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Mechanical properties of MgB2 hollow wires
Conference Paper · June 2010
DOI: 10.1063/1.3402317
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CEC-ICMC Conference, Tucson (AZ) , July 2009. To be published in Adv. in Cryogenic Eng.
MECHANICAL PROPERTIES OF MgB2 HOLLOW WIRES
L. Saglietti1, E. Perini1, A. Figini Albisetti1, G. Ripamonti1, E. Bassani2,
P. Bassani2, G. Giunchi1
1
EDISON SpA, R&D Dept.
Milano, Italy
2
CNR-IENI
Lecco, Italy
ABSTRACT
The peculiar morphology of the MgB2 wires resulting from the reactive liquid Mg
infiltration technology (Mg-RLI), characterized by an annular distribution of the
superconducting material, originates a peculiar mechanical behaviour. Monofilament
wires of different size and different thickness of the superconducting material have been
subjected to tensile and flexural deformation, in order to gain a comprehensive
description of the mechanical behaviour of the superconducting material. A model based
on a simplified description of the mechanical behaviour in term of few parameters aids
the interpretation of the various experimental measurements and represents a tool to
evaluate the mechanical behaviour of different specifically sized wires. Other variables
entering in the mechanical model take into account the filling of the internal hole with the
metallic Mg and the strength of the external metallic sheath. The present mechanical
analysis gives hints on the limits of applicability of the react-and-wind process in magnet
manufacturing with this kind of wires. A further output of the study is to find the
variation of the intrinsic mechanical properties of the MgB2 material as a function of the
Carbon doping, obtained via SiC doping.
KEYWORDS: MgB2, superconducting wires, mechanical properties, infiltration process
PACS: 74.70.Ad
INTRODUCTION
The most used superconducting wires are based on NbTi alloy, a Low Temperature
Superconductor (LTS) metallic in nature (Tc = 9 K) which is quite easily drawn to obtain
wires of micrometric diameter. Their ductile characteristics and their mechanical strength
allow a multiplicity of stranded cabling options and also greatly favour the magnet
manufacturing. On the contrary, all other superconducting materials, up until today
discovered, have very poor mechanical properties. Among the A15 superconductors
family, intermetallics in nature, the Nb3Sn material, even if very useful for its relatively
higher critical temperature (Tc = 18 K) and higher critical magnetic field, has an intrinsic
brittleness that curbs a wide use of this material. Indeed the brittleness of Nb3Sn prevents
the drawing operations in the superconducting state and forces its use, in magnet
manufacturing, by the “wind-and-react” technique. With the advent of the High
Temperature Superconductors (HTS), ceramic in nature, the same issue of the brittleness
raised, even if it was mitigated from the introduction of the Powder in Tube (PIT)
process. All the cuprate wires derived by PIT process use silver sheaths and have very
limited mechanical strength, so also in this case the strength issue remains unsolved.
With the advent of the MgB2, a material which allows a PIT process with more
robust metallic sheaths, like steel or nickel or nickel-copper alloy, the mechanical issues
are greater reduced with respect to the HTS cuprates. Some mechanical properties of the
MgB2 PIT wires have been reported in the literature, with particular emphasis to the
effect of the thermal treatment. Some attempt to make industrial-like magnets by the
“react-and-wind” process using MgB2 PIT wires have been also performed [1,2], even if
the wires performances, in term of critical currents, were not optimal. An alternative
process to the MgB2 PIT manufacturing is represented by the Liquid Mg Reactive
Infiltration (Mg-RLI) process [3], which leads to special hollow wires. The critical
current density of this kind of wire is quite high, due to the high density and connectivity
of the superconducting material. The mechanical properties of this kind of wire are very
important to choose the technology needed for the production of windings for magnets
and other applications. In this paper we present the first measurements of the mechanical
strength of these peculiar wires, both in form of unreacted precursors and as full reacted
superconducting wires. The aim is to find the range of applicability of the friendlier
“react-and-wind” process for this class of wires.
