The given invention generally relates to magnesium-based
alloys and, more definitely, to composition and structure of deformable magnesium-based
alloys with the improved combination of strength, deformability and corrosion resistance
at a room temperature.
Magnesium belongs to the group of light metals and, naturally,
is attractive as a constructional material. However it has rather low mechanical
characteristics connected with limited quantity of slip planes at plastic deformation
in h.c.p. (hexagonal close packing) crystalline structure. Besides magnesium has
low corrosion resistance in natural conditions because of strong chemical activity.
A unique way of practical using of magnesium is creation of alloys on its basis.
Mechanical and corrosion properties of any metals essentially depend on presence
into them of other metal elements, which can generate variety intermetallic connections
and the solid solutions that may work various influences upon the specified properties.
Agency of alloying elements on properties of magnesium-based alloys is well investigated
in binary systems, but in multi-component alloys their aggregate effects can appear
complex and in advance unpredictable. Therefore the choice of alloying elements
and their proportions in an alloy are the controlling factor.
The main alloying elements in industrial magnesium alloys
are: aluminum, zinc, lithium, yttrium, manganese, zirconium, rare-earth metals (RE)
and their combinations.
Mechanical properties of magnesium alloys, as well as of
other metal alloys, are controlled by change: an operating combination of known
mechanisms of hardening (solid solution, precipitation strengthening, deformation
hardening, grain-boundary hardening, etc.) and mechanisms of plastic deformation
as due to alloy building, so also/or by change an alloy condition (temper).
Alloying elements and alloy structure also influence simultaneously
its other properties, including corrosion resistance. Corrosion ratio of magnesium
and magnesium alloys also strongly depends on magnesium's degree of purity. For
example, in 4 % water solution of sodium chloride a corrosion ratio of magnesium
with purity of 99.9 wt. % is in hundreds times more, than magnesium with purity
of 99.99 % wt, see
Timonova M.A. Korrosia I zaschita magnievix splavov. M. Metallurgija, 1977,
152 p. in Russian
Besides, some impurities can change a possible solubility
range of other impurities. So, addition of aluminum in a magnesium-based alloy increases
the influence of other alloying elements on corrosion ratio of alloy [see above].
Distribution of alloying elements and impurities, structure of chemical combination,
which they form, affect also the big influence on corrosion ratio of magnesium alloys
and their uniformity. Besides corrosion ratio of magnesium alloys depends on a condition
of an alloy - deformed, aged, in full or in part annealed etc.
Alloy of the invention are supposed to be used mainly in
the field of temperatures 0 - 50 °C and within the practical applications demanding
good deformability and improved corrosion resistance. Therefore the previous development
in the field of improvement of mechanical and corrosion properties of magnesium
alloys will be considered below only under the specified temperature conditions.
Data on improvement of strength, creep resistance and corrosion resistance of magnesium
alloys at elevated and high temperatures will be considered only partially, though
authors are well familiar with them. It is so, because, though the improved strength
of such alloys will be kept and at room temperatures, but their plastic characteristics
in these conditions can strongly go down.
Below, at the description of properties of magnesium alloys,
the range of temperatures from 0 up to 50 °C, and the quantitative contents
of alloying elements in percentage on weight will be always implied by, if will
not be other special provision.
Plenty of magnesium-based alloys are fabricated now, and
their compositions are chosen depending on concrete applications.
The most part of magnesium-based alloys can be conditionally
divided into several groups, according to prevailing alloying elements. There are
groups of Mg-Li, Mg-Al, Mg-Zn and Mg-RE alloys, where RE - rare-earth metals.
Alloys are also subdivided into classes within the specified
groups, according to additional alloying elements. For example, under specification
- Alloys of type LAE (Mg-AI-RE) are within Mg-Li group;
- Alloys of type: AM (Mg-Al-Mn), AZ (Mg-Al-Zn), AE (Mg-Al-P3M) are within of Mg-Al
- Alloys of type ZK (Mg-Zn-Zr) and alloys ZE (Mg-Zn-P3M) are within Mg-Zn group;
- Most known alloys of type WE (Mg-Y-Nd-Zr) are within Mg-RE group.
