,. 

DOE/NASAl0563-1 

NASA CR-175074 

Effect of Boron on Tensile 

Properties of B2 FeAI 

M. Crimp and K. Vedula 

Case Western Reserve University 

February 1986 

Prepared for 

NASA-CR-175074 

19860012185 

llBRARV COpy 

APR 22 1Y86 

LANGLEY RESEARCH CENTER 

LIBRARY, NASA 

HAMPTON, VIRGINIA 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Lewis Research Center 

Under Grant NAG 3-563 

for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

, Office of Vehicle and Engine R&D 

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NF01238 
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Effect of Boron on Tensile· 

Properties of 82 FeAI 

M. Crimp and K. Vedula 

Case Western Reserve University 

Cleveland, Ohio 44106 

February 1986 

Prepared for 

DOE/NASAl0563-1 

NASA CR-175074 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

Under Grant NAG 3-563 

for 

U.S. DEPARTMENT OF ENERGY 

Conservation and Renewable Energy 

Office of Vehicle and Engine R&D 

Washington, D.C. 20545 

Under Interagency Agreement DE-AI01-77CS51040 

SUMMAltY 

EFFECT OF BORON ON TENSILE PROPERTIES OF B2 FeAl 

M. Crimp and K. Vedula 

Department of Metallurgy and Materials Science 

Case Institute of Technology, Case Western Reserve University 

Cleveland, Ohio 44106 

Small additions of boron have been shown to improve the room temperature 

ductility of the intermetallic compound Ni Al in recent studies. The boron is 

believed to segregate to the gr.ain boundar~es ana strengthen them, allowing 

the inher.ent ductility of the grains to be achieved. 

The present study was undertaken to see if boron has a similar effect on 

the low temperature tensile properties of the equiatomic intermetallic 

compound FeAI. A binary alloy without boron is compared with an alloy 

containing 0.78 at % B (0.2 wt %) B, by tensile testing over the temperature 

range of 300 K to 640 K. Both alloys were processed by powder metallurgy. 

Results showed that 0.78 at% B addition to FeAl does indeed change the 

room temperature fr.acture mode from inter.granular to transgranular, 

suggesting a strengthening of grain boundaries. The alloy containing bo~on 

is, however, still brittle at room temperature. A slight decrease in the 

ductile to br.ittle transition temperature is, nevertheless, observed. In 

addition a significant increase in strength of the alloy is observed with 

boron addition. 

1. INTRODUCTION 

Iron aluminide, FeAl, is an ordered intermetallic compound having the B2 

(CsCI) crystal structure. This aluminide has recently been the object of 

considerable interest as a potential structural material for moderate to high 

temper.ature applications. 

A major problem with this aluminide is that although single crystal FeAl 

displays consider.able ductility at room temperature [1-4], polycrystalline 

materials are brittle [S]. One possible r.eason which has been suggested for 

this brittleness is the lack of sufficient operative slip systems at low 

temperatures resulting in a failure to satisfy Von Mises criterion. Some of 

the studies on single crystals and polycrystals [1,3,4,6], however, have 

determined that at room temperature, {110}<111> slip occurs, which would 

result in sufficient independant slip systems (five) to satisfy the Von Mises 

criterion. Cross-slip would, however, be restricted if slip were confined to 

the {110} planes. It has been suggested [7,8] that in ordered crystal 

structures, inability to cross slip may prevent spreading of slip across 

grain boundaries resulting in brittle fracture even If five independant slip 

systems are available. 

Another possible reason for room temperature brittleness in spite of the 

I 
presence of five independant slip systems may be grain boundary decohesion 

before a sufficient number of slip systems can be activated. This may be 

particularly applicable to ordered alloys since certain grain boundar.y 

structures may be less stable than others as a result of wrong neighbor 

considerations [9]. 

