U.S. patent application number 12/594508 was filed with the patent office on 2010-05-13 for heat-resistant magnesium alloy.
This patent application is currently assigned to KABUSHIKI KAISHA TOYOTA JIDOSHOKKI. Invention is credited to Kyoichi Kinoshita, Manabu Miyoshi, Tsukasa Sugie, Motoharu Tanizawa.
Application Number | 20100116378 12/594508 |
Document ID | / |
Family ID | 39808082 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100116378 |
Kind Code |
A1 |
Sugie; Tsukasa ; et
al. |
May 13, 2010 |
HEAT-RESISTANT MAGNESIUM ALLOY
Abstract
A heat-resistant magnesium alloy according to the present
invention includes Mg, a major component; a first alloying element
"M1" being any one or more members that are selected from the group
consisting of Al and Ni; a second alloying element "M2" being any
one or more members that are selected from the group consisting of
Mn, Ba, Cr and Fe; and Ca; and it has a metallic structure
including: Mg crystalline grains; plate-shaped precipitated
substances being precipitated within grains of the Mg crystalline
grains; and grain-boundary crystallized substances being
crystallized at grain boundaries between the Mg crystalline grains
to form networks that are continuous microscopically. Since the
plate-shaped precipitated substances exist within the Mg
crystalline grains, the movements of dislocation within the Mg
crystalline grains are prevented, and accordingly it becomes less
likely to deform. Moreover, since the grain-boundary crystallized
substances, which form the networks, are present continuously
microscopically at the grain boundaries between the Mg crystalline
grains, the strength at the grain boundaries improves. The
heat-resistant magnesium alloy according to the present invention
in which both of the Mg crystalline grains' granular interior and
the grain boundaries between the Mg crystalline grains are
strengthened exhibits high mechanical characteristics even in
high-temperature regions.
Inventors: |
Sugie; Tsukasa; (Aichi-ken,
JP) ; Kinoshita; Kyoichi; (Aichi-ken, JP) ;
Tanizawa; Motoharu; (Aichi-ken, JP) ; Miyoshi;
Manabu; (Aichi-ken, JP) |
Correspondence
Address: |
Locke Lord Bissell & Liddell LLP;Attn: IP Docketing
Three World Financial Center
New York
NY
10281-2101
US
|
Assignee: |
KABUSHIKI KAISHA TOYOTA
JIDOSHOKKI
Aichi-ken
JP
|
Family ID: |
39808082 |
Appl. No.: |
12/594508 |
Filed: |
February 1, 2008 |
PCT Filed: |
February 1, 2008 |
PCT NO: |
PCT/JP2008/052085 |
371 Date: |
October 2, 2009 |
Current U.S.
Class: |
148/406 |
Current CPC
Class: |
C22C 23/00 20130101;
C22C 23/02 20130101 |
Class at
Publication: |
148/406 |
International
Class: |
C22C 23/02 20060101
C22C023/02; C22C 23/00 20060101 C22C023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2007 |
JP |
2007-097760 |
Claims
1. A heat-resistant magnesium alloy being characterized in that it
includes: magnesium (Mg), a major component; a first alloying
element "M1" being any one or more members that are selected from
the group consisting of aluminum (Al) and nickel (Ni); a second
alloying element "M2" being any one or more members that are
selected from the group consisting of manganese (Mn), barium (Ba),
chromium (Cr) and iron (Fe); and calcium (Ca); and it has a
metallic structure including: Mg crystalline grains; plate-shaped
precipitated substances being precipitated within grains of the Mg
crystalline grains; and grain-boundary crystallized substances
being crystallized at grain boundaries between the Mg crystalline
grains to form networks that are continuous microscopically.
2. The heat-resistant magnesium alloy as set forth in claim 1,
wherein said precipitated substances comprise a Laves-phase
compound with type-"C15" crystalline structure.
3. The heat-resistant magnesium alloy as set forth in claim 1,
wherein said precipitated substances are precipitated parallel to
the {001} plane of Mg crystal.
4. The heat-resistant magnesium alloy as set forth in claim 1,
wherein said grain-boundary crystallized substances comprise an
Mg-"M1"-Ca-system compound.
5. The heat-resistant magnesium alloy as set forth in claim 1,
wherein said grain-boundary crystallized substances comprise a
mixed-crystal phase of a Laves-phase compound with type-"C14"
crystalline structure and a Laves-phase compound with type-"C36"
crystalline structure.
6. A heat-resistant magnesium alloy as set forth in claim 5,
wherein said mixed-crystal structure includes the type-"C14"
crystalline structure more than the type-"C36" crystalline
structure.
7. The heat-resistant magnesium alloy as set forth in claim 1
having fine particles that include "M2" within said Mg crystalline
grains.
