Patent Publication Number: US-5833772-A

Title: Silicon alloy, method for producing the alloy and method for production of consolidated products from silicon

Description:
This is a continuation of application Ser. No. 08/436,384, filed as PCT/NO93/00171 Nov. 17, 1993 published as WO94/11138 May 26, 1994, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to silicon based aluminium and titanium-containing alloys and powder-based products produced from such alloys. The invention further relates to a method for producing silicon based aluminium- and titanium-containing alloys and a method for producing shaped articles from such alloys. 
     BACKGROUND ART 
     Silicon has up till now been used as a raw material for producing silanes, electronic products and as an alloying element for steel and aluminium. When used as an alloying element for steel, silicon is normally added in the form of ferrosilicon in amounts normally below 4% by weight of silicon. When used as an alloying element for aluminium and aluminium alloys silicon is added as elemental silicon. The content of silicon in aluminium alloys varies, but may, for aluminium-silicon alloys, be added in an amount of maximum 20% by weight of the alloys. 
     Elemental silicon is very brittle and lacks ductility. Addition of silicon to, for example, aluminium alloys thus causes an increased brittleness of the alloys when the silicon content exceeds about 20% by weight. As far as the inventors know, silicon-based alloys do not exist which have such properties that the alloy can be used for structural purposes. 
     Silicon has, however, a number of properties which makes use of silicon-based alloys very interesting for structural applications. Silicon has a low density of 2.3 g/cm 3  and a high melting point of 1410° C. Silicon based alloys having a sufficient ductility and strength would thus have a number of advantages compared with other light metals such as, for example Al, Ti, Mg and Be. This relates particularly to properties like high stiffness in relation to weight, low thermal expansion, high resistance to corrosion, high resistance against erosion, and use at higher temperatures than other light metals. 
     In the following table some properties for silicon compared to the same properties for Mg, Al, Ti, and stainless 18/8 steel are shown. 
     
