Specificity as an aspect of liquid metal embrittlement

Scripta METALLURGICA

Vol. 8, pp. 519-526, 1974 Printed in the United States

Pergamon Press, Inc.

SPECIFICITY AS AN ASPECT OF LIQUID METAL EMBRITTLEMENT

Francis A. Shunk and William R. Warke Metallurgical and Materials Engineering Department I lllnols Institute of Technology Chicago, Illinois 60616

(Received March 8, 1974)

The phenomenon known as liquid metal embrittlement (LME) refers to the occurrence of a deterioration of the mechanical properties of a solid metal or alloy subjected to some minimum tensile stress when this solid is wetted by a liquid metal environment. Typically, this deterioration is measured in terms of a loss of ductility or true fracture stress and extends over a range of test temperatures so that the LME behavior of a given couple (solid-liquid combination) can be characterized by an embrlt. . . .

,~o~lttl~t

tlement trough as depicted schematically in Figure 1. The

em~ittle~nt

location and sharpness of the lower and upper transition tem-

FRAaURE STRESS

peratures are complex functions of a number of material, environmental and test variables such as grain size, strain rate, strength level and the respective compositions of the liquid and the solid. However, no report has appeared in the literature of the lower transition temperature being above the melting paint of the liquid metal, nor of the upper transition temperature being below this temperature.

ItED'UOION

In addition to this so-called embrittlement trough, a

OF .(~REA

second characteristic of LJ~E, which is often referred to and TEMPEllATI.,IRE

FIG. 1 Schematic representation of the LME embrittlement trough

which has been the subject of much discussion, is specificity. This term was employed by Rostoker et al. (1) to describe the observations shown in Table 1. According to this table, AIand Ti- alloys are embrlttled at 30°C by a Hg-3% Zn amal-

gam and this same liquid alloy embrittles neither Mg-alloys nor steel. Embrittlement ("E" in the table) was indicated if "brittle fracture in bending with no apparent prior plastic deformation" was observed. As implied from this table, specificity means that embrittlement of a given solid metal is not always observed in the presence of a liquid metal; or, alternatively, any given solid metal is embrittled only by certain, specific liquids. An additional, commonly quoted example of specificity is thatzinc is embrlttled by mercury but that cadmium is not embrittled by this particular liquid. Specificity as a concept and as an experimentally determined factor leads to a number of practical and theoretical problems. In this paper, an attempt is made to examine some of these problems and to consider whether or not specificity is a real aspect of liquid metal embrlttlement.

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TABLE 1. Behavior of Various Liquid Metals on Common Engineering Alloys (2) Test Temperature (oc) 30

50

125

180

210

250

260

300

325

350

380

450

475

Bi

TI

Cd

Pb

Zn

Te

Liquid Metal Eng. Metal

Hg*

Ga

Na

In

Li

Se

Sn

AI AIIoys

E

E

E

E

N

N

E

N

N

N

N

E

Mg Alloys

N

N

E

N

N

N

N

N

N

N

N

E

Steel Ti Alloys

N

N

N

E

E

N

N

N

N

E

N

E

E

E

N

N

N

N

N

N

N

N

E

N

N

N

*Hg-3% Zn amalgam E - embrittlement N - non-embrittlement Specificity Tabulations The tabulation of Rostoker et al. (1) which is reproduced as Table 1 was the first systematic presentation of data pertinent to specificity. The description of the experimental technique employed by these investigators indicates that the results are reported for a simple "go-no go" bend test at a single test temperature which was roughly

50°C above the respective

melting points. Subsequent investigations have confirmed the embrittlement

of steel by cadmium (2), indium (3) and zinc (3) but have also shown that steel is indeed embrlttled by lead (4,5) and by tin (2). It seems likely that the respective single test temperatures employed by Rostaker et al. (1) were too high for the steel-tin and steel-lead couples and were greater than the upper transition temperatures. As a result of this, the conclusions drawn from this cursory study are, at best, open to question. Subsequently, Stoloff (6) and Carlston (7) have published reviews of LME in which specificity tables are presented and discussed. These tables were adaptations and expansions of the table of Rostaker et al. (1). It is interesting to note that in the course of time the designations "AI Alloy", "Mg Alloy", "Steel", and "Ti Alloy" were abbreviated to AI, Mg, Fe and Ti, thus implying that the solid metals were the pure elements rather than engineering alloys. In view of the variations indicated and possible confusion resulting from the literature cited above, it was deemed desirable to conduct a search of the literature to determine if other investigators had obtained results contrary to the existing tabulations and to provide a comprehensive tabulation of those solid metalliquid metal couples which have been tested. Such a tabulation is necessary before an evaluation can be made of the subject of specificity. The results of this literature search are shown in Table 2* for "non-embrlttllng" couples and in Table 3* for "embrlttllng" couples. Many couples do not appear in either table simply because they haven't been tested or at least such tests have not been reported.

