US3454435A - Method of treating precipitation hardened alloys - Google Patents

Description

July 8', 1969 l A. l.1. JAcoBs 3,454,435

METHOD OFl ITREATING PRECIPITATION HARDENED ALLOYS Filed April 18. 1966 United States Patent O 3,454,435 METHOD OF TREATING PRECIPITATION HARDENED ALLOYS Alvin J.. Jacobs, Canoga Park, Calif., assignor to North American Rockwell Corporation, a corporation of Delaware Filed Apr. 18, 1966, Ser. No. 543,094

Int. Cl. C21c l/26, 1/18; C22f 1/04 U.s. ci. 14s-12.7 7 claims ABSTRACT OF THE DISCLOSURE This invention concerns a method of improving the strength and stress corrosion resistance of precipitation hardened alloys by introducing dislocations into said hardened alloy by a deformation process such as explosive shock loading, forging, rolling or extrusion.

1958, Public Law 85-568 72 st aS amended. at' 426 42 USC 2451),

One of the properties common to precipitation hardened alloys is their capacity to be hardened b tation reaction from a solid solution. Precipitsationiicaiirden ed alloys particularly include a large number of aluminum alloys and thus the description of this invention will be with reference mainly to the aluminum alloys The precipitation reaction takes place during the aging of the s olid solution, which is in a supersaturated state after being cooled rapidly 4from an elevated temperature The precipitation aluminum alloys have nominal compositions of .20 to 1.2 weight percent Si, .30 to 1.0 weight percent Fe, .40 to 6.5 weight percent Cu, .10 to 1.2 weight percent Mn, .002 to 3.7 Weight percent Mg, up to 40 Weight percent Cr, .l to 6.1 weight percent Zn a maxirifurithof .t20 weilght percent Ti, and a maximuin of .15

er race eements mainder being aluminum.such as beryllium With the re- The principles of the invention will be b s tood with relation to the description of the 703785 geil;- itation hardened aluminum alloy. This alloy has a nominal composition of a maximum of .50 weight percent Si a maximum of V.70 Weight percent Fe, 1.2 to 2.0 weight percent Cu, a maximum of .30 Weight percent Mn, 2.1

8 to .40 weight percent Cr,

to 2.9 weight percent Mg, .l

n, a maximum of .20 weight 5.1 to 6.11 weight percent Z percent i, and a maximum of .15 o 'i' such as beryllium, with the remaindroltnldirligtraalieniiinnms If maximum hardening is to be achieved, the temperatures and d urations of solution annealing and aging become peculiar to the 7075 alloy and to each of the other precipitation hardened alloys. For Example for what is known as the 7075 alloy, the solution annealing and aging temperatures are 870 F. to 890 F. and 240 F. for T-6 temper to 260 F., respectively. The corresponding times for the annealing and aging are ten minutes to one hour, and 23 to 28 hours respectively. When the heat treatment is carried out under the above conditions, the 707,5 alloy attains the maximum strength and hardness that it is capable of attaining as a result 0f heat treatment alone and is known as a T-6 temper. At the Same time, however, this particular `alloy having the maximum strength is highly susceptible to stress-corrosion crack- Patented July 8, 1969 ICC ing. This problem of stress corrosion was recognized in the art and an overage heat treatment commonly referred to as the T73 temper, of the basic material was developed as described in U.S. Patent 3,198,676. As indicated in the patent, the overaging of the basic alloy is accomplished by heating the alloy to 3l5-380 F. for a period of 2-100 hours using the equation Hardening elements present Zr, W, Mo. Thickness V and (tg) of section,

Percent Zn Mn Cr inch K value Over 2 8.4)(107 Over 2 6.0)(107 Under 2.6)( l0a Under )i 2.0)(10 Under $4 2.6) 10Ll M-l 1.4)(108 1.1-1 5.0)( 100 14-1 3.8)( 107 Over 1 3.8 108 Over l 1.5)(108 Over l 1.5)(108 Thus, the treatment to obtain the improved alloy as set lforth in the referred to patent is incorporated herein by reference. The additional heating of the basic 7075- T-6 to the T73 temper serves to overage it, that is, prolong the age. However, by overaging the material, an approximately 14 percent decrease in the yield strength results. This decrease in strength lwas the penalty paid for the elimination of stress corrosion failure problems.

Thus an object of this invention is to provide a precipitation hardened alloy which maintains high strength properties while being resistant to stress corrosion.

Another object of this invention is to provide a method for controlling the properties of precipitation hardened allo s.

