CA1058424A - Aluminium alloys - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An aluminium alloy is provided which consists of 2 to 12% by weight of chromium, 0.2 to 3.0% by weight of iron, the balance being aluminium apart from minor proportions of impurities and incidental elements wherein most of the chromium is present as a metastable solution in the aluminium lattice in the form of fine particles having dimensions of 5 microns or less, and which contains a precipitate phase of iron rich zones the major proportion of which have dimensions of 200 .ANG. or less, the presence of large intermetallic particles, particularly at grain boundaries, being at a minimum. This alloy may be produced by an evaporation deposition process. Alloys according to the present invention have impact strength, fatigue properties, elevated temperature tensile strength, creep resistance and corrosion resistance superior to those of aluminium alloys used heretofore, and are suitable for aerospace applications, e.g. airframes.

Description

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The present invention is concerned with evaporation-condensation alloys, that is a~loys produced by evaporation-condensation processes.
In accordance with the present invention there is provided an evaporation-condensation aluminium alloy consisting of 2 to 12% by weight of chromium, 0.2 to 3.0% by weight of iron, the balance being aluminium apart from minor proportions of impurities and residual elements~wherein most ; of the chromium is present as a mestastable solution in the aluminium Lattice in the form of fine particles having dimensions of 5 microns or less, and which contains a precipitate phase of iron rich zones the major proportion of which have dimensions of 200X or less, and preferably 50 or less, the presence of large intermetallic particles,particularly at grain boundaries, being at a minimum.
Impurities and incidental elements which may be present in alloys of the present invention may include a total of up to about 0.5% by weight of any one of the following'' nickel, cobalt, silicon, copper, zinc, gold, silver, oxygen, magnesium, cadmium, tin, manganese, titanium, molybdenum, carbon and beryllium.
Advantageously aluminium alloys of the present invention consist of 4 to 10% by weight of chromium, 0.3 to 2.0% by weight of iron, the balance being aluminium apart from minor proportions of impurities and incidental elements, and preferably consist of 5 to 9% by welght of chromium ~nd 0.6 to 1.5% by weight of :Lron. In a particularly preferred embodiment of the present invention an aluminium alloy consists of 5 to 8% by weight of chromium, 0.8 to 1.3% by weight of iron, the balance being aluminium apart from minor proportions of impurities and incidental elements, a ma~or part of the chromium content 'being present as a mestast-able solid solution in the aluminium lattice in the form of fine particles having dimensions of 5 microns or less, and having a precipitate phase of iron rich zones a major proportion of which have dimensions of 50 3Q or less. Preferably substantially all of the iron rich zones have dimensions of 50~.

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~0584'Z4 The microstructures which are characteristic of the alloys of the present invention~ which may require appropri~te working treatment to achieve~are not obtainable by conventional melting casting, forging or solution heat treatment and precipitation techniques.
The techniques of producing alloys ~y deposition from the vapour phase is described in UK Patent Specification 1~206,585.
A process of producing an evaporation-condensation alloy of the present invention includes the stepæ of evaporating the consti-tuents of the alloy from a heated source means within a vacuum or low pressure system, depositing the constituents of the alloy upon a temperature controlled collector until a required thickness ; iB deposited and opening the vacuum or low pressure system and removing the deposit from the collector in a condition capable of undergoing metallurgical working.
Alloys in accordan~e with the present invention may have very useful mecharlical properties after suitable working in order to consolidate them. In particular they may be strong and ductile at room temperature and have impact strength~ Young's modulus~
fatigue properties, elevated temperature tensile strength~ creep resistance and corrosion behaviour much better than ~ose of other aluminium alloy~ commonly u~ed heretofore.
It has been found that the microstructure of alloys obtained by evaporation-condensation varies considerably with the temperature at which the collector is controlled. For example the microstructure characteristic of alloys of the present invention i8 obtained when _, -: .
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the collector temperature is controlled at about 260 C. When the collector temperature is maintained at about 370 C an easily worked, high strength alloy may be produced in which the iron and chromiu~
are substantially entirely in the form of a precipitated phase or phases of fine particles, a major proportion of which have dimensions of about 2000 A or less. Such an alloy may have low porosity but may have a tendency to poor corrosion properties. At collector temperatures between about 260 C and 370 C alloys with microstructures intermediate these two types are formed. At collector temperatures below about 260 C~ for example about 170C~
a more porous deposit without the precipitate phase of iron rich zones is obtained but working~ ie pressing and or rolling~ may remove the porosity and heat treatment causes precipitation of the iron rich zones.
It has been discovered that the metastable solution of chromium in the aluminium lattice when originally deposited is in the form of narrow elongated grains having a diameter of 5~m or less. After working, for example hot pressing or hot rolling~these grains can be converted to flat extended plate like grains having a thickness of 5~ or less. Preferably theæe dimen~ion~ of the grains are 1/um or less.