Alloys of more complex composition, which cannot be carried
unequivocally to a concrete class under specification ASTM, are described in various
patents. The purpose of their creation is improvement of the certain characteristics
Mg-Li eutectic alloys are the most plastic alloys of magnesium
(for example, Pat. No.
DE 3922593, 1991-01-24
). According to binary phase diagram Mg-Li (
Freeth W. E., Raynor, G. V., J. Inst. Metals 82, 575-80, 1954
), there is an HCP alpha-phase in an alloy at the lithium contents up to
5, 7 %, which is peculiar to pure magnesium. At lithium contents more than 10 %,
the beta-phase, having b.c.c. (body-centered cubic) structure, prevails in an alloy.
The possible quantity of sliding systems and, thus, formability of alloys increases
in this case. In tensile test at a room temperature elongation of alloy Mg-11 Li
reaches 39 % and UTS - 104 MPa (in
U.S. Pat. No. 6,838,049
Lack of magnesium-lithium alloys is low strength and reduction
in corrosion resistance of an alloy due to presence of chemically active lithium.
Mg-Li alloys are alloyed in addition for increase of strength
and corrosion stability. Aluminum and zinc are often added in Mg-Li alloys for increase
of strength and corrosion resistance of them. The addition of 4 - 10 % of aluminum
and up to 2 % zinc leads to an quite good combination of strength and formability
of Mg-Li-Al-Zn alloys.
Master's Thesis Hsin-Man Lin "Effects of Aluminum Addition on Properties of
Magnesium-Lithium Alloys", Date of Defense 2004-07-15
, it is shown, that addition 0.6 % Al into alloy Mg-9Li can "obviously
increase the mechanical strength and corrosion resistance, and maintain the properties
of elongation at temperatures below 200 °C and any speeds of deformation".
However presence of aluminum, also as well as zinc, in
Mg-Li alloys reduces their formability at room temperature, which is the main advantage
of these alloys. Such changes are essential adverse effect of presence of the specified
elements in Mg-Li alloys.
There are offered also another combination of alloying
elements in alloys on the base of Mg-Li.
JP Pat. No. 8-23057B
yttrium addition is offered for increase in strength of Mg-Li alloy, but
presence of one more active element in an alloy reduces, in addition, corrosion
resistance of such alloys.
U.S. Pat. No.6,838,049
is described "a magnesium alloy formed at a room temperature with excellent
resistance of corrosion". Its composition includes from 8.0 up to 11.0 % of lithium,
from 0.1 up to 4.0 % of zinc, from 0.1 up to 4.5 % of barium, from 0.1 up to 0.5
% Al, and from 0.1 up to 2.5 % Ln (a total sum of one or more lanthanides) and from
0.1 up to 1.2 % Ca with the balance, which is being Mg and inevitable impurity.
Authors consider that "Ba forms an intermetallic compound (Mg.sub.17 Ba.sub.2) with
Mg. Because Mg.sub.17 Ba.sub.2 precipitates at a temperature of 634.degree. C. which
is close to 588.degree. C., which is the Mg-Li eutectic reaction temperature, but
higher than this reaction temperature, it acts as a nucleus when the alpha- and
beta-phases precipitate, providing for refinement and uniform dispersion of alpha-
However, though barium has b.c.c. lattice, but it has a
low solubility in Mg and formed intermetallic, which reduce an initial plastic characteristics
of Mg-Li alloys.
U.S. Pat. No. 5,059,390
"A dual-phase magnesium-based alloy consisting essentially of about 7-12%
lithium, about 2-6% aluminum, about 0.1-2% rare earth metal, preferably scandium,
up to about 2% zinc and up to about 1% manganese" is offered. The alloy exhibits
improved combinations of strength, formability and/or corrosion resistance.
JP Pat. No. 9,241,778 (1997-09-16
) the magnesium alloy for use as the constructional material is offered.
Alloy contains up to 40 % Li and one more additive from the following: up to 10
% Al, up to 4 % Zn, up to 4 % Y, up to 4 % Ag and up to 4 % RE.
