THE DEFORMATION AND FRACTURE OF BORSICGD

REINFORCED TITANIUM MATRIX COMPOSITES

Karl M. Prewo Research Scientist Kenneth G. Kreider

Program Manager, Composite Materials United Aircraf't Research Laboratories

East Hartford, Connecticut

Introduction

Metal matrix composites are presently being considered for a wide range of engineering applications. When compared with resin matrix systems, these composites are less sensitive to elevated temperature, are less anisotropic in modulus and strength, and have the ability to form strong joints with primary metal struc­tures. Boron aluminum is, currently, the most widely used metal matrix system. However, because of the strength limitations of the aluminum matrix particularly at elevated temperatures, com­posites having a titanium matrix may prove superior for applications above 600°F. The investigation discussed below has shown that BORSIC fiber reinforced titanium matrix composites can be fabri­cated with axial strengths and moduli superior to monolithic titanium and transverse strengths and moduli superior to boron aluminum.

Experimental Method

Composite fabrication was carried out using polystyrene (fugi­tive binder) bonded tapes and the hot press diffusion bonding procedure. Tapes were fabricated by winding the fiber over titanium foil and spraying with a polystyrene-xylene mixture which hardened to bond fiber and foil together, A single tape was also fabricated using a polystyrene binder plus Ti-6Al-4V powder mixture. Diffusion bonding was performed in a two stage process. The fugitive binder was removed by holding the composite unconsolidated laminate at

2333

2334 K. M. PREWO AND K. G. KREIDER

84o°F for 30 minutes. The second stage of the procedure was the diffusion bonding of the composites for 30-60 minutes at elevated temperature, 1380-1600°F, at pressures of from 10~15 x 103 psi. The diffusion bonding was performed in vacuum of approximately 5 x lo-6 torr.

The materials used in composite fabrication included both 4.2 mil and 5.7 mil diameter BORSIC fiber. Matrices of commercial purity (75A) titanium, Ti-6Al-4V and Beta III titanium were used. The Beta III titanium is a Crucible Steel alloy having a composi­tion of 11.5% Mo, 6.0% Zr, 4.5% Sn and is noted for its high strength, superior fracture toughness and excellent cold form­ability. Foils of 2 mil thickness were used with the 4.2 mil fiber while 4 mil foils were used with the 5.7 mil fiber.

Unidirectionally reinforced tensile specimens were prepared and tested at both 0° and 90° to the fiber a.xis. The specimens were parallel sided and 0.25 in. wide, Aluminum doublers were bonded onto the specimens for gripping, leaving 1.0 in. to 1.5 in. long gage lengths·. Specimen mounted extensometers and strain gages were used to measure modulus and strain to failure. Tests performed at elevated temperature, as well as those performed at room temperature, were in air with 20 minutes at temperature prior to testing allotted for system thermal equilibrium to be achieved.

Impact testing was performed using notched Charpy type specimens.

Notched tensile testing was performed on 0.8 in. wide center notched panels. Notches were introduced by electrodischarge machining. No further notch tip preparation was used prior to tensile testing or considered necessary since notch acuity is determined by individual fiber breaks.

Results and Discussion

Axial Tension

The results of axial tensile tests performed at 70°F are pre­sented in Table I. Data for the Ti-6Al-4V matrix composites include results for 5.7 and 4.2 mil diameter BORSIC reinforced composites while the other two matrices were used only with the 4.2 mil fiber because of a shortage of foil. Hot pressing tem­perature was varied from 1380°F to 1600°F and it was found that fiber breakage during consolidation at the lowest temperature caused low composite strengths. Little difference was found between the performance of composites fabricated at the two higher temperatures. The choice of fabrication temperature, between 1470 and 1600°F, was made to maximize transverse composite strength and it was found that

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES 2335

Table I. Axial Tensile ProEerties of BORSIC-Titanium Tested at 70°F

Volume Ultimate Fiber Fraction Bonding Tensile Elastic Failure Diam. Fiber Temp. Strength Modglus Strain ~mils) ~%) (OF~ ~103Esi) ~10 Esi) (%)