In preparing these tables, it was found

*References to specific entries in Tables 2 and 3 will be provided on request.

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TABLE 2. Summary of Non-Ernbrittllng Couples PACL-

element (nominally pure) alloy commercial laboratory

Solid ~ , J Bi

P

Cd

P

Pb

P

Zn

P

Mg

CA

AI

P

Ga

Na

In

Li

Se

Sn

Bi

TI

Cd

Pb

Zn

Te

P

P

P

P

P

P

P

P

P

P

P

P

b

b

b

b

b

b

b

b

b

b

b

b

b

b

b

b

cte

a

CA Au

P

b

b

Cu

CP

cte

C

Fe

P

b

b

LA

b

b

CA Ti

limited testing (known or inferred) one test temperature (inferred) no published information on test also reported as embrittllng

A

Hg P

abce-

b

CA

C

b

b

b

b

b

b

C

b

b,e

b

b

b,e

b

b

b

b

b

b

b

desirable to distinguish among the pure solid metals, commercially pure elements, commercial alloys and "laboratory alloys" produced on a small scale from relatively pure melting stock, and between pure elemental liquids and liquid alloys. Why these distinctions have been made is apparent from the report that pure AI is embrlttied by Sn-rich Sn-Zn alloys but not by pure Sn and also the report that pure iron is not embrittled by pure Hg but iron-based binary alloys (>4 w/o AI) are embrittled by pure Hg. For the non-embrittling couples (those for which the absence of deterioration of mechanical properties has been observed and reported) shown in Table 2, various levels of confidence in the determination of non-embrittlement are indicated. The "a" designation is used for couples which appear to have been subjected to a reasonable amount of testing; "b" and "c" suggest a significantly lower level of confidence due to lack of testing or lack of reported test information; and "e" is used to show that the couple has also been reported as embrlttling. The extremely limited testing of the couples in Table 2, as well as the indicated conflicting reports, suggests strongly that these reports are not definitive. Specificity Criteria On the basis of their data (Table 1), Rostoker et al. (1) suggested that embrittlement was likely to be observed if the solid and the liquid satisfied certain empirical criteria. These criteria were that embrlttlement was favored when the two metals in the couple (a) exhibited little mutual solubility and (b) did not form any stable intermediate phases. However, these investigators, and others subsequently, have noted that both of these criteria are violated by some embrittlement couples and that some couples which were designated nonembrittling satisfied both of them.

¢J-i

TABLE 3. Summary of Embrittlement Couples P - element (nominally pure) A - alloy

~iquid I So I id",,,,J

Hg P

Sn

P

x x

Bi

P

Cd

P

Zn

P

P

P

In

Na

A

P

P

Li A

P

Sn P

Bi A

P

A

TI

Cd

P

P

Pb P

A

Zn

Te

Sb

Cu

P

P

P

P

X

X

X

LA

Z~

X

Mg

CA

AI

P

x

CA

x

Ge

P

Ag

P

Cu

CP

x

LA

x

CA

x

P

x

x

(/3 ¢'3

X

x

Ga

Cs

C - commercial L - laboratory

t:~

X

X

X

X

X

X

t'~ X

X

X

>

X

X

LA

Ni

LA

X

X

x(.'P)

[-~

X X

CA Fe

LA CA Pd Ti

×(?)

P x

X X

P

X

LA

X

CA

X

X

X

X

X

< O

oo Z O

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Westwood et al. (8) have observed that embrlttlement appears to be favored when the electronegativities of the liquid and solid, respectively, are the same, or nearly the same (for example, a difference of 0.1 units on the Paullng (9) scale). And, conversely, they suggest that embrittlement is not favored when the electronegativities are significantly different (for example, 0.4 units on the Paullng (9) scale). These investigators have pointed out that the "electronegativity difference" criterion may be regarded as a semi-quantltative statement of the "no intermediate phases" criterion of Rostoker et al. (1). Some additional observations concerning the electronegativity difference criterion are reported by Kamdar (10). As might be expected, there are several exceptions to this suggested correlation which is tested in Table 4. This table gives the difference in the Pauling electronegatlvities between the elemental (pure) solids and liquids which are embrittlement couples (see Table 3). In addition to the obvious variation from the electronegativity difference criterion exhibited by couples having an alkali metal as the liquid as well as the -0.3 to +0.4 range of differences for the other couples, the elemental non-embrlttlement couples may be considered.

(The elements are arranged in the table such that the

electronegativity increases from left-to-right and from top-to-bottom).

