GB2262539A - Titanium containing magnesium alloy produced by vapour quenching. - Google Patents

Abstract

Magnesium based alloy containing 0.5 to 47% of titanium by weight is produced as a solid solution alloy by vapour quenching. In specific embodiments a source of titanium is evaporated using an electron beam. The ingredients are vapourised from separate sources to avoid the problem of titanium insolubility in magnesium melts. Preferred ranges for titanium are 5 to 40% and 15 to 28% by weight. The alloy exhibits corrosion resistance particularly at higher levels of titanium. Alloys containing 15 to 28% titanium with either approximately 5% aluminium or approximately 1% silicon are disclosed. The alloys may contain other elements conventional as ingredients in magnesium based alloys.

Description

TITANIUM CONTAINING MAGNESIUM ALLOY PRODUCED BY VAPOUR QUENCHING This invention relates to alloys which are based upon magnesium and contain titanium, which are produced by the technique of vapour quenching. These alloys are intended to yield improved resistance to corrosion in comparison with existing alloys of magnesium.

Vapour quenching is a known physical vapour deposition technique which is used primarily for the production of metal films or coatings but is also known as a route to the production of alloys having metastable supersaturated solid solutions on a scale suitable for quite significant structural applications. This invention is concerned of course with the latter application of vapour quenching technology. The process is performed by evaporating alloy constituents from individual or combined sources to produce a flux of vapours, and causing these vapours to be condensed upon a collector which is controlled in temperature to a value which causes extremely rapid quenching of the impinging vapours. All this takes place within a vacuum chamber.A general description of the vapour quenching process for the production of alloys is given in two articles by Bickerdike et aZ in the International Journal of Rapid Solidification, 1985, volume 1 pp 305-325; and 1986, volume 2, pp 001-019. The collector deposit obtained by vapour quenching can be utilised in various product forms. It can be taken from the collector and processed intact e.g. by forging, rolling, or hot pressing to yield a monolithic product or it can be comminuted to particulate form and subsequently canned and consolidated by hot isostatic pressing or extrusion.

Magnesium is the lightest of the structural metals and has significant potential utility for aerospace applications, but magnesium alloys have somewhat fallen out of favour for use in such applications, partly because of poor compression properties, but principally because of their poor corrosion resistance. Conventional magnesium alloys (that is those produced by ingot metallurgy methods) have generally poor corrosion resistance due primarily to the water-solubility of the Mg(OH)2 film which forms on the alloy in damp environments. This problem of surface corrosion is exacerbated by electrochemical effects as magnesium has a large negative electrode potential.In commercial purity alloys the presence of particles containing common impurity elements such as iron, copper and nickel which have a less negative electrode potential leads to a susceptibility to pitting and general corrosion. Magnesium alloys are also subject to electrochemical corrosion when joined to other structural metals as most have a significantly different electrode potential.

Major improvements in the corrosion resistance of early magnesium alloys were obtained by the addition of small quantities of manganese which reduced the damaging effects of iron, the main impurity, and certain other impurities by combining these within relatively benign intermetallic compounds. More recently further improvements in corrosion resistance have been achieved by reducing the number of particles present in the alloys through improvements in alloy purity.

In addition, a magnesium alloy has been recently developed which is produced by a rapid solidification method to a composition by weight of Mg(base) - 5A1 - 5%Zn - 5Nd and is claimed to give corrosion properties which are superior to any other currently available wrought magnesium alloy. The weight loss of this alloy in a saline corrosion test is comparable to the widely used 2000 series aluminium alloys, but the alloy does not reveal any reduction in electrode potential in comparison to magnesium so it is still susceptible to electrochemical corrosion.

It is an object of this invention to provide a magnesium alloy which provides an improvement in corrosion resistance beyond the level available in current commercial alloys. To achieve this a reduction in both susceptibility to weight loss in saline environments and to electrochemical corrosion (by reduction in its electrode potential) is desirable.