None 0 As Recvd, 160.0 15.5 8.0 None 0 1470 166.o 7.0 None 0 1600 156.o 15.2 9.0 5,7 23 1470 174.o

191.0 5. 7. 45 1380 133.0

133.0 5,7 43 1600 180.0 33.0 0.52

186.o 176.o CY 33.0 0.52 167.0* 1 32.0

5,7 48 1600 170.0 ® 172.0* 2 0.625

4.2 30 1380 139.0 30 1470 146.o 47 1470 144.o 46 1470 154.o 46 1470 153.0 46 1470 172.0

75A Ti Matrix

4.2 47 1470 146.o 37,0 48 1470 123.0 31.0 38 1380 99.0 35.0

99,0

Beta III Matrix

None 0 As Recvd. 143.0 4.2 48 1470 179.5 38.o

162.0 50 1470 146.o 34.o

126.o 33.0 129.0 35.0

*Indicates fibers extracted from the comEosite and tensile tested

2336 K. M. PREWO AND K. G. KREIDER

in every case it was possible to achieve axial tensile strengths nearly equal or superior to the unreinforced matrix. In the case of the 5.7 mil fiber, all composite strengths exceeded the Ti-6Al-4V matrix strength.

A typical axial stress-strain diagram for 43% 5,7 mil BORSIC reinforced Ti-6Al-4V is presented in Figure 1 along with the trans­verse and unreinforced matrix stress strain curves. The axial elastic modulus of the specimen, E11, is eq~al to 33 x 106 psi which agrees well with a value of 33.8 x 10 psi predicted by a rule of mixtures calculation based on a modulus of 58 x 106 psi for BORSIC and 15.5 ·x 106 psi for Ti-6Al-4V. The proportional limit occurs at approximately 60,000 psi composite stress, however, the remainder of the curve exhibits a near linear slope of approximately 29 x 106 psi which indicates that the matrix has not fully yielded plastically. Just prior to the ultimate, a small number of serra­tions in the curve indicate the onset of final composite failure.

As indicated in Table I, fibers were extracted, by use of an acid solution, from two composites and tensile tested. The com­posites are numbered with superscripts Q) and @ in the table. The average extracted fiber strengths are 378,000 psi and 333,000 psi respectively while the standard deviations are 85,000 and 106 ,000 psi. Calculations of bundle strength, crb, for these fibers, based on the method described by Corten (1), result in bundle strengths of 236,000 psi and 200,000 psi respectively. These values can be used in a simplified rule of mixtures calculation of the composite strength, CTc, based on the bundle strength (cr0 ), the volume fraction fiber (Vr), the volume fraction matrix lVm), the composite failure strain (ec) and the matrix elastic modulus (~)

Predicted values of composite strength of 182,000 psi and 191,000 psi, for composites Q) and @ respectively, are 10% greater than the measured strengths of 167,000 psi and 172,000 psi.

Composite axial tensile strength is presented in Figure 2 as a function of test temperature. At l000°F the composite strength is approximately equal to that of heat treated Ti-6Al-4V at that tem­perature, 100,000 psi (2).

Fractured axial tensile specimens were examined using scanning electron microscopy. Figure 3 illustrates the fact that inter­facial debonding occurs between matrix and fiber. Fiber pull out lengths of up to one fiber diameter were also noted.

Transverse Tension

The results of composite transverse tensile testing are pre­sented in Table II. Composite transverse tensile strengths of 5.7

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES

·;;; Q.

"' 2 '

"' "' ... ... ,_ "'

200

180

160

140

120

100

80

60

40

20

o·

MATRIX TO 9.0%

EMAT. = 15.5 x 106 psi

E 11 = 33.2 >< 106 psi

E22 = 29.0 x 106 psi

0"-~'-----'~---'-~~~~~~~~~-'---'

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 STRAIN - 3

2337

Fig. 1. Tensile stress-strain curves for 43% - 5.7 mil BORSIC reinforced Ti-6Al-4V.

200

"' 180 "' 160 ~ a.

