WO1986006748A1

WO1986006748A1 - Alloy toughening method

Info

Publication number: WO1986006748A1
Application number: PCT/US1986/001050
Authority: WO
Inventors: Roberto J. Rioja; Diana K. Denzer
Original assignee: Aluminum Company Of America
Priority date: 1985-05-17
Filing date: 1986-05-15
Publication date: 1986-11-20
Also published as: EP0222002A1; EP0222002B1; EP0222002A4

Abstract

A method of treating a metallurgical object containing metastable featureless regions adversely affecting toughness, comprising heating the object for transforming the regions at least sufficiently out of their metastable state to improve toughness. A method of treating metal particles containing metastable featureless regions which adversely affect toughness when the particles are bonded together to form a metallurgical object, comprising heating the particles for transforming the regions at least sufficiently out of their metastable state to improve toughness in metallurgical objects formed by bonding the particles together.

Description

ALLOY TOUGHENING METHOD Background of the Invention Metallurgical objects produced from rapidly cooled metal have been burdened by low toughness. The cause of this low toughness was not known.

Summary of the Invention It is an object of the invention to provide a method for toughening metallurgical objects produced from rapidly cooled metal components. We have discovered that metastable, featureless regions in rapidly cooled metal adversely affect toughness.

We achieve this as well as other objects which will become apparent from the discussion that follows, according to the present invention, by providing: a method of treating a metallurgical object containing metastable featureless regions adversely affecting fracture toughness, comprising heating the object for transforming the regions at least sufficiently out of their metastable state to improve fracture toughness; and, a method of treating metal particles containing metastable featureless regions which adversely affect fracture toughness when the particles are bonded together to form a metallurgical object, comprising heating the particles for transferring the regions at least sufficiently out of their metastable state to improve fracture toughness in metallurgical objects formed by bonding the particles together.

Brief Description of the Drawings Figure 1 , composed of Figures 1a to 1d, are photomicrographs of a powder used in the invention. Figures 2 to 4 are plots of data. Detailed Description Featureless Regions The present invention concerns a treatment of metallurgical objects containing certain metastable, featureless regions. The treatment improves fracture toughness.

Instances in the literature where the term "featureless" is used to refer to these regions are as follows:

Location in Reference Citation of Reference

Col. 4, line 21 U.S. Pat. No. 3,899,820, 8/19/85

E.g. lines 7&8, abstract RapidlyQu'dMetalsIII, 1,73-84, 1978 E.g., the title Met. Trans. A, V.15 A, 1/ 84, pp 29-31 Intro., 2nd. para., Iine2 Scrip.Met'ica,V18, 1984,pp905-9

Intro., 2nd. para., Iine6 Scrip. Met'ica,V18, 1984, pp911-6

E.g., page 26 MatResSocSympProc,V28,1984,pp21-7

Pg. 148, top left col. Mat. Sci.&Eng.,V65, 1984, pp145-56

3rd. para., Iine2 43rdAnMt'gElecM'scopSoc,'85,pp32-3 These featureless regions are crystalline. This is evident alone in the title of the second-listed reference, "Rapidly Quenched Crystalline Alloys". It is also evident from what is believed to be the pioneer article on these regions, entitled "Observations on a Structural Transition in Aluminum Alloys Hardened by Rapid Solidification" by H. Jones, Mater .Sci.Eng., 5 (1969/70), pp. 1-18. Thus, in the Summary of the article by Jones, refrence is to X-ray diffraction alpha-Al line broadening, and shift, in zone A regions ("zone A regions" is synonymous to "featureless regions", as can be observed, for instance, in the references antedating Jones, as cited in the preceding paragraph), such indicating that discussion is of crystalline material. The featureless regions result from rapid cooling. Figure 1 illustrates the phenomenon of featureless regions. In Figure 1a, taken using optical microscopy, the featureless regions appear white as compared to the other regions which have a texture that appears to be black specks on a gray background. Note that the smaller particles tend to be completely featureless, an effect of the higher cooling rate experienced by the smaller particles. The scanning electron microscopy photographs of Figures 1b-1d further illustrate the featureless regions, which appear uniformly gray as compared to the remaining, dendritically textured regions. Figures 1b and 1d show again the smaller, completely featureless regions. Figure 1c shows in particularly good detail that the particle has a featureless half-moon region on its lower side. This is an aspect which also shows in Figures 1a and 1b, namely that higher cooling rates in some parts of a particle versus slower cooling rates in other parts can lead to a situation where the particle will be featureless in the rapidly cooled parts and textured in the slower cooled parts.

