US3240593A - Corrosion resistant magnesium alloys having a grain-refined structure - Google Patents

Description

3,240,593 ING M h 1966 A. SCHNEIDER ETAL CORROSION RESISTANT MAGNESIUM ALLOYS HAV A GRAIN-REFINED STRUCTURE Filed June 1, 1962 (Hm-s;

all 700 6 b at 760 6 q S Zrccuss achm S ierzdel Mg ATTORNEYS United States Patent 3,246,593 GURROSION RESISTANT MAGNESEUM ALLGYS HAVING A GRAlN-REFHNED STRUCTURE Armin Schneider, Gottingen, Georg Strauss, lll'ermulheirn,

near (Cologne, and .loachim Stendel, Bruhl, near Cologne, Germany, assignors to lKnapsaclr-Griesheim Alrtiengesellschaft, Knapsaclr, near Cologne, Germany, a corporation of Germany Filed June 1, 1962, Ser. No. 199,306 Claims priority, application Germany, June 2, 1961,

9 Claims. a. rs-rss The present invention relates to magnesium alloys which in addition to an especially good corrosion resistance have a fine-grained solidification structure and, associated therewith, Very good mechanical properties.

The purest nonferrous metals often exhibit the best corrosion resistance. However, they must be alloyed with other metal components if their mechanical properties are to be improved. In the case of magnesium, a content of about 1.5% by weight manganese in a binary alloy Mg-Mn does not impair its corrosion resistance. The improvement in mechanical properties is, however, very moderate owing to the coarse structure of solidification.

It is known that a fine-grained cast structure can be imparted to metals by means of foreign germs (carbides, silicides, borides, etc.) which are either stirred in solid ground form into the melt, or are produced therein from salt mixtures that form such germs by reaction with the melt. The exact dosage of germs is rather difiicult in these processes because the essential feature is not so much the quantity by weight or is mol percent of foreign germs but their number and uniform distribution in the solidifying melt. Overdosages must therefore be added whereby besides small particles large heterogeneous particles are inevitably introduced into the melt. In this working method, for reasons of security, the amount of germs necessary for germination is sometimes exceeded by powers of ten. During the solodification, the excess of foreign germs and foreign particles, especially relatively large particles, is however, forced to the grain boundaries and may considerably impair the technological properties of the metals owing to brittleness, notch effect, etc. The greatest disadvantage offered by these known grain refining processes resides, however, in the fact that the grain refining agents are not dissolved in the melt. In the case of waiting times which cannot practically be avoided in foundry practice, these refining agents deposit at the bottom and thus become inactive.

Mg-alloys containing a relatively large proportion of other metal components, for example Al and Zn, solidify with a fine-grained structure, but they have considerably poorer corrosion-resisting properties than pure magnesium or the binary Mg-Mn-alloy.

It is also known that Mg-alloys contain as undesired ingredients a small proportion of Si which emanates from the pure magnesium. Attempts have been made to keep the content of silicon in magneisum and its alloys as low as possible because silicon, especially when present in rather large amounts, reduces increasingly the corrosion resistance.

For all these reasons, it has been extremely unexpected that a silicon-containing magnesium alloy prepared by the process of the present invention besides having a very fine-grained solidification structure and good mechanical properties exhibits a resistance to corrosion better than alloys hitherto lciown.

It has been found that stable metal silicides are formed in the melt by admixing liquid magnesium as the solvent 3,24%,593 Patented Mar. 15, l fifi metal with silicon on the one hand and with at least one metal of subgroups 4 to 7 of the Periodic Table of the elements on the other.

If, for example, the silicon-containing magneisum is alloyed with titanium and/or manganese, titanium silicides and optionally manganese silicides are formed.

The stoichiometrical composition of the metal silicides which are in the state of equilibrium with the melt corresponding to their solubility product, whose formation depends on the temperature of the melt, is a function of the concentration in which the silicon or metal is used.

Since the solubility of the above metal silicides depends substantially on the temperature, they crystallize on cooling and solidification of the melt in an extremely finegrained form, the tiny crystal germs in their uniform state of distribution acting as an ideal grain-refining agent.