"' 140 2 "'~ ::c 120 ,_ C> "' z 100 ... ... ,_

80 .o. AXIAL "' 0 TRAN SYER SE ~

"' 60 z

~ ... ,_

40

0 20

00 400 800 1200 TEST TEMPERATURE - 0 f

Fig. 2. Tensile strength of 43% - 5.7 mil BORSIC reinforced Ti-6Al-4V as a function of .test temperature.

2338 K. M. PREWO AND K. G. KREIDER

Table II. Transverse Tensile Pro:eerties of BORSIC-Titanium Tested at 70°F

Volume Ultimate Fiber Fraction Bonding Tensile Elastic Failure Diam. Fiber Temp. Strength Modglus Strain

(%) ~OF) ~103Psi) (10 :esi)

Ti-6Al-4V Matrix

5.7 23 1470 47.5 48.3

40-43 1600 54.o 29.0 o.46 56.0 48.o 31.0 0.28 54.o o.42 49.5 0.34 50.5 0.25 37.1 0.23

47 1600 43.0 28.0 0.24 44.8 40.7

5.7 13 1600* 49.5 0.30 55.2 0.30

4.2 34 1470 15.3 17.6

22 1470 47.5 48.3

Beta III Matrix

4.2 46 1470 30.6 22.0 31.0 22.0

50 1470 31.0 22.0 31.0 22.0

*Fabricated using foil :elus :eowder matrix

mil BORSIC reinforced composites reach values well above 50,000 psi and are superior to those of the 4.2 mil reinforced composites. This relates to an observed difference in composite failure mode. The 4.2 mil BORSIC fibers split longitudinally during composite failure so that the composite transverse fracture surface is com­posed predominantly of split fibers. This is similar to the frac­ture morphology of 4.2 mil BORSIC-6061 aluminum composites (3,4) and agrees with the observation of the low transverse tensile strength of 4.2 mil BORSIC fibers (5). The fracture surfaces of 5,7 mil BORSIC reinforced Ti-6Al-4V composites, however, are com­posed primarily of matrix failure and fiber-matrix interfacial · failure. The 5.7 mil BORSIC fibers do not split as readily and have a higher transverse tensile strength, than the 4.2 mil fibers (5).

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES 2339

Figure 4, taken by scanning electron microscopy, illustrates the transverse fracture surfaces of three 5,7 mil BORSIC reinforced Ti-6Al-4V specimens tested at 70°F, 6oo°F, and l000°F. At 70°F and 6oo°F some longitudinal fiber splitting is visible and emanates from the composite edges where specimen preparation by diamond abrasive wheel cutting damaged the brittle BORSIC fiber ends. At l000°F, however, only matrix and interfacial failure modes are visible.

A typical transverse tensile stress strain curve for 43% 5,7 mil BORSIC - Ti-6Al-4V is presented in Figure 1. The composite transverse elastic modulus, E2~' of 29 x 10° .psi is not much less than the value of E11 (33 x 10 psi), eliminating the large aniso­tropy associated with many unidirectionally reinforced composites. The much lower transverse modulus of the Beta III titanium matrix composites is associated with the low elastic modulus of this matrix alloy when continuously furnace cooled from elevated temperature.

The composite transverse tensile strength is given in Figure 2 as a function of test temperature. The composite transverse strength, at all temperatures tested, was equal to 30% of the axial composite strength and retained a value of approximately 30,000 psi at l000°F. Transverse strength was proportional to the net area of titanium matrix in the composite fracture surface,

Several transverse tensile specimens were fabricated using powder and foil Ti-6Al-4V matrix in attempt to improve transverse properties. The composite mode of fracture and strength, however, did not change significantly.

Notched Tension

Center notched 5,7 mil BORSIC reinforced tensile specimens were tested in both the axial and transverse orientations. Notches con­sisted of 0.2 inch diameter circular holes and 0.2 inch long slots. The notch tip radius of the slots was 0.003 inch. Tensile test data are presented in Table III. All specimens failed in the plane of the notch and the net tensile strength of all specimens, both axial and transverse, was reduced due to the notch.

