Alloy Toughening Method

FIG. 1, composed of FIGS. 1a to 1d, are photomicrographs of a powder used in the invention.

FIGS. 2 to 4 are plots of data.

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:

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, reference 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. FIG. 1 illustrates the phenomenon of featureless regions. In FIG. 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 FIGS. 1b-1d further illustrate the featureless regions, which appear uniformly gray as compared to the remaining, dendritically textured regions. FIGS. 1b and 1d show again the smaller, completely featureless regions. FIG. 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 FIGS. 1aand 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.

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. Pat. Nos. 4,379,719 and 4,464,199.

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.degree.-950.degree. 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 obtain temperature uniformity.

Fast heating appears to be best (via induction heating), since this will prevent coarsening, for instance dispersoid coarsening.

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.

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.degree. F. followed by cooling to 725.degree. F. extrusion temperature) compared to results without heat treatment (i.e. the billet was heated directly to the 725.degree. F. extrusion temperature and then extruded). Processing in going from extruded bar to sheet was the same in both instances.

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.

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.

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 5and 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 FIG. 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 lb) 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.degree. 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.degree. 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 .times.30.5 cm (12 in.) length was machined from the billet and subjected to a uniformizing treatments of 1 minute at 850.degree. F. and 1 minute at 900.degree. F. Heating was done using an induction furnace operating at 60 H.sub.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.degree. F. or 900.degree. F. at which point temperature was controlled at 850.degree. F. and 900.degree. F. for the 1 minute holding time.

The extrusion charge was then air-cooled to 725.degree. F. and extruded as a bar of 5 cm (2 inches) .times.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.degree. F. for 1 min.

Properties for both alloys are recorded in Table I. Results from Table I are shown graphically in FIG. 2. Note the strength toughness relation for the two different alloys.

Extruded bar of Example I was rolled at 600.degree. 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 .times.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.sub.c per ASTM B646 and E561 were used to compare sheet toughness. These results are shown in Table II. FIG. 3 shows the graphic representation of the strength/fracture toughness, K.sub.c, relationships for representative samples of Table II, while FIG. 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.

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.