Recently, considerable effort has been devoted to the study of 

polycrystalline Ni3Al [6], an ordered intermetallic compound with the L12 

crystal structure [10,11]. This is a FCC ordered crystal structure and has 

enough independent slip systems for polycrystalline deformation. However, 

polycrystal1ine alloys were found to be brittle and fractured in an 

intergranular manner. Additions of small amounts of boron, changed the 

fracture mode from intergranu1ar to transgranular and dramatically made these 

alloys ductile at room temperature. TIle amount of ductility was found to be 

strongly related to the amount of boron. In addition, the mode of failure was 

found to change fr.om intergranular to transgranular with the addition of 

boron. It has been suggested [12] that boron acts to modify the intrinsic 

nature of the atomic bonds at grain boundaries resulting in greater grain 

boundary strength and hence the inherent ductllity of the individual grains 

of Ni3Al can be obtained without causing the grain boundaries to separate. 

A possibility, therefore, exists that po1ycrystalline FeA1 which appears 

to demonstrate {ll0} (111) slip with adequate number of independent slip 

systems, may also be failing in a brittle manner at room temperature due to 

weak grain boundaries. By analogy with Ni3Al, addition of boron may be 

expected to result in a change in the fracture mode as well as an improved 

ductility at r.oom temperature. A test of this hypothesis form8 the focus of 

this paper. 

Another interesting observation in the previous studies [1,3,4,6] on 

FeAl is a change in slip system to {110}<100> at approximately 0.40 Tm. The 

exact temperature of the transition was found to be a function of the 

orientation of the single crystals and the alloy stoichiometry. Polycrysta1s 

of FeA1 exhihit a ductile to brittle transition at about this same 

temperature. Although no clear correlation between the change in dominant 

slip system and the onset of ductility has been established, it is possible 

that the ductility may be a result of the observation that the critical 

resolved shear stress in the (lOa> direction decreases more rapidly than in 

the (111) direction. 

The purpose of the current investigation, hence, was a preliminary study 

of the effect of boron on the low temperature mechanical behavior of 

polycrysta1line FeAl. Samples contqining 0.78 at% B have been studied at 

temperatures ranging from 300 to 680 K and compared with binary FeA1. 

2. MATERIALS AND PROCEDURE 

Materials for this study were made from preal10yed FeAl powder mixed 

with boron powder in a Vee blender. The powders were canned in 2-inch 

diameter steel containers, evacuated, sealed and extruded at a 16:1 area 

reduction ratio at 1250 K. The extruded bars were then cut into 2-inch 

sections and the can removed by grinding. A homogenization treatment was then 

2 
performed at 1373 K for 48 hours In an argon atmosphere, followed by furnace 

cooling. A few of the specimens received a longer treatment for 100 hours at 

1423 K. Tensile bars were then machined by center less grinding. To avoid any 

residual effects from the machining, the sample bars were electropolished in 

a 10% perchloric acid - 90% methanol solution, maintained at a temperature of 

253 K and at a voltage of 10V. 

The chemical analysis of the two alloys shows the binary alloy (referred 

to as the Fe-AI alloy) to contain 51.5 at% Fe and 48.5 at% Al and the ternary 

alloy (referred to as the Fe-Al-B alloy) to contain 53.1 at% Fe, 46.1 at% Al 

and 0.78 at% B. The slight difference in stoichiometry of the two alloys is 

not considered to be very critical in this study. 

_lThe tensile specimens were tested in vacuum at a strain rate of -3x10-3 

sec at temperatures ranging fr.om 300 K to 680 K. Fracture surfaces were 

examined using scanning electron micl'."oscoPY' The substl'."ucture of untested 

samples were examined using a Phillips 400T tl'."ansmission electron micr.oscope 

operated at 120 kV. TEM specimens were prepared by electropolishing in a twin 

jet polisher using a 33% Irno3 - Methanol solution. The solution was 

maintained at a temperature of 253 K and at a voltage of 30V. 