THE MgB2 HOLLOW WIRES MANUFACTURING
The Mg-RLI technology was successfully applied to produce MgB2
superconducting wires that, among their interesting superconducting properties, have the
peculiar characteristics to be hollow [4]. With regard to wires manufacturing, the Mg-RLI
technology includes the drawing of a precursor billet made by an internal Mg rod
surrounded by boron powders, and encased in a metallic sheath. The draw ability of this
peculiar billet, even in presence of a quite brittle metal like Mg, has been proved to be
feasible at least up to diameters of the order of few tenth of mm, with a good
reproducibility of the starting ratio between the components and of their circular
morphology [5]. At laboratory level, we can obtain prototype precursor wires, several
hundred meters long. Some attempt to draw multifilamentary (7 filaments) wires with the
same process was successful, even if more critical processing conditions are involved in
order to obtain long length wires of this kind. A thermal treatment of about 1 hour, at
temperatures ranging from 680° C to 800° C (in Ar atmosphere to prevent oxidation of
the external sheath) transforms the precursor in a superconducting wire. Such a treatment
can be done either batch wise or in a continuous way. In the past years we have
elucidated the key parameters which govern the superconducting quality of these wires
[6]. Among others we recall: a) the type of the boron powders, generally amorphous, but
of various degree of purity and of grain size; b) the presence of additional doping
powders, mainly carbonaceous species; c) the Mg to B ratio. In the present study we
consider two typical wires, that we can consider representative of the actual state of the
art of the Mg-RLI technology.
The wires were produced in our lab starting from a billet of 25 mm of diameter,
drawn to submillimeter size. The type A) wires are made up by almost pure reactants Mg
(rod) and B, inserted in the billet constituted by the external Monel 400 metallic sheath
and by an internal thin lining of Nb. For the type B) wires, the pure B powder is mixed
with 10wt% nanosized SiC powders. The metallic lining is the same as for type A).
The reaction was performed in a tubular furnace on a wire length of about 1 m and
at temperatures of 680°, 710° and 800° C, for about 1h. Codes A0, A1, A2, A3 and B0,
B1, B2, B3 are used here to identify the wires where the letter categorizes the type of
wire and the number indicates its state and diameter, according to: 0 = precursor; 1 =
reacted, 0.35(A) or 0.38(B) mm ; 2 = reacted, 0.8 mm; 3 = reacted, 1.1(A) or 1.0(B) mm.
A typical cross section of the reacted wires is reported in FIGURE 1. The percentages of
the various area components of the precursor and of the reacted wires are given in
TABLE 1. These values can be considered as mean values due to the variability of the
processing.
THE MECHANICAL TESTS
Tensile
We have used a MTS dynamometer equipped with a mechanical extensometer and
a load cell of 2 kN as maximum force. The tested wire specimens have been cut from the
longer wires in pieces 100 mm long. The samples have been mounted on the
dynamometer with pneumatic clamps; the tests were displacement-controlled, with a rate
of 0.3 mm/min.
Flexural
Three-point flexural measurements on wires with large diameter (0.8 mm or larger)
were performed employing the MTS dynamometer. A DMA apparatus was utilized for
the smaller diameter wires.
FIGURE 1 – An optical image of the MgB2 hollow wire, resulting from the Mg-RLI process: A-type wire
(A3 of the text)
TABLE 1 - Geometrical parameters of the wires
Area
Fill factor
Wire Area
Area
Area
Area
Hole (%)
type
Monel+Nb (%)
B (%)
Mg (%)
MgB2 (%)
A0
60-70
20-15
15
A1
60-70
25-15
15
~ 25-15
B0
55-60
30
15-10
B1
55-60
30-25 (*)
15-20
~ 30-25
(*) In the doped wires part of the MgB2 sector is occupied from the residues of the doping compound (i.e.