U.S. Pat. No.5,238,646
the method of preparation of the alloy having the improved combination
of strength, formability and corrosion resistance is offered. The specified alloy
includes: approximately 7-12 % of lithium; approximately 2-7 % of aluminum; approximately
0.4-2 % RE; approximately up to 2 % of zinc; and approximately up to 1 % of manganese,
balance magnesium and impurity. Purity of magnesium taken for a basis of an alloy
is 99.99 %. Authors ranked yttrium and scandium also to group of rare earth metal.
Though they have an identical structure of external electronic shells of atoms with
metals of RE group and similarity of some chemical properties, but they shell be
distinguished from RE, according to Standard ASTM, in their differing characteristics
JP Pat. No. 2000 282165
Mg-Li alloy with the improved corrosion resistance is offered. The alloy
contains up to 10.5 % Li and magnesium with concentration of iron <= 50 p.p.m.,
which can be achieved "by carrying out the melting of the alloy in a crucible coated
with chromium or chromium oxide".
Mg-Al alloys (classes AM, AZ and AE) are the most widespread
in practice group of magnesium alloys. However, though they also show the better
corrosion resistance and higher strength, than Mg-Li alloys, but they are much less
One of methods of corrosion resistance increasing of magnesium
alloys is reduction of a contents level of Fe, Ni and Cu. According to
L. Duffy (Materials World, vol. 4, pp. 127-30, 1996
), corrosion ratio of alloy AZ91 E (salt fog tests) is in 100 times less,
than for alloy AZ91C, owing to higher purity (0.1 % Cu, 0.01 % Ni, 0.3 % max the
others - in alloy AZ91C and 0.015 % Cu, 0.001 % Ni, 0.005 % Fe, 0.3 % max the others
- in alloy AZ91 E).
U.S. Pat. No. 2005 0,129,564
it is offered the alloy containing: of 10 to 15 % Al, 0.5 to 10 % Sn,
0.1 to 3 % Y, and 0.1 to 1 % Mn, the balance being Mg and inevitable impurities.
U.S. Pat. No. 6,395,224
the alloy, which includes magnesium as a main component, boron of 0.005
% or more, manganese of 0.03 to 1 %, and substantially no zirconium or titanium
is offered. This magnesium alloy may further include aluminum of 1 to 30 % Al and/or
zinc of 0.1 to 20 %. Because of appropriate amounts of boron and manganese contained
in the magnesium alloy, the grain of the magnesium alloy is refined.
U.S. Pat. No. 2005 0,095,166
it is offered "Heat resistant magnesium alloy for casting", witch includes
6-12 % of aluminum, 0.05-4 % of calcium, 0.5-4 % of rare earth elements, 0.05-0.50
% of manganese, 0.1-14 % of tin, balance are magnesium and inevitable impurities.
Data about plastic characteristics of an alloy at room temperatures are not resulted.
Among Mg-Zn alloys are the most known: alloys of class
ZK (magnesium-zinc-zirconium), having good durability and plasticity at a room temperature;
alloys of class ZE (magnesium-zinc-RE), having average durability; alloys of class
ZH (magnesium-zinc-thorium), having high room-temperature yield strength in aged
condition (T5). The magnesium alloys, containing thorium, are not made now, because
of their weak radio-activity.
U.S. Pat. No. 2001 6,193,817
it is offered a magnesium base alloy for high pressure die casting (HPDC),
providing good creep and corrosion resistance, comprises: at least 91 % of magnesium;
0.1 to 2 % of zinc; 2.1 to 5 % of a rare earth metal component; 0 to 1 % of calcium.
Alloys of type WE (Mg-Y-Nd-Zr) are the most known among
alloys Mg with RE. These alloys possess quite good formability and the increased
corrosion resistance. According to the specification of the manufacturer (Magnesium
Elektron Ltd., Manchester, England) elongation of alloy ELEKTRON WE43 CASTINGS can
achieve 17 % at a room temperature, and corrosion ratio is equal 0.1-0.2 mg\cm2\day
(ASTM B117 salt spray test) or 0.1 mg\cm2\day (sea water immersion test).