The best results are obtained when the concentration of MezSi in the melt is such that silicides of the general formula Me Si are obtained, where Me is a metal of subgroups 4 to 7 of the Periodic Table, since these silicides act as germs due to isotypism.

The formation of that phase is associated with a further advantage: Numerous tests have shown that in customary magnesium alloys it is due to the formation of Mg Si that these alloys are rendered so sensitive to corrosion, since Mg Si reacts with water with the intermediate formation of hydride followed by a decomposition with the evolution of hydrogen, as it is known, for example, from the Zintel phases.

In a magnesium alloy meit, which contains both silicon and at least one metal Me of subgroups 4 to 7 of the Periodic Table of the elements, Mg Si cannot form due to the chemical affinity of the metals Me for the silicon which is greater than that of magnesium, if, in the range of existence of the corresponding metal silicides of the general formula Me Si the melt contains Si and the metal Me in the same or a greater proportion than corresponds to the solubility product of the corresponding metal silicides at the casting temperature of the melt. The molar ratio of Me:Si should be equal to or greater than 523..

The metal representatives which are especially suitable for use in this invention are titanium and manganese, but representatives such as vanadium, chromium, zirconium and hafnium, which also form stable silicides of the type Me Si may likewise be used.

If the concentrations of silicon and metals of subgroups 4 to 7 are greater than would correspond to the solubility product of a Me Si compound, the silicides will precipitate from the melt and deposit with the resultant formation of an equilibrium in the homogeneous melt, which corresponds to the saturation concentration of the particular silicide. This equilibrium concentration remains constant as long as the temperature of the melt is maintained. On cooling and solidification, the silicides percipitate again, but they are then retained in the allov in the form of tiny crystal germs and in uniform distribution and act as highly active grain refining agent.

The advantages offered by the present invention reside in the preparation of a magnesium alloy which, irrespective of its having a corrosion resistance greater than the magnesium alloy hithert considered to possess the greatest resistance to corrosion, exhibits an extremely finegrained cast structure imparting to the alloy excellent technological properties, for example, tensile and tearing strength.

In order to produce such effect, it is often sufficient to use very minor amounts of the above metals, for example 10" to 10- percent by weight titanium. A further advantage offered by the present process would thus appear to reside in the saving of cost for material.

When manganese is used in a proportion greater than 0.8% by weight, especially good results are obtained by incorporating into the alloy an additional amount say of about 0.001 to about 0.1% by weight titanium. Such alloys may also contain about 0.3 to 10% by weight zinc, whereby the strength values and resistance to corrosion are considerably improved. The addition of still further metals, such as zinc, aluminum, silver or cadmium which form no stable silicides but improve the general technological properties, do not impair the effect produced.

Alloys containing, for example, about 0.3 to 0.8% by weight manganese besides about 0.05 to 0.5% by weight silicon, corresponding to the saturation concentration of Mn Si in liquid magnesium at its solidification temperature, are distinguished by their very fine cast structure.

More particularly, the present invention is concerned with the preparation of silicon-containing magnesium alloys having improved properties, especially the grain fineness of the cast structure, strength properties and resistance to corrosion, the alloys being prepared by incorporating therewith at least one phase of the general formula Me Si whereby the concentration ratios of Me and Si in the alloy melt are within the range of existence of that phase, and the Me Si phase is used in a concentration which corresponds approximately to the solubility products of the metal silicides concerned in the alloy melt at casting temperature, Me being a metal of subgroups 4 to 7 of the Periodic Table of the elements. All the Si contained in the alloy practically may and preferably will appear therein in the form of a metal silicide of the general formula Me Si According to a further embodiment of this invention, the silicon-containing alloys may also contain as additional alloy components, for example, zinc, cadmium, aluminum and silver which form no stable metal silicides, but enable the range of existence of the Me Si phase to be varied, if desired or necessary.