8. The heat-resistant magnesium alloy as set forth in claim 1
including: Ca in an amount of from 2% by mass or more to 4% by mass
or less; said first alloying element "M1" in an amount of from 0.9
or more to 1.1 or less by mass ratio with respect to Ca ("M1"/Ca);
said second alloying element "M2" in an amount of from 0.3% by mass
or more to 0.6% by mass or less; and the balance comprising Mg and
inevitable impurities; when the entirety is taken as 100% by
mass.
9. The heat-resistant magnesium alloy as set forth in claim 8
including said second alloying element "M2" in an amount of from
0.3% by mass or more to 0.5% by mass or less.
10. The heat-resistant magnesium alloy as set forth in claim 1
including: Ca in an amount of from 1.235 atomic % or more to 2.470
atomic % or less; said first alloying element "M1" in an amount of
from 1.34 or more to 1.63 or less by atomic ratio with respect to
Ca ("M1"/Ca); said second alloying element "M2" in an amount of
from 0.13 atomic % or more to 0.27 atomic % or less; and the
balance comprising Mg and inevitable impurities; when the entirety
is taken as 100 atomic %.
11. The heat-resistant magnesium alloy as set forth in claim 10
including said second alloying element "M2" in an amount of from
0.15 atomic % or more to 0.25 atomic % or less.
12. The heat-resistant magnesium alloy as set forth in claim 1,
wherein: said first alloying element is Al; and said second
alloying element is Mn.
Description
TECHNICAL FIELD
[0001] The present invention is one which relates to a
heat-resistant magnesium alloy that are capable of withstanding
services under high loads and at high temperatures.
BACKGROUND ART
[0002] Magnesiumalloy, which is much more lightweight than aluminum
alloy is, is about to come to be used widely for aircraft material,
vehicle material, and the like, from the viewpoint of weight
saving. However, in magnesium alloy, since the strength and heat
resistance are not sufficient depending on applications, further
improvement of the characteristics has been sought.
[0003] Hence, in Japanese Unexamined Patent Publication (KOKAI)
Gazette No. 2004-162,090, and in Japanese Unexamined Patent
Publication (KOKAI) Gazette No. 2004-232,060, there are disclosed
magnesium alloys in which calcium (Ca) and aluminum (Al) are
contained in adequate amounts. In these literatures, since Ca--Al
compounds and Mg--Ca compounds crystallize or precipitate at the
grain boundaries between the Mg crystalline grains in the magnesium
alloys, the movements of dislocations are held back. As a result,
the magnesium alloys undergo creep deformations less even in
high-temperature regions, and therefore exhibit good heat
resistance. Further, in the aforementioned magnesium alloys, Mn is
solidified into the Mg crystalline grains, and thereby the
magnesium alloys are subjected to solid-solution strengthening.
DISCLOSURE OF THE INVENTION
[0004] The metallic structure of alloy affects its characteristics
greatly. Accordingly, in order to obtain a magnesium alloy that
possesses strength and creep resistance being sufficient for
services at high temperatures, it is necessary to adapt the types
and amounts of additive elements into adequate ones in order to
control the metallic structure.
[0005] It is an object of the present invention to provide a
magnesium alloy, both of whose crystalline grains' interior and
crystalline grain boundaries are strengthened and which therefore
exhibits good heat resistance, by means of controlling the metallic
structure of the magnesium alloy using adequate alloying
elements.
[0006] Specifically, a heat-resistant magnesium alloy according to
the present invention is characterized in that it includes:
[0007] magnesium (Mg), a major component;
[0008] a first alloying element "M1" being any one or more members
that are selected from the group consisting of aluminum (Al) and
nickel (Ni);
[0009] a second alloying element "M2" being any one or more members
that are selected from the group consisting of manganese (Mn),
barium (Ba), chromium (Cr) and iron (Fe); and
[0010] calcium (Ca); and
it has a metallic structure including:
[0011] Mg crystalline grains;
[0012] plate-shaped precipitated substances being precipitated
within grains of the Mg crystalline grains; and
[0013] grain-boundary crystallized substances being crystallized at
grain boundaries between the Mg crystalline grains to form networks
that are continuous microscopically.
[0014] Note that, in the present description, the "networks that
are continuous microscopically" take on network structures
(three-dimensionally mesh structures) macroscopically, and are
states in which crystals exist continuously even inside the
networks (see FIG. 2). Therefore, the following are not involved:
discontinuous states whose interior is constituted of small
crystals, even though they take on network structures (see FIG.
3).
[0015] Since the heat-resistant magnesium alloy according to the
present invention includes the second alloying element "M2," it has
the plate-shaped precipitated substances within the grains of the
Mg crystalline grains, and the grain-boundary crystallized
substances, which form the networks that are continuous
microscopically, at the grain boundaries, as will be detailed
later. Since the plate-shaped precipitated substances exist within
the Mg crystalline grains, the movements of dislocation within the
Mg crystalline grains are prevented, and accordingly it becomes
less likely to deform. Moreover, since the grain-boundary
crystallized substances, which form the networks, are present
continuously microscopically at the grain boundaries between the Mg
crystalline grains, the strength at the grain boundaries improves.
