CA1056621A - Amorphous alloys with improved resistance to embrittlement upon heat treatment - Google Patents

Abstract

INVENTION: AMORPHOUS ALLOYS WITH IMPROVED
RESISTANCE TO EMBRITTLEMENT UPON
HEAT TREATMENT

INVENTORS: DONALD E. POLK
RANJAN RAY
LANCE A. DAVIS

ABSTRACT OF THE DISCLOSURE
An improvement in resistance to embrittlement upon heat treatment of amorphous metal alloys containing iron, nickel, cobalt and/or chromium in the temperature range of about 200° to 350°C is achieved by including boron or boron plus at least one metalloid element of carbon, silicon and aluminum in a total amount of about 15 to 25 atom percent of the alloy composition.
The alloys of this invention find use as razor blades and in applications requiring high thermal stability and increased mechanical strength.

Description

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AMORPHOUS ALLOYS WITH IMPROVED RESISTANCE TO EMBRITTLEMENT
UPON HEAT TREAT~ENT
BACKGROUND OF THE INVENTION
1. Field of the Invention The invention relates to amorphous metal alloy com positions, and, in particular, to amorphous alloys containing iron, nickel, cobalt and/or chromium having improved resistance to embrittlement upon heat treatment.

2. Description of the Prior Art -- ~ .
Investigations have demonstrated that it is possible to obtain solid amorphous metals for certain alloy compositions.
An amorphous substance generally characterizes a non-crystalline or glassy substance, that is, a substance substantially lacking any long range order. In distinguishing an amorphous substance from a crystalline substance, X-ray diffraction measurements are generally suitably employed. Additionally, transmission electron ~ micrography and electron diffraction can be used to distinguish - I between the amorphous and the crystalline state.
An amorphous metal produces an X-ray diffraction pro-file in which intensity varies slowly with diffraction angle.
Such a profile is qualitatively similar to the diffraction profile of a liquid or ordinary window glass. On the other hand, a crystal-line metal produces a diffraction profile in which intensity varies rapidly with diEfraction angle.
These amorphous metals exist in a metastable state.
Upon heating to a sufficiently high temperature, they crystallize . . .
with evolution of a heat of crystallization, and the X-ray diffrac-tion profile changes from one having glassy or amorphous character-~; istics to one having crystalline characteristics.
It is possible to produce a metal which is totallyamorphous or which comprises a two-phase mixture of the amorphous and crystalline state. The term "amorphous metal", as employed herein, refers to a metal which is at least 50~ amorphous, and '' ;. .:

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preferably 80% amorphous, but which may have some fraction of the ~
material present as included crystallites. ~ -Proper processing will produce a metal alloy in the amor-phous state. One typical procedure is to cause molten alloy to be spread thinly in contact with a solid metal subs-trate such as copper or aluminum so that the molten alloy loses its heat to the substrate. When the molten alloy is spread to a thickness at about 0.002 inch, cooling rates of the order of 106C/sec are achieved. See, for example, R.C. Ruhl, Vol. 1, Materials Science 10 and Engineering, pp. 313-319 (1967), which discusses the depen-dence of cooling rates upon the conditions of processing the molten alloys. Any process which provides a suitable high cooling rate, as on the order of 105 to 106C/sec, can be used. Illustrative examples of procedures which can be used to make the amorphous metals are the rotating double roll procedure described by ~I.S.
Chen and C.E. Miller in Vol. 41, Review of Scientific Instruments, pp. 1237-1238 (1970) and the rotating cylinder technique described by R. Pond, Jr. and R. Maddin in Vol. 245, Transactions of the ; Metallurgi__l Soclety~ AIME, pp. 2475-2476 (1969).
Novel amorphous metal alloys have been disclosed and claimed by H.S. Chen and D.E. Polk in U.S. Patent 3,856,513, issued December 24, 1974. These amorphous alloys have the formula MaYbZC, where M is at least one metal selected from the group consisting of iron, nickel, cobalt, chromium and vanadium, Y is at least one element selected from the group consisting of phosphorus, boron and carbon, Z is at least one element selected from the group consisting of aluminum, antimony, beryllium, germanium, indium, tin and silicon, "a" ranges from about 60 to 90 atom percent, "b" ranges from about 10 to 30 atom percent and "c" ranges from about 0.1 to 15 atom ~ 30 percent. These alloys have been found suitable for a wide variety ; of applications, including ribbon, sheet, wire, powder, etc.
Amorphous alloys : `