Mg2Si in our case), so that the real MgB2 content can be estimated to be about 90% of the given values.
These tests were displacement- and load-controlled, respectively. The length
adopted for the specimens was about 15 mm because the distance (L) between the two
external fixed points was 10 mm (FIGURE 2).
To evaluate the stress and strain from the measured flexural forces F and from the
arrow, u, which describes the reached curvature of the sample wire, the following
relations have been used:
2
⎛L⎞
2
⎜ ⎟ +u
2
bending radius: R = ⎝ ⎠
2u
; bending strain: ε ≈
stress values, respectively: σ max,flex =
FL
⎛d⎞
π⎜ ⎟
⎝2⎠
3
and
σ ≈
d
2R
;
σ max d
4t
where d is the wire diameter and t is the thickness of the Monel+Nb+MgB2 wall.
The last relation is only an approximate evaluation of the stress actually acting on
the rim of Monel+Nb+MgB2.
RESULTS
Tensile tests
The typical tensile curves for a precursor wire and a reacted wire are shown in
FIGURE 3.
Unreacted precursors
The precursor wires behave elastically, with Young modulus ranging between 100
and 120 GPa, until the yielding stress, whose value is of 600-700 MPa. The plastic region
is limited if compared with Monel’s one, and the strain to rupture is generally below 1%.
u
L = 10 mm
FIGURE 2 – Flexural tests equipments: MTS dynamometer (top); scheme of the DMA apparatus (bottom).
Note that the same support and loading anvil were employed for both equipments.
This phenomenon can be due to the hardening of the sheath consequent to the
drawing process. The annealing process of the precursor wire (400° C, 30 minutes) only
partially enhances the wire ductility. The mechanical behaviour of the precursor wires is
independent by the wire diameter, and well follows the rule of mixtures:
Etot = E1f1 + E2f2 + E3f3
where Ei and fi are the Young modulus and the fraction of area occupied by the Monel
sheath, the boron powder and the inner magnesium. The calculated values well fit the
experimental data if the contribution of the boron powder (i.e. its modulus E) is assumed
to be equal to zero.
The reacted superconducting wires
As well depicted in FIGURE 3, the behaviour of the reacted hollow wires differs
from the previously described one: in this case, the linear elastic region is followed by a
sudden change in the slope of the curve, which corresponds to a region of damaging of
the inner MgB2.
The Young modulus values for the reacted wires spread from 50 to 110 GPa: this
scattering of values may be due to the different monel/MgB2 areas ratio and to the
presence of residual Mg in the hole. The correlation between the modulus and the area
percentages of the components still exists, but the rule of mixtures does not fit very well,
in particular for the thinner wires, whose Young modulus are far lower than predictable.
This behaviour can be due to the hollow configuration, which may be associated to a high
radial striction of the reacted wires with respect to the unreacted, more pronounced for
the thinner wires.
700
unreacted precursor
600
stress (MPa)
500
400
300
reacted wire
200
100
0
0
0,5
1
1,5
strain (%)
FIGURE 3 – Stress-strain curves for unreacted precursor and reacted hollow wires.
260
stress (MPa)
240
225
220
220
elastic limit
200
215
210
205
180
200
160
195
0
0,5
1
0,2
strain (%)
(a)
(b)
(c)
FIGURE 4 – (a) Criterion to define the stress and strain elastic limit; (b) detail of the σ(ε) curve, around
the elastic limit and at the first crack of the wire; c) SEM micrographs of a cracked wire
In order to define an empirical parameter useful to describe a reacted wire and its
range of use, we chose to replace the conventional yield stress (0.2% yielding) with a
more general “elastic limit”: this stress/strain limit is defined (FIGURE 4) as the
intersection between the curve and a vertical line moving from the point where the
tangents to the two regions (elastic and non-elastic) of the stress-strain curve meet up.