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After suitable working alloys of the present invention may be obtained with a tensile strength of at least 45 tonf/ir.2.
Apparatus suitable for the production Or alloys in accordance with the p~esent in~ention is illustrated in the accompanying dra~inGs i~
which:-Figure 1 is a schematic cross-sectional representation, ~ igure 2 is a perspective view of a controllably heated source means, ~ igure 3 i~ a shaped ingot of fead metal~ and ,~ Figure 4 is a perspective view Or a temperature controllable collector.
Referring now to ~igure 1 which is a schematic representation, a vacuum or low pressure vessel 10 is evacuated by a conventional vacuum pump 11 and a pres~ure gauge 12 is provided to monitor the pressure. A heated source means 13 is provided with metal by a metal supply 14 and a temperature controllable collector 15 is provided upon which metal eraporated from the heated source means 1~ may be deposited.
A removable shuttar 16 is provided operated by a handle 17 outside the vacuum vessel 10. The metal supply means 1~ is preferably provided ~c with a vacuum lock so that it can be charged without breakirg the vacuum in the vacuum vessel 10, and in duplicate so that one may be charged while the other is in operation and continuous operation thus achieved, Any suitable heated source means may be used but prererably the , controllably heated source means disclosed in U.K. Patent Specification 1,440,921 (published June 30, 1976) is us,ed. Figure 2 is a perspective view of one embodiment of a controllably heated source means and is shown empty so that the B

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~OS84Z4 intelnnl structure i~ visible, A meltin~ compartnlent 20 heated by an electron gun 21 connects with a mixin~ compartment 22 throueh a channel 23 which contain~ A partition 24 ~ith a slit 25 thcrein, The mixing co~partment 22 con~ects with a channel 2~ with an evaporation compartment 27 heated by an electron gun 28, The compartments are enclosed in a copper cooling jacket 29 provided with copper pipes 30 ~or the circulation of cold water, the entrance and exit Or which is not sho~n. The material of the compartments is heat and corrosion-resistant ceramic.
Metal may be supplied to the heated source means 13 in the form Or discs obtained by casting a cylindrical ingot, turning it on a lathe and cuttine it into evenly sized discs. One suitable feed mechanism ; is described in U.K. Patent Specification No. 1,434,Q16, puhlished April 28, 1976. However preferably metal is supplied in the form of an ingot as illustrated in Figure 3. Such an ingot is cast in such a manner that the necks 80 solidify first and early in the solidification so that each of the pieces 81 has substantially the nominal composition of the original material. This can be lowered into a melting vessel in such a manner that it melts one piece at a time and is degassed.
Suitable specific temperature controllable collectors 15 are described in UK Patent Specification 1,279,975 and U.K. Patent Specifica-tion No. 1,433,753, published April 28, 1976. ~lowever a preferred form of collector is disclosed in U.K. Patent Specification No. 1,443,144 (published July 21, 1976) and one embodiment is shown in perspective view ' in Figure 4. The collector 15 comprises a thick metal plate 110 having a surface 111 for the deposition of the alloy and on the reverse surface 112 two longitudinal ridges 113. A metal is selected which has a similar coefficient of thermal expansion as the alloy to be deposited. Copper bars 114 are bolted in pairs to the ridges by means of a single bolt 115 per pair.