The process used for making such alloy comprises using an alloy mixture containing about 0.0001 to 3.0% by weight of at least one metal of subgroups 4 to 7 of the Periodic Table of the elements and about 0.01 to 0.5% by weight of silicon, the remainder being magnesium, on the condition that the concentration ratios of Me and Si in the alloy melt are within the range of existence of the Me Si phase, and that they correspond to the solubility products of the metal silicides concerned in the alloy melt at casting temperature. The alloy melt mixture may also contain about 0.3 to 10% zinc. The metal representatives are advantageously manganese and/ or titanium, zirconium, vanadium or chromium, the titanium, for example, being incorporated in the form of a Zn-Ti pre-alloy into the alloy melt mixture.

In carrying out the process of this invention the concentrations, in the starting melt, of at least one metal Me of subgroups 4 to 7 of the Periodic Table of the elements and of silicon are selected so that the solubility product corresponding to the phase Me Si is reached or exceeded at the casting temperature of the melt. Within the range of existence of the Me Si -phase, the molar ratio of MezSi in the alloy melt mixture is then for example, 5:3, whereby the formation of Mg Si is avoided and substantial linkage of the Si-content as Me Si is favored.

It is advantageous to use as metal representative manganese, whereby the solubility product of the concentration of Me Si in the melt at about 760 C. is greater than about 1.5 10 and simultaneously the concentrations of Mn and Si are within the range of existence of the phase Mn Si in view of the fact that the ratio of the concentrations of MnzSi are greater than about 0.5 and smaller than about 10, the concentrations being expressed in percent by weight, calculated on the whole melt mixture. In other words, it is advantageous to use manganese as metal Me whereby the concentrations of manganese and silicon must be so selected that-corresponding to the test results indicated in the accompanying drawingthe solubility product of the Mn Si -phase is reached or exceeded. This is the case with Mn-concentrations lower than 0.8% and the corresponding Si-concentrations. At 700 C., a Mn-content of 0.7% calls for a minimum concentration of Si of 0.15%; a Mn-content of 0.3% calls for a minimum concentration of Si of 0.4%. Analogously, a fine-grained cast structure has been found in alloys containing, for example: (a) 0.3% Mn and 0.4% Si or (b) 0.5% Mn and 0.15% Si.

If the concentrations of Mn and Si are situated below the straight line of the saturation concentration of Mn Si (cf. the accompanying drawing) and if, therefore, the solubility product of Mn Si is not obtained on solidification of the melt, the melt solidifies in coarse grains. A coarse cast structure has been found, for example, in alloys containing: (a) 0.06% Mn and 0.13% Si or (b) 0.07% Mn and 0.05% Si. If the Mn-contents of a Mg-Mn-Si alloy are greater than 0.8 to 1.0%, a coarse structure is obtained: thus, for example, with an alloy containing (a) 1.0% Mn and 0.08% Si or (b) 1.3% Mn and 0.04% Si. As has been acknowledged by experiments the explanation for this phenomenon resides in the precipitation of phases such as, for example, Mn Si, a or B-manganese mixed crystals which have no germinating action contrary to Mn Si With manganese contents above 0.8% by weight, for example between about 0.8 and 2.0% by weight, zinc may be added in amounts ranging from about 0.5 to 5.0% by weight, preferably about 1.0 to 2.0% by weight. Moreover, titanium and/or zirconium may be used as an additional metal component in a proportion of about 0.0001 to 0.1% by weight.

The following examples serve to illustrate the invention, but they are not intended to limit it thereto:

EXAMPLE 1 and the concentrations being indicated in percent by weight.

When the concentration ratio of MnzSi is greater than 10, the Mn-containing phase Mn Si is concerned. The following Table I illustrates the results obtained in a test series:

Table 1 Percent by weight; Concentration [Mn] Grain Test No. product [Si] structure Mn Si 0. 3 0. 4 1. 6X10- 0. 75 Fine. 0.5 0.18 1 8X10- 3.80 Do. 0.06 0.13 6 X10- 0. 46 Coarse. 0.07 0.05 5 X10 1.40 D0. 1.0 0.08 2 X10 12.5 D0. 1. 3 0.04 2. 4X10- 32. 5 D0.

Corresponding conditions apply to all the elements of subgroups 4 to 7 of the Periodic Table, which are capable of producing the Me Si -phase and are added to the Mgalloy melt. It should be noted that the above phase must be used in a certain minimum amount in order to produce sufficient grain refining.