3. RESULTS 

The micl'."ostructul'."es of both the alloys after the homogenization 

tl'."eatment reveal that the grains are equiaxed, indicating that 

recrystallization has occurred. Pl'."ior particle boundaries decorated by oxide 

pal'."ticles and some residual porosity al'."e also evident, as illustl'."ated in 

Figure 1. No difference is found between the optical microstructures of the 

Fe-AI specimens given slightly different heat treatments. The Fe-Al-B 

specimen displays a structure which appears to contain a significant number 

of second phase particles which are suspected to be borides. A detailed 

evaluation of these particles forms part of an ongoing study. 

A tr.ansition from br.ittle behavior at low temperatures to ductile 

behavior at high temperatures is cleal'."ly evident in both the alloys. 

Engineering stress- strain curves for the two alloys at different 

temperatures are compared in Figure 2 and the tensile properties measured are 

reported in Table I. The Fe-AI tensile specimens fractured in a totally 

brittle manner up to 600 K, whereas at 640 K, a significant amount of 

ductility is observed. One specimen exhibited - 8% elongation to fracture and 

another specimen (the one which was heat treated longer) tested at the same 

temperature of 640 K resulted in - 55% elongation to fracture after extensive 

necking elongation. The alloy containing boron, Fe-Al-B, remains brittle at 

300 K, but begins to show some ductility at 560 K, suggesting a modest 

decrease in the ductile to brittle transition temperature with boron addition. 

The addition of boron substantially increases the strength of Fe-AI, in 

addition to slightly lowering the ductile to brittle transition temperature. 

This is observed not only in the low temperature brittle fracture regime, but 

also in the higher temperature ductile fracture regime where both the yield 

stress and ultimate tensile strength are higher for the samples containing 

boron. 

3 
Of particular importance, however, is the observation that boron 

addition results in a change in the fracture mode from intergranu1ar to 

transgranu1ar. Figure 3 shows typical fracture surfaces from specimens tested 

below and just above the ductile to brittle transition temperature for both 

alloys (i.e at 300 K and 640 K). The Fe-A1 specimens display totally 

intergranu1ar fracture at 300 K whereas the Fe-A1-B specimens display totally 

transgranu1ar cleavage fracture at 300 K. At temperatures just above the 

ductile to brittle transition temperature (640 K), the intergranular mode for 

Fe-AI and the transgranu1ar mode for Fe-A1-B are still maintained even though 

some ductility is observed. The specimen of Fe-A1 which showed the large 

ductility at 640 K demonstrated typical dimple rupture features as 

illustrated in Figure 4. 

Typical dislocation substructures observed with TEM are shown in Figure 

5. The structure of the Fe-A1 consists of dislocations with definite 

orientations. The dislocations display sharp jogs and no evidence of faulting 

is observed. The addition of boron, however, has a dramatic effect on the 

dislocation substructure. The dislocations display smoother jogs and less of 

a tendency to lie in specific orientations. In addition, a considerable 

number of features were observed which are interpreted to be stacking faults. 

4. DISCUSSION 

The most significant finding from this study is that the addition of 

boron has resulted in strengthening of the FeAl grain boundaries relative to 

the grains as evidenced by the change in fracture mode from intergranular to 

transgranular fracture. This result is similar to the effect of boron in 

Ni3Al. It is possible to only speculate that this effect is due to boron 

segregating at the grain boundaries, until it can be verified by Auger 

Spectroscopy: However, no measurable ductility at room temperature is 

observed, inspite of this fracture mode change. One possibility is that the 

boron addition and/or stoichiometry of the alloy are not at the optimum 

values. The sensitivity of this kind of fracture transition to slight changes 

in composition has been very clearly established in Ni3Al. The level of boron 

in this study (0.78 at%) is believed to be much higher than that needed for 

grain boundar.y cohesion alone. Another possibility, as discussed in the 

introduction, is the difficulty of cross-slip contributing to early crack 

formation before plastic deformation spreads across grain boundaries. 