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                                       18/8                               
           Si    Mg      Al      Ti    steel                              
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Density (g/cm.sup.3)                                                      
             2.3     1.7     2.7   4.5   8.1                              
E-module (GPa)                                                            
             113     44      71    106   200                              
Stiffness/density-ratio                                                   
             4.8     2.1     2.6   2.4   2.5                              
Melting point (°C.)                                                
             1410    650     660   1660  1400                             
Thermal expansion                                                         
             2.5     26      23    10    18                               
(10.sup.-6 /K)                                                            
Thermal conductivity                                                      
             84      160     190   19    14                               
(J/smK)                                                                   
Heat capacity (J/gK)                                                      
             0.71    1.03    0.90  0.53  0.48                             
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     DISCLOSURE OF INVENTION 
     It is an object of the present invention to provide silicon-based alloys having such a ductility and strength that the alloys can be used for structural purposes and where the alloys still have the good properties of silicon. 
     Thus, according to a first aspect the present invention relates to a rapidly solidified silicon-based alloy, which alloy contains 2-40% by weight Al, 2-45% by weight Ti, 0-10% by weight of one or more of the elements V, Cr, Mn, Fe, Ni, Co, 0-1% by weight of one or more of the elements B, Sr, P, the rest, except for normal impurities, being silicon in an amount of at least 35% by weight. 
     According to a preferred embodiment, the silicon alloy contains 10-30% by weight Al and 3-15% by weight Ti. 
     According to another preferred embodiment the silicon alloy contains 2-10% by weight Al and 25-40% by weight Ti. 
     The alloy according to the present invention preferably contains boron in an amount of 0.01-0.1% by weight, and/or phosphorous in an amount of 0.01-0.05% by weight and/or strontium in an amount of 0.05-0.5% by weight The content of the elements V, Cr, Mn, Fe, Ni and Co is preferably between 1 and 3% by weight. 
     The rapidly solidified alloy preferably has a primary grain size of less than 50 micron and more preferred less than 10 micron. In order to obtain a highest possible strength and ductility it is particularly preferred that the solidified alloy and precipitated intermetallic phases have a primary grain size of less than 1 micron. 
     According to a second aspect, the present invention relates to a method for production of rapidly solidified silicon-based alloy, said method being characterized in that it is provided a molten alloy containing 2-40% by weight Al, 2-45% by weight Ti, 0-10% by weight of one or more of the elements V, Cr, Mn, Fe, Ni and Co, 0-1% by weight of one or more of the elements B, Sr and P, the rest, except for impurities, being silicon in an amount of at least 35% by weight, which melt is solidified at a rate of at least 10 3  ° C./second. 
     According to a preferred embodiment the melt is solidified at a rate of between 10 4  ° and 10 6  ° C./second. 
     The solidification is preferably done by melt spinning or by gas atomization. It is, however, within the scope of the present invention to use other known methods to achieve a sufficiently high solidification rate. 
     According to a third aspect, the present invention relates to a method for producing consolidated articles from a rapidly solidified silicon-based alloy wherein the rapidly solidified silicon-based alloy containing 2-40% by weight Al, 2-45% by weight Ti, 0-10% by weight of one or more of the elements V, Cr, Mn, Fe, Ni, Co, 0-1% by weight of one or more of the elements B, Sr and P, the rest, except for impurities, being silicon in an amount of at least 35% by weight, is crushed and milled to a particle size below 500 microns and formed to articles by means of powdermetallurgical methods, whereafter the formed articles are hot consolidated. 
     According to a preferred embodiment the rapidly solidified silicon-based alloy is milled to a particle size below 200 microns before the articles are formed. 
     Forming of articles and consolidation of the formed articles are done by conventional powdermetallurgical methods. It is preferred to use hot isostatic pressing, but it is within the scope of the present invention to use for example cold isostatic pressing followed by sintering, hot single axial pressing, forging, extruding and injection casting followed by sintering. 
     It has surprisingly been found that the consolidated articles made from the silicon-based alloy according to the present invention have very high compression strength and a sufficiently high ductility such that the products can be used for structural purposes. 
     By rapid solidification of the silicon-based alloy according to the present invention a very fine grained material is obtained which has an exceptional good distribution of intermetallic phases in the material and very small grains. It is assumed that it is this combination which give the material its high ductility and high strength. By hot consolidation of the articles according to the present invention, it is important to use such a combination of temperature and pressure that the finished products become sufficiently dense and that grain growth during the consolidation process does not affect the properties of the material. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     A silicon alloy containing 25% by weight of Al, 5% by weight of Ti, the rest except for normal impurities being silicon, was melted in a vacuum furnace and cast in the form of rods. The rods were used as a raw material for melt spinning. By the melt spinning the rods were melted and cast to thin sheets or ribbons with a solidification rate of above 10 4  ° C./second. The ribbons were milled in a closed mill to a particle size of less than 200 microns. 
     The alloy particles were thereafter filled into a cylinder-shaped mould having a diameter of 1 cm and a height of more than 1 cm. The alloy particles were thereafter pressed for two hours using single-axial pressure of 40 MPa and at a temperature of 700° C. 
     The produced articles were thereafter tested by compression. The ultimate strength was mesured to 878 MPa and the change in length during compression was 7%. 
     The results show that the produced alloy has a very high compression strength and a compression length comparable to fiber-reinforced aluminium. 
     EXAMPLE 2 
     Five alloys were made in powder form using the same procedure as described in Example 1. 
     Alloy 1: 25% by weight Al, 5% by weight Ti, 0.01% Sr, the rest being silicon. 
     Alloy 2: 15% by weight Al, 5% by weight Ti, the rest being silicon 
     Alloy 3: 35% by weight Al, 5% by weight Ti, the rest being silicon. 
     Alloy 4: 25% by weight Al, 5% by weight Ti, the rest being silicon. 
     Alloy 5: 5% by weight Al, 35% by weight Ti, the rest being silicon. 
     Alloys 1 through 5 were subjected to hot pressing and the fracture strength and compression length were measured. The results are shown in Table I. 
     
                       TABLE I                                                     
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                      Fracture                                            
Hot pressing parameters                                                   
                      Strength Compression                                
Alloy                                                                     
     Temp (°C.)                                                    
               Time (min.)                                                
                         Load (kg)                                        
                                (MPa)  Length %                           
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1    700       120       530    1196   7.5                                
2    700       120       420    926    5.4                                
3    625       120       420    723    5.4                                
4    700       120       420    978    7.7                                
5    1125      120       420    664    5.9                                
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     Table I shows that the hot pressed products had a very high strength and a good compression length.