Some of the alloying ingredients utilised in alloys other than magnesium alloys have minimal or no solubility within magnesium in the melt because of the low boiling point of magnesium and the binary phase diagrams of magnesium with these materials are unknown. We have been able to produce magnesium alloys incorporating a variety of such ingredients using a vapour deposition process by evaporating the magnesium base and the alloying ingredients from separate sources and combining the vapour streams to yield an alloyed deposit on the collector. We have found that magnesium - titanium alloys produced by this method are capable of yielding a particularly good resistance to corrosion, USA patent 4264362 discloses a prior magnesium - titanium alloy produced in the solid state by a mechanical alloying process.In contrast to the alloys of this invention, the alloys disclosed in that specification were alloys in which neither of the two elements was present in solid solution within the other (the two being in the state of a mechanical mixture of the elementsj and the alloy product was a supercorroding material intended to react quickly with sea water to provide exothermic heating.

The invention claimed herein is a magnesium based alloy produced by vapour quenching which comprises 0.5 to 47 of titanium by weight. and within which the titanium is substantially held in solid solution in the alloy as deposited.

Preferably the alloys have a titanium content in the range 5 to 40.

We have demonstrated a marked improvement in corrosion resistance in these alloys compared to prior art materials and compared to alternative research materials with different alloying ingredients such as aluminium, chromium and silicon.

The magnesium alloy may contain, in addition to the titanium, other ingredients such as those used in prior art magnesium alloys within the following weight limits: manganese up to 6%; aluminium up to 13%; zinc up to 7%; zirconium up to 5%; neodymium up to 6%; commercial mixed rare earths up to 5%; yttrium up to 6%; silver up to 3%; thorium up to 5%; lithium up to 10%; and silicon up to 2%.

Preferably the titanium content of the alloy does not exceed 40% in alloys intended for duties where low density is important for at higher titanium contents the alloy density approaches that obtainable from other alloys such as aluminium-lithium alloys. But it should be noted that the resistance to corrosion and the electrode potential improves with increasing titanium content so a high titanium content might be preferred for this reason.

A most preferred range of titanium in the alloy is 15 to 28%. Practical ternary alloys might include titanium in the aforementioned preferred range with either approximately 5% aluminium (to yield a physically stronger magnesium based material still having good corrosion resistance) or else approximately 1% silicon (to increase corrosion resistance still further).

Alloys within the scope of the claims have demonstrated thermal stability (as ascertained by differential scanning calorimetry) up to 230 degrees Celsius and can be processed from comminuted particulate form by first canning the particulate and then extruding or hot isostatic pressing this.

The invention is further described below by reference to examples of materials made to varying compositions by two alternative vapour quenching methods and by reference to the drawings, of which: Figure 1 is a schematic cross sectional view of one form of small scale evaporation equipment; and Figure 2 is a schematic cross sectional view of a second form of evaporation equipment.

Those alloy examples which follow and are designated with the prefix "V" have been produced using the apparatus depicted in Figure 1. The apparatus has separate evaporators for the magnesium and titanium charges. The evaporator for the titanium charge comprises a water-cooled copper crucible 1 with a underside opening 2 through which is fed a rod charge of titanium 3 and there is an electron beam gun 4 which is beam steered so that the electrons impinge upon the topmost end of the rod charge 3 to cause this to be melted locally. A stream of titanium vapour 5 issues from the melt pool in an upward direction towards a collector 6 and the flux of vapours is controlled by adjustment to the power setting of the electron beam gun 4.

The evaporator for the magnesium base constituent of the alloy is a heated block 7 of metal of a generally "U" shaped configuration which is located above the level of the titanium source crucible 1 and is of a size and is positioned such that it encompasses the rising flux of titanium vapour on three sides thereof. Block 7 has a inwardly directed slot 8 within which solid charge pieces 9 of magnesium are placed and the block 7 is heated by means of a radiant heater 10. extending through the mass of the block, to a temperature at which magnesium vapour issue forth from the charge pieces 9 by sublimation. These vapours issue forth laterally through the slot opening to converge upon the rising flux of titanium vapour 5 and vapour mixing takes place by mingling of the vapour streams and by atomic collisions.Some of the intermixed vapours rise upwards to condense upon the collector 6 to yield a deposit 11.

The collector is heated to and maintained at a suitable temperature to ensure that the impinging vapours are quenched to produce an appropriate microstructure in the alloy deposit 11. Between the collector plate 6 and the sources of vapours there is a shutter 12 which is closed in the warm up period of operation of the equipment and opened when the equipment stabilises. This is a safeguard measure intended to avoid unwanted collection of deposits having the wrong composition.

