Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C23/00—Alloys based on magnesium
  • C22C23/06—Alloys based on magnesium with a rare earth metal as the next major constituent

Definitions

• This invention relates to gadolinium-containing magnesium alloys, particularly those which possess high strength combined with corrosion resistance, and an optimised balance of strength and ductility.
• the described alloys also have exceptional high temperature performance for magnesium alloys.
• the alloys of the present invention have been developed as extrusion alloys, but can be rolled to produce sheets and are also suitable for forging and machining. Although they can be cast successfully to form billets, these alloys are not as suitable to use as shape casting alloys in processes such as die casting or sand casting as other magnesium alloys due to a tendency to form cracks.
• the Russian patent SU1010880 teaches about magnesium alloys containing yttrium and gadolinium, optionally with zirconium.
• the two specific alloys discussed in the patent specification have the mechanical properties summarised in Table 2.
• Alloy Composition Yield Stress UTS Elongation (MPa) (MPa) (%)- 6% Y, 8-10% Gd, 0.3-1. 0% Mn 378-390 393-442 4.4-9.8- 6. 5% Y, 3. 5-5.5% Gd, 0.15-0.7% Zr 353-387 397-436 4.0-6.0
• the Japanese patent JP10147830 teaches that an alloy containing l- ⁇ 6 wt% Gd and 6-12 wt% Y produces good strength at high temperature. Zirconium in an amount of up to 2 wt% can also be present.
• JP9263871 also discusses the addition of Ca and other lanthanides, but we have found that the addition of Ca and certain lanthanides is very deleterious to these types of alloys.
• the Chinese patent CN1676646 purports to teach that a broad range of alloys containing 1-6 wt% Y, 6-15wt% Gd, 0.35-0.8 wt% Zr and 0-1.5 wt% Ca can be extruded to produce extrudates of good strength, but there is little specific description of the alloys of the Examples and no clear demonstration of the utility of the described alloys near the limits of the claimed range.
• the alloys of the present invention will generally have corrosion rates of less than 100 mils per year (mpy) in the industry standard ASTM B117 salt -fog test, and preferably less than 50 mpy. Since the above prior art does not mention the corrosion performance of the described alloys and so it can be assumed that this feature of the described alloys was in line with conventional alloys, i.e. inferior to that of the alloys of the present invention and greater than a corrosion rate of 50 mpy.
• the amount of zinc is such that the ratio of the weight of zinc to the weight of zirconium is preferably less than 2:1, and more preferably less than 0.75:1,
• lanthanides viz. lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium and ytterbium, in an aggregate amount of less than at 0.2 at%, and preferably less than 0.1 at%,
• the balance being magnesium, with any other element being present in an amount of no more than 0.2 at%, preferably no more than 0.1 at%, and more preferably being present only as an incidental impurity.
• soluble heavy lanthanides are defined as elements with atomic numbers 65 to 69 inclusive and 71.
• Soluble heavy lanthanides are those which display substantial solid solubility in magnesium. They are terbium, dysprosium, holmium, erbium, thulium and lutetium. These elements are characterised by all of them having the same hexagonal close packed metallic structure as possessed by yttrium and magnesium, and by having a metallic radius of between 0.178nm and 0.173nm. They also exist only in a trivalent state when oxidised, which thus distinguishes them from elements such as europium and ytterbium which show both tri- and bivalency and do not show any appreciable solid solubility in magnesium. When present the aggregate level of soluble heavy lanthanides should be greater than 0.1 at% in order ot contribute significantly to the mechanical properties of the alloy.
• a particularly preferred soluble heavy lanthanide is erbium.
• the ratio is between 1.25:1 and 1.75:1 for alloys which contain from 2.3 to 4.6 at% in total of gadolinium and at least one of soluble heavy lanthanide or yttrium. Outside this range either the strength and/or the ductility of the alloys declines. This decline becomes noticeable when the total amount of gadolinium, soluble heavy lanthanide and yttrium is below 2.0 at% and above 5.0 at%.
• a grain refining element can be added in an amount up to its solid solubility limit in the alloy.
• a preferred such element is zirconium. This can be added with increasing amounts generally improving the alloy's yield stress and elongation-to-failure properties. For such an effect at least 0.03 atomic per cent of zirconium should be present, and the maximum amount is the solid solubility limit of Zr in the alloy which is generally at about 0.3 atomic percent . However with both high and low levels of zirconium corrosion resistance may decline.
• the most preferred composition for a zirconium containing alloy of the present invention is 5.5 to 6.5 wt% Y, 6.5 to 7.5 wt% Gd and 0.2 to 0.4 wt% Zr, with the remainder being magnesium and incidental impurities.
• the level of zirconium should be from 0.3 to below 0.35% by weight in order to pass the 50 mpy salt-fog test. It has been found that the presence of small amounts of zinc are beneficial to the corrosion performance of the alloys of the present invention, but that as the level of zinc is increased the alloy's corrosion performance deteriorates.
• the level of zinc should be from 0.07 to below 0.5at%.
• the ratio of zinc to zirconium should not exceed 2:1, and should be preferably less than 0.75:1.
• Any lanthanide other than the required soluble heavy lanthanide or yttrium should be present in a total amount of less than 0.2 atomic per cent, and preferably below 0.1 at%, otherwise there is interference with the formation of the desired at least one indeterminate ternary phase as described above.
• any other element should be present in an amount of no more than 0.2 at%, preferably no more than 0.1 at%, and more preferably be present only at an incidental impurity level .
• the alloys of the present invention may be used for extrusions, sheet, plate and forgings . Additionally they may be used for parts machined and/or manufactured from extrusions, sheet, plate or forgings .
• a magnesium alloy DF8791 was produced containing 3.04 at % in total of yttrium and gadolinium, where the yttrium to gadolinium ratio was 1.52:1. Additionally it contained 0.15 at% zirconium, with other elements being at impurity levels.
• Another magnesium alloy, DF8961 was produced containing 2.65 at% in total of yttrium and gadolinium, with an yttrium to gadolinium ratio of 1.46:1. Additionally, it contained 0.12 at% Zr and 0.08 at% Zn, with other elements being at impurity levels.
• Another magnesium alloy DF9380 was produced containing a a 3.03 at% of a mixture of erbium, gadolinium and yttrium with a soluble rare earth plus yttrium to gadolinium ratio of 1.38:1. Additionally it contained 0.125 at% zirconium.
• All these alloys possessed yield stresses greater than 300MPa and elongations-to-failure greater than or equal to 10%.
• DF8915 had a significantly higher ratio of 3.9:1 and this produced a reduced yield stress of only 250MPa.
• DF9386 and DF8758 both had a significantly lower ratio of 0.72:1 and 0.93:1 respectively. These low ratios had the effect of reducing the ductility of these alloys to below 5% to levels that are commercially unacceptable for this type of product.
• a further alloy magnesium alloy DF9381 was produced containing 2.99 at% of a mixture of ytterbium, gadolinium and yttrium with a soluble rare earth plus yttrium to gadolinium ratio of 1.39:1. Additionally it contained 0.121 at% zirconium.
• the ytterbium in this alloy is not a soluble heavy lanthanide, and as a result of its addition to the alloy the strength of the alloy was reduced to unacceptably low levels.
• a further set of test alloys were produced to examine the effect of zirconium on corrosion for the alloys of the present invention.
• Melts DF9382a to DF9382e all had the same composition except for varying levels of zirconium. Alloy DF9382a shows that if the material is zirconium free (i.e. below detectable limits with standard industrial spark emission spectroscopy) the corrosion rate is above the acceptable level of 50 mils per year corrosion in the standard salt fog test. Further, at higher levels of zirconium for this alloy, DF9382b and DF9382c also show this poor behaviour. However at levels of zirconium between 0.03 at % (0.1 wt %) and 0.12 at % (0.4 wt%) good corrosion performance is achieved. This is demonstrated by DF9382d and DF9382e.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Forging (AREA)
• Powder Metallurgy (AREA)