Tshe method of this invention involves the strengthening process applied externally to the alloy after or just before age hardening is completed. An example of the strengthening process would be explosive shock loading whereby the material is plastically deformed and strengthened `without an ensuing change in dimensions. Other deformation processes such as rolling, forging, extrusion, etc. could be utilized to obtain strengthening as long as they are applied after age hardening is completed or just before the maximum hardening is attained. However, as will beexpla-ined, if a reduction in stress corrosion alone is desired, then the deformation can occur during overaging. The explosive hardening, :for example, or use of another deformation process in treating or hardening mater-ials is in itself old; however, as will be explained in detail, the conditions under which the methods are utilized and the results obtained are heretofore unkown and serve as a substantial advancement in the art of treating precipitation hardening materials.

The heart of this invention relates to the ability to Aregulate and thus improve properties of precipitated alloys through the understanding of dislocations in the material. A dislocation can be defined as the microscopic boundary between deformed and undeformed portions of a material. Plastic deformation of an alloy occurs by the movement of the dislocation. The T-6 alloy, for example, contains dislocations as observed in the electron microscope, while the T73 does not contain such dislocations. In the T-6 temper, the dislocations which result from the quenching of the material are anchored in place by many small precipitate particles formed during aging. In other words, the dislocations in the T-6 are created du-ring the quenching operation. When aging transpires following the quenching the line precipitate particles formed vin the alloy initially precipitate on the dislocations and exert an attractive force on the dislocations thus setting such dislocations in place. Stress corrosion exists in the T-6 temper through the interaction of anchored dislocations with pitted precipitate particles in the grain boundaries. This increases the stress concentration at the pit site. It is believed the invention will be better understood from the .following detailed description and drawings in which:

FIG. l is a schematic cross-section of a precipitation hardened alloy.

FIG. 2 is a schematic representation of -a single grain.

FIG. 3 is a graph depicting the strength of an alloy versus age time.

FIG. 4 is a schematic of a means for explosively shocking an alloy.

FIG. 5 discloses schematically the specimen arrangement in FIG. 4.

Referring now to FIG. 1, to better explain the mechanism involved in the invention, there is shown a schematic of a cross-section of the granular structure of a precipitate hardened alloy. Shown are a plurality of `grains 11 of the alloy. A grain is comprised of the solid solution of the alloy which in the specific example would be the 7075 alloy previously set forth. Within each grain are several lar-ge precipitate particles 13 and numerous smaller particles 14. The precipitate is MgZn2. The precipitates in the grains are called intrag'ranular precipitates. The MgZnz precipitates out during the aging step in making the alloy and serves as one of the primary hardening factors. Shown in the ligure is an enlarged grain boundary 15 lwhich is the boundary between the layers of grains 11. In the boundaries are disposed a plurality of precipitates 17 which are called intergranular precipitates. 'One of the precipitates, particularly 19, disposed at the outer edge of the specimen and at the boundary has been corroded away, portion 21, and thus is pitted. The precipitate MgZn2 is thus susceptible to attack by corrosion in salt environments. This corrosion will leave a pitting on the adjacent surface of the material. As shown, the specimen is under stress. The stress is shown by the two arrows pulling in opposite directions with regard to the particular boundary 15. Grains 23 are used to depict dislocations. The dislocations 25 are shown as lines in the grains. This is similar to the observation using an electron microscope to observe granular structures. As shown, the lines are curved or bowed due to the stress placed upon the dislocations. A relatively straight dotted line 27 is shown to depict the dislocation prior to the material being treated by shock force, for example. The dislocation as shown by the dotted lines were rigid and held in place and would not give under stress in the manner as shown by lines 25. This will be explained more in detail in the following description. When the material is initially quenched prior to aging, dislocations will appear in the grains, as seen in the enlarged grain of FIG. 2. The dislocation, as previously indicated, will appear as a line 27 in the grain 23. The line 27, which is the dislocation, will run from two large precipitate particles or from a surface 29 as shown to a large precipitate particle 31. The dislocation, in other words, is a discontinuity in the structure o'f the grain. When precipitation occurs, small precipitate particles 33 will precipitate out along the dislocation 27 serving to lock the dislocation in place in a Arigid fixed manner. When a precipitate particle is corroded at 21 in FIG. 1, the stress of the pit and the stress of the dislocation combined with the external stress to the material will cause a `crack to emanate from the pit. The reason for this phenomenon relates to the fact that there is no means by which the material can relieve the stress induced therein. This relief of the stress is normally accomplished by the movement or bending of the dislocations as shown by the curved lines 25 in FIG. 1. When the dislocations are rigidly affixed as shown in FIG. 2 by the precipitate particles or the dotted line 27 in FIG. l, they cannot be readily moved or give during stress. Thus, there is no means `for relieving the stress in the structure `and a crack will result emanating from a pit surface.