In the use of the above-described apparatus in the production of the magnesium-titanium alloys exemplified below, the apparatus was controlled to the following parameters: collector temperature 125 to 2000C electron beam gun power 2-3kW magnesium source temperature approximately 5500C vacuum chamber pressure around 5 x 10-5 torr Variation of composition is achieved through alteration of the respective rates of generation of the titanium and magnesium vapours.

The electron beam gum power is varied within the range given above to increase or decrease the flux of titanium vapours and the temperature to which the magnesium source is heated is varied (but remaining below the melting temperature) to increase or decrease the flux of magnesium vapour. Simultaneous manipulation of both these provides control over the alloy deposition rate.

Using different sources for the two separate ingredients as described above overcomes the problems of melt insolubility of one ingredient within the other and overcomes also problems which migh otherwise be caused by gross differences between the vapour pressures exhibited by different ingredients. Ternary and more complicated alloys can be produced by using a pre-alloyed rod source containing both the titanium and the other ingredients.

The remaining alloys of the examples which follow were produced using the equipment depicted in Figure 2 and these alloy examples are identified with a prefix of "AS". More detailed information regarding this form of equipment is given in UK patent application 9022449.4.

This second form of equipment comprises an inner evaporation crucible 31 surrounded by a another evaporation crucible 32. A charge 33 of the titanium is contained within the crucible 31. This crucible is open topped and the charge of titanium within it is heated by means of an electron beam 37. This electron beam 37 is focussed and steered by means of a magnets 38 to impinge upon the uppermost surface of the charge 33 to heat it to a temperature at which an appreciable flux of titanium vapours escapes from the charge. The other evaporation crucible 32 hold a charge 34 of magnesium within it and this charge is heated by a radiant heater 39. Crucible 32 has a lid 40 and there are nozzles 41 present in the lid through which a flux of magnesium vapours issues forth. The two crucibles are placed as depicted (one within the other) beneath a collector 35.The nozzles 41 are so directed as to cause the individual streams of magnesium vapours to converge towards each other on paths which intersect the direct path between the other evaporator 31 and the collector 35 so that vapour mixing between the two species of vapours occurs before either species lands upon the collector. A deposit 36 of alloy is built up upon the lower surface of the collector 35. The lid 40 is heated by a separate radiant heater 42 to a higher temperature than that of the magnesium charge. Moreover the nozzles are of re-entrant form as depicted in the figure and by this combination of re-entrant form and higher temperature condensation within or upon the nozzle of magnesium vapours is minimised.The nozzles 41 serve the dual function of directing the streams of magnesium vapours and choking the volume flow-rate somewhat allowing the magnesium charge to be superheated to yield sufficient vapour pressure to disperse surface oxide contaminants from the surface of the melt without saturating the deposit with an excess of magnesium.

In the deposition of the claimed magnesium-titanium alloys by this second form of equipment the evaporator 32 is charged with pieces of magnesium and heated to produce a melt of metal at a temperature of approximately 750 degrees Celsius. The temperature of the titanium charge 33 within evaporator 31 is not controlled directly, instead control of titanium evaporation rate and hence of alloy deposit composition is achieved by variation of electron beam power up to a maximum power of 20 kW. Ternary and more complex alloys can be produced by this second equipment by using a different form of inner crucible 31 accepting an under-fed rod source in the manner of the first type of equipment, and using a pre-alloyed titanium alloy rod charge of appropriate composition.

Vapour-quenched magnesium-titanium binary alloys to the following compositions have been produced by means of the two equipments described above. All compositions are given in proportions by weight.

