US20090162242A1

US20090162242A1 - Heat resistant magnesium alloy and production process thereof

Info

Publication number: US20090162242A1
Application number: US12/338,747
Authority: US
Inventors: Kenshi Inoue; Yuichi Ienaga
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2007-12-21
Filing date: 2008-12-18
Publication date: 2009-06-25
Also published as: JP2009149952A

Abstract

Provided are a heat-resistant magnesium alloy which has at the same time both high strength and high ductility even under high temperature environment and is also inexpensive, and a production process of the heat-resistant magnesium alloy. The heat-resistant magnesium alloy includes, in relation to the total amount of the alloy, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities, wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, an a-Mg phase and an intermetallic compound Mg₃Y₂Zn₃phase are finely dispersed, and a long period stacking ordered structure phase is formed in a three-dimensional network shape. The heat-resistant magnesium alloy can be produced by melting a metal material having the above-described composition at temperatures within a range from 650 to 900° C., pouring the molten metal material into a mold and cooling the molten metal material at a rate of 10 to 10³K/sec.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a heat-resistant magnesium alloy and a production process thereof.
2. Description of the Related Art
Magnesium is lighter in weight than iron and aluminum, and hence the use of magnesium as a lightweight substitute for members made of a steel iron material or an aluminum alloy material is being investigated. Common magnesium alloys are, however, degraded in mechanical properties such as tensile strength and elongation in a high temperature range from 200 to 250° C., and cannot attain a high temperature strength comparable to the strength of heat-resistant aluminum alloys such as the cast AC8B-T6 material and the wrought A4032-T6 material.
Accordingly, there have hitherto been proposed various heat-resistant magnesium alloys which have at the same time both high strength and high ductility even under high temperature environment. Among such heat-resistant magnesium alloys, for example, known is a magnesium alloy produced by casting a Mg alloy having a composition composed of, in relation to the total amount of the alloy, 1 to 4 at % of Zn, 1 to 4.5 at % of Y and 0.1 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, and by subsequently applying a plastic processing to the cast alloy. This heat-resistant magnesium alloy has an alloy structure containing an intermetallic compound Mg₃Y₂Zn₃and Mg₁₂ZnY exhibiting a long-period structure and can attain high strength and high ductility under high temperature environment (Japanese Patent Laid-Open No. 2006-97037).
However, the above-described heat-resistant magnesium compound further needs a plastic processing after casting, and the plastic processing needs a large amount of energy to increase the production cost. Accordingly, demanded is a heat-resistant magnesium alloy which has at the same time both high strength and high ductility under high temperature environment and is also inexpensive.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, by overcoming these disadvantages, a heat-resistant magnesium alloy which has at the same time both high strength and high ductility even under high temperature environment and is also inexpensive.
In order to achieve such an object, the heat-resistant magnesium alloy of the present invention includes, in relation to the total amount of the alloy, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities, wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, an α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase are finely dispersed, and a long period stacking ordered structure phase is formed in a three-dimensional network shape.
The heat-resistant magnesium alloy of the present invention has the above-described composition, additionally includes the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase both finely dispersed therein and the long-period layered structure phase formed therein in a three-dimensional network shape, and consequently, can have at the same time both high strength and high ductility even under high temperature environment of 200 to 250° C. When the content of Zn is less than 1 at % or exceeds 3 at % and the content of Y is less than 1 at % or exceeds 3 at %, in relation to the total amount of the heat-resistant magnesium alloy, any or both of strength and ductility become insufficient.
The inclusion of Zr within the above-described range in the heat-resistant magnesium alloy of the present invention enables the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase to be made fine and at the same time enables the intermetallic compound Mg₃Y₂Zn₃to be prevented from being made coarse in the course of casting and cooling. The content of Zr less than 0.01 at % in relation to the total amount of the alloy cannot attain the effects to make fine the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase and to prevent the intermetallic compound Mg₃Y₂Zn₃from being made coarse in the course of casting and cooling, and the content of Zr exceeding 0.5 at % leads to a state of saturation reached by the effect to make fine the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase.
The heat-resistant magnesium alloy of the present invention is required to have the composition ratio Zn/Y between Zn and Y falling within the range from 0.6 to 1.3 for the purpose of including therein, without failure, both of the intermetallic compound Mg₃Y₂Zn₃and the long-period layered structure. When the composition ratio Zn/Y between Zn and Y is less than 0.6 or exceeds 1.3, the heat-resistant magnesium alloy does not include any or both of the intermetallic compound Mg₃Y₂Zn₃and the long period stacking ordered structure as the case may be.
The heat-resistant magnesium alloy of the present invention is required to include an alloy structure in which the fine particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase are located between the long period stacking ordered structure phases formed in a three-dimensional network shape. When the particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase are made coarse and such coarse particles destroy the three-dimensional network-shaped structure of the long period stacking ordered structure phase, it is impossible to attain high strength and high ductility under the above-described high temperature environment.
The heat-resistant magnesium alloy of the present invention including the above-described alloy structure can be advantageously produced by melting at temperatures within a range from 650 to 900° C. a metal material including, in relation to the total amount of the metal material, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, and by pouring the molten metal material into a mold and thereafter by cooling at a rate of 10 to 10³K/sec. When the melting temperature is lower than 650° C., the individual metal components constituting the metal material cannot be uniformly melted, and when the melting temperature exceeds 900° C., the individual metal components are lost partially by evaporation. When the cooling rate is less than 10 K/sec, the particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase become coarse, and the three-dimensional network-shaped structure of the long period stacking ordered structure phase cannot be obtained. On the other hand, it is technically difficult to set the cooling rate so as to exceed 10³K/sec.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating the shape of a cast product of a magnesium alloy according to the present invention;