For example, the electronegativlty dif-

ferences for the non-embrittled couples AI-Sn, Cd-Hg, and Fe-Hg are +0.3, +0.2, and +0.1, respectively. TABLE 4. Electronegativity Differences for Elemental Embrittlement Couples

SolldX~ AI

Cs

Na

Li

Zn Cd

Ga

Cd

In

0.1

0.2

0.2

0.0 -1.0

Pb

Sn

0.2

0.2

TI

Bi

-0.1

Sb

0.3

0.1 0.1(7)

Ge

-0.2

Ni

-0.8

Sn

-0.1

-0.1

0.0

0.0

0.0

0.1

-0.9

0.1

0.0 -0.2

Ag

0.1

-0.3

0.0

Bi Pd

Hg 0.4

Fe

Cu

Cu

0.0 -1.0

-0.9

-0.1

0.0

0.0

-1.2 Practical Problems with Specificity As mentioned above, the concept of specificity leads to certain practical problems. The first of these is

the question of how much testing over what range of variables must be performed in order to state that a couple is non-embrittling. Consider, for example, the case of mercury as a liquid metal environment. It is of course convenient when testing various solid metals in mercury to perform those tests at ambient temperatures. Pure aluminum tested in mercury at room temperature was found to be only slightly embrittled and at one time Westwood and his co-workers considered that the LME effect of other elements on aluminum could be studied by dissolving them in mercury. The mercury in this case was called an "inert carrier" for the second element. Further research however revealed that the AI-Hg couple was embrittllng with an upper transition temperature between 0°C and room temperature (11).Addltions of other elements such as gallium to the mercury shifted the

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transition to temperatures above ambient so that brittle behavior at room temperature was observed. This exampie demonstrates the danger of single temperature testing when dealing with a ductile-brlttle transition type of mechanical behavior. Consideration of the pertinent literature will lead to the conclusion that similar cases may be represented by the reports of non-embrlttlement of cadmium (12), iron (13), and gold (14) by mercury. When the additional variables of grain size and strain rate are brought into the picture, the situation becomes still more complex. In cases of non-embrittlement there will always be the question of whether embrlttlement would be found if the grain size were slightly greater or the strain rate higher. In other words, having observed no embrlttlement for a particular set of experimental parameters and lacking a theory which predicts non-embrittlement (see below)~ one is then left with the difficult problem of experimentally establishing, with reliability, a negative observation which involves at least six experimental variables. Another practical problem is the question of how much deterioration of mechanical properties is necessary for a couple to be considered to be embrittled. Speclflclty~ as normally dlscussed~ is a go-no go concept; a given couple is either embrlttllng or non-embrlttling. In the case of many high-strength commercial structural alloys, the embrlttlement is severe and virtually zero ductility, with a fracture stress at or near the yield stress, is observed.

However, when working with ductile, low-strength pure metals, the embrlttlement may not be at

all severe. As mentioned above, the AI-Hg couple was, for a time, considered to be inert in spite of a small loss in ductility relative to pure, bare AI. In much the same veint there exists 0 question with regard to composition, in a number of cases, it has been found that although a commercial alloy based on a particular element was embrlttled by a certain liquid metal, the pure base metal was not. So, it may be asked whether it is reasonable to assume that if Fe-Si and Fe-AI alloys are embrlttled by mercury (13)~ then iron should also be embrittled by this liquid metal if tested under the proper conditions. Parallel situations no doubt exist for 2024-T4 aluminum alloy and pure aluminum, and for beryllium copper and pure copper. As one moves in the direction from alloy to pure metal, embrlttlement becomes more difficult due to the lower strength and the absence of stable barriers to dislocation motion (15) and changes in sllp characteristics (13). It is tempting to reason that these effects are purely mechanical and not related to the influence of the environment on the base metal and that the metal that is fracturing is basically the same in the alloy and in the pure metal but this rationalization is hardly satisfactory. In short, the current experimental basis for specificity in LME is considered to be tenuous and arbitrary. Specificity in LME Theories Theories which have been proposed to account for I.ME (1, 16, 17) do not explicitly encompass specificity. Indeed, each of the proposed theories is capable~ with appropriate assumptions, of either incorporating or denying the existence of specificity in LME. There are basically two widely accepted theories for I.ME: the surface energy approach and the bond-breaklng approach. The surface energy approach is basically an adaptation of the Perch and Stables theory (19) of hydrogen embrlttlement and was applied to I.ME by Rostoker and his coworkers (1) and more recently by Ichinose and Oouchi (16).