As a result, the heat-resistant magnesium alloy according to the
present invention exhibits high mechanical characteristics even in
high-temperature regions. That is, in the magnesium alloy according
to the present invention, the mechanical characteristics in
high-temperature regions are improved by strengthening it not only
within the Mg crystalline grains' granular interior but also at the
grain boundaries between the Mg crystalline grains.
[0016] Said precipitated substances can desirably comprise a
Laves-phase compound with type-"C15" crystalline structure.
Moreover, said precipitated substances can desirably be
precipitated parallel to the {001} plane of Mg crystal.
[0017] Said grain-boundary crystallized substances, which form the
networks that are continuous microscopically, can desirably
comprise an Mg-"M1"-Ca-system compound. Moreover, said
grain-boundary crystallized substances can desirably comprise a
mixed-crystal phase of a Laves-phase compound with type-"C14"
crystalline structure and a Laves-phase compound with type-"C36"
crystalline structure; on this occasion, it is allowable that said
mixed-crystal structure can include the type-"C14" crystalline
structure more than the type-"C36" crystalline structure.
[0018] When the precipitated substances are precipitated parallel
to the {001} plane of Mg crystal, the movements of dislocation on
the sliding plane of hexagonal Mg crystal are suppressed. When the
grain-boundary crystallized substances comprise a mixed-crystal
phase of a Laves-phase compound with type-"C14" crystalline
structure and a Laves-phase compound with type-"C36" crystalline
structure, compounds, which constitute the networks, do not undergo
any phase separation, and consequently turn into single crystals
virtually in appearance (see FIG. 4), the area of the
crystalline-grain boundaries between crystalline grains that
constitute the networks, and the number of the crystalline grains
that constitute the networks become minimum.
[0019] Note that the aforementioned "type-`C14`," "type-`C15`," and
"type-`C36`" are codes in accordance with a magazine,
"STRUKTURBERICHTE," and express three similar basic crystalline
structures that are represented by MgZn.sub.2, MgCu.sub.2 and
MgNi.sub.2 of the Laves phases.
[0020] Further, it is desirable that it can have fine particles
that include said second alloying element "M2" within said Mg
crystalline grains.
[0021] The heat-resistant magnesium alloy according to the present
invention can preferably include: Ca in an amount of from 2% by
mass or more to 4% by mass or less; said first alloying element
"M1" in an amount of from 0.9 or more to 1.1 or less by mass ratio
with respect to Ca ("M1"/Ca); said second alloying element "M2" in
an amount of from 0.3% by mass or more to 0.6% by mass or less; and
the balance comprising Mg and inevitable impurities; when the
entirety is taken as 100% by mass.
[0022] Alternatively, the heat-resistant magnesium alloy according
to the present invention can preferably include: Ca in an amount of
from 1.235 atomic % or more to 2.470 atomic % or less; said first
alloying element "M1" in an amount of from 1.34 or more to 1.63 or
less by atomic ratio with respect to Ca ("M1"/Ca); said second
alloying element "M2" in an amount of from 0.13 atomic % or more to
0.27 atomic % or less; and the balance comprising Mg and inevitable
impurities; when the entirety is taken as 100 atomic %.
[0023] Heat-resistance magnesium alloys, which possess metallic
structures that are desirable from the viewpoints of mechanical
characteristics at high temperatures, are obtainable by setting the
content proportions of the first alloying element, second alloying
element and Ca that the heat-resistance magnesium alloy according
to the present invention contains to appropriate ranges.
[0024] Note that the "heat resistance" being referred to in the
present specification is one that is evaluated by mechanical
properties of magnesium alloy in high-temperature atmospheres
(creep characteristics or high-temperature strengths that are
determined by means of stress relaxation tests or axial-force
retention tests, for instance).
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a metallic-structure photograph in which a cross
section of a test specimen being labeled #01 was observed with a
metallographic microscope.
[0026] FIG. 2 is a metallic-structure photograph in which an
observational sample being labeled #01 was observed with a
transmission electron microscope (or TEM).
[0027] FIG. 3 is a metallic-structure photograph in which an
observational sample being labeled #C1 was observed with a TEM.
[0028] FIG. 4 is a dark-field
scanning-transmission-electron-microscope (or DF-STEM) image on the
observational sample being labeled #01.
[0029] FIG. 5 is a DF-STEM image on the observational sample being
labeled #C1.
[0030] FIG. 6 is a TEM image on the observational sample being
labeled #01, and an electron diffraction pattern thereof (the
incident direction being <110>).
[0031] FIG. 7 is another TEM image on the observational sample
being labeled #01, and another electron diffraction pattern thereof
(the incident direction being <111>).