EXAMPLE 2 The following Table 11 illustrates the improvement in resistance to corrosion associated with a refinement in cast structure. The corrosion tests were carried out in a 30 liter bath with a 3% sodium chloride solution at a temperature of i0.2 C.

The Mg-specimens were placed into eudiorneter tubes reliably connected with the liter bath. The Mg-specimens were cylinders 1 cm. wide and 1 cm. high turned under identical conditions on a precision turning lathe (without lubricant; feed 0.096 mm./revolution; speed: 640 revolutions/min.) and cut oif to ensure that cylinders having an equal surface and surface structure were obtained. Before the specimens were introduced into the corrosion bath, they were washed with petroleum ether and handled exclusively with pincers to avoid the formation of greasy films. The gas evolution was measured as a function of the time, recalculated to normal conditions.

The following Table II indicates the results obtained in a further test series:

Table II Composi- Cc. gas evolved after hours (as tion of measure indicating the resist- Test alloy ance to corrosion) Grain N0. (weight structure percent) 1 Ptge Mg 14 33 53 72 92 111 Coarse 48 85 120 150 Fine.

3 7 10 14 20 Coarse.

5 7 8 10 13 Coarse to fine.

. 3 4. 5 G 8 10 Fine, without Zn, coarse. I i 1. 5 2. 5 3.5 4. 5 5. 5 Do. 2Z0 0.4 SL 3 5. 5 8 11 14 17 Fine.

0.003 Ti. 1 8.g7ZMn [104 sin 14 24 35 43 54 64 Do. 0.002 Ti. 0.07 Mn--- gfi t: 9 16 23 30 38 45 Coarse, fine.

The following statements are deduced from tests 1 to 15 listed in Table II.

Test 1 indicates the results obtained with customary, commercially pure magnesium, whereas tests 2 and 3 were carried out with the customary alloy AM 503. Tests 2 and 3 show that Mn used in a concentration such as in AM 503 admittedly improves the corrosion resistance as compared with pure Mg, but the alloys solidify with a coarse structure as before.

Test 7 indicates that by adjusting Mn and Si-concentrations so that a Me Si -compound precipitates a finegrained structure is obtained with less than about 0.7% by weight Mn, but the resistance to corrosion of pure Mg of 99.9% from which the alloy was made is not fully reached.

As shown in Test 13, a good corrosion resistance associated with a fine-grained solidification structure can be obtained, for example, by adding titanium to enable the Ti Si -phase to be precipitated. The incorporation of titanium with the aid of a Ti-Zn pre-alloy leads unequivocally to a fine grain structure and improves the resistance to corrosion.

Test 13 thus establishes that the formation of the Ti Si -phase rendered possible in this test suffices for refining the grain structure even if, due to a Mn-excess with respect to the residual silicon, the Mn Si-phase would have been expected to form rather than the Mn Si -phase.

Tests 11 and 12 show that the addition of Zn to Si-containing Mg-Mn alloys of the type AM 503 is advantageous especially when about or a little more than 1.0% zinc is added. Smaller zinc additions of less than about 0.8% admittedly improve the resistance to corrosion, but they have no substantial grain refining eifect as demonstrated in Tests 9 and 10 which is contrary to expectation.

It must be assumed therefore that increased zinc additions in the order of at least 1% by weight, provided that the Mg-Mn alloy contains suflicient silicon, have probably the effect to enlarge the range of existence of the Mn Si phase so that the grain-refining Mn Si phase does not even form if the Mn is substantially in excess related to the Si in that Mn Si -phase, that is to say between about 0.8 and about 2.0% by weight, for example at or above about 1.0% by weight, so that the Mn si-phase would usually be expected to form.

In summarizing, it is stated that a grain-refined struct-ure can be imparted to Mg-alloys containing relatively small amounts of Si and Mn in a proportion of less than about 0.8 to 1.0% by weight with the proviso that the ratio of Mn to Si is within the range of existence of the Mn Si -phase, although such alloys do not exhibit optimal properties as regards corrosion resistance Si-containing Mg-alloys containing more than about 1.0% by weight Mn but Si in a proportion as specified above and little zinc have a fine grain structure associated with a considerably improved resistance to corrosion. Si-containing Mg-alloys containing more than about 1.0% Mn and more than about 1.0% zinc exhibit a fine grain structure and optimum corrosion resistance associated with very good strength properties.