The values for the assumed stress limit (evaluated by following this criterion) range
between 150 and 210 MPa, while the strain limits go from 0.2 - 0.3%, for the large
diameter wires, to 0.4 - 0.5% for the thinner wires. This behaviour of the thinner wires
represents the only evident correlation between the tensile properties and the diameter of
the wires coming out of the measurements.
Nevertheless, the tensile strength seems to be correlated to the reaction temperature,
as an increase in T induces a decrease in the stress/strain limits (FIGURE 5).
The adopted criterion identifies the point in which the curve leaves the linearity,
corresponding to a region in which the phenomena of the yielding of the sheath and
cracking of the MgB2 are starting. The MgB2 breaks stepwise (Fig. 4b), and the
development of cracks is well visible in the curves after the elastic limit (sudden strain
increments with reduction of the load). Regions in which the MgB2 has failed correspond
to the localized yielding of the external sheath. The peaks corresponding to the cracks are
not detectable in the curves before reaching the elastic limit. The failure of the wires
(FIGURE 4c) is due to ductile failure in the external Monel sheath.
Flexural tests
Flexural measurement data, obtained in terms of force vs. displacement, have been
converted in stress-strain curves, in order to compare the flexural behaviour with the
tensile. The typical stress-strain flexural curves for the unreacted and the reacted wires
are reported in FIGURE 6. The strain reported is related to the outermost lamina of the
wire. Flexural stress values are larger than the ones from tensile tests. This can be
explained because the latter are engineering stresses (loads divided by the total wire area,
hole included) while the former are calculated as mean values, acting over the effective
area of Monel+MgB2. The relative behaviour of the unreacted and reacted wires mimics
that one of the tensile tests, even if the elastic area for reacted wires seems to be reduced.
Flexural measurements have been exploited mainly for the calculation of the
minimum bending radii the wires can undergo without damaging (TABLE 2).
250
250
s tres s (MP a)
s tres s (MP a)
TT
200
200
150
150
wire B : 1 mm diam.
wire B : 1 mm diam.
100
100
0
0
0,2
0,2
0,6
0,6
s train (% )
s train (% )
0,4
0,4
0,8
0,8
1
1
FIGURE 5 - Dependence of the tensile stress from the reaction temperature of the wire B3 (1mm diam.)
1600
1400
unreacted wires
<stress> (Mpa)
1200
1000
800
hollow wires
600
400
200
0
0
0,5
1
1,5
2
2,5
3
3,5
Strain (%)
FIGURE 6 – Flexural test for an unreacted wire and for a reacted wire
The minimum bending radius has been obtained for each wire from the critical
displacement value corresponding with an irreversible damage of the wire. In the case of
precursor wires, this damage has been linked to the plastic deformation, while for reacted
wires the displacement considered is the one of the first crack.
As predictable, the minimum bending radius increases with the wire diameter. The
measurements also indicate that an increase of the reaction temperature leads to a
weakening of the wires, with the resulting increase in the minimum bending radius
values. Doping seems also to have a negative influence, as mean values for wire B are
higher than for wire A.
Jc MEASUREMENTS
Critical current measurements have been performed at 4.2 K, in a liquid He bath,
with 4 points method. Due to the high disturbances of the V(I) curves, caused by
recovered flux jumping we have used, as approximate criterion for Ic, the current which
corresponds to the irreversible quenching of the wire. For these current values, generally
the electric field is of the order of several µVolt/cm.
TABLE 2 – Minimum bending radius, derived from the flexural tests
.
minimum bending radius (mm)
reacted 680° C (**)
reacted 710° C (**)
35
50
80
85
wire
precursor (*)
reacted 800° C (**)
A-0.35
30
A-0.8
32
A-1.1
46
115
B-0.38
30
55
65
B-0.8
34
80
120
B-1.0
45
87
105
140
(*) bending radii shorter than reported values lead the precursor to go beyond its elastic limit.