An attempt is made here to corr.elate the increase in strength and slight 

decrease in the ductile to brittle transition temperature with addition of 

boron. The stress required to initiate plastic deformation in any alloy 

typically decreases with temperature as illustrated in Figure 6. Included on 

the figure are near horizontal lines representing intergranular fractur.e 

stress for the case of weak grain boundaries and transgranular cleavage 

stress for the case of strong grain boundaries. The stress for intergranu1ar 

brittle fracture will be lower than the stress for transgranular brittle 

fracture. The competition between the stress required to initiate plastic 

deformation and the stresses needed for either intergranu1ar or transgranular 

brittle fracture determines the fracture mode at any temperature. If the 

stress required to initiate intergranular or transgranular brittle fracture 

is less than that required to cause plastic flow, the specimen will break in 

4 
a brittle manner. Whereas, if the stress required to initiate plastic flow is 

lower than the b~itt1e f~acture stress, as it is at higher temperatures, then 

the specimen exhibits ductility. Hence, Figure 6 schematically illustrates 

how the ductile to brittle transition temperature is obtained at the 

intersections between these curves. 

Strengthening of the grain boundaries by addition of boron can result in 

a significant decrease in the ductile to brittle transition temperature if 

the stress required to initiate plastic deformation (the curve for Fe-A1) 

remains unchanged as illustrated by points A and B in Figure 6. Point A 

represents the transition temperature when the grain boundaries are weak and 

hence brittle fracture occurs intergranu1ar1y. Point B represents the 

transition temperature when the grain boundaries are strengthened so that 

brittle fracture occurs by transgranular failure rather than intergranular 

failure. 

However, if the matrix also becomes stronger, the curve for the stress 

required for plastic deformation will shift upwards (curve for Fe-Al-B) and 

the fracture transition temperature shifts to a higher value (point C). 

thereby neutralizing some of the advantage gained by strengthening the grain 

boundaries. 

Using this approach, it is postulated that the higher strength of the 

Fe-A1-B matrix (as evident from Figure 2) has reduced the effect of grain 

boundary strengthening on the ductile to brittle transition temperature in 

the alloys investigated. The grain boundaries, however, remain stronger than 

the matrix resulting in a transgranular fracture mode and a significantly 

stronger alloy. 

The observation of intergranular fracture in Fe-Al even at temperatures 

where some ductility starts appearing, is probably a reflection of a delicate 

balance between the grain boundary strength and the grain matrix strength. 

Although the grains deform initially without grain boundary decohesion, work 

hardening can strengthen the grains relative to the grain boundaries, 

resulting in intergranu1ar failure. Higher temperatures, where work hardening 

cannot be expected to make the grains stronger than the grain boundaries, 

will therefore result in typical ductile fracture features. The fact that one 

specimen tested at 640 K showed extensive ductility with ductile fracture 

features whereas another specimen tested at the same temperature showed less 

ductility with intergranular fracture features, is believed to be either a 

result of changes in the material caused by the slightly different heat 

treatment or a result of experimental scatter in the properties at 

temperatures very close to the transition temperature. 

The transgranular fracture of Fe-Al-B at 560 K where some ductility is 

observed is probably a result of a similar balance between the transgranular 

cleavage stress and the stress required for plastic deformation of the matrix. 

Based on the results of the limited TEM information it is only possible 

to speculate on the reasons for the increase in the strength of the matrix 

with the addition of boron. As discussed earlier, the addition of boron 

resulted in a dislocation structure containing stacking faults as opposed to 

5 
a non-faulted structure in the material without boron. The presence of these 

stacking faults suggests a lowering of stacking fault energy (the nature of 

the stacking faults being unknown at present), making it more difficult for 

slip to occur and possibly resulting in the observed increase in matrix 

strength. 

5. CONCLUSIONS 

The addition of a small amount of boron to FeAl has been shown to result 

in a change in fracture mode from intergranular to transgranular at low 

temperatures. The fracture, however, remains brittle at room temperature. The 

strength of the alloy is increased very significantly and the ductile to 

brittle transition temperature is lowered slightly with boron addition. These 

results are rationalized in terms of grain boundary strengthening due to 

boron segregation at grain boundaries and strengthening of the matrix. 