It has been found, however, that the explosive shock treatment of the material will cause two events to occur. The first event is that the shock forces induce some dislocations in the material to move and thus relieve the stress at the pit site. The second and perhaps more important event is the introduction of additional dislocations within the grains. The introduction of these dislocations after the precipitation has occurred means that the dislocations will not be locked rigidly in place by precipitates forming thereon. The newly introduced dislocations themselves serve as means to relieve the stress induced in the material. Plastic flow depends on the presence of dislocations in the structure. lf no dislocations are present before the stress is applied, the stress will introduce dislocations. However, if the stress required to fracture the material is less than that which would introduce dislocations, then the material will not have plastic flow and will crack. By this invention, the introduction of free dislocations before stress is applied serves to enable plastic liow to occur in material that would normally crack because its fracture stress is less than that required to generate dislocations. This highlights what is believed to be the most important concept derived from the herein invention, that is, the inducement of free dislocations within a precipitation-hardened alloy after precipitation has occurred.

Turning now to the T73 temper where as previously indicated overaging has transpired, the initial dislocations had completely dissipated through the aging process. In other words, the inspection of a T73 type alloy with an electron microscope will indicate the presence of n0 dislocations in the structure. As can thus be expected, there is no means by which a crack can easily initiate or propagate from a pitted surface so that stress corrosion is n0 longer a problem. However, partly because of the absence of the dislocations in the material, the strength of the T73 has decreased as previously indicated by 14 percent. By means of the herein invention, dislocations are induced in the T73 material after the overaging process which dislocations will be able to relieve stress and encourage plastic flow because they are not anchored so that the strength will be significantly increased, as will be shown in the specific examples Without a decrease in stress-corrosion resistance.

It is believed that the principles of the invention will be even better understood with a description of the chart of FIG. 3. The chart shows the conditions for obtaining the T-6 temper. As can be seen, the hardness or strength of the material increases over the length of time of the heat treat to a maximum at the T-6 point. During this time period and at the temperature involved, the precipitate particles remain very ne Within the structure. The ne precipitation lends to the high strength of the material. When the T-6 temper material is shock treated, for example, the tendency of this tempered alloy to stress corrode is substantially decreased for the reasons previously given; namely, the freeing of the dislocations from the precipitated material and the introduction of additional dislocations to absorb the stresses. However, when the T-6 condition is shocked, overaging will transpire even at room temperature and this material will progress on a steep overaging curve (dashed). One of the reasons for the enhanced overaging from the shock treatment of the T-6 temper is that the shocking causes a structure which is more conducive to the conglomeration of the precipitated particles and as shown on the chart, the conglomeration in the region between T-6 and T73 tends to decrease the hardness of the material. As a result, in order to obtain maximum strength it is most desirable to shock the material prior to the T-6 state as indicated on the chart at a point of approximately 80 percent of maximum hardness. Thus, the tendency will be towards the completion of aging and not overaging so that the material progresses on the curve toward the T-6 state, not over into the region where conglomeration transpires, and thus a decrease in hardness. As a result, the introduction of new dislocations in the material without overaging will tend to result in an alloy having maximum strength.