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A copyer bloc~ 116 i~ hel~ b~tween each ~)Rir Or plat~3 114 by end plates 117 positioned outside th0 copper bars 114 and having bolts 118 passing throu~h the copper block 116. In(lividual copper blocks 116 are hollow, having inlet and outlet pipes ll9 and 120.
The shank of the bolt 115 has a lo~er coerficient of thermal expansion than the material of the ridge 113 so that when the assembly heats up the bars 114 are pulled more tightly on the ridges 113 ensuring efficient thermal contact. Similarly the bolts 118 have a lo~er coefricient of thermal expansion than copper so that efficent thermal contaot i9 ensured between the bars 119 and copper block 116. The collector plate 110 is also provided with a thermocouple 121 by which the temperature Or the surface 111 can be monitored. Heaters 122 to pre-heat the plate 110 are also provided. Leads to thermocouple 121 and heaters 122 are not shown. A safety device is provided at the top of each bar 114 to prevent the collector falling into the source 13 should the bolts 118 looseD for any reason. The sarety device consists Or a washer 124 extending beyond the edges of the bar 114 and held in place by a bolt 123.
In use the thickness Or the bars 114 are preselected accordine to the required operating temperature of the collector. For example, increasing . .
the thicknes~ of the ~ars 114 increases the heat flux whioh they c~rry and thu~ for a given thermal input lowere the temperature of the oollector plate.
110. Similarly the copper blocks 116 can be positioned close to the plate 110 to remove heat more quickly again resulting in a relatively lower plate temperature, During operation Or the apparatus minor adjustments can be made by varying the rate of flo~ and/or temperaturë of tho ooDlir6 rluid, whioh i9 prerersbly wster.

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In a typical deposition the coll~ctor is of aluminium alloy, such as duralumin, and is polished and cleaned before deposition begins.
Cleaning may be carried out by washing with detergent, rinsing, drying and heating to about 250C, or any suitable alternative process. One suitable process is by glo~ discharge cleaning, as disclosed in U.K
Patent Specification No. 1,447,754, published September 2, 1976. The metal charge is als~ washed ; with detergent~ rinsed and dried,- The desired qnantitie~ o~ metal~charge are then loaded into the container or containers Or the heated souroe means, and the reed magazine. The relative concentrations Or aluminium:chromium:
iron in the starting material in the compartment 27 are not of c~urge the same as the nominal concentrations required in the condensed alloy, due to the widely difrering fugacitles of the metals. However the initial concentrations re~uired to produce an alloy Or the present invention may easily be ascertained by those experienced in the art.
- The apparatus is then assembled and the system evacuated, generally to about 1 to 2 x 10 5 torr. The collector 15 is then preheated to the reguired operating temperature, ~or example by heaters 122, and is then maintained as near to this temperature as possible throughout -~
the entire deposition e*periment. The temperature Or the heated source ; means 13 is then raised until the charge is evaporating ~ast~ ~or example by means Or electron gun~ 21 and 28, however the ~hutter 16 is kept in place until splashing o~ the charge has essentially stopped. The shutter 16 is then removed so that deposition on to the collector 15 may take place, extra charge rrom the metal supply 14 being admitted at suitable regular intervals. Deposition is terminated when a desired thickness has been deposited ~ the collector by switching o~ the ; , . ' ` 8_ . . .
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1058~Z4 electron guns~allo~ing the collector to cool and opening the vacuum chamber. The deposit may then be removed from the collector by sr~
suitable means. For thick deposit4 a band-~aw may be used whilst for thin deposits the Qpplication Or a parting agent to the ~urface Or the collector prior to deposition may ~llow easy peeling Or the deposit from the collector.
Alloys o~ the present invention require mechanical working by any suitable working technique in order to consolidate them prior to use.
Advantageously the ~orking temperature should not exceed the temperature at ~hich the collector -~s maintained during deposition. Suitable working techniques to consolidate and thus remove porosity may include pressing and rolling or extrusion and be followed by shaping. Other techniques may include annealing and~or stretching to remove internal stress.
The ~ollowing Examplesdescribe speciPio alloys within the present invention and processes by which they may be produced and are given by way o~ example only, e~cept Example 2 which is of the production of an alloy not ha~ing the desired structure.
E~AMPLE 1 A crucible of the type illustrated in ~igure 2 was used. The following materials were loaded in the melting compartment 20, the mixing compartment 22 and the e~aporation chamber 27 respectively:-945 grams Al 1 ~ Cr ingot 945 grams 99, ~ Al plate 22 706 erams Al 1~k Cr ingot 63 grams Swedish iron 27 1650 grams 99.~ Al plate 480 grams Cr arc-melted buttons 533 ~rams Swedish iron A feed magazine, (the metal supply means 14) was loaded with 148 9 - .

