deformation after the interruption
was found to cause the increase in the stress that is proportional to the interruption time. After
the deformation renews, the increase disappears.
160
180
200
220
240
260
280
300
320
340
0.00 0.04 0.08 0.12 0.16 0.2 0.24 0.28
Elongation, mm
Tensile stress, MPa deformation to the 1st interruption
subsequent deformation to the
2nd interruption
subsequent deformation after the
2nd interruption
continuous deformation
Fig. 5. Tensile deformation curves for the [100] Mo-7 wt% Re single crystals
subjected to noncontinuous and continuous deformation at a rate of 1.1⋅10-3 s
-1.
4. CONCLUSIONS
1. Molybdenum single crystals subjected to tensile deformation are characterized by the
parabolic strengthening curve with so-called low-temperature deformation regime. In terms
of the regime, several equivalent slip systems operate from the beginning of deformation.
This is confirmed by studies of the surface relief of deformed samples.
2. The alloying of molybdenum with rhenium changes the low-temperature deformation
regime to the mediate-temperature regime with unidirectional slip.
3. The alloying of molybdenum with rhenium changes the dependence of the properties on
the crystallographic direction. The [110] Mo single crystal is characterized by minimum
strengthening and maximum plasticity, whereas the [100] Mo-7 wt% Re single crystals
exhibit the maximum plasticity.
4. In the case noncontinuous tensile deformation of the molybdenum-rhenium alloy, each
interruption causes the increase in stresses that is proportional to the time of interruption
and disappears when the deformation renews.
BIBLIOGRAPHY
1. SAVITSKII, E.M., BURKHANOV, G.S., Single crystals of refractory and rare metals and
alloys. Moscow : Nauka, 1972. 259 p.
2. POVAROVA, K.B., BANNYKH, O.A., ZAVARZINA, E.K. Low and high-rhenium
alloys: properties, production, and treatment. In Proceedings from Internal Symposium
“Rhenium and rhenium alloys”. Orlando : TMS. Miner., Metals, Mater, 1997, pages 691-
705.
METAL 2007 22.-24.5.2007, Hradec nad Moravicí
___________________________________________________________________________
7
3. SAVITSKII, E.M., BURKHANOV, G.S., BOKAREVA, N.N. Orientational dependence
of tensile deformation of molybdenum single crystals. In Single crystals of refractory and
rare metals. Moscow : Nauka, 1971, pages 171-176.
4. KOTTREL, A. Kh. Dislocations and plastic flow in crystals. Moscow : Metallurgizdat,
1958. 267 p.