In this approach, the grain size dependence of the brittle fracture stress is used to obtain an

effective surface energy for fracture which is then related to the various interfacial energies involved in the process (grain boundary energy, solld-vapor surface energy and solld-liquld surface energy). It is thus implied that I.ME will be favored by good wetting or by low dihedral angles where grain boundaries meet the liquidsolid interface.

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The bond-breaking model (17,18) seeks to extend the surface energy approach by putting the energy into terms of the processes occurring at a growing crack tip. The model envisions atoms of the liquid metal being adsorbed at the crack tip and in some way influencing the bond between the two solid metal atoms at the crack tip in such a way as to lower the bond energy and flatten the interatomic potential energy curve. This change results in a lowering of the peak force required to break the bond in question which in turn is reflected in a decrease in the observed fracture energy, ¢ :

where 11 is called a coefficient of embrittlement and is the square of the ratio of the force required for bondbreaking in the presence and in the absence of the liquid metal atom, ~ is a parameter which is a measure of crack tip blunting due to plastic flow and Yo is the surface energy for the fracture in the absence of any aggressive environment. While this model appears to give a better picture of the atomistics of the fracture process, this theory does not encompass the concept of specificity. There is nothing in this theory that says why one atomic species changes the interatomic potential (or force) while another liquid metal does not. It may be assumed that a particular liquid had decreased the interatomic force at the equilibrium spacing but only after the fact. There is no aspect of the bond-breaking theory which would predict such a decrease. Conclusion It is the conclusion of this critical examination of specificity as an aspect of liquid metal embrittlement that it is a feature that is of questionable validity, difficult to demonstrate and unaccounted for theoretically. Acknowledgments The work reported here was supported by the Department of Defense under Project THEMIS (Contract No. DAAA-25-69-C0608) through Frankford Arsenal. It is to be submitted by one of the authors (FAS) in partial fulfillment of the requirements for the Ph. D. degree in the Department of Metallurgical and Materials Engineering at the Illinois Institute of Technology, Chicago, Illinois 60616. References 1.

W. Rostoker, J. M. McCaughey, and H. Markus, "Embrittlement by Liquid Metals", 162 pp., Reinhold Publishing Corp., New York (1960).

2.

W.R. Warke, P. Gordon, and J. C. Lynn, First Annual Tech. Rpt. on Contract No. DAAA-25-69C0608, Illinois Institute of Technology, Chicago (1970).

3.

W.R. Warke, P. Gordon, and J. C. Lynn, Second Annual Tech. Rpt. on Contract No. DAAA-25-69C0608, Illinois Institute of Technology, Chicago (1971).

4.

S. Mostovoy and N. N. Breyer, Trans. ASM 61 (1968) 219.

5.

W.R. Warke, K. L. Johnson and N. N. Breyer, in "Corrosion by Liquid Metals", J. E. Draley and J. R. Weeks (eds.), p. 417, Plenum Press, New York (1970).

6.

N . S . Stoloff in "Surfaces and Interfaces I1", J. J. Burke et al. (eds.), p. 157, Syracuse University Press, Syracuse (1968).

7.

R.C. Carlston, Naval Res. Revs. 18 (9) (1965) 1.

8.

A . R . C . Westwood, C. M. Preece, and M. H. Kamdar, in "Fracture", Vol. 3, H. Liebowitz (ed.), p. 589, Academic Press, New York (1971).

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L. Pauling, "The Nature of the Chemical Bond", 3rd ed., p. 93, Cornell University Press, Ithaca (1960).

10.

M . H . Kamdar, Phys. Star. Sol. (A)4 (1971)225.

11.

C . M . Preece and A. R. C. Westwood, Trans. ASM 62 (1969) 418.

12.

M . H . Kamdar and A. R. C. Westwood, Phil. Mag. 15 (1967) 641.

13.

N . S . Stoloff et al., in "Environment Sensitive Mechanical Behavior", AIME Met. Soc. Conf., Vol. 35, p. 613, Gordon and Breach, New York (1967).

14.

C . M . Preece and A. R. C. Westwood, in "Corrosion by Liquid Metals", J. E. Draley and J. R. Weeks (eds.), p. 441, Plenum Press, New York (1970).

15.

M . H . Kamdar and A. R. C. Westwood, in "Environment Sensitive Mechanical Behavior", AIME Met. Soc. Conf., Vol. 35, p. 581, Gordon and Breach (1967).

16.

H. Ichinose and C. Oouchi, Trans. Japan Inst. Metals 9 (1968) 35.

17.

N . S . Stoloff and T. L. Johnston, Acta Met. 11 (1963) 251.

18.

A . R . C . Westwood and M. H. Kamdar, Phil. Mag. 8 (1963) 787.

19.

N . J . Perch and P. Stables, Nature 169 (1952) 842.