Alloys In general, any alloy containing featureless regions can be treated according to the invention.

A preferred Al alloy consists essentially of 4 to 12% Fe, 2 to 14% Ce, remainder Al. Fe combines with Al to form intermetallic dispersoids and precipitates providing strength at room temperature and elevated temperature. Ce combines with Fe and Al to form intermetallic dispersoids which provide strength, thermal stability and corrosion resistance. Further information concerning this alloy is contained in U.S. Patent Nos. 4,379,719 and 4,464,199.

Uniformizing With respect to strength, such as yield or tensile strength, our uniformizing heat treatment, within the featureless regions, represents an overaging. This heating step of the invention for the above preferred Al alloy will generally be in the range 750-950°F for 10 seconds to 4 hours. However, at lower temperatures, longer time may be suitable. This could be of advantage in the case of large billets, in order to obtain temperature uniformity. Fast heating appears to be best (via induction heating), since this will prevent coarsening, for instance dispersoid coarsening.

Deformation In the heating to effect the uniformizing of the invention, the featureless particles are stabilized and they become deformable. Deformation after the uniformizing treatment, for instance deformation in the form of compaction, extrusion or rolling, will provide a more uniform microstructure, with improved bonding between powder particles. Improved interparticle powder bonding further increases toughness and resistance to crack propagation.

Illustration The following Table A illustrates results achieved by procedure according to the present invention (with heat treatment, i.e. 1 to 3 minutes at 900°F followed by cooling to 725°F extrusion temperature) compared to results without heat treatment (i.e. the billet was heated directly to the 725 °F extrusion temperature and then extruded). Processing in going from extruded bar to sheet was the same in both instances.

TABLE A

Comparative Examples

With Heat Treatment^a Without Heat Treatment

Toughness Strength Toughness Strength

Extrusions 21.4 50.9 13.7 55.1

Sheet 720^c 70.2 405^c 73.7

^a 1 min at 900°F b Toughness = Ksi·in^½, Strength = Ksi cSheet toughness given in unit propagation energy (UPE) in-lb/in²

In the case of the extrusion, there was a 56% increase in toughness for an 8% decrease in yield strength. For the sheet, toughness was increased 78% for an 5% decrease in yield strength. Advantages

The invention improves toughness and thermal stability in metallurgical objects based on rapid solidification processes. It is expected that creep behavior will also be improved. Further illustrative of the invention are the following examples.

Example I Rapidly solidified aluminum alloy powder of composition 8.4% Fe, 4.0% Ce, rest essentially aluminum, had featureless regions resulting from rapid cooling during formation of the powder. To make the powder, a pot of such composition was alloyed by adding high purity alloying elements to high purity aluminum. The melt was passed through a filter and atomized using high temperature flue gas to minimize the oxidation of the alloying elements. During atomization, the powder was continuously passed through a cyclone to separate the particles from the high velocity air stream. The majority of powder particles had diameters between 5 and 40 micrometers. Powder was screened to retain only less than 74 micrometers size powder and fed directly into a drum. Besides Fe, Ce, and Al, the powder had the following percentages of impurities: Si 0.14, Cu 0.02, Mn 0.04, Cr 0.01, Ni 0.02, Zn 0.02, Ti 0.01. The powder was found to have featureless regions in about the same quantity and distribution as shown in Figure 1. The particle size distribution of the powder was 4.4% in the range 44 to 74 micrometers and 95.4% smaller than 44 micrometers. Average particle diameter was 15.5 microns as determined on a Fisher Subsieve Sizer. Billet was made from this powder by cold isostatic pressing to approximately 75% of theoretical density. Each 66 kg (145 1b) cold isostatic compact was encapsulated in an aluminum container with an evacuation tube on one end. The canned compacts were placed in a 658 K (725°F) furnace and continuously degassed for six hours, attaining a vacuum level below 40 microns. Degassed and sealed compacts were then hot pressed at 725°F to 100 percent density using an average pressure of 469.2 MPa (68 ksi).