FIG. 2 is a sectional view along the II-II line in FIG. 1;

FIG. 3 is electron reflection images of the alloy structures of heat-resistant magnesium alloys according to the present invention;

FIG. 4 is a view illustrating the shape of the specimens cut out, from the cast products shown in FIGS. 1 and 2, for the measurement of mechanical properties; and

FIGS. 5 and 6 are electron reflection images of the alloy structures of heat-resistant magnesium alloys of Comparative Examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In present Embodiment, first, a molten alloy is obtained by melting at temperatures within a range from 650 to 900° C. a metal material including, in relation to the total amount of the metal material, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3.
Next, the molten alloy is poured into a mold to be cast. The casting is conducted, for example, by using a mold made of oxygen-free copper (C1020, purity; 4N), by adopting gravity casting and by pouring the molten alloy into the mold while the molten alloy is being maintained at temperatures within a range from 800 to 850° C. The cooling in the casting is conducted at a rate falling within a range from 10 to 10³K/sec.
Consequently, there can be obtained a magnesium alloy including, in relation to the total amount of the alloy, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, an α-Mg phase and an intermetallic compound Mg₃Y₂Zn₃phase are finely dispersed, and a long period stacking ordered structure phase is formed in a three-dimensional network shape. The magnesium alloy does not need a plastic processing after casting to be inexpensive, and is a heat-resistant magnesium alloy which can have at the same time both high strength and high ductility even under high temperature environment of 200 to 250° C.
It is to be noted that the heat-resistant magnesium alloy of present Embodiment may be an alloy including, in place of Y, a metal selected from the group consisting of Dy, Ho, Er, Gd, Tb and Tm, or may be an alloy including, in place of Zr, Ti or Hf.
Hereinafter, Examples, Comparative Examples and Reference Examples of the present invention are presented.

