Method for Manufacturing High Strength Ultra-Fine/Nano-Structured Al/Aln or Al Alloy/Aln Composite Materials

Abstract

The present invention relates to the a method for manufacturing high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composites using mechanical milling or mechanical alloying process which is conducted in the nitride-forming atmosphere such as nitrogen gas (N), ammonia gas (NH) or mixed gas including both gases, subsequent heat treatment process, and hot consolidation process. Also, high strength ultra-fine/nano-structured Al/ALN or Al alloy/ALN composite materials fabricated by the method of present invention have superior mechanical strength and heat resistance to those fabricated by conventional powder metallurgy process or liquid processes.

Description

TECHNICAL FIELD

The present invention relates to high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials, and their manufacturing process.

BACKGROUND ART

Currently aluminum matrix composite materials reinforced with aluminum oxide, silicon carbide, and etc show poor interface characteristics, such as decohesion of re-inforcements, reaction with matrix phases, and poor wettability. These problems limit the usage of aluminum matrix composite materials. Aluminum nitride is a potential re-inforcement for aluminum matrix composite materials having high elastic modulus, high thermal conductivity, good thermal stability, chemical stability and sound wettability with aluminum.

However, conventional processes, such as conventional powder metallurgy process, directed melt nitridation process, and liquid process in which aluminum nitride reinforcements are directly added to aluminum melts, show some problems like poor interface characteristics caused by the oxide layer of aluminum nitride particles, inhomogeneous distribution of reinforcements, and difficulty in controlling the reinforcement size. Therefore, the a novel method for the fabrication of aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials having homogeneous distribution of fine reinforcement particles and prevention of oxide layer at the surface of matrix and reinforcements should be developed.

DISCLOSURE OF INVENTION

Technical Problem

An object of the invention is to provide high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials exhibiting superior interfacial characteristics, and their manufacturing method, which is composed of mechanical milling or mechanical alloying process which is conducted in the nitride-forming atmosphere such as nitrogen gas (N₂), ammonia gas (NH₃) or mixed gas including both gases, subsequent heat treatment process, and hot consolidation process. Following According to the present invention, the direct nitride forming reaction is occurred in conventional mechanical milling or mechanical alloying process which is currently used. Therefore, the present invention offers the an economical method for manufacturing a high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials, because no additional process is needed.

The continuous supply of nitride-forming gases, such as nitrogen gas (N₂), ammonia gas (NH₃) or mixed gas including both gases, and the removal of residual products after nitride-forming reaction is are crucial for the present invention. The gas supplying devices, gas regulator, and flow rate controller are attached to a milling device so that the pressure of milling chamber and the flow rate of nitride-forming gases are maintained constantly.

ADVANTAGEOUS EFFECTS

As apparent from the above description, the present invention provides aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials having superior mechanical strength at ambient and elevated temperature, and heat resistance to those fabricated by conventional powder metallurgy process or liquid processes, and their fabrication method using mechanical milling/alloying process.

Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the inventions as disclosed in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the schematic diagram of the present invention.

FIG. 2 is a graph depicting the result of X-ray diffraction pattern of aluminum powders, which are fabricated by mechanical milling/alloying process in nitride-forming atmosphere, and of powders which are heat treated at 500° C., 600° C., and 900° C. for 1 hour.

FIG. 3 is a graph depicting the result of differential thermal analysis of aluminum powders which are milled in nitride-forming atmosphere.

FIG. 4 is a graph depicting the results of X-ray photoelectron spectroscopy of aluminum powders, which are fabricated by mechanical milling/alloying process in nitride-forming atmosphere, and of powders which are heat treated at 500° C., and 600° C., and 900° C. for 1 hour.

FIG. 5(a), 5(b), 5(c), and 5(d) are transmission electron micrographs of aluminum/aluminum nitride composite powders, which are fabricated by mechanical milling/alloying and subsequent heat treatment: (a) bright field image of composite powders, (b) dark field image of aluminum in composite powders, (c) dark field image of aluminum nitride in composite powders, and (d) selected area electron diffraction pattern of composite powders.