ACKNOWLEDGEMENTS 

Research supported by NASA Lewis Research Center, Cleveland, Ohio, under 

Grant No. NAG 3-563. 

6 
REFERENCES 

1) T. Yamagata and H. Yoshida, Mat. Sci. Eng.,Vol 12, p. 95 

(1973). 

2) R.C. Crawford, Phil. Mag., Vol 33, p. 529 (1976). 

3) Y. Umakoshi and M. Yamaguchi, Phil. Mag. A, Vol 41, p.S73 

(1980). 

4) Y. Umakoshi and M. Yamaguchi, Phil. Mag. A., Vol 44, p. 711 

(1981). 

5) D.K. Chatterjee and M.G. Mendir.atta, J. Met., 33, (12), p.S 

(1981). 

6) M.G. Mendirata, H. Kim and H.A. Lipsitt, Met. Trans. A., p. 

395 (1984). 

7) M.M. Shea and N.S. Stoloff, Met. Trans., Vol.5, p.755, 1974. 

8) T.L. Johnson, R.G. Dave and N.S. Stoloff, Phil. Mag., Vol.12, 

p.30S, 1965. 

9) T. Takasugi and o. Izumi, Acta Met. Vol. 31, No.8, p.1187, 

1983. 

10) C.T. Liu, C.L. White and J.A. Horton, Acta Met., Vol.33, No.2, 

p.213, 1985. 

11) C.T. Liu, C.L. White, C.C. Koch and E.H. Lee, Proc. Symp. High 

Temperature Materials Chemistry II (edited by Munir et al.), 

The Electrochemical Soc. 1983. 

12) T. Ogura, S. Hanada, T. Masumoto and O. Izumi, Met. Trans. A, 

Vol.16A, p.441 (1985). 

7 
Transverse Longitudinal 

(a) 

Transverse Longitudinal 

(b) 

Figure 1. Comparison of Optical Micrographs of the (a) Fe-AI 

(b) Fe-Al-B Alloys Illustrating Recrystallized Grains 

and Prior Particles Boundaries in Both Alloys. 

8 
(/) 

(/) 

W 

0: 

tn 

(/) 

(/) 

W 

I<.SI 

160 

12 

I<.SI 

160 

120 

0: 8 

tn 

o 

300 1<.560 I<. 

0.04 0.08 

640 I<. 

0.12 

STRAIN 

(a) 

6401<. 

0.12 

STRAIN 

(b) 

0.16 

Fe-AI-B 

0.16 

0.~5 

Strain at 

fractur. 

0.20 

0.20 

MPa 

1000 

MPa 

1000 

Figure 2. Comparison of Engineering Stress-Strain Curves for 

(a) Fe-Al and (b) Fe-Al-B Alloy. 

9 
300 K 640 K 

(a) 

300 K 640 K 

(b) 

Figure 3. Comparison of the Fracture Surfaces for the Two Alloys: 

(a) Fe-AI and (b) Fe-AI-B alloy at Two Different 

Temperatures. (Scanning Electron Micrographs, SEM). 

10 
Figure 4. SEM Fracture Surface of Fe-AI Alloys Showing Large 

Ductility and Typical Ductile Fracture at 640 K. 

(a) (b) 

Figure 5. Transmission Electron Micrographs Illustrating the Major 

Differences in the Substructure of the Two Alloys 

(a) Fe-AI and (b) Fe-Al-B. 

11 
<f) 

<f) 

W 

c.r 

I-

<f) 

Pt. A: DBTT FOR Fe-AI 

Pt. C: DBTT FOR Fe-AI-B 

BI 

I 

I 

1 

I 

I 

YIELD STRESS 

I 1 

1 I -----1-- ....J I --- 1-- ---

1 I AI 

I I I 

I I I 

I I I 

I I I 

1 1 I 

I I I 

I 1 I 

1 I I 

1 I I 

TEMPERATURE-------

Figure 6. A Schematic Repr.esentation of a Possible Explanation for 

the Obser.ved Differences in the Behavior of the Two 

Alloys. 