However, deformability of this alloy is insufficient in many cases, and the experimental
dispersion of mechanical characteristics of WE43 ingots is very great: elongation
2 - 17 % (average value is 7 %, data of the manufacturer on the base of 215 samples).
Deformed (extruded, forget) and thermo-threatened alloy WE 43 shows more stable,
but lower plasticity at a room temperature - up to 10 % (condition T5, T6).
Various changes of Mg-RE alloys composition are offered
for increase of its operating ability.
U.S. Pat. No. 2004-07-27 6,767,506
it is offered "High temperature resistant magnesium alloys", containing
at least 92 % magnesium, 2.7 to 3.3 % neodymium, 0.0 to 2.6 % yttrium, 0.2 to 0.8
% zirconium, 0.2 to 0.8 % zinc, 0.03 to 0.25 % calcium, and 0.00 to 0.001 % beryllium.
The alloy may additionally contain up to 0.007 iron, up to 0.002 % nickel, up to
0.003 % copper and up to 0.01 silicon and incidental impurities.
Interest to WE-type alloys, as to a constructional material
of vessel stents, has increased last years. For example, in
U.S. Pat. No. 2004 098108
it is offered to make vascular endoprostheses, comprising a carrier structure,
which contains a metallic material. This metallic material contains a magnesium
alloy of the following composition: magnesium: > 90 %, yttrium: 3.7-5.5 %, rare
earths: 1.5-4.4 % and balance: <1 %. In essence, this composition corresponds
to alloy WE43. However, because of insufficient plasticity of such alloy, authors
had simultaneously to offer a new stent design, which provides its working capacity
at the lowered plastic characteristics of the offered alloy.
Mechanical characteristics (tensile test, room temperature)
and corrosion ratio of some most widespread magnesium-based alloys, taken of various
accessible sources are resulted in Table 1.
Tests for corrosion behavior were carried out by the special technique: in a stream
of 0.9 % water solution of sodium chloride. Speed of stream was 50 m\min. Corrosion
ratio was determined on loss of sample's weight and through quantity of magnesium,
passed into a solution washing specimen. The data of measurements were averaged.
Such testing scheme allows continuously washing off products of corrosion from sample's
surface which, for example, deform results of corrosion ratio studying by a method
of measurement of sample's weight loss.
0.1 mg/cm2/day (sea water immersion)
0.1-0,2 mg/cm2/day ASTM B 117 salt spray test
2.6 mg/cm2/day *
< 0.13 mg/cm2/day ASTM B 117 salt spray test
< 0.13 mg/cm2/day ASTM B 177 salt spray test
Alloy of invention, Example 1
Deformed, annealed (H2)
Alloy of invention, Example 2
Deformed, annealed (H2)
Characters in titles of alloys designate: A - aluminum, E - the rare earth (RE),
K - zirconium, L - lithium, M - manganese, W - yttrium, Z - zinc, and figures -
the contents of an alloying elements approximated to an integer in percentage.
Table 1 shows that various magnesium alloys have different
combinations of mechanical and corrosion characteristics. One has higher strength,
others are less strength, but are more deformable. However, for responsible applications,
it is desirable to combine high strength and high plasticity with preservation of
sufficient level of corrosion resistance.
The purpose of the present invention is creation of new
magnesium-based alloy having improved (in comparison with existing) combination
of strength and plasticity at preservation of low corrosion ratio, peculiar to alloys
of WE- and AZ-types. For example, it is desirable to create an alloy having yield
stress (YS) more 200 MPa, tensile strength about 300 MPa and more, elongation more
than 22 % and corrosion ratio about of 0.1 mg\cm2\day (sea water immersion
test) at a room temperature.
On the basis of available data about the influence of various
alloying elements and their compounds (quantity, condition, distribution, etc.)
on magnesium properties and the carried out own experiments, authors have accepted
following preconditions for development of an offered alloy.