[0032] FIG. 8 is a TEM image on the observational sample being
labeled #C1, and an electron diffraction pattern thereof (the
incident direction being <111>).
[0033] FIG. 9 is a DF-STEM image in which the interior of Mg
crystalline grains in the observational sample being labeled #01
was observed.
[0034] Note that "#01" and "#C1" are codes for distinguishing
magnesium alloys whose compositions differed in later-described
examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] Hereinafter, the best mode for carrying out the
heat-resistant magnesium alloy according to the present invention
(hereinafter being abbreviated to as "magnesium alloy") will be
explained.
[0036] The magnesium alloy according to the present invention
includes: magnesium (Mg), a major component; a first alloying
element "M1"; a second alloying element "M2"; and calcium (Ca); and
it has a metallic structure that includes: Mg crystalline grains;
plate-shaped precipitated substances being precipitated within
grains of Mg crystalline grains; and grain-boundary crystallized
substances being crystallized at grain boundaries between the Mg
crystalline grains to form networks that are continuous
microscopically.
[0037] In the magnesium-alloy according to the present invention,
the plate-shaped precipitated substances are present within the Mg
crystalline grains. The plate-shaped precipitated substances
prevent the movements of dislocation within the Mg crystalline
grains. The deformations of crystal occur because the dislocation
moves on sliding plane. Therefore, it is allowable that they can be
plate-shaped precipitated substances that are parallel to the "c"
plane of hexagonal Mg crystal, that is, to the {001} plane of Mg
crystal. Note that the plate-shaped precipitated substances come to
exhibit a plate thickness of 2-20 nm, and that the thicker the
plate thickness is the more mechanical characteristics improve.
[0038] Moreover, it is allowable that the plate-shaped precipitated
substances can comprise a Laves-phase compound with type-"C15"
crystalline structure. The "c" plane of Mg crystal, and the {111}
plane of "C15" structure are likely to form interfaces that are
stable to each other crystallographically, and therefore it is
possible to predict that the formation of the plate-shaped
precipitated substances is facilitated. It is allowable that
compounds constituting the precipitated substances that have such
crystalline structures can be "M1"-Ca-system compounds and/or
Mg-"M1"-Ca-system compounds.
[0039] It is allowable as well that the magnesium alloy according
to the present invention can have fine particles within the
granular interior of the Mg crystalline grains. The fine particles
are present within the Mg crystalline grains, and most of them
exist around the plate-shaped precipitated substances. It is
believed that, although these fine particles are present within the
Mg crystalline grains, they are not those which contribute to the
improvement of strength inside the Mg crystalline grains. However,
the presence of the fine particles is related to the generation of
the precipitated substances (will be described later), and the fine
particles are fine particles, which include "M2," like
"M1"-"M2"-system compounds, for instance. Note that the fine
particles are sphere-shaped ones substantially and exhibit particle
diameters of 10-15 nm approximately.
[0040] In the magnesium alloy according to the present invention,
the grain-boundary crystallized substances, which form networks
that are continuous microscopically, are crystallized at the grain
boundaries between the Mg crystalline grains to be present therein.
For example, even in compositions being made by excluding the
second alloying element "M2" from that of the magnesium alloy
according the present invention, grain-boundary crystallized
substances might be crystallized at the grain boundaries between
the Mg crystalline grains, and additionally might form networks.
However, in magnesium alloys that do not include any "M2," it has
been understood that no microscopic continuity can be seen in the
grain-boundary crystallized substances that form the networks. On
the other hand, in the magnesium alloy according to the present
invention, because of including "M2," the grain-boundary
crystallized substances form the networks that are continuous
microscopically. Because of the fact that the networks are
continuous microscopically, the crystalline grain-boundary area of
compounds that constitute the networks, and the number of
crystalline grains are reduced greatly. As a result, the
grain-boundary strength is improved, and is then strengthened. On
this occasion, it is desirable that the networks of the
grain-boundary crystallized substances can cover 70% or more of the
grain boundaries between the Mg crystalline grains that are
observed linearly in a regional cross section with 400
.mu.m.times.600 .mu.m approximately in the magnesium alloy (this
value will be abbreviated to as a "covering ratio of
networks").
[0041] Moreover, it is allowable that the grain-boundary
crystallized substances can comprise a mixed-crystal phase of a
Laves-phase compound with type-"C14" crystalline structure and a
Laves-phase compound with type-"C36" crystalline structure. The
type-"C14" crystalline structure, and the type-"C36" crystalline
structure are desirable, because they are hexagonal ones to each
other and are likely to form mixed-phases. Since the Laves-phase
compounds in the mixed-crystal phase come to be approximated to the
single crystals extremely, the grain-boundary crystallized
substances are continuous microscopically; and accordingly the
crystalline-grain boundary area of crystalline grains, that is,
compounds that constitute the networks, and the number of the
crystalline grains that constitute the networks become minimum.