A cylindrical extrusion charge measuring 15 cm (6.125 in.) diameter x 30.5 cm (12 in.) length was machined from the billet and subjected to a uniformizing treatments of 1 minute at 850°F and 1 minute at 900°F. Heating was done using an induction furnace operating at 60 H_z. Temperature was measured by a thermocouple placed at an axial location about 1.2 cm (0.5 in.) from the end.

It took about 10 minutes to heat the extrusion charge from room temperature to 850°F or 900°F at which point temperature was controlled at 850°F and 900°F for the

1 minute holding time.

The extrusion charge was then air-cooled to 725°F and extruded as a bar of 5 cm (2 inches) x 10 cm

(4 inches) cross section. Another Al-Fe-Ce alloy having the composition

Al-8.4%Fe-7.0%Ce was also uniformized at 900°F for 1 min. Properties for both alloys are recorded in Table I.

Results from Table I are shown graphically in Figure 2 .

Note the strength toughness relation for the two different alloys. TABLE I Room Temperature Tensile and Fracture Toughness Test Results of Extrusions

Uniformizing

Treatment Yield Strength

Fracture Toughness Temp . Time 0. 2 % O f f s et Ten si le S t r e ng t h E l ong at i on

Sample No. Alloy º F Min. MPa (Ksi) MPa (Ksi) (%) MPa ^. m^½ (Ksi· in^½)

514295-1B A1-8.4Fe-4.0Ce Control 388 (56.2) 497 (72.0) 12.5 14.7 (13.4) 514282-1 A1-8.4Fe-4.0Ce Control 380 (55.1) 469 (68.0) 9.6 15. 1 (13.7)

514412-T A1-8.4Fe-4.0Ce 850 1 366 (53.0) 449 (65.0) 17.8 19.6 ( 17.8)

514413-1B A1-8.4Fe-4.0Ce 900 1 351 (50.9) 425 (61.6) 16.7 23.5 (21.4)

514398-2T A1-8.4Fe-7.0Ce Control 426 (61.7) 530 (76.8) 11.0 9.35 (8.5)

514416-2T A1-8.4Fe-7.0Ce 900 1 373 (54) 450 (65.2) 16.0 27.8 (25.3)

NOTES ;

Values are averages from duplicate tests. Yield and tensile strengths were measured in the longitudinal (L) direction using 0.907 cm (0.357") diameter specimens machined from the extruded product. Elongation was measured in a 3.56 cm (1.40") gauge length. Tensile properties were obtained according to ASTM B557. Fracture toughness was measured in the L-T orientation using compact tension specimens of size 1.90 cm (0.75") thick x 3.81 cm (1.50 m) x 4.57 cm (1.80"). a) Product size; 5.1 cm x 10.2 cm (2.0 in. x 4.0 in.) b) Values are Kic per ASTM E399. c) This value was not a valid Kic but a meaningful value per ASTM B645

Example II Extruded bar of Example I was rolled at 600 °F to sheet of final thickness equalling 1.60 mm (0.063 inch). Prior to rolling, the extrusion was sawed to approximately 25 cm (10 in.) lengths. Surface roughness, caused by pickup of aluminum on the extrusion dies, was eliminated by machining the extrusions to the thicknesses listed in Table III. Also listed are process parameters used to roll the Al-Fe-Ce 1.60 mm (0.063 in.) sheet. Each piece was cross rolled until the desired width, greater than 41 cm (16 inches) was obtained, followed by straight rolling to the desired thickness, 1.60 mm (0.063 inch).