- 1. The magnesium taken as a basis of an alloy should have high purity. The total
contents of impurities should no be more than 0.005 %, without taking into account
contents of Fe, Ni and Cu. The contents of these elements, affect the most adverse
influence on corrosion characteristics of magnesium, should be limited no more than
0.001 % of everyone.
- 2. The alloy should contain alloying elements in the quantities, which are not
essentially exceeding their solubility in solid magnesium, according to known binary
- 3. Purity of alloying elements should not be worse than 99.98 % (only metal
impurities are considered).
- 4. Authors have chosen basic alloying elements, which appreciably improve one
of characteristics of alloy in considered combination (strength, plasticity, corrosion
resistance) and which influence minimally unfavorably on other alloy properties
- 5. For use in medical purposes, the alloy of invention should not contain, in
appreciable quantities, the elements that affect adverse influence on human's or
animal's organism (Zn, Th, Sr, Cd, Al, etc).
- 6. It is necessary to add in alloy the elements that affect modifying influence
(grain-refining) on its structure and providing grain size in initial ingot no more
- 7. For additional (besides an alloying) improvement in combination of mechanical
and corrosion characteristics of offered alloy, it is suggested to use it in ultra
fine-grained condition with the average grain size no more than 3 microns. The specified
grain structure may be created by processing of an initial ingot or preliminary
extruded slab with application of developed by authors method of programmed intensive
plastic deformation in a combination with programmed heat treatment [
Physitcheskoe metallovedenie beryllium, I. Papirov, G. Tikhinsky, 1968, Atomizdat,
Moscow, in Russian
]. Methods of pressure processing of preform should be applied for this
purpose, which will be providing prevalence of torsional or shear stresses in a
On the basis of the aforesaid authors have chosen following
alloying elements for magnesium-based alloy as the preferred embodiment(s) of the
Scandium has a limit of solubility in solid magnesium about
29 %. According to laboratory findings of authors, addition of scandium into magnesium
within the limits of up to 8 % provides creation of solid solution Mg-Sc that increases
its plasticity and strength. In the interval of scandium concentration from 3 up
to 8 % corrosion ratio of Mg-Sc alloy in water solution of sodium chloride increases
slightly. Precipitation of Mg-Sc phase is possible during high-temperature processing
of magnesium alloys with the big contents of scandium. However, very thin intermetallic
bond in the form of the plates, formed in a direction <1120> in a basal plane,
is distributed non-uniformly and do not make any hardening at a room temperature
when the main mechanism of deformation is basic sliding [
Buch F., Mordike B.: Microstructure, Mechanical Properties and Creep Resistance
of Binary Magnesium Scandium Alloys. In: Magnesium 97 (Eds. Aghion, E., Eliezer,
D.), MRI, Beer Sheva 1998, p. 163-168
Besides scandium also is the strong modifier grain structure
of magnesium ingots.
Yttrium has the limit of solubility in magnesium about
2 % at room temperature. Addition up to 3 % of yttrium into magnesium increases
strength of an alloy without essential reduction in its plasticity and corrosion
Rare earth (RE) metals influence on properties of magnesium
alloys depends on their solubility in it and their melt point. Solubility RE in
solid magnesium changes from practically zero (La) up to 7 percent (Lu). Metals
from group with nuclear numbers from 64 (Gd) up to 71 (Lu) have melting temperatures
and limits of solubility in magnesium higher, than metals of cerium group. Introduction
up to 3 % refractory RE in a magnesium alloy raises creep and corrosion resistance
of an alloy, and reduces micro porosity of multi-component alloy at its melting.
Zirconium, as is well-known, is a basic element, which
crushes grain size in magnesium alloys during an ingot production. The fine-grained
ingot is easier exposed to preliminary and subsequent deformation.
In accordance with the foregoing objectives, as preferable
embodiments, authors offer the following magnesium-based alloy having the improved
combination of mechanical and corrosion characteristics at room temperature. Alloy
consists essentially of: magnesium base with purity not less 99. 995 %, scandium
from 1 up to 10 %, preferable 2.5-6 %, yttrium from 0.1 up to 3 %, preferable 2-2.5
%, rare earth from 1 up to 3 %, zirconium from 0.1 up to 0.5 %, preferable 0.3-0.4
%. Contents Fe, Ni and Cu do not exceed 0.001 % of everyone, the total contents
of incidental elements and impurities do not exceed 0.005 %.