FIG. 6 is a graph depicting the variation of hydrogen contents in aluminum/aluminum nitride composite powders, which are fabricated by mechanical milling/alloying process, during subsequent heat treatment at 540° C. and 560 580° C.

BEST MODE FOR CARRYING OUT THE INVENTION

Mechanical milling or mechanical alloying process used in the present invention is a solid-state powder processing technique involving repeated welding, fracturing, and the rewelding of powders due to repeated collisions between milling media, such as steel balls and ceramic balls, and powders in milling devices like a ball mill and an attritor. Mechanical milling or mechanical alloying process has a number of advantages including grain refinement, homogeneous distribution of reinforcement, extended solid solubility, and the formation of metastable or amorphous phases, depending on the initial powder composition and processing condition. For the effectiveness of mechanical milling or mechanical alloying process, it is appropriate that the initial charge ratio of milling media to powders is 5:1 to 50:1.

After mechanical milling or mechanical alloying process in nitride-forming atmosphere, the precursor of aluminum nitride is formed. To transform this precursor into aluminum nitride, subsequent heat treatment process is needed.

The conditions of subsequent heat treatment process of composite powders should be acquired from the results of various experiments which are differential thermal analysis. X-ray photoelectron spectroscopy, X-ray diffraction analysis, isothermal heat treatment and hydrogen analysis. Subsequent heat treatment process to transform the precursors of aluminum nitride into aluminum nitride phase should be conducted at temperatures above of 400° C. or above, which is lower than the melting point of composite powders, during 0.1 to 48 hours, or is substituted for degassing process. Naturally, degassing process is conducted at conditions similar to those of heat treatment process.

The aluminum/aluminum nitride and aluminum alloy/aluminum nitride composite powders fabricated by the process previously described is consolidated by the process such as hot pressing, hot isostatic pressing, hot extrusion, and etc. The hot consolidation processes of powder materials require the degassing process which removes residual moisture, organic compounds and hydrogen. It is the necessary process for improving mechanical properties of final consolidated materials. It is composed of a canning process of powder materials, a sealing process of metal can and a evacuating process at high temperatures (below 1 10⁻¹ torr).

In the present invention, subsequent heat treatment process is substituted for the degassing process, which is conducted with under the conditions of subsequent treatment process, at temperatures above of 400° C. or above, which is lower than the melting point of composite powders, during 0.1 to 48 hours. Therefore, no additional process is needed.

The present invention gives the method for manufacturing the ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials, in which the grain size of aluminum matrix or aluminum alloy matrix, and the size of aluminum nitride reinforcements is below 100 particles are 100 or below.

Also, for improving the mechanical properties of aluminum matrix, such as strength and hardness, solid solution hardening elements like magnesium, silver and manganese could be added in the range of with 0.1 wt. % to below solubility limit; precipitation hardening elements like copper, zinc, silicon, titanium, iron, lithium, tin, chromium and zirconium could added with above solubility limit or above, rare earth elements like yttrium, cerium, lanthanum, scandium, samarium, neodymium, gadolinium, praseodymium and misch metal could be added with in the range of 0.1 wt. % to 10.0 wt. %; alloying elements like tungsten, molybdenum and cobalt could be added with in the range of 0.1 wt. % to 50.0 wt. %; or ceramic reinforcement particles like Al₂O₃, SiC and Si₃N₄ could be added with in the range of 0.1 wt. % to 50.0 wt. % in the present invention.

The present invention is described with following figures in detail in reference to the figures.