EXAMPLES 1 TO 4

In present Examples, a molten alloy was obtained by melting at a temperature of 850° C. a metal material including, in relation to the total amount of the metal material, 2 at % of Zn, 2 at % of Y, 0.2 at % of Zr and 95.8 at % of Mg. In the molten alloy, the composition ratio Zn/Y between Zn and Y was 1.0.
Next, by means of gravity casting, the obtained molten alloy was poured into a mold made of oxygen-free copper (C1020, purity: 4N) to yield a cast product of a heat-resistant magnesium alloy. This casting was conducted while the molten alloy was being maintained at temperatures within a range from 800 to 850° C.
As shown in FIG. 1, a cast product 1 obtained in present Examples was, as viewed in plan, rectangular and of a size of 46 mm×56 mm. Additionally, as shown in FIG. 2, the thickness of the cast product 1 was varied stepwise from t₁to t₄along the long side, wherein t₁=2 mm (Example 1), t₂=4 mm (Example 2), t₃=8 mm (Example 3) and t₄=16 mm (Example 4).
The cast product 1 obtained in present Examples underwent different cooling rates depending on the above-described thickness such that the cooling rate was 668 K/sec for the portion having the thickness t₁=2 mm, 166 K/sec for the portion having the thickness t₂=4 mm, 55 K/sec for the portion having the thickness t₃=8 mm, and 21 K/sec for the portion having the thickness t₄=16 mm.
The above-described rates were derived as follows. First, in place of the above-described metal material, a eutectic alloy Al_82.7Cu_17.3was cast in exactly the same manner as in present Examples except that a metal material composed of 82.7 at % of Al and 17.3 at % of Cu was used in place of the above-described metal material, and the dendrite arm spacing (DAS) of the eutectic alloy was measured. Next, the empirical formula representing the relation between the DAS and the cooling rate in the eutectic alloy was obtained. The DAS values in the portions having the thickness values of t₁to t₄of the heat-resistant magnesium alloy in present Examples were substituted into the empirical formula to derive the cooling rates in the respective portions.
FIG. 3 shows the backscattered-electron images of the alloy structures of the heat-resistant magnesium alloys obtained in present Examples. FIG. 3(A) shows the backscattered-electron image of the alloy structure of the portion having the thickness t₁=2 mm and the cooling rate of 668 K/sec, FIG. 3(B) shows the backscattered-electron image of the alloy structure of the portion having the thickness t₂=4 mm and the cooling rate of 166 K/sec, FIG. 3(C) shows the backscattered-electron image of the alloy structure of the portion having the thickness t₃=8 mm and the cooling rate of 55 K/sec, and FIG. 3(D) shows the backscattered-electron image of the alloy structure of the portion having the thickness t₄=16 mm and the cooling rate of 21 K/sec.
From FIG. 3, in any of the portions respectively having the thickness values of t₁to t₄, it was observed that the fine particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase were located between the long period stacking ordered structure phases formed in a three-dimensional network shape.
A specimen 2 having the shape shown in FIG. 4 was cut out from each of the portions respectively having the thickness values of t₁to t₄of the cast product 1 shown in FIGS. 1 and 2, and was subjected to an evaluation of the mechanical properties thereof. The specimen had a total length L=46 mm, a total width d=12 mm and a thickness=2 mm, and had a 4-mm-wide constricted portion in the center thereof, a strain gauge 3 (gauge length: 2 mm) for magnesium being adhered to each of the front and back sides of the constricted portion.
In each of present Examples, the specimen 2 was heated in air to 200° C. and 250° C. by induction heating, and was subjected to the measurements of the tensile strength and the elongation at a strain rate of 5×10⁻⁴/sec (0.12 mm/min, measurement length: 4 mm) with an electrohydraulic fatigue tester manufactured by MTS System Corp. The heating of the specimen 2 was controlled in such a way that the temperature in the 4-mm length range in the lengthwise direction of the constricted portion fell within a range of 200±2° C. when heated to 200° C. and within a range of 250±2° C. when heated to 250° C. The results thus obtained are shown in Table 1.
In each of present Examples, the specimen 2 was heated in air to 200° C. by induction heating, and the determination of the operating life thereof was conducted on the basis of separation fracture by using the electrohydraulic fatigue tester manufactured by MTS Systems Corp. at a stress ratio R=−1 (resonance) with a sinusoidal wave having a frequency of 30 to 60 Hz, and the determination of the fatigue strength was conducted on the basis of the stress amplitude at a repetition number of 1×10⁷. The heating of the specimen 2 was controlled in such a way that the temperature in the 4-mm length range in the lengthwise direction of the constricted portion fell within a range of 200±2° C. when heated to 200° C. The results thus obtained are shown in Table 1.

EXAMPLE 5

In present Example, a 2-mm-thick cast product was obtained in exactly the same manner as in Example 1 except that a metal material including, in relation to the total amount of the metal material, 1.2 at % of Zn, 1.2 at % of Y, 0.2 at % of Zr and 97.4 at % of Mg was used. In the molten alloy obtained by melting the metal material, the composition ratio Zn/Y between Zn and Y was 1.0. The cooling rate of the cast product was 668 K/sec.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the cast product obtained in present Example, and subjected to the measurements of the tensile strength and the elongation in exactly the same manner as in Example 1 except that the specimen 2 was heated in air to 250° C. by induction heating. The results thus obtained are shown in Table 1.

EXAMPLE 6

In present Example, a 2-mm-thick cast product was obtained in exactly the same manner as in Example 1 except that a metal material including, in relation to the total amount of the metal material, 3 at % of Zn, 3 at % of Y, 0.2 at % of Zr and 93.8 at % of Mg was used. In the molten alloy obtained by melting the metal material, the composition ratio Zn/Y between Zn and Y was 1.0. The cooling rate of the cast product was 668 K/sec.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the cast product obtained in present Example, and subjected to the measurements of the tensile strength and the elongation in exactly the same manner as in Example 1 except that the specimen 2 was heated in air to 250° C. by induction heating. The results thus obtained are shown in Table 1.