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are also disclosed and claimed having the formula TiXj, where T is at least one transition metal, X is at leasl: one element selected - from the group consisting of aluminum, antimony, beryllium, boron, -~
germanium, carbon, indium, phosphorus, silicon and tin, "i" ranges from about 70 to 87 atom percent and "j" ranges from about 13 to 30 atom percent. These alloys have been fo~nd suitable for wire ~; applications.
Ductility is generally desirable either to render mechani-cal applications possible or to ease the handling and processing of the product. It is known that amorphous metal alloys tend to lose ductility in bending upon heating to temperatures near which the onset of crystallization occurs (crystallization temperature).
Often, prolonged heating at lower temperatures is sufficient to induce embrittlement. Many of the amorphous alloys containing iron, nickel, cobalt and/or chromium known in the art, which include phosphorus as an aid to glass formation, tend to embrittle upon ~-heating in the temperatùre range of about 200 to 350C. While many applications involving these amorphous alloys wou]d not re-quire such heat treatment, there are specific instances where such heating would be necessary and where it would be desirable to uti-lize these alloys, many of which are relatively inexpensive compo-sitions.
SUMMARY OF THE INVENTION
In accordance with the invention, an improvement in resis-tance to embrittlement upon heat treatment of amorphous metal alloys consisting essentially of at least one element selected from the group consisting of iron, nickel, cobalt and chromium in the temp-erature range of about 200 to 350C is achieved by including boron plus at least one metalloid element selected from the group consisting of carbon, silicon and aluminum in a total amount of about 15 to 25 atom percent oE the alloy composition. The ~ ~3~

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amorphous metal alloys of this invention consist essentially of the composition MaXb, where M is at least one element of iron, nickel, cobalt and chromium, X is boron plus at least one element of carbon, silicon and aluminum, "a" ranges lrom about 75 to 85 atom percent and "b" ranges from about lS to 25 atom percent. Pre ferably, X is about 60 to 80 percent boron, and "b" ranges from about 17 to 22 atom percent. These alloy compositions may include up to about 20 atom percent, as in the order of about 5 to 15 atom percent, of chromium, and up to about 30 atoM percent, as in the order of about 15 to 25 atom percent, of cobalt.
Also in accordance with the invention, an improvement in resistance to embrittlement upon heat treatment is also obtained for alloys which consist essentially of the composition M'aBb, where M' is at least three elements selected from the group con-sisting of iron, nickel, cobalt and chromium, the amount of each of iron, nickel and cobalt rangin~ from about 20 to 35 atom percent, and preferably from about 20 to 30 atom percent,and the amount of chromium ranging from about 5 to 20 atom percent, and : -preferably from about 5 to 15 atom percent, and where "a" ranges from about 75 to 85 atom percent and "b" ranges from about 15 to 25 atom percent. Preferably, "bl' ranges from about 17 to 22 atom percent.
The alloys in accordance with the invention do not be-come brittle to bending upon heating to temperatures typically ~ employed in subsequent processing. These alloys are also char--~ acteri~ed by increased mechanical strength.
I The amorphous metal alloys in accordance with the .1 invention are fabricated by a process which comprises forming a melt of the desired composition and quenching at a rate of about 105 to 106C/sec by casting molten alloy onto a .