FIG. 1 is the schematic diagram of the present invention. In the step of powder preparation (P1), the powder of aluminum and alloying elements is prepared. Alloying elements is added in the form of individual element powders or master alloy powders. In the step of mechanical milling or mechanical alloying in nitride-forming atmosphere (P2), the mechanical milling or mechanical alloying is conducted for the nitride formation. The atmosphere in the milling container is composed of nitride-forming gases, such as nitrogen gas (N₂), ammonia gas (NH₃) or mixed gas including both gases. The devices for the continuous supply of nitride-forming gases and the removal of residual products after nitride-forming reactions are attached. The step of the subsequent heat treatment process for the formation of aluminum nitride (P3) is substituted for the degassing process in the present invention. In the step of hot consolidation (P4), the composite powders is are consolidated into the form of bulk products.

FIG. 2 is a graph depicting the results of X-ray diffraction pattern of aluminum powders, which are fabricated by the mechanical milling/alloying process in nitride-forming atmosphere, and of powders which are heat treated at 500° C., 600° C., and 900° C. for 1 hour. No aluminum nitride peak is observed in the as-milled powders and the powders which are heat treated at 500° C. The powders which are heat treated at 600° C. show evidence of aluminum nitride formation. The aluminum nitride peaks are more sharp and obvious in the powders heat treated at 900° C. From this result, it is clear that the precursor of aluminum nitride is formed in as-milled powders and a suitable heat treatment process is needed.

FIG. 3 is a graph depicting the result of differential thermal analysis of aluminum powders which are milled in nitride-forming atmosphere. Two exothermic peaks are observed at 430° C. and 565° C. It is thought that these peaks are related to the trans-formation of aluminum nitride precursor into aluminum nitride. To verify the binding status of nitrogen atoms, X-ray photoelectron spectroscopy is conducted.

FIG. 4 is a graph depicting the results of X-ray photoelectron spectroscopy of aluminum powders, which are fabricated by mechanical milling/alloying process in nitride-forming atmosphere, and of powders which are heat treated at 500° C., and 600° C., and 900° C. for 1 hour. It is found that nitrogen atoms have Al—N, N—H and N—H₂ type bonds in as-milled powders, N—H₂ type bonds are disappeared through isothermal heat treatment at 500° C., and only Al—N type bonds is existing after isothermal heat treatment above 600° C.

FIG. 5(a), 5(b), 5(c), and 5(d) are transmission electron micrographs of aluminum/aluminum nitride composite powders, which are fabricated by mechanical milling/alloying and subsequent heat treatment. Those show that the size of aluminum grains and aluminum nitride phases is below 200 nm. The composite powders have ultra-fine structure or nanostructure.

FIG. 6 is a graph depicting the variation of hydrogen contents in aluminum/aluminum nitride composite powders, which are fabricated by mechanical milling/alloying process, during subsequent heat treatment at 540° C. and 560 580° C. In the case of 540° C., no hydrogen is detected after 3 hours and in the case of 560 580° C., no hydrogen is detected after 2 hours. This These results show that the precursor of aluminum nitride in as-milled powders is transformed into aluminum nitride completely.

Mode for the Invention

Now, the high strength ultra-fine/nano-structured aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials according to the present invention will be described in more detail, with reference to the following examples.

Example 1

In accordance with the present invention, the mixture of aluminum powders and master alloy powders with the composition of Al-50 wt. % Mg are used as starting materials. Initial composition of starting materials is Al-4 wt. % Mg. Then mechanical alloying process is conducted in ammonia gas atmosphere with the condition in which the final volume fraction of aluminum nitride is 25%. The final bulk product of Al-4 wt. % Mg/25 vol. % AlN composite materials is manufactured through cold compacting process, degassing process and hot extrusion process.

Example 2

In accordance with the present invention, the mixture of aluminum powders and titanium powders are used as starting materials. Initial composition of starting materials is Al-5 wt. % Ti. Then mechanical alloying process is conducted in ammonia gas atmosphere with the condition in which the final volume fraction of aluminum nitride is 25%. The final bulk product of Al-5 wt. % Ti/25 vol. % AlN composite materials is manufactured through cold compacting process, degassing process and hot extrusion process.