12 
Temp. 

300 K 

560 K 

600 K 

640 K(1) 

640 K(2) 

Table I 

Results of Tensile tests on Fe-Al and Fe-Al-B 

as a function of TemperatuE~· -1 

Constant strain rate of -3 x 10 sec • 

y.S. 

89 

71 

Fe-AI 

T.S. F.S. 

92 

85 

77 

80 

EI% 

8 

55 

Fe-AI-B 

y.S. T.S. F.S. 

141 

137 143 

120 130 

116 129 

y.s. 0.2 % offset yield strength for specimens which 

exhibited plastic yielding. 

EI% 

<1% 

2.5 

11 

T.S. Ultimate tensile strength for specimens which showed 

maximum load followed by necking elongation. 

F.S. Stress at which brittle fracture occurred before 

yielding for brittle specimens. 

OR 

Stress at which ductile fracture occurred before 

necking elongation for ductile specimens. 

El% Per.cent elongation to failure in a gage length of 

3 cm. for ductile specimens. 

13 
1. Report No. 2. Government Accession No. 

NASA CR-175074 

4. Title and Subtitle 

Effect of Boron on Tensl1e Propertles of B2 FeAl 

7. Author(s) 

M. Crlmp and K. Vedu1a 

9. Performing Organization Name and Address 

Case Western Reserve Unlvers1ty 

Cleveland, Oh10 44106 

12. Sponsoring Agency Name and Address 

U.S. Department of Energy 

Off1ce of Vehlc1e and Englne R&D 

Washlngton, D.C. 20545 

15. Supplementary Notes 

3. Recipient's Catalog No. 

5. Report Date 

February 1986 

6. Performing Organization Code 

8. Performing Organization Report No. 

10. Work Unit No. 

11. Contract or Grant No. 

NAG 3-563 

13. Type of Report and Period Covered 

Contractor Report 

14. Sponsoring Agenct:eode~epO rt No. 

DOEINASA/0563-1 

Flna1 Report. Prepared under Interagency Agreement DE-AIOl-77CS51040. Project 

Manager, Robert H. T1tran, Materla1s Dlv1s10n, NASA Lew1s Research Center, 

Cleveland, Ohl0 44135. 

16. Abstract 

Small add1t1ons of boron have been shown to 1mprove the room temperature 

ductlllty of the 1ntermetal11c compound N13Al 1n recent studles. The boron ls 

be11eved to segregate to the galn boundarles and strengthen them, a110w1ng the 

lnheret duct1l1ty of the gralns to be achleved. 

The present study was undertaken to see 1f boron has a s1m11ar effect on 

the low temperature tenslle propert1es of the equ1atomic lntermetal11c compound 

FeAl. A blnary alloy w1thout boron ls compared with an alloy contalnlng 

0.78 at % B (0.2 wt %) B, by tensl1e test1ng over the temperature range of 

300 K to 640 K. Both alloys were processed by powder metallurgy. 

Results showed that 0.78 at % B addltlon to FeAl does lndeed change the 

room temperature fracture mode from 1ntergranu1ar to transgranu1ar, suggestlng a 

strengthen1ng of gra1n boundarles. The alloy contalnlng boron 1s, however, 

still brittle at room temperature. A s11ght decrease ln the ductl1e to brltt1e 

trans1t10n temperature 1s, nevertheless, observed. In addit10n a s1gn1flcant 

1ncrease 1n strength of the alloy 1s observed w1th boron add1tlon. 

17. Key Words (Suggested by Author(s)) 

Intermetal11c; Ordered B2; Tens11e; 

Ductl1e; Boron; Iron a1umln1de 

18. Distribution Statement 

Unclass1f1ed - un11mlted 

STAR Category 26 

DOE Category UC-25 

19. Security Classif. (of this report) 

Unc1assH1ed 

20. Security Class if. (of this page) 

Unc1assHled 

21. No. of pages 

15 

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