When the unshocked T-6 tempered material is further treated, as for example 350 for an additional ten hours, the T73 temper is derived. This material, as previously indicated, is virtually free of any dislocations which have disappeared due to the conglomeration of the precipitate particles and other factors.4 However, the T73 is in an overaged state in that the 7075 alloy has passed its point of maximum hardness as shown on the curve. As shown, to achieve the T73 condition, the materials were heated an additional ten hours at 350 F. during which time the line precipitated particles conglomerated into larger particles. This results in a decrease in strength, but, the liner the precipitation, the stronger the material. In other words, in the T-6 condition, the maximum amount of fine precipitated particles has been achieved through heat treating. After that point, the particles which have precipitated out from the solution will conglomerate. In the process of conglomerating, however, the particles free the dislocations that had been induced during the quench so that they disappear at the point that the T73 condition is reached. As shown on the curve, however, in order to achieve this point a decrease in hardness results. At the Y T73 condition, as indicated, virtually all dislocations have disappeared. Shocking at this point introduces, as previously explained, new dislocations which are not pinned in by precipitate particles. These disloctaions through interactions with each other as well as with precipitates in the material serve to increase the strength to a point almost equivalent of T-6. The shocking prior to the T-6 condition, or even slightly after the T-6 condition, serves in addition to introducing dislocations in the material and freeing dislocations that are pinned in by precipitate, to cause new material to precipitate from the solution. At the T-6 condition essentially the maximum amount of precipitate has occurred from heat treating alone. To get more precipitate out of the solution then, other processes such as the shock loading referred to must be resorted to. Thus, by causing more precipitate to come out of solution at a point when the material is shock loaded prior to the T-6 condition, the strength is additionally increased. Turning now to a point intermediate between the T-6 and T73 as indicated in the graph of FIG. 3, shocking at this location will probably give the best combinay tion of strength yand stress corrosion resistance. At the T73 virtually all the precipitate available will have come out of solution; however, it will be in a relatively larger conglomerate form, thus making the material not as strong as T-6. An intermediate point between T-6 and T73 is shown. The shocking will cause a small amount of additional material in a finer form to precipitate out of the solution. It will also raise the strength of the material to a point at least equal to if not greater than the strength derived at T-6, since the material has not overaged too far. 'Stress corrosion resistance will be great due to the fact of the introduction of many free dislocations for relieving the stresses. Thus, at an intermediate state as is shown, there should be upon shock loading, no stress corrosion problem and a material having a strength at least equal, if not greater, than the T-6 condition.

The shock loading of the alloys was accomplished using a plain wave generator with driver and spall plates. The arrangement produces a flat uniaxial Wave which is reflected outside of the specimen being shock loaded. The T73 specimens were submerged in liquid nitrogen prior to being shocked. This precaution |was to prevent overheating and thus further overaging of the T73. The following specific examples will serve as a better indication of the results obtained.

Example I With reference to FIGS. 4 and 5 there is shown the device used in the examples. As is shown, two specimen plates 41 and 43 of the 7075 alloy are surrounded by a plurality of spall plates 45, all plates being situated on an anvil base plate 47. A stand 48 is constructed about the anvil plate and serves to support a driver plate 49 having an explosive slab or plain wave generator 51 resting thereon with an initiator 53 connected thereto.

The driver plate 49 is supported on a slight edge between two plastic stops 55 on the stand 47 at an inclination angle of a. The stops are blown olf when the explosive is discharged. The angle a is chosen so that the driver plate strikes lthe specimen instantaneously across its whole surface. In the examples the plain wave generator utilized a .328 thick slab of DuPont EL506A sheet explosive containing 8.0 grams of PETN per square inch. The driver plate was 9 x 9 x .125 sheet of aluminum alloy. The specimen plates were of an inch thick and were 21/2" x 3%. An angle of inclination of a=12 degrees was found to yield a plain impact of the driver plate against the specimen plate at a velocity of 2.150 mm./ microsecond. The spall and anvil plates were of machined mild steel. This is a higher impedance material than the 7075 aluminum of the specimens so that as a result, attenuation of the shock wave in the specimen in consequent spalling was effectively prevented. All internal mating surfaces as well as the top and bottom surfaces of the specimens and spall plates were ground to a 32 RMS finish to ensure a close fitting assembly. The following Table I compares the tensile properties of unshocked 7075-T6 and -T73 to those of shocked T73.

TABLE I-TENSILE PROPERTIES OF UNSHOCKED 7075-T6 .IIIITOMPARED TO THOSE OF -T73 SHOCKED AT 204 0.2% tensile Ultimate ten- Elongation (per 1 Average values from four tests. 2 Average values from two tests.

Note that in Table I there was an eighty percent recovery of .2 percent tensile yield strength as a result of the shock loading of the T73 at 204 kilobars. A higher shock pressure will result in complete recovery of the strength.

In this example, stress corrosion tests were performed on 7075 alloys. The tests were of the ultimate-immersion type. In the test a specimen is immersed for ten minutes in a 31/2 percent NaCl solution and is then exposed to the air for fifteen minutes. The cycle is repeated until a specimen fails or until a thirty day period is over, whichever occurs lirst. The test is set forth in an ASTM specification, B-l92-44-T. The specimens according to this test are placed in a frame which is tightened until a strain corresponding to 75 percent of the yield strength of the particular specimen is reached. Strain is measured by an extensometer and then on a strain recorder. Specimens loaded in this manner are essentially free of all but uniaxial tensile stresses. The results of the tests of the shocked and unshocked T-6 and T73 specimens are shown in the following Table II.