Strength tests have shown that an alloy prepared by this invention and containing 2.2% by weight Zn, 1.36% by weight Mn, 0.002% by Weight Ti, 0.004% by weight Zr and 0.06% by weight Si have the following properties associated with a very fine grain structure: a tensile strength of 19.7 kp./mm. at an elongation of 16% and a 0.2% yield point of 9 kp./mm. Similarly good effects could only be obtained with the conventional very costly Mg-Zr alloys which contain considerably greater proportions of Zr than the Si-containing alloys according to this invention.

We claim:

1. A magnesium base alloy having an improved grainrefined cast structure, improved strength properties and an improved corrosion resistance, said alloy consisting essentially of a silicide of the general formula Me Si wherein Me is a metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, chromium and manganese, and the balance magnesium, the concentrations of the silicon and the metal Me in the alloys corresponding at least to the concentrations of the solubility products of the Me Si within the range of existence of said Me Si at the casting temperature of the alloy melt, and the Si-content of the alloy being smaller than the proportion of silicon required for the formation of Mg Si in the cast alloy.

2. The magnesium alloy of claim 1 wherein the alloy contains 0.0001 to 3.0% by weight of Me, and 0.01 to 0.5% by Weight of silicon.

3. The magnesium alloy of claim 1 wherein the alloy contains further metals, the silicides of which are outside the general formula Me Si 4. The magnesium alloy of claim 1 wherein the alloy contains Me and silicon in a molar ratio of Me to Si of at least 5 :3.

5. The magnesium alloy of claim 3 wherein the further metal is a metal selected from the group consisting of zinc, aluminum, cadmium and silver.

6. The magnesium alloy of claim 1 wherein the alloy contains 0.3 to 0.8% by weight manganese and 0.05 to 0% by weight silicon.

7. The magnesium alloy of claim 5 wherein the alloy contains 0.8 to 3.0% by weight manganese, 0.001 to 0.1% by weight titanium, 0.01 to 0.5% by weight silicon and 0.3 to 10.0% by weight zinc.

8. The magnesium alloy of claim 5 wherein the alloy contains 0.8 to 2.0% by weight manganese, 1.0 to 2.0% by weight zinc, 0.01 to 0.5% by weight silicon, and 0.0001 to 0.1% by weight of titanium.

9. The magnesium alloy of claim 5 wherein the alloy contains 0.8 to 2.0% by weight manganese, 1.0 to 2.0% by weight zinc, 0.01 to 0.5% by weight silicon, and 0.0001 to 0.1% by Weight of Zirconium.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Light Metal Age, August 1945, pages 22-23.

DAVID L. RECK, Primary Examiner.

Claims (1)

1. A MAGNESIUM BASE ALLOY HAVING AN IMPROVED GRAINREFINED CAST STRUCTURE, IMPROVED STRENGTH PROPERTIES AND AN IMPROVED CORROSION RESISTANCE, SAID ALLOY CONSISTING ESSENTIALLY OF A SILICIDE OF THE GENERAL FORMULA ME5SI3 WHEREIN ME IS A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, VANADIUM, CHROMIUM AND MANGANESE, AND THE BALANCE MAGNESIUM, THE CONCENTRATIONS OF THE SILICON AND THE METAL ME IN THE ALLOYS CORRESPONDING AT LEAST TO THE CONCENTRATIONS OF THE SOLUBILITY PRODUCTS OF THE ME5SI3 WITHIN THE RANGE OF EXISTENCE OF SAID ME5SI3 AT THE CASTING TEMPERATURE OF THE ALLOY MELT, AND THE SI-CONTENT OF THE ALLOY BEING SMALLER THAN THE PROPORTION OF SILICON REQUIRED FOR THE FORMATION OF MG2SI IN THE CAST ALLOY.

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