Abstract:
The invention relates to high mechanical strength magnesium alloys and to a process for producing these alloys by fast solidification and consolidation by drawing generally exceeding 400 or 500 MPa, an elongation at break of generally at least 5% and a chemical composition by weight within the following limits: 
     
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     Aluminium        2-11%Zinc             0-12%Manganese        0-1%Calcium        0.5-7%Rare Earths    0.1-4%______________________________________ 
     with the main impurities and the residue being magnesium, their structure being constituted by grains with a mean size below 3 μm and intermetallic compounds with a size below 2 μm precipitated at the grain boundaries.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is linked to claims 1 and 2 of the main French patent application 88-02885 and relates to high mechanical strength magnesium alloys and to their production process. 
     SUMMARY OF THE INVENTION 
     The alloys of the invention have a breaking load of at least 290 MPa, but more particularly at least 400 MPa and an elongation at break of at least 5% and which, in combination, have the following characteristics: 
     a weight composition between the following limits: 
     
         ______________________________________Aluminium     2-11 and preferably                       3 to 9%Zinc          0-12 and preferably                       0 to 3%Manganese     0-1 and preferably                       0.1 to 0.2%Calcium       0.5-7 and preferably                       1 to 7%Rare Earths (RE)         0.1-4 and preferably                       0.5 to 2.5%______________________________________ 
    
     with the following contents of the main impurities: 
     
         ______________________________________  Silicon        &lt;0.6%  Copper        &lt;0.2%  Iron  &lt;0.1%  Nickel        &lt;0.01%______________________________________ 
    
     the remainder being magnesium; 
     a mean grain size below 3 μm; 
     they are constituted by a homogeneous matrix reinforced by particles of intermetallic compounds precipitated at the grain boundaries Mg 17  Al 12 , optionally Al 2  Ca, as a function of the Ca concentration, Mg 32  (Al,Zn) 49  if Zn is present in the alloy, Mg-RE and/or Al-RE, as a function of the content and/or the nature of the rare earth, said particles having a mean size below 2 μm and preferably below 0.5 μm. This structure remains unchanged after keeping for 24 h at 300° C. When Mn is present, it is an at least quaternary element and its minimum weight content is preferably 0.1%. 
     Such alloys also have an improved corrosion resistance. Thus, unlike the alloys described in the main French patent application 88-02885 and its first certificate of addition 89-01913, which have local corrosions (e.g. pitting, corrosion in the form of wear ridges and grooves, etc.) which can lead in the long term to weakness areas, they have a corrosion which is at least as low, but which is also more homogeneous. Thus, in the requisite proportions, the alloys according to the invention contain both calcium and rare earths, particularly Y (included here as a RE), Nd, Ce, La, Pr or misch metal (MM). These additions make it possible to improve the mechanical characteristics of the magnesium-based alloys obtained after rapid tempering and compaction by drawing, including drawing temperatures which can reach or even exceed 350° C., whilst still retaining an interesting level for the characteristics. Such a property in particular makes it possible to increase the drawing or extrusion rates and speeds, the alloy being able to withstand the heating resulting therefrom without losing its characteristics, so that the productivity levels can be improved. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the final alloy, the calcium can be in the form of dispersoids of Al 2  Ca precipitated at the grain boundaries and/or in solid solution. The particles of the intermetallic compound Al 2  Ca appear when the Ca concentration is adequate. Their size is below 1 μm and preferably below 0.5 μm. There is no need for Mn to be present. This also applies with respect to the RE, the dispersoids appear as from certain concentrations inherent in each of the rare earths. It is also possible for other intermetallic particles, e.g. based on Al and Mn and which are of a very small size (approximately 40 to 50 nanometers) to be dispersed in the magnesium grains. 
     According to the invention, the alloys are obtained by the processes and different embodiments described in the main patent, which form an integral part of the present description. The alloy in the liquid state undergoes a fast solidification at a speed at least equal to 10 4  K sec -1  and generally below 10 6  K sec -1 , so as to obtain a solidified product, whereof at least one of the dimensions is below 150 μm, said product then being directly consolidated by precompacting and compacting or by direct compacting, compacting taking place at between 200° and 350° C. It is preferable for the solidified product to undergo no other conditioning operation such as grinding before being consolidated by precompacting and/or compacting, said operation possibly reducing the mechanical characteristics of the consolidated alloy obtained. 
     The rapid cooling for the solidification can either be obtained by casting in strip form on a so-called &#34;hyper-tempering on roller&#34; apparatus, which is conventionally constituted by a vigorously cooled drum on to which is cast the metal; or by melting an electrode or a liquid metal jet, the liquid metal then being mechanically divided or atomized and sprayed onto a vigorously cooled surface which is kept free; or by atomization of the liquid alloy in an inert gas jet. 
     The first two procedures make it possible to obtain a solid in the form of strips, scales or small plates, whilst the latter gives powder. These processes are described in detail in the main patent application and do not form part of the present invention as such. The rapidly solidified product can be vacuum degassed at a temperature equal to or below 350° C. prior to consolidation. 
     The consolidation, which is also described in the main application is performed, according to the invention, directly on the solidified products and in particular directly on the scales or plates. In order to preserve the fine, original structure obtained by fast solidification, it is necessary to ensure that there is no long exposure to high temperatures. Therefore tepid drawing or extrusion is used, which makes it possible to minimize the high temperature passage time. 
     The drawing temperature is between 200° and 350° C. The drawing ratio is generally between 10 and 40 and preferably between 10 and 20. The ram advance speed is preferably between 0.5 and 3 mm/sec, but can also be higher. 
     As described in the main application, prior to consolidation, the solid product can be directly introduced into the press container, or following precompacting at a temperature at the most 350° C. with introduction into a sheath made from Mg or its alloys, or Al or its alloys, which is itself introduced into the said container. 
     As a variant, it is possible to perform other compacting processes not leading to a rise in the temperature of the product beyond 350° C. These optional processes include hydrostatic drawing, forging, rolling and superplastic forming. 
     Thus, the process according to the invention unexpectedly makes it possible to obtain a consolidated magnesium alloy which, as has already been described, has a fine structure (grain smaller than 3 μm) reinforced by intermetallic compounds and the excellent mechanical characteristics remain unchanged in the same way as the structure of said alloy, after keeping for a long time at a temperature reaching and even exceeding 350° C. The corrosion resistance is improved in uniformity and weight loss (which is reduced). 
    