TABLE II-STRESS-CORROSION PROPERTIES OF UN- SHOCKED 7075-T6 AND -T73 COMPARED TO THOSE OF SHOCKED (AT 204Kb) -T AND -T73, AND 7075 SOLUTION- TREATED/SHOCKED/AGED TO -T6.

Time to failure in alternate immersion test conducted in 3% percent aqueous solu- Shocked 7075-T 73 30 without failure.3 30 without failure.3

1 All specimens taken in the short-transverse direction of forged billet.

2 Results the same in four tests.

3 Results the same in two tests.

As seen in Table II, shocking solution treated 7075 alloy prior to aging to the T-6 condition did not improve the stress-corrosion resistance of this unshocked material. It should also be noted that shocking the T-6 alloy raises its stress-corrosion resistance to a level of that of the T73. Finally it should be observed that shocking the T73 does not impair its stress-corrosion resistance. Thus, it can be seen from results of these examples that by shocking the T73 condition stress-corrosion is not aifected, yet the strength is greatly improved. In T73, as is indicated in the graph of FIG. 3, shocking at this location will give an excellent combination of strength and stress-corrosion resistance. At the T73 virtually all precipitate available will have come out of solution, However, it will be in a relatively larger conglomerate form, thus making the material not as strong as T-6. As a result, an intermediate point between T-6 and T73 is shown as giving maximum results. The shocking at this point will cause additional material in a iiner form to precipitate out of the solution. It will also raise the strength of the material to a point at least equal to, if not greater than, the strength derived at T-6, since the material has not aged too far. Stresscorrosion resistance will be great due to the introduction of the dislocations for relieving the stresses. Thus, at an intermediate point, as shown, there should be upon shock loading no stress corrosion problem and a material having a strength at least equal if not greater than the T-6 condition.

As can be seen, a new approach to relieving stress-corrosion problems in precipitation hardened alloys has been set forth. It is well known in the prior art to work materials of the nature described and, in fact, similar alloys have even been subjected to explosive forces. However, no one has ever utilized the placement of dislocations in precipitated hardened alloys to prevent stress-corrosion, while maintaining good strength. Prior t the herein invention, for example, it was felt that to have the hardness of the T-6 condition, one must sacrifice stress-corrosion resistance. Only by decreasing the strength, such as in the T73 condition, could the stress-corrosion problem be resolved. Alternatively, it would have been presumed that any strengthening of the T73 condition would cause stresscorrosion. This invention discloses for the rst time how one can now, by particular utilization of energy to dispose dislocations within the alloys, control both the stresscorrosion and strength of the material. Thus, though shock loading has been previously utilized to harden or form materials, it has never been utilized in the unobvious manner described as an effective means for controlling stress-corrosion and maintain strength properties in precipitation hardened alloys.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by Way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this inventino being limited only by the terms of the appended claims.

I claim:

1. A method of improving the strength and stress corrosion resistance of precipitation hardenable alloys comprising the steps of heat treating to precipitation harden the alloy, and

introducing dislocations into said alloy by a deformation process at a point after about percent of maximum hardness has been achieved.

2. The method of claim 1 wherein dislocations are introduced homogeneously by any one of the deformation processes of explosive shock loading, forging, rolling or extrusion of said alloy.

3. The method of claim 1 wherein said alloy is an aluminum alloy.

4. A method of improving the strength and stress corrosion resistance of a precipitation hardened aluminum alloy having a composition in weight percent of a maximum of .2O to 1.2 Si, maximum of .30 to 1.0` Fe, .40 to 6.5 Cu, maximum of .10 to 1.2 Mn, -002 to 3.7 Mg, up to .40 Cr, .10 to 6.1 Zn, maximum of .20` Ti, maximum of .15 of trace elements, with the remainder being Al, cornprising:

first heat treating said alloy to obtain precipitation of MgZn2, then introducing dislocations into said heat treated alloy by a deformation process after about 80 percent of maximum hardness has been achieved.

5. The method of claim 4 wherein said dislocations are introduced homogeneously by any one of the deformation processes of explosive shock loading, rolling, forging or extrusion.

6. The method of claim 4 additionally comprising:

further heating said alloy for a period of from 2 to 111 hours at 315 to 380 F. to an overaged condition prior to said introduction of dislocations.

7. The method of .claim 6 wherein said dislocations are introduced homogeneously by any one of the deformation processes of explosive shock loading, rolling, forging or extrusion.

References Cited UNITED STATES PATENTS 8/1939 Deutsch l48-12.7 8/1965 Sprowls et al. 148-159

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