EXAMPLE 7

In present Example, a 2-mm-thick cast product was obtained in exactly the same manner as in Example 1 except that a metal material including, in relation to the total amount of the metal material, 1.2 at % of Zn, 2 at % of Y, 0.2 at % of Zr and 96.6 at % of Mg was used. In the molten alloy obtained by melting the metal material, the composition ratio Zn/Y between Zn and Y was 0.6. The cooling rate of the cast product was 668 K/sec.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the cast product obtained in present Example, and subjected to the measurements of the tensile strength and the elongation in exactly the same manner as in Example 1 except that the specimen 2 was heated in air to 250° C. by induction heating. The results thus obtained are shown in Table 1.

EXAMPLE 8

In present Example, a 2-mm-thick cast product was obtained in exactly the same manner as in Example 1 except that a metal material including, in relation to the total amount of the metal material, 2.6 at % of Zn, 2 at % of Y, 0.2 at % of Zr and 95.2 at % of Mg was used. In the molten alloy obtained by melting the metal material, the composition ratio Zn/Y between Zn and Y was 1.3. The cooling rate of the cast product was 668 K/sec.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the cast product obtained in present Example, and subjected to the measurements of the tensile strength and the elongation in exactly the same manner as in Example 1 except that the specimen 2 was heated in air to 250° C. by induction heating. The results thus obtained are shown in Table 1.

COMPARATIVE EXAMPLE 1

In present Comparative Example, a cast product of a magnesium alloy was obtained in exactly the same manner as in Example 1 except that a metal material including, in relation to the total amount of the metal material, 2 at % of Zn, 2 at % of Y and 96 at % of Mg was used.
The cooling rate of the cast product obtained in present Comparative Example was 21 K/sec in a portion having the thickness t₄=16 mm.
FIG. 5 shows the electron reflection image of the alloy structure of the magnesium alloy obtained in present Comparative Example. From FIG. 5, it was observed that the particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase were made coarse, and the three-dimensional network-shaped structure of the long-period layered structure phase was destroyed.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the portion having the thickness of t₄=16 mm of the cast product obtained in present Comparative Example, and subjected to the evaluation of the mechanical properties in exactly the same manner as in above-described Examples. The results thus obtained are shown in Table 1.

COMPARATIVE EXAMPLE 2

In present Comparative Example, a molten alloy was obtained by melting a metal material including, in relation to the total amount of the metal material, 2 at % of Zn, 2 at % of Y, 0.2 att of Zr and 95.8 at % of Mg at a temperature of 850° C. In the molten alloy, the composition ratio Zn/Y between Zn and Y was 1.0.
Next, a cast product of a heat-resistant magnesium alloy was obtained as follows: the obtained molten alloy (molten metal) was poured into a mold made of soft steel having an inner diameter of 320 mm and a height of about 1000 mm; the mold was rotated for 5 to 60 seconds with the longitudinal axis thereof as a rotation axis in such a way that the circumferential velocity of the outermost circumference of the molten metal was 400 to 1000 mm/sec, then rotated in the reverse direction for 5 to 60 seconds, and in this way the forward and reverse rotations were repeated to solidify the molten metal to yield the cast product of the heat-resistant magnesium alloy. The cooling rate of the cast product 1 obtained in present Comparative Example was 1 K/sec.
FIG. 6 shows the backscattered-electron image of the alloy structure of the magnesium alloy obtained in present Comparative Example. From FIG. 6, it was observed that the particles of the α-Mg phase and the intermetallic compound Mg₃Y₂Zn₃phase were made coarse, and the three-dimensional network-shaped structure of the long period stacking ordered structure phase was destroyed.
Next, a specimen 2 having the shape shown in FIG. 4 was cut out from the cast product obtained in present Comparative Example, and subjected to the evaluation of the mechanical properties in exactly the same manner as in Example 1. The results thus obtained are shown in Table 1.

REFERENCE EXAMPLE 1

In present Reference Example, in exactly the same manner as in Example 1 except that a heretofore known magnesium alloy ZE63A-T6 material was used, the tensile strength of the magnesium alloy at 200° C. was measured. The result thus obtained is shown in Table 1.

REFERENCE EXAMPLE 2

In present Reference Example, in exactly the same manner as in Example 1 except that a heretofore known aluminum alloy A4032-T6 material was used, the mechanical properties of the aluminum alloy were evaluated. The results thus obtained are shown in Table 1.