1.27 cm (0.5 in.) width x 5.08 cm (2.0 in.) gage length tensile specimens were prepared and tested to give results as shown in Table II. Sheet tensile strength was determined per ASTM E8 and E23. The Alcoa-Kahn tear test (see "Fracture Characteristics of Aluminum Alloys," J. G. Kaufman, Marshall Holt, Alcoa Research Laboratories, Technical Paper No. 18, pp. 10-18, 1965) and fracture toughness K per ASTM B646 and E561 were used to compare sheet toughness. These results are shown in Table II. Figure 3 shows the graphic representation of the strength/fracture toughness, K_c, relationships for representative samples of Table II, while Figure 4 provides a corresponding presentation from Table II in the form of toughness indicator, or unit propagation energy, against yield strength. The superiority of sheet treated according to the present invention compared to the ingot metallurgy representatives is apparent. It is to be noted that for a given alloy, the tradeoff between strength loss and toughness improvement is a function of time and temperature during the uniformizing treatment. TABLE II Room Temperature Tensile and 1 Fracture Toughness 1.60mm (0.063 In.) Sheet

Treatment Tear Test Fracture Toughness, kc Temp Time Yield , Strength Tensile Strength Elonga- in.-1b/

Sample No. Alloy ·F Min MPa Ksi MPa Kai tion % kJ/m² in.² MPa√m Ksi√in. Valid^h

514295-2B A1-8.3Fa-4.0Ce Control 508 73.7 546 79.1 6.8 70.9 405^b 122.7 HI.7 Tee

554314 A1-8.3Fe-4.0Ce Control 523 75.8 575 83.4 10.0 68.9 395

514388-2 Al-8.3Fe-4.0Ce Control 524 76.0 561 81.3 6.5 69.2 395^f

514412-BR Al-8.3Fe-4.0Ce 850 10 477 69.2 513 74.3 5.8 125.6 715^c 180.8 164.5 No 514413-1BR Al-8.3Fe-4.0Ce 900 1 484 70.2 518 75.1 6.0 125.7 720^d 191.2 174.0 No 514408-2BR A1-8.3Fe-4.0Ce 900 10 424 61.6 460 66.7 8.0 135.5 775 168.1 153.0 No 554311 A1-8.3Fe-4.0Ce 850 60 432 62.6 483 70.0 10.0 135.6 775 214.5 195.0 No

514398-2T A1-B.4Fe-7.0Ce Control 579 84.1 622 90.2 6.5 0 0^g

514416-2TR Al-8.4Fe-7.0Ce 900 1 519 75.4 549 79.6 8.2 117.3 670^o 98.9 90.0 Yes

7075-T6 - - 517 74.9 568 82.3 11.2 50.7 290 70.8 64.4 Yes 7075-T73 - - 416 60.3 494 71.6 10.6 89.2 510 - 2024-T81 - - 482 69.8 512 74.2 6.6 29.7 170 - 2024-T6 - - 367 53.2 464 67.2 9.2 48.1 275 -

NOTES: a) All teats were done in the L-T orientation. Sheet thickness varies from 1.60 to 1.78 as. (0.063" to 0.070") except;

554311 which haa a nominal thickness of 1.42 mm (0.056"). Al-Fe-Ce tenalle and tear test results are averages of duplicate testa, Kc results are single tests. 7075 and 2024 reaults ara averages of 2-10 tests. b) One of the dupllcatea underwent rapid t diagonal fracture (UPE may be estimated and slightly high; Included in average). c) Both tests: diagonal fracture (tear strength and UPE may be slightly high; Included in average). d) One of the dupllcatea underwent diagonal fracture (tear strength snd UPE may be slightly high;1 Included in average). e) One of the duplicates underwent rapid fracture (UPE was estimated, but not Included in average shown). f) One test: rapid and diagonal fracture - curve not reliable (energy near zero; not included in average shown). g) Crack growth was unstable. h) Invalidities are due to specimen size, i.e., specimen was not large enough to provide enough recoverable elastlc energy to produce unstable crack growth In an elastic-stress field.