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chill wheel or into a quench fluid. Improved physical and mechanical properties, together with a greater degree of amorphousness, are achieved by casting the molten alloy onto a chill wheel in a partial vacuum having an absolute pressure of less than about 5.5 cm of Hg.
DETAILED DESCRIPTION OF THE :[NVENTION
___ _ ___ _ __ _ _ The thermal stability of an amorphous metal alloy is an important property in certain applications. Thermal stability is characterized by the time-temperature transEormation behavior of an lO alloy, and may be determined in part by DTA (differential thermal analysis). As considered here, relative thermal stability is also indicated by the retention of ductility in bending after thermal ~; treatment. Amorphous metal alloys with similar crystallization behavior as observed by DTA may exhibit dlfferent embrittlement behavior upon exposure to the same heat treatment cycle. By DTA
measurement, crystallization temperatures, Tc, can be accurately r determined by slowly heating an amorphous metal alloy (at a~out 20 to 50C/min) and noting whether excess heat is evolved over a limited temperature range (crystallization temperature) or whether ``~
20 excess heat is absorbed over a particular temperature range (glass transition temperature). In general, the glass transition temperature Tg is near the lowest, or first, crystallization temperature, TCl, and, as is con~entional r is the temperature at which the viscosity ranges from about 1013 to 10l4 poise.
Most amorphous metal alloy compositions containing iron, nickel, cobalt and/or chromium which include phosphorus, among other metalloids, evidence ultimate tensile strengths of about 265,000 to 350,00G psi and crystallization temperatures of about 4t)0 to 460C.
For example, an amorphous metal alloy having the composition Fe76Pl6C4Si2A12 (the subscripts are in atom percent) has an - ultimate tensile strength of about 310,000 psi and a crystallization ':

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temperature of about 460C; an amorphous metal alloy having the composition Fe30Ni30Co20Pl3B5Si2 has an ultimate tensile strenyth of about 265,000 psi and a crystallization temperature of about 415C; and an amorphous metal alloy having the composition Fe74 3Cr4 5Pl5 gC5Bo 3 has an ultimate tensile strength of about 350,000 psi and a crystallization temperature of 446C. The ther-mal stability of these compositions in the temperature range of about 200 to 350C is low, as evidenced by a tendency to embrittle after heat treating, for example, at 330C for 5 minutes. Such heat treatment is required in certain specific applications, such as curing a coating of polytetrafluoroethylene on razor blade edges.
In accordance with the present invention, the resistance to embri-ttlement upon heat treatment o~ these alloys in the tempera-ture range of about 200 to 350C Eor several minutes is improved by replacing phosphorus with boron or boron plus at least one of the metalloid elements of carbon, silicon and aluminum.
Specifically, the amorphous metal alloys of the invention consist essentially of the composition MaXb, where M is at least I one element of iron, nickel, cobalt and chromium, X is boron plus at least one element of carbon, silicon and aluminum, "a" ranges from about 75 to 85 atom percent and "b" ranges from about 15 to ;~

