Rapidly Solidified Magnesium Base Alloy Sheet

This invention relates to a sheet product of magnesium base metal alloy made by rapid solidification of the alloy, to achieve good mechanical properties.

Magnesium alloys are considered attractive candidates for structural use in aerospace and automotive industries because of their light weight, high strength to weight ratio, and high specific stiffness at both room and elevated temperatures.

The application of rapid solidification processing (RSP) in metallic systems results in the refinement of grain size and intermetallic particle size, extended solid solubility, and improved chemical homogeneity. By selecting the thermally stable intermetallic compound (Mg.sub.2 Si) to pin the grain boundary during consolidation, a significant improvement in the mechanical strength [0.2% yield strength (Y.S.) up to 393 MPa, ultimate tensile strength (UTS) up to 448 MPa, elongation (El.) up to 9%] can be achieved in RSP Mg-Al-Zn-Si alloys, [S.K. Das et al., U.S. Pat. No. 4,675,157, High Strength Rapidly Solidified Magnesium Base Metal Alloys, June, 1987]. The addition of rare earth elements (Y, Nd, Pr, Ce) to Mg-Al-Zn alloys further improves corrosion resistance (11 mdd when immersed in 3% NaCl aqueous solution for 3.4.times.10.sup.5 sec. at 27.degree. C.) and mechanical properties (Y.S up to 435 MPa, UTS up to 476 MPa, El. up to 14%) of magnesium alloys, [S.K. Das et al., U.S. Pat. No. 4,765,954, Rapidly Solidified High Strength Corrosion Resistance Magnesium Base Metal Alloys, August, 1988].

The alloys are subjected to rapid solidification processing by using a melt spin casting method wherein the liquid alloy is cooled at a rate of 10.sup.5 to 10.sup.7 .degree. C./sec while being solidified into a ribbon. That process further comprises the provision of a means to protect the melt puddle from burning, excessive oxidation and physical disturbance by the air boundary layer carried with the moving substrate. The protection is provided by a shrouding apparatus which serves the dual purpose of containing a protective gas such as a mixture of air or CO.sub.2 and SF.sub.6, a reducing gas such as CO or an inert gas, around the nozzle while excluding extraneous wind currents which may disturb the melt puddle.

The as cast ribbon is typically 25 to 100 .mu.m thick. The rapidly solidified ribbons are sufficiently brittle to permit them to be mechanically comminuted by conventional apparatus, such as a ball mill, knife mill, hammer mill, pulverizer, fluid energy mill. The comminuted powders are either vacuum hot pressed to about 95% dense cylindrical billets or directly canned to similar size. The billets or cans are then hot extruded to round or rectangular bars at an extrusion ratio ranging from 14:1 to 22:1.

Magnesium alloys, like other alloys with hexagonal crystal structures, are much more workable at elevated temperatures than at room temperature. The basic deformation mechanisms in magnesium at room temperature involve both slip on the basal planes along <1,1,2,0> directions and twinning in planes (1,0,1,2) and <1,0,-1,1> directions. At higher temperatures (>225.degree. C.), pyramidal slip (1,0,-1,1) <1,1,2,0> becomes operative. The limited number of slip systems in the hcp magnesium presents plastic deformation conformity problems during working of a polycrystalline material. This results in cracking unless substantial crystalline rotations of grain boundary deformations are able to occur. For the fabrication of formed magnesium alloy parts, the temperature range between the minimum temperature to avoid cracking and a maximum temperature to avoid alloy softening is quite narrow.