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• 2006
  • 2006-09-13 GB GBGB0617970.9A patent/GB0617970D0/en not_active Ceased
• 2007
  • 2007-09-12 JP JP2009527892A patent/JP5309031B2/ja not_active Expired - Fee Related
  • 2007-09-12 CN CN2007800336858A patent/CN101512029B/zh not_active Expired - Fee Related
  • 2007-09-12 WO PCT/GB2007/003491 patent/WO2008032087A2/en not_active Ceased
  • 2007-09-12 EP EP07804280A patent/EP2074236B1/en not_active Not-in-force
  • 2007-09-12 RU RU2009113576/02A patent/RU2450068C2/ru active
  • 2007-09-12 KR KR1020097007372A patent/KR101350126B1/ko not_active Expired - Fee Related
  • 2007-09-12 BR BRPI0716895-0A patent/BRPI0716895A2/pt not_active IP Right Cessation
  • 2007-09-12 CA CA2663605A patent/CA2663605C/en not_active Expired - Fee Related
  • 2007-09-13 TW TW096134268A patent/TWI426137B/zh not_active IP Right Cessation
• 2009
  • 2009-03-04 IL IL197400A patent/IL197400A/en active IP Right Grant
  • 2009-03-12 US US12/402,918 patent/US20090175754A1/en not_active Abandoned

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