[0042] Moreover, it is desirable that the grain-boundary
crystallized substances can comprise an Mg-"M1"-Ca-system compound.
Since Mg.sub.2Ca has a type-"C14" crystalline structure, it is
assumed that a mixed-crystal phase of a type-"C14" crystalline
structure and a type-"C36" crystalline structure is formed by
solidifying "M1" into Mg.sub.2Ca. In this instance, it is allowable
that the mixed-crystal phase can include the type-"C14" crystalline
structure more than the type-"C36" crystalline structure.
[0043] The magnesium alloy according to the present invention that
has the metallic structure as described above includes: magnesium
(Mg), a major component; a first alloying element "M1"; a second
alloying element "M2"; and calcium (Ca).
[0044] For the first alloying element "M1," it is possible to use
at least one member that is selected from the group consisting of
aluminum (Al) and nickel (Ni). Although not only Al but also Ni are
elements that react with Ca to form compounds and take on a
type-"C15" Laves structure, a mixed-crystal phase of a type-"C14"
Laves structure and a type-"C36" Laves structure is formed under
such a condition that Mg.sub.2Ca, which takes on a type-"C14" Laves
structure, is dominant, because Al and/or Ni are dissolved into
Mg.sub.2Ca For the second alloying element "M2," it is possible to
use at least one member that is selected from the group consisting
of manganese (Mn), barium (Ba), chromium (Cr) andiron (Fe). The
reason why it is possible to use these elements as "M2" can be
explained by means of structural changes of the magnesium alloy
according to the present invention in the cooling process.
[0045] It was understood from the cooling curve when casting a cast
product comprising the magnesium alloy according to the present
invention by a general solidifying process (air cooling) that three
temperature-halting points (the respective temperatures are labeled
"T1," "T2" and "T3"; and "T1">"T3," and "T2">"T3") appear.
When the molten-metal temperature reaches a primary-crystal
temperature (i.e., a temperature at which the solidification
begins: "T1"=from 600.degree. C. or more to 620.degree. C. or
less), primary-crystal Mg crystallized. Moreover, when it reaches
"T2," it is predicted that "M1" and "M2" react to generate fine
particles of "M1"-"M2"-system compounds, high-temperature-generated
compounds. Next, when it reaches the eutectic temperature "T3," the
grain-boundary crystallized substances, which form the networks,
crystallize along with eutectic Mg. However, as a result of
Carrying out an elementary analysis on the fine particles of the
resulting cast product, it was found that "M2" was included therein
more than the theoretical value. Specifically, in regions of low
temperatures that are much lower than "T3," it is possible to
predict that "M1" is spewed out from the fine particles (or
"M1"-"M2"-system compounds), and that the spewed-out "M1" forms
compounds with Ca and then precipitates being accompanied by the
agglomeration of Ca that dissolves into the Mg crystalline
grains.
[0046] Therefore, it is necessary that not only the second alloying
element "M2" can react with the first alloying element "M1" at high
temperatures that are higher than "T3" but also it can be less
likely to dissolve into Mg. Because of such reasoning, it is
possible to use at least one member that is selected from the group
consisting of manganese (Mn), barium (Ba), chromium (Cr) and iron
(Fe), especially from among the transition elements. These elements
exhibit atomic radii being comparable with each other, and take on
similar crystalline structures; further they react with "M1" to
generate the compounds in comparatively high-temperature regions,
to be concrete, between "T1" and "T3" alone.
[0047] Note that the magnesium alloy according to the present
invention includes at least one species of the aforementioned first
alloying elements and second alloying elements, respectively. It is
also allowable that it can include one species of them as for the
first element and second element, respectively; and it is even
allowable that it can include plural species of them as for either
one of them or both of them.
[0048] It is preferable that the magnesium alloy according to the
present invention can include: Ca in an amount of from 2% by mass
or more to 4% by mass or less; said first alloying element "M1" in
an amount of from 0.9 or more to 1.1 or less by mass ratio with
respect to Ca ("M1"/Ca); said second alloying element "M2" in an
amount of from 0.3% by mass or more to 0.6% by mass or less; and
the balance comprising Mg and inevitable impurities; when the
entirety is taken as 100% by mass. Alternatively, it is preferable
that the magnesium alloy according to the present invention can
include: Ca in an amount of from 1.235 atomic % or more to 2.470
atomic % or less; said first alloying element "M1" in an amount of
from 1.34 or more to 1.63 or less by atomic ratio with respect to
Ca ("M1"/Ca); said second alloying element "M2" in an amount of
from 0.13 atomic % or more to 0.27 atomic % or less; and the
balance comprising Mg and inevitable impurities; when the entirety
is taken as 100 atomic %
[0049] When "M1"/Ca is less than 0.9 by mass ratio (namely, being
less than 1.34 by atomic ratio), it is not preferable because the
content of Ca is so great that the castability deteriorates. On the
other hand, when "M1"/Ca surpasses 1.1 by mass ratio (namely,
surpassing 1.63 by atomic ratio), it is not preferable because the
grain-boundary crystallized substances are less likely to turn into
a mixed-crystal phase, and because crystalline grains, which are
constituted of type-"C36" Laves structure alone, are likely to be
formed so that they undergo phase separation. Further, when
type-"C36" crystalline structure is exposed to high temperatures,
it is likely to undergo phase transition to type-"C15" crystalline
structure (Scripta Materialia 51 (2004) 1005-1010). Since the
type-"C15" crystalline structure is likely to undergo massive
agglomeration in high-temperature regions, and since it does not
form the networks of the crystallized substances, networks which
are continuous microscopically, the mechanical characteristics at
high temperatures lower remarkably. A more preferable "M1"/Ca value
can be from 0.95 or more to 1.05 or less (namely, being 1.42-1.56
by atomic ratio).