Rolling of metals is the most important metal-working process. More than 90% of all the steel, aluminum, and copper produced go through the rolling process at least one time. Thus, rolled products represent a significant portion of the manufacturing economy and can be found in many sectors. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of metals at very high speeds in a continuous manner. The primary objectives of the rolling process are to reduce the cross section of the incoming material while improving its properties and to obtain the desired section at the exit from the rolls. The main variables which control the rolling process are (1) the roll diameter, (2) the deformation resistance of the metal, (3) the friction between the rolls and the metal, and (4) the presence of front tension and back tension. The friction between the roll and the metal surface is of great importance in rolling. Not only does the friction force pull the metal into the rolls, but it also affects the magnitude and distribution of the roll pressure. The minimum thickness sheet that can be rolled on a given mill is directly related to the coefficient of friction. By far the largest amount of rolled material falls under the general category of ferrous metals, including carbon and alloy steels, stainless steels, and specifically steels. Nonferrous metals, including aluminum alloys, copper alloys, titanium alloys, and nickel base alloys also are processed by rolling. Rolled magnesium alloy products include flat sheet and plate, coiled sheet, circles, tooling plate and tread plate. The commercially available rolled magnesium alloy sheets include AZ31B, HK31A, HM21A. AZ31B is a wrought magnesium base alloy containing aluminum and zinc. This alloy is most widely used for sheet and plate and is available in several grades and tempers. It can be used at temperatures up to 100.degree. C. Increased strength is obtained in the sheet form by strain hardening with a subsequent partial anneal (H24 and H26 temper). HK31A is a magnesium base alloy containing thorium and zirconium. It has relatively high strength in the temperature up to 315.degree. C. Increased strength is obtained in sheet by strain hardening with a subsequent partial anneal (H24 temper). HM21A is a magnesium base alloy containing thorium and manganese. It is available in the form of sheet and plate usually in the solution heat-treated, cold-worked, and artificially aged (T8) and (T81) tempers. It has superior strength and creep resistance and can be used up to 345.degree. C. Good formability is an important requirement for most sheet materials.

Work on metalworking of formed magnesium parts made from rapidly solidified magnesium alloys is relatively rare. Busk & Leontis [R.S. Busk and T.I. Leontis, "The Extrusion of Powdered Magnesium Alloys", Trans. AIME. 188 (2) (1950), pp. 297-306.] investigated hot extrusion of atomized powder of a number of commercial magnesium alloys in the temperature range of 316.degree. C. (600.degree. F.)-427.degree. C. (800.degree. F.). The as-extruded properties of alloys extruded from powder were not significantly different from the properties of extrusions from permanent mold billets.

In the study reported by Isserow and Rizzitano [S. Isserow and F.J. Rizzitano, "Microquenched Magnesium ZK60A Alloy", Int'l. J. of Powder Met. & Powder Tech., 10, (3) (1974), pp. 217-227.] on commercial ZK60A magnesium alloy powder made by a rotating electrode process, extrusion temperatures varying from ambient to 371.degree. C. (700.degree. F.) were used. The mechanical properties of the room temperature extrusions were significantly better than those obtained by Busk and Leontis, but those extrude at 121.degree. C. (250.degree. F.) did not show any significant difference between the conventionally processed and rapidly solidified material. However, care must be exercised in comparing their mechanical properties in the longitudinal direction from room temperature extrusions since they observed significant delamination on the fracture surfaces; and properties may be highly inferior in the transverse direction.

U.S. Pat. No. 4,938,809 to Das et al. entitled "Superplastic Forming of Rapidly Solidified Magnesium Base Metal Alloys", discloses a method of superplastic forming of rapidly solidified magnesium base metal alloys extrusion to a complex part, to achieve a combination of good formability to complex net shapes and good mechanical properties of the articles. The superplastic forming allows deformation to near net shape.

There remains a need in the art for a method of rolling magnesium alloy rolling stock extruded or forged from a billet consolidated from powders made by rapid solidification of the alloy and the sheet product to achieve good mechanical properties.

The present invention provides a method of rolling magnesium base alloy sheet from rolling stock extruded or forged from a billet consolidated from powders made by rapid solidification of the alloy. Generally stated, the alloy has a composition consisting of the formula Mg.sub.bal Al.sub.a Zn.sub.b X.sub.c, wherein X is at least one element selected from the group consisting of manganese, cerium, neodymium, praseodymium, and yttrium, "a" ranges from about 0 to 15 atom percent, "b" ranges from about 0 to 4 atom percent, "c" ranges from about 0.2 to 3 atom percent, the balance being magnesium and incidental impurities, with the proviso that the sum of aluminum and zinc present ranges from about 2 to 15 atom percent.