Alloy of the specified composition is received by direct
fusion of magnesium with preliminary prepared master alloy from the specified alloying
elements in high-frequency induction furnace in atmosphere of high purity argon
and in high purity graphite crucible. Melt is poured out in cooled steel mold with
a special daubing by a method of bottom teem.
The prepared ingot further is subjected to pressure treatment
by the developed by authors method of programmed intensive plastic deformation (for
example, by different-channel angular extrusion) at temperatures 250-350 °C
in combination with programmed heat treatment. At achievement of micro-hardness
Hµ more than 110 kg/mm2, preform is subjected to an annealing at
temperature 270-320 °C.
Preform prepared by the above-stated method, further is
subjected to usual industrial schemes reception of sheets, rods, wire, tubes, etc.
for produce final products.
For example the alloy material is well qualified for stents.
The alloy material has the capability of a desired deformation regarding to a specific
application. Furthermore the grain size is adjustable for tuning the strength characteristics.
Examples of preferred embodiments
Alloy consists essentially of: magnesium with purity of
99.997 % with addition of 4.2 % scandium, 2.4 % yttrium, 3.0 % the rare earth, 0.4
% zirconium. Contents Fe, Ni and Cu were not exceed 0, 001 % of everyone, the contents
of incidental elements and impurities do not exceed 0,005 %.
The alloy was received by direct fusion of magnesium with
preliminary prepared master alloy from the specified elements in high-frequency
induction furnace in an atmosphere of high purity argon and in high purity graphite
For full dissolution of all components, alloy was stood
in crucible at temperature 720 °C within 30 minutes and then was poured out
in cooled steel mold with a special daubing by a method of bottom teem.
The received ingot (diameter of 50 mm) was extruded at
temperature 340 °C with an extrusion ratio of 3:1.
The received semi-finished product has been subjected to
deformation by different-channel angular extrusion at temperature 320 °C, number
of cycles of extrusion - 12, with intermediate annealing at temperature 275 °C
through 2 - 3 cycles (at achievement of micro-hardness Hµ of 110
Samples have been cut out from the received extrudate for
tensile test at room temperature and tests for corrosion behavior (in a stream of
0.9 % water solution of sodium chloride. Speed of stream was 50 m\min).
Mechanical properties (after annealing at temperature 320
°C within one hour): YS=240 MPa, UTS=320 MPa, elongation=25 %.
Corrosion ratio (it is obtained by measurement of weight loss of specimens and quantitative
definition of the magnesium, which has passed in a solution, through the fixed time
intervals) - 2.1 mg/cm2/day.
Results of tests show that the alloy of the invention with
the specified composition has the best combination of mechanical and corrosion properties
in comparison with the most widespread industrial alloys of magnesium (see Tabl.1).
The ingot on the basis of magnesium with purity of 99.995
%, with addition of 10.0 % scandium, 1.4 % yttrium, 2.0 % of rare earth (mainly
- gadolinium) and 0.5 % zirconium was received by the method specified in an example
Then the ingot had been subjected to deformation by alternation
of cycles extrusion with extrusion ratio 2,5:1 and swage out till initial diameter
(one cycle) at temperatures 300-340 °C, number of cycles - 5, with intermediate
annealing at temperature 275 °C.
Samples have been cut out from the received preparation
for mechanical tests and tests for corrosion (in a stream of 0.9 % water solution
of sodium chloride. Speed of stream was 50 m\min).
Mechanical properties (after annealing at temperature 290
°C within one hour): YS=210 MPa, UTS=290 MPa, elongation=29 %. Corrosion ratio
(in stream) - 2.9 mg/cm2/day.
Results of tests show that the alloy of the invention of
the specified structure has the best combination of deformability and corrosion
properties at satisfactory strength in comparison with the most widespread industrial
alloys of magnesium (see table 1).