(**) as for reacted wires, bending radii shorter than reported values correspond to fractures in the MgB2.
TABLE 3 - Critical current dependence from the tensile load applied to the wire before the measurement.
WIRE B,
1 mm diam
T = 800° C
as reacted
σ = 76 MPa
σ = 127 MPa
σ = 152 MPa
σ/σ LIMIT
0
0.6
1
1.2
IC (A) (*)
@ 4.2K
S.F.
450
380
350
280
TABLE 4 - Mechanical properties of different Nb3Sn and MgB2 wires
Wire
Nb3Sn
E (GPa)
110-125
σy(MPa)
20-30
εy (%)
0.025
ε MAX (%)
<0.5
Ref.
7
MgB2 –PIT
(ex situ - Fe sh.)
MgB2 –PIT
(in situ - Fe sh.)
MgB2- RLI
(in situ– Monel)
120-150
110-140
0.2
-
8
60
200
0.2
0.5
9
60-100
150-200
0.2 – 0.4
0.4
As reported in TABLE 3, the degradation of Ic already starts before reaching the
elastic limit: for stress corresponding to the assumed stress limit, the Ic decrease is
anyway moderate, but it increases to almost 40% for stresses 20% higher than the stress
limit.
COMPARISON WITH OTHER SUPERCONDUCTORS
The comparison between the mechanical properties of MgB2 hollow wires and of
other superconducting wires, in particular commercial Nb3Sn and PIT-MgB2, allows
understanding the peculiarity of the MgB2 wires obtained by the Mg-RLI process. In
particular (TABLE 4), the yielding stress for Nb3Sn wires is greatly lower than for MgB2
wires, and RLI wires show to have similar mechanical strength to the PIT-MgB2 wires;
the strain εy is not conventionally fixed at 0.2% in the present study, as it can vary
corresponding to the maximum strain the wires can bear.
CONCLUSIONS
The given mechanical analysis of the precursor wires and of the reacted hollow
MgB2 wires, produced by the Mg-RLI process, has elucidated several important
applicative characteristics. The precursor wires can withstand stresses of the order of 500
MPa, without exceeding the elastic limit and, in the same limit, their minimum bending
radius is of about 30 mm, for a thickness of 0.35 mm. This means that for a lower
bending radius some plastic deformation of the external sheath can be foreseen, but
without catastrophic damage of the wires, at least up to 1% of strain.
The reacted wires, characterized by the presence of dense MgB2 of annular shape,
show a fragile behaviour after the elastic limit that brings to discontinuities in the stressed
wires. This damage can be easily detected by eyes as ripples on the metallic external
sheath, in correspondence of the sharp MgB2 fractures. The elastic limit of the reacted
wires has a large spreading of values in the stresses as well as in the strains. The main
correlation extracted by the tensile measurements, on a large number of samples, is that
the higher strain limits, of the order of 0.5%, can be associated to the smaller diameter
wires and the limiting stresses are inversely proportional to the reaction temperature of
the wires. This last behaviour evidences once more that the mechanical characteristics of
the wires are largely dominated by the metallic external sheath. Indeed the Monel, used
as sheath material for these wires, presents a well known reduction of its tensile strength
as a function of the annealing temperature.
The minimum bending radius, corresponding to the elastic limit of the reacted
MgB2 wires having diameters of the order of 1 mm, is about 140 mm, indicating that a
“react & wind” process for the magnet manufacturing can be safely applied at diameters
of the order of 400 – 500 mm. The elastic limit of the wires, here defined, appears a valid
parameter also to characterize the superconducting transport properties of these wires.
We have verified that for applied tensile loads 20% larger than the wires’ limiting stress,
more than 50% of their initial critical current can be measured.
In comparison with other competing superconducting materials, we can underline
far better mechanical characteristics of the MgB2 wires with respect to Nb3Sn and, among
the MgB2 materials obtained by PIT process, a substantially similar behaviour.
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