The magnesium alloys used in the present invention are subjected to rapid solidification processing by using a melt spin casting method wherein the liquid alloy is cooled at a rate of 10.sup.5 to 10.sup.7 .degree. C./sec while being formed into a solid ribbon. That process further comprises the provision of a means to protect the melt puddle from burning, excessive oxidation and physical disturbance by the air boundary layer carried with the moving substrate. Said protection is provided by a shrouding apparatus which serves the dual purpose of containing a protective gas such as a mixture of air or CO.sub.2 and SF.sub.6, a reducing gas such as CO or an inert gas, around the nozzle while excluding extraneous wind current which may disturb the melt puddle.

The alloy elements manganese, cerium, neodymium, praseodymium, and yttrium, upon rapid solidification processing, form a fine uniform dispersion of intermetallic phase such as Mg.sub.3 Ce, Al.sub.2 (Nd, Zn), Mg.sub.3 Pr, Al.sub.2 Y, depending on the alloy composition. These finely dispersed intermetallic phases increase the strength of the alloy and help to maintain a fine grain size by pinning the grain boundaries during consolidation of the powder at elevated temperature. The addition of the alloying elements, such as: aluminum and zinc, contributes to strength via matrix solid solution strengthening and by formation of certain age hardening precipitates such as Mg.sub.17 Al.sub.12 and MgZn.

The sheet of the present invention is produced from rolling stock extruded or forged from a billet made by compacting powder particles of the magnesium base alloy. The powder particles can be hot pressed by heating in a vacuum to a pressing temperature ranging from 150.degree. C. to 275.degree. C., which minimizes coarsening of the dispersed, intermetallic phases, to form a billet. The billet can be extruded or forged at temperatures ranging from 200.degree. C. to 300.degree. C. The extrusion ratio ranges from 12:1 to 20:1. The extrusion or forging has a grain size of 0.2-0.3 .mu.m, dispersoid size of 0.01-0.04 .mu.m. The extrusion or forging can be rolled to 0.020" thick sheet by pre-heating the rolling stock to a temperature ranging from 200.degree. C. to 300.degree. C. Rolling is carried out at a rate ranging from 25 to 100 rpm. During rolling the roll gaps are adjusted to produce a thickness reduction of 2 to 25% per pass. The rolling process is repeated one or more times under the above conditions until the sheet thickness required is obtained. The sheet of the present invention has a strong (0001) texture, with subgrain size of 0.1-0.2 .mu.m, dispersoid size of 0.02-0.04 .mu.m, and network of dislocation.

The sheet of the present invention possesses good mechanical properties: high ultimate tensile strength (UTS) [up to 449 MPa (65 ksi)] and good ductility (i.e., >5% tensile elongation) along the rolling direction at room temperature. These properties are far superior to those of commercially available rolled magnesium sheets. The sheets are suitable for applications as structural components such as heat rejection fins, cover, clamshell doors, tail cone, skin in helicopters, rocket and missiles, spacecraft and air frames where good corrosion resistance in combination with high strength and ductility are important.

The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description and the accompanying drawings, in which:

FIG. 1 is a macrograph of a 0.02" thick rolled sheet of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1.

FIG. 2a and FIG. 2b are optical micrographs of rolled sheet of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 at a low and high magnification.

FIG. 3 is a dark field transmission electron micrograph of a sheet of Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 rolled at 300.degree. C., illustrating the formation of dislocation network within subgrains due to plastic deformation.

FIG. 4 is a scanning electron micrograph of sheet of Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 rolled at 300.degree. C., illustrating the intragranular subgrain structure as a result of dynamic recovery.

FIG. 5 is a bright field transmission electron micrograph of extrusion of Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1, illustrating the absence of dislocations.