REFERENCE EXAMPLE 3

In present Reference Example, in exactly the same manner as in Example 1 except that a heretofore known aluminum alloy AC8B-T6 material was used, the mechanical properties of the aluminum alloy were evaluated. The results thus obtained are shown in Table 1.

TABLE 1

Plate				Fatigue
thick-	Cooling	Tensile		strength
ness	rate	strength (MPa)	Elongation (%)	(MPa)

	(mm)	(K/sec)	200° C.	250° C.	200° C.	250° C.	200° C.

Ex. 1	2	668	238	223	9.6	8.5	80
Ex. 2	4	166	229	198	14.7	15.7	75
Ex. 3	8	55	212	190	8.2	15.8	75
Ex. 4	16	21	207	199	10.1	14.4	70
Ex. 5	2	668	—	228	—	11.3	—
Ex. 6	2	668	—	232	—	7.7	—
Ex. 7	2	668	—	236	—	10.3	—
Ex. 8	2	668	—	233	—	11.3	—
Com.	16	21	173	178	5.7	7.3	—
Ex. 1
Com.	—	1	150	142	16.6	24.2	—
Ex. 2
Ref.	—	—	130	—	—	—	—
Ex. 1
Ref.	—	—	221	171	5.7	7.0	80
Ex. 2
Ref.	—	—	246	153	1.5	5.8	66
Ex. 3

As can be clearly seen from Table 1, the heat-resistant magnesium alloys of Examples are larger in strength and elongation as compared to the magnesium alloys of Comparative Example 1 and Reference Example 1, and have at the same time both high strength and high ductility under high temperature environment of 200 to 250° C.
On the other hand, as for the cooling rate, the heat-resistant magnesium alloy of Comparative Example 2 obtained with a cooling rate of 1 K/sec has a large elongation but cannot be said to have a sufficient strength, and as can be clearly seen from Table 1, the adoption of the cooling rates of 10 to 10³K/sec enables to obtain heat-resistant magnesium alloys which have at the same time both high strength and high ductility even under high temperature environment of 200 to 250° C., as in the individual heat-resistant magnesium alloys in Examples.
Additionally, as can be clearly seen from Table 1, the heat-resistant magnesium alloys of present Examples are comparable in fatigue strength to the aluminum alloy A4032-T6 material of Reference Example 2, and superior in fatigue strength to the aluminum alloy AC8B-T6 material of Reference Example 3. Additionally, as can be clearly seen from Table 1, as for the tensile strength, the heat-resistant magnesium alloys of Examples are free from such a large strength decrease occurring on going from 200° C. to 250° C. as seen in the aluminum alloys of Reference Examples 2 and 3, and largely exceed in the tensile strength at 250° C. the aluminum alloys of Reference Examples 2 and 3.

Claims

1. A heat-resistant magnesium alloy comprising, in relation to the total amount of the alloy, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities, wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3, an α-Mg phase and an intermetallic compound Mg₃Y₂Zn₃phase are finely dispersed, and a long period stacking ordered structure phase is formed in a three-dimensional network shape.

2. The heat-resistant magnesium alloy according to claim 1, wherein the tensile strength thereof in a temperature range from 200 to 250° C. falls within a range from 190 to 240 MPa.

3. The heat-resistant magnesium alloy according to claim 1, wherein the elongation thereof in a temperature range from 200 to 250° C. falls within a range from 8 to 16%.

4. The heat-resistant magnesium alloy according to claim 1, wherein the fatigue strength thereof at 200° C. falls within a range from 70 to 80 MPa.

5. A production process of a heat-resistant magnesium alloy comprising:

obtaining a molten alloy by melting at temperatures within a range from 650 to 900° C. a metal material including, in relation to the total amount of the metal material, 1 to 3 at % of Zn, 1 to 3 at % of Y and 0.01 to 0.5 at % of Zr with the balance composed of Mg and inevitable impurities, wherein the composition ratio Zn/Y between Zn and Y falls within a range from 0.6 to 1.3; and

casting the molten alloy by pouring into a mold and thereafter by cooling at a rate of 10 to 10³K/sec.

6. The production process of a heat-resistant magnesium alloy according to claim 5, wherein the casting is conducted by means of gravity casting.

7. The production process of a heat-resistant magnesium alloy according to claim 5, wherein the pouring into the mold is conducted while the molten alloy is being maintained at temperatures within a range from 800 to 850° C.

8. The production process of a heat-resistant magnesium alloy according to claim 5, wherein the cooling is conducted at a rate of 20 to 670 K/sec.