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disc~ of 64 mm diameter, oonsisting alternatively of discs of 9.1-9.5% Cr in Al weighing 74g each, and 99.8% Al discs weighing 53g each. All the oharge was first washed with detergent, rinsed and dried. A
duralumin block collector of the type shown in Figure 4 was placed with its lower surface 360 mm above the evaporation chamber 27 of the crucible, the removable shutter 16 being positioned between. The collector had previously been polished washed and dried.
The vacuum ~hamber containing crucible, collector and feed magazin0 was pumped out to about 2 x 10 torr. Ths collector was then heated to 320 ~ and when the shutter was opened the current in the collsctor heaters was reduced and the collector temperature maintained as near-to 320C as possible, ie from 308C to 323C, during the rest of the e~periment.
; The electron gun 21 was switched on and the beam was focussed on the metal in the melting compartment 20; the accelerating voltage was 18 Kv and the emission current about 300 mA. ~he second electron gun 28 was switched on, and the beam was focussed on to the metal in the evaporating compartment 27 of the crucible; acceler~ting voltage 15.5 Kv, emission : current about 250 mA. The voltages on both guns were kept constant and the emission currents were gradually increased to melt the crucible charge without too much splashing. After about 70 minutes the emission ; current of electron gun 21 had reached 1 amp, and splashing had essentially stopped. ~hree minutes later the shutter was moved away to allow deposition of evaporated metal on the collector lower surfaoe. ~wo minutes after this the mechanical feeding of discs from the feed ; magazine into cruoible chamber 20 was started, one diso being introduoed about every 100 seDonds, until after a further period of 3 hours 50 minutes a total of 139 disos had been introduoed. The deposition was ' '` 10 , . . .

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then termin~ted by switching ofr the electron guns, the collector ~85 allowed to cool, and the vacuum chamb~r was opened. During the feedin~ Or the discs the ernission current Or gun 21 was abo,ut 1.~6 amps and that oP gun 28 was 520-620 mA.
The deposit was removed ~rom the collector with a band-saw. The chromium and iron contents near the central region o~ the deposit were: Gr 4.8~ to 6.5~, ~e 1.~o to 0.~0.
Slabs cut from the deposit were ~orked to 3heet by pressing ~ollowed by rolling, using pressing tem~eratures in the range 20C to 260C
and rolling tem~erature nominally in the range 20C to 230C. For, example one piece was pressed at 20C rrom an initial thickness o~ 0.47 tnch to a thickness o~ o.~6 inch and rolled at 20C down to a thickness o~ 0.052 inch. It had a room temperature tensile strength in tnis oondition o~ 44 ton*/ins2 ~ith an elongation of 5~. Another piece ~as pressed at 250C from an initial thickness o~ 0.75 inch to a thickness o~ 0.30 inch and then rolled to a final thickness o~ 0.057 inch. It had a room temperature tensile strength o~ 43 ton ~irs2 and an elongation , Or ~. The Young's modulus was in both cases about 11~5 x 106 psi.
- A third piece ~as pressed at 200C ~rom a thickness of 0.55 inch to 0.32 inch and then rolled at 200 C and ~elpw to o.o6~ in¢h. It had a tensile strength o~ 43 ton~/ins at room temperature, elongation ~, and a tensils streneth o~ 23 ton~/ins at 300 &, elon~ation 10~o.
A ~ourth pieoe was pressed at 250 C ~rom 0.71 inoh to 0,25 inch and rolled at 230C to 0.058 inch. It had a tensile strength at room temperature oP 45 tonP/ins , elongation l~o~ and a tensile strength at 200C o~ 37 ton~/in, elongation ~0~