In accordance with the present invention a sheet is produced from a rolling stock extruded or forged from a billet consolidated from rapidly solidified alloy powders. The alloy consists essentially of nominally pure magnesium alloyed with about 0 to 15 atom percent aluminum, about 0 to 4 atom percent zinc, about 0.2 to 3 atom percent of at least one element selected from the group consisting of manganese, cerium, neodymium, praseodymium and yttrium, the balance being magnesium and incidental impurities, with the proviso that the sum of aluminum and zinc present ranges from about 2 to 15 atom percent. The alloy is melted in a protective environment, and quenched in a protective environment at a rate of at least about 105.degree. C./sec by directing the melt into contact with a rapidly moving chilled surface to form thereby a rapidly solidified ribbon. Such alloy ribbons have high strength and high hardness (i.e., microVickers hardness of about 125 kg/mm.sup.2). When aluminum is alloyed without addition of zinc, the minimum aluminum content is preferably above about 6 atom percent.

The alloy has a uniform microstructure comprised of a fine grain size ranging from 0.2-1.0 .mu.m together with precipitates of magnesium and aluminum containing intermetallic phases of a size less than 0.1 .mu.m. The mechanical properties [e.g. 0.2% yield strength (YS) and ultimate tensile strength (UTS)] of the alloys of this invention are substantially improved when the precipitates of the intermetallic phases have an average size of less than 0.1 .mu.m, and even more preferably an average size ranging from about 0.03 to 0.07 .mu.m. The presence of intermetallic phases precipitates having an average size less than 0.1 .mu.m pins the grain boundaries during consolidation of the powder at elevated temperature with the result that a fine grain size is substantially maintained during high temperature consolidation and secondary fabrication.

The as cast ribbon is typically 25 to 100 .mu.m thick. The rapidly solidified materials of the above described compositions are sufficiently brittle to permit them to be mechanically comminuted by conventional apparatus, such as a ball mill, knife mill, hammer mill, pulverizer, fluid energy mill, or the like. Depending on the degree of pulverization to which the ribbons are subjected, different particle sizes are obtained. Usually the powder comprises of platelets having an average thickness of less than 100 .mu.m. These platelets are characterized by irregular shapes resulting from fracture of he ribbon during comminution.

The powder can be consolidated into fully dense bulk parts by known techniques such as hot isostatic pressing, hot rolling, hot extrusion, hot forging, cold pressing followed by sintering, etc. Typically, the comminuted powders of the alloys of the present invention are vacuum hot pressed to cylindrical billets with diameters ranging from 50 mm to 279 mm and length ranging from 50 mm to 300 mm. The billets are preheated and extruded or forged at a temperature ranging from 200.degree. C. to 300.degree. C. at a rate ranging from 0.00021 m/sec to 0.00001 m/sec.

The microstructure obtained after consolidation depends upon the composition of the alloy and the consolidation conditions. Excessive times at high temperatures can cause the fine precipitates to coarsen beyond the optimal submicron size, leading to a deterioration of the properties, i.e. a decrease in hardness and strength. The alloys of the extrusion or forging, from which the sheet of the invention rolled, have a very fine microstructure, which is not resolved by optical micrograph. Transmission electron micrograph reveals a uniform solid solution phase ranging from 0.2-1.0 .mu.m in size, together with precipitates of very fine, binary or ternary intermetallic phases which are less than 0.1 .mu.m and composed of magnesium and other elements added in accordance with the invention. At room temperature (about 20.degree. C.), the extrusion or forging of the invention has a Rockwell B hardness of at least about 55 and is more typically higher than 65. Additionally, the ultimate tensile strength of the extrusion or forging of the invention is at least about 378 MPa (55 ksi).

Samples cut from the extrusions or forgings can be rolled using conventional rolling mills, for example: two-high mill with 5" diameter steel rolls, at temperatures ranging from 200.degree. C. to 300.degree. C. with intermediate annealing at temperatures the same as roll temperature. The roll speed ranges from 25 rpm to 100 rpm. The reduction of thickness in the sample in each pass ranges from about 2 to 25%; and preferably from about 4 to 10%. The rolling process is repeated at least once and, typically, from 5 to 20 or more times until the desired sheet thickness is achieved. At room temperature (about 20.degree. C.), the sheet (0.016" thickness) of the invention has a yield strength of 455 MPa (66 ksi), ultimate tensile strength of 483 MPa (70 ksi) and elongation of 5% along the rolling direction, which are superior to those of commercially available rolled magnesium alloy sheet. The sheet of the present invention has a strong (0001) texture, with subgrain size of 0.1-0.2 .mu.m, dispersoid size of 0.02-0.04 .mu.m, and network of dislocation. The sheets are suitable for applications as structural components such as heat rejection fins, cover, clamshell doors, tail cone, skin in helicopters, rocket and missiles, spacecraft and air frames where good corrosion resistance in combination with high strength and ductility is important.