Example 3

In accordance with the present invention, the mixture of aluminum powders and zinc powders are used as starting materials. Initial composition of stalling materials is Al-5 wt. % Zn. Then mechanical alloying process is conducted in ammonia gas atmosphere with the condition in which the final volume fraction of aluminum nitride is 25%. The final bulk product of Al-5 wt. % Zn/25 vol. % AlN composite materials is manufactured through cold compacting process, degassing process and hot extrusion process.

The compression tests are conducted at room temperatures and 200° C., and their results are shown in table 1. From examples 1 to 3, the aluminum alloy/aluminum nitride composite materials have superior yield strength to those of conventional aluminum alloys and aluminum matrix composite materials. Especially, Al-4 wt. % Mg/25 vol. % AlN composite material shows the drastic increase of yield strength at room temperature. These results of the aluminum/aluminum nitride composites is are due to the refinement of aluminum matrix grains and reinforcement particles and the homogeneous distribution of aluminum nitride particles.

TABLE 1
Comparison of Yield Strength of Various Samples
		Yield Strength	Yield Strength
		at 25° C.	at 200° C.
Example	Compositions	(MPa)	(MPa)
1	Al-4 wt. % Mg/	957	328
	25 vol. % AlN
2	Al-5 wt. % Ti/	390	241
	25 vol. % AlN
3	Al-5 wt. % Zn/	440	225
	25 vol. % AlN
Comparative		330
Example
 comparative example: Min Zhao, Gaohui Wu, Dezhi Zhu, Longtao Jiang and Zuayong Dou: Materials Letters, 58(2004) p. 1899.

INDUSTRIAL APPLICABILITY

As apparent from the above description, the present invention provides aluminum/aluminum nitride or aluminum alloy/aluminum nitride composite materials having superior mechanical strength at ambient and elevated temperature, and heat resistance to those fabricated by conventional powder metallurgy process or liquid processes, and their fabrication method using mechanical milling/alloying process.

Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the inventions as disclosed in the accompanying claims.

Claims

1. Method for manufacturing high strength ultra-fine nano-structured aluminum and aluminum nitride or aluminum alloy and aluminum nitride composite materials, wherein aluminum powder or mixture powder of the aluminum and alloying element is mechanically milled and mechanically alloyed in an atmosphere of NH3 gas containing nitrogen for inducing nitrification reaction so as to produce a precursor of aluminum nitride; subsequent heat treatment is performed to produce aluminum and aluminum nitride or aluminum alloy and aluminum nitride composite powder; this composite powder is subjected to a hot forming process to produce composite material.

2. The method of claim 1, wherein the mechanical milling and mechanical alloying are performed in a vessel in which the atmosphere of NH3 gas containing is maintained.

3. The method of claim 1, wherein the subsequent heat treatment for production of aluminum nitride is performed at a temperature of 400° C. to the melting temperature of the composite powder for 0.1˜48 hours.

4. The method of claim 1, wherein the subsequent heat treatment in the hot forming process is a high-temperature degassing process.

5. The method of claim 1, wherein the grains of aluminum or aluminum alloy matrix and the aluminum nitride reinforcement particles have a ultra-fine nano-structure with a size equal to 10 μm or less through the mechanical milling or mechanical alloying process.

6. The method of claim 1, wherein to aluminum matrix, solid solution hardening elements like magnesium, silver and manganese could be added in the range of 0.1 wt. % to solubility limit, precipitation hardening elements like copper, zinc, silicon, titanium, iron, lithium, tin, chromium and zirconium could added with solubility limit or above, rare earth elements like yttrium, cerium, lanthanum, scandium, samarium, neodymium, gadolinium, praseodymium and misch metal could be added in the range of 0.1 wt. % to 10.0 wt. %, alloying elements like tungsten, molybdenum and cobalt could be added in the range of 0.1 wt. % to 50.0 wt. % or ceramic reinforcement particles like Al2O3, SiC and Si3N4 could be added in the range of 0.1 wt. % to 50.0 wt. %.