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, ~ , ~0~3424 Specimens pressed at 200 to 250 & from 0.55 inch to 0.25 inch and rolled at 230C to o.o6 inch had the following mechanical properties:-~ a) Fat~ (te~ted at fatigu0 cycle p+0,9p where p = stress) (1) Holed te3t piece (elastic stres~ concentration factor ~ = 2.6) ht peak stress (1.gp) of 25000 lbf/in2-Sample unbrokeD after 2.9 x cycle3.
At peak stress o~ 25500 lbf/in2 Sample unbroken after 1 x 10 cycles.
~2) Plain test piece At peak stress of 40,000 lbf/ir. :Sample unbroken after 5.3 x 107 cycles.
Results obtained indicate a ~atigue strength about 3~ greater than standard aluminium aircraft alloys(for example 2024-T3 :an Al-Cu-M~
alloy).
(b) Creep Stres~ for 0.1~ total plastic strain in 100 hours at 251 & = 10 tonf/in2. `
Stress for 0.1~ total plastic strain in 100 hours at 223 C = 15 tonf'~in2 Stress for 0~1~o total pla~tic strain in 100 hours at 183 C = 20 tonf/in .
Stress ~or 0.1~ total plastic strain in 1000 hr~ at 195C ~ 14 tonf/in2.
Results obtained indicate that this alloy has about a factor o~ 2 advantage in stress, and, for a 3tre~s of 20 ton~/in a 70 C aàvantage in temperature over a standard aluminium air¢ra~t alloy(for example CU001-1C~

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c ImPact Impact properties were me~sured using miniature Cha.~y test pieces, 2.8 mm x 2.B nun x 40 mm unnotched (UN) or notched (N) with 45 notch, 0.6 mm deep and 0.15 mm root radiu~.

2esults obtained were UN 5.5 to 6.2 rt lb unbroken - N 0.9 to 2.5 ~t lb unbroken These impact strengths are comparable YJith the titanium alloys rMI

3~a (Ti-6Al-4V) and IblI 685 (Ti-6Al-5~r).

d Corrosion . _ ___ .qaight loss in 5~ aqueous NaCl at 36 C.
ht condensation rate o~ 1.5 ~0.5 mls/hr over a sample area o~ 80 cm2 for 6 week~ exposure the weight loss is less than 0.45 m ~ om , which is similar to that of pure aluminium.
NOT~: The chemical compositio~ o~ each test piece is not known precisely but it is in, or near to the range Or composition given above.

An experirnent was carried out essentially as described in example 1, except that the crucible only had two interconnected chambers - an evsporation chamber and a ~eed chamber. One electron beam played on the metal in the evaporation chamber, and thermal conduction occurred ~rom this chamber into the ~eed chamber su~ricient to melt the ~eed.
The collector temperature was held at ~56 C to 374 & during deposition.
The deposit composition near the central region was: Cr 6~o to a.~; Fe 0. ~ to 1.4%.
Se~eral pieces of the deposit were worked as in exarnple 1. Thus one piece was pressed at 230C ~rom 0.46 inch to 0.14 inch in thickness and then rolled at 210C to 0.054 inch. Its room temperature tensile strength was 43 tons/in , elongation ~o, Young's Modulus 11 x 10 psi.

., , ' Another piece ves rolled l~arm, without prior pressing, from a thickness Or 0.33 inch to 0.0~4 inch. rt had a room te;nperature tensile strength o~ 40 tons/in , elon~ation ~,~, Young's A!odulus 12 x 10 psi.
EX~5'L~ 3 ~ deposit ~as made by the method described in example 1, except that the collector ~las heated initially to 260 5 and held, during deposition, in the temperature range 252 C to 258C. The a~ount o~
metal evanorated was 9.2 kg in about 3 hours 40 minutes. The composition o~ the deposit near the central region v~as: Cr 7.6 to 7.~0; Fe 0.99 to 1.1490. One piece of the deposit was pressed at 260 C to 230 C
~rom a thickness of 0.47 inch to 0.20 inch, and then rolled at 235 C
to 250C down to a thiclcness of o.o63 inch. In this condition the room temperature tensile strength ~las 47 tonr/in Ylith an elongation of a~, and the tensile strength at 200C was 40 ton~/in with an elongation Or ~0.
~XAMPLE 4 This deposit was ~ormed under the same conditions as those given in example 3, except that the feed was introduced as ingot as illustrated in Fig 3 and ~hich was lowered ~rom a vertical stock as contained in an evacuated tower as described in British Patent Specification No. 1,434,016.
The reed stack contained 7.9 XB Or ingots Or composition Al, ~ Cr, 1.5~ Fe.
There were rour len~ths o~ ineot held one below the other by iron wire.
The centre Or the deposit had the follo~;ing composition: Cr 7.5% Fe ~.q~o.
The deposit was cut of~ and small pieces were worked and tested as described in Example 1, and exhibited similar mechanical properties.