The following examples are presented in order to provide a more complete understanding of the invention. The specific techniques, conditions, materials and reported data set forth to illustrate the invention are exemplary and should not be construed as limiting the scope of the invention.

Ribbon samples were cast in accordance with the procedure described above by using an over pressure of argon or helium to force molten magnesium alloy through the nozzle onto a water cooled copper alloy wheel rotated to produce surface speeds of between about 900 m/min and 1500 m/min. Ribbons were 0.5-2.5 cm wide and varied from about 25 to 100 .mu.m thick.

The nominal compositions of the alloys based on the charge weight added to the melt are summarized in Table 1 together with their as-cast hardness values. The hardness values are measured on the ribbon surface which is facing the chilled substrate; this surface being usually smoother than the other surface. The microhardness of these Mg-Al-Zn-X alloys of the present invention ranges from 140 to 200 kg/mm.sup.2. The as-cast hardness increases as the rare earth content increases. The hardening effect of the various rare earth elements on Mg-Al-Zn-X alloys is comparable. For comparison, also listed in Table 1 is the hardness of a commercial corrosion resistant high purity magnesium AZ91D alloy. It can be seen that the hardness of the present invention is higher than commercial AZ91D alloy. The alloy has a uniform microstructure comprised of a fine grain size ranging from 0.2-1.0 .mu.m together with precipitates of magnesium and aluminum containing intermetallic phases of a size less than 0.1 .mu.m.

Rapidly solidified ribbons were subjected first to knife milling and then to hammer milling to produce -40 mesh powders. The powders were vacuum outgassed and hot pressed at 200.degree. C. to 275.degree. C. The compacts were extruded at temperatures of about 200.degree. C.-300.degree. C. at extrusion ratios ranging from 12:1 to 22:1. The compacts were soaked at the extrusion temperatures for about 20 mins. to 4 hrs. Tensile samples were machined from the extruded bulk compacted bars and tensile properties were measured in uniaxial tension at a strain rate of about 5.5.times.10.sup.-4 /sec at room temperature. The tensile properties together with Rockwell B (R.sub.B) hardness measured at room temperature are summarized in Table 2. The alloys show high hardness ranging from 65 to about 81 R.sub.B.

Most commercial magnesium alloys have a hardness of about 50 R.sub.B. The density of the bulk compacted samples measured by conventional Archimedes technique is also listed in Table 2.

Both the yield strength (YS) and ultimate tensile strength (UTS) of the present alloys are exceptionally high. For example, the alloy Mg.sub.91 Zn.sub.2 Al.sub.5 Y.sub.2 has a yield strength of 66.2 ksi and UTS of 74.4 ksi which is similar to that of conventional aluminum alloys such as 7075, and approaches the strength of some commercial low density aluminum-lithium alloys. The density of the magnesium alloys is only 1.93 g/c.c. as compared with the density of 2.75 g/c.c. for conventional aluminum alloys and 2.49 g/c.c. for some of the advanced low density aluminum-lithium alloys now being considered for aerospace applications. Thus, on a specific strength (strength/density) basis the magnesium base alloys provide a distinct advantage in aerospace applications. In some of the alloys ductility is quite good and suitable for engineering applications. For example, Mg.sub.91 Zn.sub.2 Al.sub.5 Y.sub.2 has a yield strength of 66.2 ksi, UTS of 74.4 ksi, and elongation of 5.0%, which is superior to the commercial wrought alloy ZK60A, and casting alloy AZ91D, when combined strength and ductility is considered. The magnesium base alloys find use in military applications such as sabots for armor piercing devices, and air frames where high strength is required.