[0050] When the content proportion of the second alloying element
"M2" is less than 0.3% by mass (namely, 0.13 atomic %), it is not
preferable because it is impossible to retain the "M1," which
constitutes the precipitated substances in the cooling step (or
solidifying step), as compounds so that the precipitated substances
are not precipitated sufficiently. Moreover, it is not preferable
because many "M1" reside without ever combining with "M2" so that
crystalline grains, which possess type-"C36" Laves structure alone
that does not take on any mixed-crystal structure as the
grain-boundary crystallized substances, are likely to be formed,
and so that they undergo phase separation. On the other hand, when
the content proportion of "M2" surpasses 0.6% by mass (namely, 0.27
atomic %), it is not preferable because compounds that contain "M2"
are precipitated within the grain-boundary crystallized substances
so that they might possibly cut off the networks. The lower limit
of a more preferable content proportion of "M2" can be 0.34% by
mass (namely, 0.15 atomic %) or more. The upper limit of a more
preferable content proportion of "M2" can be 0.55% by mass (namely,
0.25 atomic %) or less, and can much more preferably be 0.5% by
mass (namely, 0.23 atomic %) or less.
[0051] Ca is an element that forms type-"C14" and type-"C36" Laves
structures together with Mg. When a content proportion of Ca is
less than 2% by mass (namely, 1.235 atomic %), it is not preferable
because the precipitated substances and grain-boundary crystallized
substances are not generated sufficiently so that the effect of
improving the heat-resistant characteristic is not sufficient. On
the other hand, when the content proportion of Ca surpasses 4% by
mass (namely, 2.470 atomic %), it is not preferable because the
generation amounts of the precipitated substances and
grain-boundary crystallized substances become too great so that
problems might arise in post-processes. A more preferable content
proportion of Ca can be from 2.5% by mass or more to 3.5% by mass
or less (namely, from 1.54 atomic % or more to 2.16 atomic % or
less).
[0052] The magnesium alloy according to the present invention is
not limited to those made by ordinary gravity casting and pressure
casting, but can even be those made by die-cast casting. Moreover,
even the casting mold being utilized for the casting does not
matter if it is sand molds, metallic molds, and the like. Although
even the solidification rate in the solidifying step is not limited
in particular, it is allowable to let it stand to cool in air
atmosphere.
[0053] Beginning with the fields of space, military and aviation,
applications of the magnesium alloy according to the present
invention can be extended to various fields, such as automobiles
and home electric instruments. In reality, however, it is all the
more suitable that, taking advantage of its heat resistance, the
magnesium alloy according to the present invention can be utilized
in products being utilized in high-temperature environments, such
as engines, transmissions, compressors for air conditioner or their
related products that are put in place within the engine room of
automobile, for instance. To be concrete, the following can be
given: cylinder heads, cylinder blocks and oil pans of internal
combustion engine; impellers for turbocharger of internal
combustion engine, transmission cases being used for automobile and
the like, and so forth.
[0054] So far, the embodiment modes of the heat-resistant magnesium
alloy according to the present invention have been explained,
however, the present invention is not one which is limited to the
aforementioned embodiment modes. It can be conducted in various
modes to which modifications, improvements, and the like, which one
of ordinary skill in the art can carry out, are performed, within a
range not departing from the scope of the present invention.
[0055] Hereinafter, while giving specific examples, the present
invention will be explained in detail.
[0056] Two kinds of test specimens whose contents (or addition
amounts) of Al, Ca and Mn in magnesium alloys were varied were
made, and then not only their metallic structures were observed but
also a stress relaxation test was carried out.
[0057] (Making of Test Specimens)
[0058] A chloride-system flux was coated onto the inner surface of
a crucible being made of iron that had been preheated within an
electric furnace, and then a weighed pure magnesium base metal,
pure Al, and an Mg--Mn alloy, if needed, were charged into it and
were then melted. Further, weighed Ca was added into this molten
metal that was held at 750.degree. C. (i.e., a molten-metal
preparing step).
