deformation and fracture properties of damage tolerant in-situ titanium matrix composites

Applied Composite Materials 4: 361–374, 1997. 361c© 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Deformation and Fracture Properties of DamageTolerant In-situ Titanium Matrix Composites

S. DUBEY and W. O. SOBOYEJODepartment of Materials Science and Engineering, The Ohio State University, Columbus, Ohio43210, U.S.A.

T. S. SRIVATSANDepartment of Mechanical Engineering, The University of Akron, Akron, Ohio 44325, U.S.A.

(Received 17 December 1996; accepted 21 May 1997)

Abstract. This paper discusses the tensile response and fracture toughness of in-situ titanium alloymetal matrices discontinuously-reinforced with whiskers of titanium boride which were success-fully produced by ingot metallurgy techniques. Additions of elemental boron resulted in a nearuniform dispersion of the rod-like titanium boride (TiB) reinforcements in the alloy matrix. Suchcomposites have engendered considerable scientific and technological interest due to their attrac-tive combinations of improved mechanical properties and low manufacturing cost. The improvedelastic moduli of the composites are explained using shear lag and rule-of-mixtures theories. Theincreased strengths of the in-situ composites are rationalized by considering the combined effectsof deformation restraints imposed by the stiff whiskers and strengthening contributions arisingfrom the substructure that evolves from the presence of additional dislocations.

1. Introduction

Engineering the development and successful emergence of a new generationof civil and military aerospace vehicles which can travel at higher speeds, forlonger ranges, withstand greater payload capacity, offer better fuel economy andhave improved operational capabilities, necessitates the need for more efficientengines, improvements in airframe design and use of high performance materials[1]. While substantial improvements in aircraft design and engine performancehave been realized [1], it is generally held that design with currently availablecommercial materials alone will not meet the demand for a significant improve-ment in structural efficiency for the newer generation of aerospace vehicles suchas the advanced tactical fighter (ATF) [2, 3].

Due to their increased temperature capabilities, enhanced by their low density,brittle-phase reinforced metal matrices have emerged as attractive and potential-ly viable alternatives to the traditional and never generation monolithic alloys[3–6]. The metal matrices are reinforced with high strength, high modulus andbrittle second phases which can be either continuous (in the form of fibers) ordiscontinuous (in the form of whisker, platelet and particulate) reinforcements.From a technological viewpoint, the incorporation of discontinuous reinforce-

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ments into a titanium alloy metal matrix results in appreciable improvementsin: (i) specific modulus [E/ρ] and specific strength [σ/ρ], (ii) wear resistance,(iii) structural efficiency, (iv) reliability, and (v) control of physical propertiessuch as coefficient of thermal expansion [7–15], thereby affording the possibilityof improved mechanical performance in comparison to the unreinforced matrix.Given that titanium is now an everyday engineering material with a price perunit volume between the stainless steel and nickel-base superalloys, the overallpicture of either a whisker or particulate-reinforced titanium is construed as oneof a ‘well-matured’ matrix, but still an ‘immature’ composite.

Recent years have seen the development and emergence of both continuously-reinforced and discontinuously-reinforced titanium-matrix composites [TMCs].These composites offer dramatic improvements in maximum temperature capabil-ities in regions where previously only nickel-base alloys were used. Furthermore,the TMCs have provided an opportunity to extend the operating temperaturerange of monolithic titanium alloys in advanced aerospace products. Besides, itis interesting to note that the density of TMCs is only half that of the contendingnickel-base alloys [16]. There have been many potential applications for TMCs inareas spanning from aerospace to automotive engineering [17]. Integrally bladedcompressor rings, shafts, ducts, fan components and structural rods are but a fewof the components for which the use of TMCs has been proposed. Titanium-basedcomposites have been envisioned for the aircraft skin, internal structure and coolengine parts of future commercial and military aircraft [18], and for componentsof the exhaust nozzle of the High Speed Civil Transport Aircraft [HSCT] [17, 18].The Toyota Motor Corporation has utilized discontinuously-reinforced titanium-matrix composites [DRTMCs] for the manufacture of exhaust and intake valves,connecting rods and other automobile engine components [19]. The primary con-cern that limits the extended use of TMCs, due primarily to their matrices, istheir high temperature capability and environmental resistance viewed from thestandpoints of both oxygen embrittlement and external oxidation at temperatureshigher than 535◦C [1]. Other causes for concern are the conjoint and mutuallyinteractive influences of large coefficient of thermal expansion (CTE) mismatchbetween the hard, brittle and elastically deforming reinforcing phase and thesoft, ductile and plastically deforming metal matrix, and the compatibility andreactivity of the reinforcement phase with the metal matrix.