25 atom percent. Examples include Fe77B15C5SilA12, Fe60Crl8Bl5-2 and Fe28Ni2gco2oBlgc2si2Al2 ~ or these amorphous metal alloys, ease of glass formation occurs where "b" ranges from about 17 to 22 atom percent. Opti-mum thermal stability is achieved with compositions where about 60 to 80 percent of X is boron. Accordingly, such compositions s~ are preferred.
~ The properties of corrosion resistance, hardness, and ;- 30 mechanical strength are improved in these amorphous metal alloys ~ ' :
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alloy composition. Preferably, chromium is present in an amount ; of about 5 to 15 atom percent of the total alloy composition ~or optimum improvement.
Glass formation is aided in these amorphous metal alloys by including up to about 30 atom percent o~ cobalt. Preferably~
cobalt is present in an amount of about 15 to 25 atom percent of - the total alloy composition for optimum ease of glass formation.
The amorphous metal alloys of the invention also con-sist essentially of the composition M'aBb, where M' is at least three elements selected from the group consisting of iron, nickel, cobalt and chromium, the amount of each of iron, nickel and cobalt ranging from about 20 to 35 atom percent, and preferab:ly from about 20 to 30 atom percent, and the amount oE chromium ranging from about 5 to 20 atom percent, and preferably from about 5 to 15 atom percent,and where "a" ranges Erom about 75 to 85 atom percent and "b" ranges from about 15 to 25 atom percent. PreEerably, "b"
ranges from about 17 to 22 atom percent. Examples include s Fe Ni 0co20B20~ Fe30Ni3oco23Bl7~ Fe3oNi32 20 18 Fe25Ni25co2ocrloB2o ! 20 Such amorphous metal alloys containing boron evidence superlor strength over compositions which include phosphorus.
For example, an amorphous metal alloy having the composition Fe25Ni25Co20CrlOB8P12 has an ultimate tensile strength of 330,000 psi. Changing the metalloid content to -B16P4 increases the ultimate tensile strength to 395,000 psi. Where the metalloid content is -~20~ the ultimate tensile strength is increased to 500,000 psi. The crystalli~ation temperature is also observed to increase from 461C for Fe25Ni25Co20CrlOB8P12 Fe25Ni25Co20CrlOB20. These amorphous metal alloys in which boron is the only metalloid element are also characteriæed by high hard-ness values, which are typically about 1000 DPH.
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The amorphous metal alloys are formed by cooling a melt at a rate of about 105 to 106~/sec. A variety of techniques are available, as is now well-known in the art, for fabricating splat-quenched foils and rapicl-quenched continuous ribbons, wire, sheet, etc. Typically, a particular composition is selected, powders of - the requisite elements (or of materials that decompose to form the elements, such as ferroboron, ferrosilicon, etc.) in the de-sired proportions are melted and homogenized, and the molten alloy is rapidly quenched either on a chill surface, such as a rotating cylinder, or in a suitable fluid medium, such as a chilled brine solution. The amorphous metal alloys may be formed in air. How-ever, superior physical and mechanical properties are achieved by :~ .
forming these amorphous metal alloys in a partial vacuum with absolute pressure less than about 5~5 cm of Hg, and preferably about 100 ~m to 1 cm of Hg. The purity of all materials is that found in normal commercial practice.
While as stated earlier the amorphous metal alloys are at least 50% amorphous, and preferably at least 80~ amorphous, a substantial degree of amorphousness approaching 100% amorphous is obtained by forming these amorphous metal alloys in a partial vacuum. Ductility is thereby improved, and such alloys possess-ing a substantial degree of amorphousness are accordingly pre-ferred.
The amorphous metal alloys of the invention evidence super-ior fabricability, compared with prior art alloys. In addition to their improved resistance to embrittlement upon heat treatment, the amorphous metal alloys of the present invention tend to be more oxidation and corrosion resistant than prior art compositions.
These alloy compositions remain amorphous at heat treating con-; 30 ditions under which phosphorus-containing amorphous alloys tend .:

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to embrittle. Ribbons of these alloys find use in applications requiring relatively higher thermal stability and increased mechanical strength.
EXAMPLES
._ Rapid melting and fabrication of amorphous strips of ribbons of uniform width and thickness from high melting (about 1100 to 1600C) reactive alloys was accomplished under vacuum.
The application of vacuum minimized oxidation and contamination of the alloy during melting or squirting and also eliminated sur-face damage (blisters, bubbles, etc.) commonly observed in stripsprocessed in air or inert gas at 1 atm. A copper cylinder was mounted vertically on the shaft of a vacuum rotary feedthrough and placed in a stainless steel vacuum chamber. The vacuum chamber was a cylinder flanged at two ends ~ith two side ports and was connected to a diffusion pumping system. The copper cylinder was rotated by variable speed electric motor via the feedthrough. A
crucible surrounded by an induction coil assembly was located above the rotating cylinder inside the chamber. An induction power supply was used to melt alloys contained in crucibles made of fused quartz, boron nitride, alumina, zirconia or beryllia. The amorphous ribbons were prepared by melting the alloy in a suitable non-; reacting crucible and ejecting the melt by overpressure of argon through an oriEice in the bottom oE the crucible onto the surface :
of the rotating (about 1500 to 2000 rpm) cylinder. The meltingand squirting were carried out in a partial vacuum of about 10 4 `~ ~ m, using an inert gas such as argon to adjust the vacuum pressure.
The amorphous ribbons were subsequently tested for ultimate tensile strength~ ;
Using the vacuum melt casting apparatus described above, ....
a number of various glass-forming metal alloys were chill cast as continuous ribbons having-substantially uniform thickness and width.
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The mechanical behavior as a function of heat treatment conditions of amorphous metal alloys having compositions in accordance with the invention was compared with that of phos-phorus-containing amorphous metal alloysO All alloys were fabri-cated by the process given above. The amorphous ribbons of the alloys of this invention were all ductile in the as-quenched condi-tion, and remained so upon heat treatment within the range of about 200 to 350C for several minutes. This was in contrast with ribbons of amorphous alloys which included phosphorus as a metal-loid element. These alloys evidenced embrittlement under the , same conditions of heat treatment. The ductility of the ribbons ; in the as-quenched and heat treated conditions was determined as follows. The ribbons were bent end on end to form a loop. The diameter of the loop was gradually reduced between the anvils of a micrometer. The ribbons were considered ductile if they could be bent to a radius of curvature less than about 0.005 inch without fracture. If a ribbon fractured, it was considered to be brittle.
Table I lists the results of these tests.

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The data on mechanical and thermal properties ~ultimate tensile strength in psi; crystallization temperature in C) of some typical amorphous metal alloys in accordance with the in-vention are given .in Table II below.
TABLE II

Mechanical and Thermal Properties of Amorphous Metal Alloys Alloy Composition Ultimate Tensile Crystallization (Atom Percent) Strength (psi) Temperature (C)*
: , Fe77B15C5silA 2 486,000 510 : Fe66Crl2Blscssi2 Fe60crl8Blscssi2 438,000 578 Fe30Ni3oco2oBl8si2 478 :~
Fe28Ni30co2oBl6si4Al2 338rO00 479 Fe28Ni28Co2oBl8c2si2Al2 320,000 490 Fe28Ni30co2oBl8c2Al2 280,000 457 Fe30Ni3oco2oBl7 3 442 ' Fe25Ni25co2ocrloB2o 485 . ` Fe3 oN i 3 oC 2 3B 17 . ~ .
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Claims (4)

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

1. An amorphous metal alloy that is at least 50 percent amorphous, characterized in that the alloy consists essentially of the composition M'a and Bb, where M' is at least three elements selected from the group consisting of iron, nickel, cobalt and chromium, the amount of each of iron, nickel and cobalt ranging from about 20 to 35 atom percent and the amount of chromium ranging from about 5 to 20 atom percent and B is boron and where a ranges from about 75 to 85 atom percent and b ranges from about 15 to 25 atom percent, said alloy being resistant to embrittlement upon heat treatment in the temperature range of about 200° to 325°C for at least 5 minutes.

2. The amorphous metal alloy of claim 1 in which the amount of each of iron, nickel and cobalt ranges from about 20 to 30 atom percent and the amount of chromium ranges from about 5 to 15 atom percent.

3. The amorphous metal alloy of claim 1 in which b ranges from about 17 to 22 atom percent.

4. The amorphous metal alloy of claim 1 consisting essentially of a composition selected from the group consisting of Fe30Ni30Co20B20,Fe30Ni30Co23B17,Fe30Ni32Cr20B18 and Fe25Ni25Co20Cr10B20.