Samples cut from the extrusions were cross rolled using two-high mill with 5" diameter rolls at temperatures ranging from 200.degree. C. to 300.degree. C. with intermediate annealing at temperatures the same as roll temperature. The roll speed ranges from 25 rpm to 100 rpm. The reduction of thickness in the sample in each pass is about 0.01". FIG. 1 shows a macrograph of rolled sheets of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 with thicknesses of 0.02". Tensile samples were machined from the sheet and tensile properties were measured in uniaxial tension along the sheet rolling direction at a strain rate of about 5.5.times.10.sup.-4 /sec at room temperature. The tensile properties measured at room temperature along with their hardness are summarized in Table 3. At room temperature (about 20.degree. C.), 0.016" thick sheet of Mg.sub.92 Zn.sub.2 Al.sub.15 Nd.sub.1 has a yield strength of 455 MPa (66 ksi), ultimate tensile strength of 483 MPa (70 ksi) and elongation of 5% along the rolling direction; 0.095" thick sheet of Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 has a yield strength of 490 MPa (71 ksi), ultimate tensile strength of 490 MPa (71 ksi) and elongation of 6%, which are superior to those of commercially available rolled magnesium alloy sheet.

The microstructure of rolled sheet of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 was examined by optical micrography using conventional metallographic technique. FIG. 2a and FIG. 2b shows distorted or fibered powder particular structure in rolled sheet, which is a microstructure resulting from plastic deformation at elevated temperature. The grain structure of sheet is very fine and can not be resolved by optical metallography. The rolled sheet and extrusion were prepared for transmission electron microscopy (TEM) by ion milling. FIG. 3 shows a dark field transmission electron micrograph of sheet rolled at 300.degree. C., illustrating the development of an intragranular subgrain structure due to dynamic recovery. In this structure, tangled and network of dislocations formed within the subgrain with the grain size about 0.1-0.2 .mu.m, dispersoid size of 0.02-0.04 .mu.m. FIG. 4 is a scanning electron micrograph, also illustrating the subgrain structure. As a comparison, FIG. 5 shows a bright field transmission electron micrograph of extrusion, which has a grain size of 0.2-0.3 .mu.m, dispersoid size of 0.01-0.04 .mu.m, with absence of dislocation.

The process of rolling can be described in simple terms as a compression perpendicular to the rolling plane and a tension in the rolling direction. In simple slip, the compression will rotate the active slip plane such that its normal moves toward the stress axis. Like other close-packed hexagonal metals, the most closely packed plane in magnesium is the (0001) basal plane and the close-packed directions are <1,1,-2,0>. The slip is most likely to occur on the basal plane in the <1,1,-2,0> direction.

The texture development of the sheet product (0.016" thick) of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 rolled at temperatures ranging from 200.degree. C. to 300.degree. C. was investigated using X-ray diffraction (XRD) with Cu K.alpha. radiation at 40 kV and 30 mA. Table 4 shows the formation of a strong (0001) texture normal to the rolled sheet (i.e. basal plane parallel with the rolling plane) with intensity about 10 times of the intensity of the extrusion of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 during hot rolling. The preferred orientation resulting from plastic deformation is strongly dependent on the slip and twinning systems available for deformation, but it is not affected by processing variables such as roll diameter, roll speed, and reduction per pass. The formation of texture results in an increase in strength and a decrease in ductility. The low ductility of rolled sheet can be improved by annealing.

Tensile samples were machined from sheet alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 and annealed at temperatures ranging from 325.degree. C. to 350.degree. C. for 2 hours and then quenched in water. Tensile properties were measured in uniaxial tension along the sheet rolling direction at a strain rate of about 5.5.times.10.sup.-4 /sec at room temperature. The tensile properties measured at room temperature are summarized in Table 5. At room temperature (about 20.degree. C.), 0.075" thick sheet of alloy Mg.sub.92 Zn.sub.2 Al.sub.5 Nd.sub.1 has a yield strength of 304 MPa (44 ksi), ultimate tensile strength of 407 MPa (59 ksi) and elongation of 14% along the rolling direction; which are superior to those of commercially available rolled magnesium alloy sheet. The sheets are suitable for applications as structural components such as fins, cover, clamshell doors, tail cone, skin in helicopters, rocket and missiles, spacecraft and air frames where good corrosion resistance in combination with high strength and ductility is important.