[0059] After fully stirring this molten metal to melt the raw
materials completely, it was held calmly at the same temperature
for a while. During this melting operation, a mixture gas of carbon
dioxide gas and SF.sub.6 gas was blown onto the molten metal's
surface in order to prevent the burning of Mg, and the flux was
sprayed whenever being deemed appropriate.
[0060] The thus obtained various alloy molten metals were poured
into a metallic mold with a predetermined configuration (i.e., a
molten-metal pouring step), and were then solidified in air
atmosphere (i.e., a solidifying step). Thus, test specimens with 30
mm.times.300 mm.times.40 mm were made by means of gravity casting.
The obtained test specimens were labeled #01 (an example including
Mn), and #C1 (a comparative example not including Mn). The chemical
compositions of the respective test specimens were specified in
Table 1. Note that, in the magnesium-alloy compositions being given
in Table 1, the balances are Mg, respectively.
TABLE-US-00001 TABLE 1 Magnesium-alloy Magnesium-alloy Composition
Composition (% by mass) (atomic %) Test Specimen Al Ca Mn Al Ca Mn
Al/Ca #01 3 3 0.5 2.75 1.85 0.23 1.49 #C1 3 3 -- 2.74 1.85 --
1.48
[0061] Note that, in Table 1, "% by mass" and "atomic %" are used
as the units for the alloy compositions being labeled #01 and #C1.
Here, the values that used the unit, "% by mass," were the charged
quantities in the molten-metal preparing step, and those values
were converted into the "atomic %."
[0062] (Observation on Metallic Structure)
[0063] Test specimens #01 and #C1 were observed with a
metallographic microscope or transmission electron microscope
(TEM).
[0064] FIG. 1 is a metallic-structure photograph in which a cross
section of the test specimen being labeled #01 was observed with a
metallographic microscope. The Mg crystalline grains (bright
parts), and the grain-boundary crystallized substances (black
parts) that existed like networks at the grain boundaries between
the Mg crystalline grains were observed. Note that, although not
being shown diagrammatically, a metallic-structure photograph being
similar to FIG. 1 was obtained even when a cross section of the
test specimen being labeled #C1 was observed. That is, in either
one of the test specimens, network-shaped grain-boundary
crystallized substances were observed macroscopically.
[0065] Next, in order to observe micro-fine constructions of the
metallic structures, the respective test specimens were adapted
into a flake-shaped observational sample, respectively, and were
then observed using a TEM.
[0066] FIG. 2 and FIG. 3 are metallic-structure photographs in
which the observational samples being labeled #01 and #C1 were
observed with the TEM. In both of them, crystalline grain
boundaries in which two or more crystalline grains of
primary-crystal Mg neighbor to each other were observed. In FIG. 2
(#01), the grain-boundary crystallized substances (black parts)
were grown as a lamellar shape, and were continuous. In FIG. 3
(#C1), the grain-boundary crystallized substances were interrupted
partially, and were discontinuous. Note that the covering ratio of
networks in #01 was about 90%.
[0067] Moreover, FIG. 4 and FIG. 5 are a dark-field
scanning-transmission-electron-microscope (DF-STEM) images in which
the grain-boundary crystallized substances in the observational
samples according to #01 and #C1 were observed, respectively. In
the test specimen being labeled #01, no phase separation was seen
as shown in FIG. 4; whereas, in the test specimen being labeled
#C1, phase separation was seen as shown in FIG. 5. When an
elementary mapping was carried out with respect to the DF-STEM
images of FIG. 4 and FIG. 5 by means of energy-dispersion-type
X-ray spectroscopy (EDX), Mg, Al and Ca were distributed uniformly
in FIG. 4 (#01); whereas the concentration of Al was high in the
crystalline grains, which were agglomerated granularly to undergo
phase separation, in FIG. 5 (#C1). And, the electron diffraction of
type-"C36" crystalline structure was obtained from the crystalline
grains with high Al concentrations. On the other hand, the
electron-diffraction pattern of type-"C14" crystalline structure
was obtained mainly from the crystals in which each of Mg, Al and
Ca was distributed uniformly in FIG. 4 and FIG. 5; however, the
diffraction spot of type-"C36" crystalline structure, which
coincided with the twofold cycle to type-"C14" crystalline
structure, appeared partially, even though they did not undergo any
phase separation. Specifically, it was understood that the crystals
in which Mg, Al and Ca were distributed uniformly were a
mixed-crystal phase of type-"C14" crystalline structure and
type-"C36" crystalline structure, and were virtually single
crystals visually. Therefore, in the test specimens being labeled
#01, the grain-boundary crystallized substances forming the
networks were continuous microscopically, and they virtually turned
into single crystals visually. On the contrary, in the test
specimen being labeled #C1, although the grain-boundary
crystallized substances formed networks macroscopically, the
networks were discontinuous microscopically, and the Laves-phase
compounds, which comprised type-"C36" crystalline structure alone
and had undergone phase separation, were present.