Alloy designation Composition VM49 Mg base - 9.5% Ti VM46 Mg base - 21% Ti VM47 Mg base - 26.5% Ti VM48 Mg base - 37% Ti VM44 Mg base - 47% Ti AS19 Mg base - 1.5%Ti AS20 Mg base - 2.OTi AS18 Mg base - 7.4%Ti AS21 Mg base - 8.4%Ti AS23 Mg base - 15.3%Ti AS22 Mg base - 22.OTi AS14 Mg base - 27.5%Ti Tensile tests have been performed on specimens prepared from certain of the alloys. The alloy deposit was first removed from the collector then comminuted to particles, canned then hot isostatically pressed and subsequently extruded at 180 degrees Celsius.Tensile properties are as follows: Alloy 0.2% Tensile elongation Reduction proof stress strength (%) in area (MPa) (MPa) (%) AS 21 154 226 25 31 AS 23 212 283 3.4 10 AS 22 243 317 1.6 2.5 Corrosion resistance tests have been performed on various examples of the claimed alloy and also a comparison specimen of pure magnesium prepared by the same vapour quenching route. These materials were all tested in the as-deposited condition. Some other comparison materials were tested also in their normal commercially available form. These additional comparison materials are listed below: WE 43 (nominal compostion: Mg base - 4%Y - 3%Nd + other rare earths - 0.5%or); AZ91E (nominal composition: Mg base - gAl - 0.5%Zr - 0.3%Mn); and EA55RS a rapid solidification route alloy (nominal composition: Mg base - 5%Al 5%Zn - 5%Nd).The corrosion resistance tests comprised a total immersion of standard coupons of the material in 600mM/l sodium chloride solution for a period of 7 days followed by cleaning (in accordance with ASTM G1-81) to remove corrosion products and weighing to determine weight change. A rate of corrosion in terms of weight loss per day was calculated (mgXdm2 per day) and from this a corrosion rate expressed in terms millimetres per year - representative of the rate of corrosion penetration - was derived using the following expression: Corrosion rate = corrosion rate x 0.0365/density of alloy (mm/year) (mg/dm2 per day) Results of the corrosion resistance tests are given below:: Alloy Corrosion rate (mm/year) As 19 0.30 AS 20 0.33 AS 18 weight gain AS 22 0.03 AS 14 weight gain VM 44 0.005 pure magnesium 0.49 WE43 (commercial corrosion 0.42 resistant Mg alloy) AZ91E ( ditto ) 0.242 EA55RS ( ditto ) 0.14 NOTES t weight gain indicates that corrosion products are stable.

2 AZ91E relies for its corrosion resistance upon its high purity and this low value of corrosion rate may not be representative of resistance to corrosion in practical situations where contact with contaminants cannot be avoided.

The other aspect to corrosion resistance is the electrode potential exhibited by the material. This value of electrode potential has been measured for the alloy examples listed below. The values of electrode potential given represent the open circuit potential (relative to a standard saturated calomel electrode) exhibited by the alloys in a 600my/1 solution of sodium chloride after 30 minutes immersion therein.

Material Open-circuit potential pure Mg -1675mV VM49 -1378mV VM46 -1360mV VM47 -1354mV VM44 -1301mV The experimental results documented above indicate that alloys containing titanium even at low levels such as 1.5% produce a useful degree of corrosion resistance compared to both pure magnesium and prior art alloy WE43. At higher levels of titanium, such as 7.4% plus, the alloys demonstrate a marked improvement in corossion resistance compared with the best comparative materials AZ91E and EA55RS. Significant reductions in the high negative electrode potential of pure magnesium are achieved with titanium levels as low as 8 to 11%. The strength and ductility are reasonable for binary magnesium based alloys. Additions of Zn (up to 2Ne) or Al (up to 10;.) or Si (up to 2%) can provide significant strengthening with little adverse effect on predicted corrosion resistance. Also Zr might give a useful grain refinement effect with consequential strengthening or toughening and no significant predicted adverse effect on corrosion resistance. Mn might be expected to yield some increase in corrosion resistance but no significant increase in strength.

Claims (6)

1. A magnesium based alloy produced by vapour quenching which comprises 0.5% to 47 of titanium by weight, and within which the titanium is substantially held in solid solution in the alloy as deposited.

2. A magnesium based alloy as claimed in claim 1 comprising 5% to 40% of titanium by weight.

3. A magnesium based alloy as claimed in claim 1 comprising 15% to 28% of titanium by weight.

4. A magnesium based alloy as claimed in any one of the preceding claims additionally comprising ingredients other than titanium and magnesium from within the group which follows and within the stated individual proportional limits by weight: manganese up to 6%; aluminium up to 13%; zinc up to 7%; zirconium up to 5%: neodymium up to 6%; commercial mixed rare earths up to 5%; yttrium up to 6;; silver up to 3%; thorium up to 5%; lithium up to 10%; and silicon up to 2%.

5. A magnesium based alloy as claimed in claim 3 or claim 4 comprising approximately 5r aluminium.

6. A magnesium based alloy as claimed in claim 3 or claim 4 comprising approximately 1% silicon.