Specimen Sizes:

Tensile: Sheet thickness x 1.27 cm (0.5") wide specimen. Elongation waa measured i,n 5.08 cm (2.0") gauge length.

Tear Test: Kehn-type, sheet thickness x 3.65 cm (1.44") x 5.72 cm (2.25").

Fracture Toughness: Center-crack, sheet thickness x 40.6 cm (16.0") x 111.8 cm (44.0").

TABLE III Process Parame t ers Used To Roll 1.60mm (0.063 in.) Al-Fe-Ce Sheet

Rolling Temperature Extrusion Thickness Sheet ThicknessSample No K F cm in. mm in.

514295- 2B 589 600 4.72 1.86 1.59 0.0625 554314 616/589 650/600* 4.45 1.75 1.55 0.061 514388-2 589 600 2.51 0.988 1.65 0.065

514412-BR 589 600 5.08 2.0 1.68 0.066 514413-1BR 589 600 5.08 2.0 1.69 0.0665 514408-2BR 589 600 5.08 2.0 1.70 0.067 554311 616/589 650/600* 4.45 1.75 1.37 0.054

514398-2T 589 600 4.65 1.83 1.54 0.0605

514416-2TR 589 600 4.76 1.875 1.60 0.063

* Extrusions were heated to 616°K (650°F) for the first rolling reductions and 589°K (600°F) for subsequent reductions.

* * * * *

Unless noted otherwise, percentages herein are on a weight basis.

While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Claims

1. A method of treating a metallurgical object containing metastable, featureless regions adversely affecting toughness, comprising heating the object for transforming the regions at least sufficiently out of their metastable state to improve toughness.

2. A method as claimed in claim 1, the heating being sufficient to provide at least a 10% improvement in toughness.

3. A method as claimed in claim 1, the heating being sufficient to provide at least a 20% improvement in toughness.

4. A method as claimed in claim 1, the heating being sufficient to provide at least a 30% improvement in toughness.

5. A method as claimed in claim 1, the object comprising an aluminum alloy.

6. A method as claimed in claim 5, the object comprising an aluminum alloy of the class referred to as non-heat treatable or dispersion hardened.

7. A method as claimed in claim 6, the object comprising bonded powder.

8. A method as claimed in claim 7, the object comprising a dispersion hardened, bonded powder.

9. A method as claimed in claim 8, the alloy consisting essentially of 4 to 12% iron, 1 to 8% rare earth metal, balance aluminum.

10. A method as claimed in claim 9, the alloy consisting essentially of 6 to 10% iron, 2 to 6% cerium, balance aluminum.

11. A method as claimed in claim 1, further comprising deforming the object following the heating.

12. A method of treating metal particles containing metastable featureless regions which adversely affect toughness when the particles are bonded together to form a metallurgical object, comprising heating the particles for transforming the regions at least sufficiently out of their metastable state to improve toughness in metallurgical objects formed by bonding the particles together.

13. A method as claimed in claim 12, the heating being sufficient to provide at least a 10% improvement in toughness.

14. A method as claimed in claim 12, the heating being sufficient to provide at least a 20% improvement in toughness.

15. A method as claimed in claim 12, the heating being sufficient to provide at least a 30% improvement in toughness.

16. A method as claimed in claim 12, the particle comprising an aluminum alloy.

17. A method as claimed in claim 16, the particle comprising an aluminum alloy of the class referred to as non-heat treatable.

18. A method as claimed in claim 17, the particle comprising bonded powder.

19. A method as claimed in claim 18, the particle comprising a dispersion hardened, bonded powder.

20. A method as claimed in claim 19, the alloy consisting essentially of 4 to 12% iron, 1 to 8% rare earth metal, balance aluminum.

21. A method as claimed in claim 20, the alloy consisting essentially of 6 to 10% iron, 2 to 8% cerium, balance aluminum.