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10589~2 EXA~ 5 This deposit was made under the same conditions as those used in exr~mple 4, but a crucible charge and ingot reed stack richer in iron ~ere used in order to obtain a deposit with a higher iron content.
Thus:

meltin{~ cha~ber 2081g Swedish iron 162g arc-melted Cr buttons 3008g 99.~ Al plate mixing chamber 22391g Swedish iron 137g Cr buttons 622g Al plate evaporation chamber 271260e Swedish iron 445g Cr buttons 2000g Al plnte l~e ~eed stack contained 8.2 kg of' Al, 5~c Cr 2.5% Fe ingot, l~e eollector WQS held at 262C to 273C during deposition, The eentral region of' the deposit had the ~omposition 6,~ to 8,5~ Cr, 3.7% to 4.3~ Fe, balanee Al, The deposit was lrrorked and tested as in previous examples, af'ter having be n cut f'rom the colleetor.

This deposit ~qas similar to that given in example 5 but ~qith lower Cr and Pe eontents, The erucible and f'eed eharges were as follows:-20 47g Fe 95B Cr 3020~ Al 22237g Fe 79g Cr 710g Al 27765g ~
256B Cr 2300g Al :

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' 1058~4 T~lc L`ecd stack containo~l f~.5 kg of ingot of compositiorl 3% Cr1.~% ~`c balancc Al.
The composition of the central region of the deposit was 3.1% to 3. ~0 Cr, 1. ~ to 1.5~0 Cr balance Al. The deposit was ~orked and tested as in pre~ious examples after having been cut off the collector.
It should be noted that the materials produced in accordance with the above described Examples suffered from a certain amount of porosity, which resulted in cracks appearing at the edge~ of the sample as these were worked. Such cracked portions were cut off and discarded before further working or use as test samples.

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Claims (13)

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows

1. An evaporation-condensation aluminium alloy which consists of 2 to 12% by weight of chromium, 0.2 to 3.0% by weight of iron, the balance being aluminium apart from minor proportions of impurities and residual elements wherein most of the chromium is present as a metastable solution in the aluminium lattice in the form of fine particles having dimensions of 5 microns or less, and which contains a precipitate phase of iron rich zones the major proportion of which have dimensions of 200.ANG. or less, the presence of large intermetallic particles being at a minimum.

2. An alloy as claimed in claim 1 which consists of 4 to 10% by weight of chromium and 0.3 to 2.0% by weight of iron.

3. An alloy as claimed in claim 1 which consists of 5 to 9% by weight of chromium and 0.6 to 1.5% by weight of iron.

4. An alloy as claimed in claim 1 which consists of 5 to 8% by weight of chromium, 0.8 to 1.3% by weight of iron and in which a major proportion of the iron rich zones have dimensions of 50.ANG. or less.

5. An alloy as claimed in claim 4 wherein substantially all of the iron rich zones have dimensions of 50.ANG. or less.

6. An alloy as claimed in claim 1, 4 or 5 wherein the metastable solution of chromium in the aluminium lattice is in flat extended plate like grains having a thickness of 5µum or less.

7. An alloy as claimed in claim 1 or claim 5 wherein the dimension of the grains is 1 µm or less.

8. An alloy as claimed in claim 1 or claim 5 and which consists of 4.8% to 6.9% by weight of chromoim and 0.8% to 1.0% by weight of iron.

9. An alloy as claimed in claim 1 or claim 5 and which consists of 6.3% to 8.0% by weight chromium and 0.9% to 1.4% by weight iron.

10. An alloy as claimed in claim 1 or claim 5 which consists of 7.6 to 7.8% by weight chromium and 0.9% to 1.14% by weight iron.

11. An alloy as claimed in claim 1 or claim 5 and which consists of 7.5% by weight chromium and 1.6% by weight iron.

12. An alloy as claimed in claim 1 or claim 5 and which consists of 6.8% to 8.5% by weight chromium and 3.7% to 4.3% by weight iron.

13. An alloy as claimed in claim 1 or claim 5 and which consists of 3.1% to 3.9% by weight chromium and 1.20% to 1.58% by weight iron.