    
     EXAMPLES 
     Several alloys were produced under fast solidification conditions identical to those used in the examples of the main application: wheel casting, peripheral wheel speed 10 to 40 m/s, cooling speed between 10 5  and 10 6  K s -1 . The strips obtained were then directly introduced into the container of a drawing or extrusion press in order to obtain a consolidated alloy on which characterization tests were carried out: microscopic examination, measurement of the mechanical characteristics, corrosion resistance (measured by tempering in a 5% NaCl solution over 3 days). 
     Table 1 gives the operating characteristics for the drawing process and the characteristics of the alloys obtained: 
     Hv=Vickers hardness expressed in kg/mm 2   
     TYS=yield strength measured with 0.2% elongation in MPa 
     UTS=breaking load in MPa (ultimate tensile strength) 
     e=elongation at break as a % 
     Corrosion=weight loss in mg/cm 2  /day (m.c.d)--appearance of corrosion. 
     
         __________________________________________________________________________  According to  invention     According to prior art__________________________________________________________________________N° and test   20  21  22    4   23  7  9  11                                  12Alloy com-           AZ91                    AZ91         AZ91 +position                              Ca 2%wt % (1)Al      5   7   5     9   9   9  5  5  9Zn      0  1.5  0     1   1   0  0  0 0.6Mn      0   0   0    0.2  0   0  0 0.5                                 0.2Ca     6.5 4.5 6.5    0   0   1 3.7                              3.5                                  2RE     2(Nd)      1(Nd)          2(MM)(2)                 0   0   0  0  0  0T° drawing  300 300 300   200 300 200                           250                              300                                 250°C.Drawing   20  20  20    20  20  20                            20                               20                                  20ratioRam    0.5 0.5 0.5   0.5 0.5 0.5                           0.5                              0.5                                 0.5speedmm/secHv kg/mm2  132 134 138   129 105 139                           124                              100                                 125TYS (0,2)  564 535 565   457 330 500                           538                              483                                 427MPaUTS MPa  593 574 598   517 380 555                           567                              492                                 452e %     2  4.7 1.6   11.1                     20 6.9                           5.2                              8.0                                 5.4Corrosion:mg/cm2/day   0.56       0.25          0.2   0.4 0.4  0.35                           0.5                               0.65                                  0.075Corrosion  Uni-      uni-          uni-  fili                    fili                        deep                           uni-                              uni-                                 uniformtype   form      form          form  form                    form                        pitt-                           form                              form                        ing__________________________________________________________________________ (1) The residue being Mg (2) MM: Misch Metal 
    
     This table includes tests 20-21-22 illustrating the present invention, whilst tests 4-23-7-9-11-12 illustrate the prior art and are partly taken from French certificate of addition FR 89-01913. 
     Tests 4 and 23 relate to alloys treated by fast solidification and consolidation with a composition identical to that of AZ91. Tests 7-9-11-12 relate to alloys containing Ca also obtained by fast solidification and consolidation. The results obtained with regards to the corrosion and/or mechanical characteristics of these alloys are inferior to those of the alloys according to the invention. Samples 23, 4 and 7 are subject to heterogeneous corrosion with relatively high weight losses. Samples 4 and 7 also have mechanical characteristics well below those of the alloys according to the invention. Sample 11 has uniform corrosion, but a high weight loss comparable with that of alloy 20 and mechanical characteristics decidely inferior to those of the latter and also to those of alloys 21 and 22. Finally, sample 12 has an excellent corrosion resistance, but the mechanical characteristics are well below those of the alloys according to the invention. 
     Thus, according to the invention, the addition of rare earths permits a higher level for the mechanical characteristics, improves the uniformity of the corrosion (test 20-21-22) and reduces the weight loss (tests 21-22). It should be noted that the mechanical characteristics are obtained by consolidation drawing at 300° C. and that the difference compared with the prior art would increase if the drawing in the tests for the latter was carried out at such a high temperature. 
     Thus, the invention makes it possible to obtain alloys with an improved corrosion resistance (uniform corrosion and generally lower weight loss), whilst giving improved mechanical characteristics for a high drawing temperature. The latter advantage is important because such temperatures make it possible to draw sections having large dimensions and/or increase the drawing speeds, whilst still retaining good mechanical characteristics. It should also be noted that this high drawing temperature makes it possible to improve the fatigue strength of alloys according to the invention.