The axial tensile specimens exhibited a ratio of notched to unnotched net strength of from 0.70 to 0.89 depending on volume fraction fiber, notch geometry, and specimen thickness. The data are too few to draw conclusions on the·relative importance of each of these factors, however, the data do indicate that the BORSIC axially reinforced Ti-6Al-4V specimens are less sensitive to the presence of notches than BORSIC-aluminum specimens of similar geometry tested by Kreider,· et al ( 4). It was shown, in that inves­tigation, that notches of the type used herein reduce BORSIC­aluminum net composite axial tensile strength by as much as 50%.

2340 K. M. PREWO AND K. G. KREIDER

Fig. 3. Fracture surface of 5.7 mil BORSIC reinforced Ti-6Al-4V axial tensile specimen.

,.... ""iiiiiiill""' ·--::':' "" . ·-4"'"'*

"

p- I ..., ___________ __. .... _._. _!_II ()QQ!!. __

Fig. 4. Fracture surfaces of three 5,7 mil BORSIC reinforced Ti-6Al-4V transv~rse tensile specimens tested at 70°F top, 600°F middle, and l000°F bottom.

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES 2341

Table III. Tensile Stren th of Center Notched 5.7 mil BORSIC Reinforced Ti- Al-4V

Volume *Net Fraction Specimen Ori en- Tensile Notch UTS/ Fiber Thickness ta ti on Notch Strength Unnotched UTS

(%) inches (103Esi)

48 0.056 oo No 172.0 0.2 11 long slot 119.5 0.70

II 121.3

42 0.062 90° No 52.0 0.2 11 long slot 42.3 0.82

28 0.013 oo No 136. 7 No 136.o

0.2" long slot 112.0 0.82 0.2" dia. hole 122.0 o.89

28 0.013 90° No 40.4 No 36.5 o. 77

0.2 11 lone; slot 29. 7 *Tensile strength calculated on basis of maximum load divided by net cross sectional area

The reason for the lower notch sensitivity of the BORSIC-titanium specimens can be related to the observation of fiber-matrix inter­facial failure on specimen fracture surfaces, similar to the features noted in Figure 3 for unnotched specimen fracture morphol­ogy. As has been pointed out for resin matrix composites (6), the occurrence of fiber-matrix interfacial failure decreases the axial tension notch sensitivity of unia.xially reinforced specimens by decreasing the effective stress concentration at a crack tip. Fiber-matrix interfacial failure does not occur in fully consoli­dated BORSIC-aluminum and thus in that system matrix plasticity effects are the only method of "crack blunting".

Several BORSIC reinforced specimens were tensile tested while situated in an X-ray machine such that transmission radiographs could be taken during successive loading steps. As shown in the radiograph, Figure 5, fiber fracture occurred at the tips and ahead of machined notches prior to final specimen failure. The figure illustrates fractures of fiber cores, white lines, in the region adjacent to a machined circular hole (black region of figure). Matrix failure is also visible connecting the broken fibers. Fiber fractures were observed to occur, however, well ahead of matrix failure in the specimen. These observations indicate that the effective notch geometry, prior to composite failure, is quite different from that initially machined into the composite specimens.

2342 K. M. PREWO AND K. G. KREIDER

Any analysis of composite fracture toughness must take into account the new effective notch shape and size before valid generalized fracture criteria can be established,

The transverse tensile specimens are as sensitive to the presence of notches as the axial tensile specimens. The fracture mode of these specimens, as in the case of the unnotched specimens, was primarily one of matrix plus interfacial failure. The notch sensitivity of these specimens is in marked contrast to the com­plete insensitivity of both 5.7 mil BORSIC reinforced 6061 aluminum and 4.2 mil BORSIC aluminum transverse tensile specimens (4) to the presence of similar notches,

Impact

Notched Charpy impact specimens were tested in three different orientations; Q) the normal to the plane of the crack is parallel to the specimen principal fiber axis and the direction of crack propagation is transverse to the fibers, @ the normal to the crack plane is transverse to the principal fiber axis and the direction of crack propagation is parallel to the fibers, (]) the normal to the crack plane is transverse to the fiber axis and the direction of crack propagation is also transverse to the fiber axis. These specimen types have been described in more detail elsewhere for BORSIC aluminum (4), All impact specimens tested herein contained 46%-5.7 mil BORSIC in a Ti-6Al-4V matrix.