[0068] Note that, on a magnesium alloy in which the Mn content in
#01 was changed to 0.2% by mass (namely, 0.09 atomic %), the
grain-boundary crystallized substances were observed with the TEM,
though not being shown diagrammatically. According to the obtained
DF-STEM image, the massive agglomerations that were seen in #C1
(FIG. 5) decreased so that compounds extending as strip shapes came
to account for it greatly when the Mn amount increased; however, it
was understood that no continuity that was observed in #01 (FIG. 4)
was seen when the Mn content was 0.2% by mass.
[0069] FIG. 6 and FIG. 7 are TEM images on Test Specimen #01, and
FIG. 8 is a TEM image on Test Specimen #C1. In FIG. 6, the interior
of the Mg crystalline grains was observed while setting the
incident direction to <110>, whereas it was observed while
setting the incident angle to <111> in FIG. 7 and FIG. 8. In
FIG. 6 (#01), streak-shaped precipitated substances that were
parallel to the {001} plane were seen. And, from FIG. 7 in which
the observation was carried out at the same position as that in
FIG. 6 but while inclining the incident direction, the precipitated
substances were found to have plate shapes that were parallel to
the {001} plane. When the STEM-EDX analysis was carried out onto
these plate-shaped precipitated substances, Al and Ca were detected
mainly. Moreover, from the plate-shaped precipitated substances,
the electron-diffraction pattern of type-"C15" crystalline
structure that coincided with Al.sub.2Ca was obtained.
[0070] On the contrary, in FIG. 8 (#C1), no clear streak-shaped
contrast was seen. Note that, even when the same STEM-EDX analysis
as that was done for #01 was carried out, Al and Ca were hardly
detected. Therefore, the precipitated substances hardly existed in
the test specimen being labeled #C1.
[0071] FIG. 9 is a DF-STEM image in which the interior of the Mg
crystalline grains in the observational sample being labeled #01
was observed. A plurality of fine particles were seen around the
plate-shaped precipitated substances. When an elementary analysis
was carried out onto the fine particles (e.g., "B" in FIG. 9), Mn
was detected. Note that no Mn was detected even when the
plate-shaped precipitated substances (e.g., "A" in FIG. 9) were
analyzed.
[0072] (Stress Relaxation Test)
[0073] A stress relaxation test was carried out not only onto Test
Specimens #01 and #C1 given in Table 1 but also onto AXE662, AE42
and AZ91D (all as per ASTM standards), thereby examining the
magnesium alloys' heat resistance (e.g., creep resistance). In the
stress relaxation test, a process was measured, process in which
the stress, which was needed when a load was applied to a test
specimen until it exhibited a predetermined deformation magnitude,
decreased with time in the course of testing time. To be concrete,
in 150.degree. C. air atmosphere, a compression stress of 100 MPa
was loaded to the test specimens, and then the compression stress
was lowered in agreement with the elapse of time so as to keep the
displacements of the test specimens at that time constant.
[0074] In Table 2 and Table 3, the respective test specimens' alloy
compositions, and their stresses after 40 hours since the stress
relaxation test started are given. Note that, in the
magnesium-alloy compositions being given in Table 2 and Table 3,
the balances are Mg, respectively. Moreover, "RE" is a mish
metal.
TABLE-US-00002 TABLE 2 Magnesium-alloy Test Composition (% by mass)
Stress Specimen Al Zn Ca RE Mn (MPa) #01 3 -- 3 -- 0.5 92 #C1 3 --
3 -- -- 86 AXE662 6 -- 6 2 -- 83 AE42 4 -- -- 2 -- 74 AZ91D 9 1 --
-- -- 68
TABLE-US-00003 TABLE 3 Magnesium-alloy Test Composition (atomic %)
Stress Specimen Al Zn Ca RE Mn (MPa) #01 2.75 -- 1.85 -- 0.23 92
#C1 2.74 -- 1.85 -- -- 86 AXE662 5.67 -- 3.81 0.36 -- 83 AE42 3.68
-- -- 0.36 -- 74 AZ91D 8.23 0.38 -- -- -- 68
[0075] Test Specimen #01 exhibited a decrease proportion of the
loaded stress especially less, compared with those of the other
test specimens, and therefore showed high creep resistance even
under high temperatures. This is due to the following: the firm
networks, which were continuous microscopically, were formed at the
grain boundaries between the Mg crystalline grains because of the
presence of Mn; and the deformation resistance of Test Specimen #01
enlarged so that the strength thereof improved because the
movements of dislocation were suppressed by the plate-shaped
precipitated substances within the Mg crystalline grains.
* * * * *