The fracture of a type G) Charpy specimen required 5,2 ft lbs of energy. This was the highest level of energy for the three specimen types tested and is approximately equal to the energy absorbed in the fracture of a 50% 5,7 mil BORSIC reinforced 6061 aluminum specimen, The notched Charpy impact energy of Ti-6Al-4V is typically 15-18 ft lbs (2). The fracture surface, Figure 6, illustrates that for this fracture mode all fibers must be frac­tured in longitudinal tension. Fiber pullout of over several fiber diameters in length has occurred for many fibers on the surface and fiber-matrix interfacial failure is also observed. Impact specimens of type Q) were tested in two orientations with respect to the original tape planes of the composite. In one case the planes of the tapes were normal to the direction of crack propagation while in the second case the tape planes contained the direction of crack propagation. No significant difference in impact energy was noted due to this variation,

Type @ specimen impact fracture required 0. 7 ft lbs of energy. The fracture surface, Figure 7, illustrates the pre­dominant matrix and interfacial modes of failure. Type 3 speci­men fracture absorbed 0.2 ft lbs of energy and the fracture surface is. illustrated in Figure 8. In this case, with the crack propa­gating colinear with the fibers, a large amount of fiber splitting was noted to occur and emanate from the root of the original

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES 2343

Fig. 5. Radiograph of a center notched BORSIC reinforced Ti-6Al-4V specimen under load showing fractured fiber cores at the notch •

. . ·· .. ' . ' . . ~ ~, ..... : ::~. ·~. ·•·.

_,!.\ --. -, !· ... •.' , ..

Fig. 6. Fracture surface of a type Q) 46% - 5.7 mil BORSIC reinforced Ti-6Al-4V notched charpy impact specimen.

2344 K. M. PREWO AND K. G. KREIDER

Fig. 7. Fracture surface of a type ~ 46% - 5.7 mil BORSIC reinforced Ti-6Al-4V notched charpy impact specimen.

Fig. 8. Fracture surface of a type ~ 46% - 5.7 mil BORSIC reinforced Ti-6Al-4V notched charpy impact specimen.

DEFORMATION AND FRACTURE OF BORSIC REINFORCED COMPOSITES 2345

machined notch. Both the type ~ and ~ impact energies are lower than those associated with BORSIC-aluminum composites and illustrate a large anisotropy in specimen toughness that must be considered for engineering applications.

Summary

It has been shown that BORSIC reinforced titanium matrix com­posites can be fabricated with axial tensile strength, axial elastic modulus and transverse elastic modulus greater than those of monolithic titanium. This superiority is even larger when these properties are compared on a specific (property/density) basis due to the lower density of the composite. Similarly, the transverse tensile strength and modulus of BORSIC titanium, particularly at elevated temperature, are considerably greater than those of boron aluminum composites. It has been demonstrated that composite perfonnance can be rationalized on the basis of constituent materials properties and that the presence of an "inter­facial failure mode, although limiting transverse composite ·· strength, is effective in limiting material notch sensitivity.

Although larger impact energies are desirable for the design of reliable man-rated structures, it is clear that the BORSIC­titanium system can provide a useful material for lightweight stiffness and strength critical applications.

References

1. Corten, H., "Micromechanics and Fracture Behavior of Com:­posites" in Modern Composite Materials, ed. by Krock, Addison­Wesley, 1967.

2. ASM Metals Handbook, Vol. 1, 1961.

3. Prewo, K. and Kreider, K., "The Transverse Tensile Properties of Boron-Aluminum", Met. Trans., 1972.

4. Kreider, K., Dardi, L. and Prewo, K., "Metal Matrix Composite Technology", U. S. Air Force Materials Lab. Technical Report, AFML-TR-71-204, Dec. 1971.

5. Prewo, K. and Kreider, K., "The Transverse Strength of Boron Fibers", Composite Materials: Testing and Design~ STP-497,

6. Beaumont, P. and Philips, D., "Tensile Strengths of Notched Compos it es" , Jl. Comp. Mats • , Vol. 6, p 32, 1972.