The primary manufacturing processes that have been used for the synthesisof discontinuously-reinforced titanium matrix composites are:(1) The ingot metallurgy (IM) route [3, 20] in which the element forming the

discontinuous ceramic reinforcement is mixed with the molten metal, whichis subsequently cast and then subjected to mechanical deformation.

(2) The rapid solidification powder metallurgy [RSP/PM] technique [3, 5, 12,13, 21] in which the reinforcement is blended well with rapidly solidifiedtitanium powder. The resulting mixture is consolidated into a billet usingconventional powder metallurgy [PM] forming processes.

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DAMAGE TOLERANT TITANIUM MATRIX COMPOSITES 363

(3) The in-situ technique [22–27] in which conventional primary fabricationtechnique, that is, casting, is used to produce a two-phase material. Thetwo-phase material is subjected to extensive mechanical deformation by oneor more of several processes (swaging, drawing, extrucion and rolling). Theheavily deformed in-situ composite has a phase morphology quite similar tothat found in the conventionally processed matrix material.

The rapid solidification processing of Ti–B (boron) alloys with concurrentlarge undercooling and high cooling rates was found to be very effective forthe production of in-situ titanium matrix composites which contain large volumefractions of the reinforcing phase, similar to the whisker-reinforced aluminumalloy metal matrix composites [28–31]. The resultant RSP alloys have excellentcompositional homogeneity, small grain sizes and a near homogeneous distri-bution of the reinforcing phase. However, they are relatively expensive whencompared to the material produced by conventional ingot metallurgy.

In recent years, the in-situ processing route has been generally favored since itoffers a relatively low cost approach to engineering a material having impressivecombinations of strength, toughness and damage tolerance, while concurrentlyenhancing the range of potential matrix-reinforcement combinations. Conven-tional titanium alloy fabrication techniques such as extrusion, forging and rollingcan be used to deform the cast ingot to produce a wide range of product forms.

The objective of this paper is to document the influence of alloy compositionand boron content on microstructure, tensile response and toughness properties ofdiscontinuous whisker-reinforced titanium alloy metal matrix composites. Contri-butions to strengthening of the composite, from the second-phase reinforcements,are quantified using the shear-lag and rule-of-mixtures approaches. The strengthproperties of the composite are discussed in light of processing and intrinsicmicrostructural effects.

2. Material

The metal matrix composite materials (MMCs) selected for investigation in thisstudy were based on titanium-base alloys as the matrix material, discontinuously-reinforced with whiskers of titanium boride (TiBw). The discontinuous Tibw-reinforced titanium alloy metal-matrix composites were produced at Duriron Inc.(Dayton, Ohio, U.S.A.) by induction skull melting and conventional casting.Actual chemical composition (in weight percent) of the in-situ composites isgiven in Table I. The starting ingots were 70 mm in diameter. The cast ingotswere turned on a lathe to produce cylindrical billets. The billets were extrudedat the Wright Patterson Air Force Base (Dayton, Ohio, USA) to form rods withan average diameter of 20 mm. The ingot metallurgy alloy Ti-8Al-1V-1Mo-0.6B(Table I) was also produced by induction skull melting and conventional castingat Duriron, Inc. However, the final ingot had a starting diameter of 100 mm.The cast ingots were turned on a lathe to produce a cylindrical billet that was

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Table I. Actual chemical compositions (wt.-%) of in-situ titanium composites.

Alloy Al Cr Mo Fe Si Sn Zr B C O H N(ppm) (ppm)

Ti-62222-0.5B 6.0 1.7 1.7 – 0.20 2.2 1.7 0.5 0.01 0.14 45 140

Ti-62S-0.5B 6.5 – – 2.3 0.07 – – 0.50 0.01 0.12 42 172

Ti-62222-0.8B 6.2 1.8 1.8 – 0.18 2.1 2.0 0.80 0.01 0.14 50 217(700◦C/2h/AC)

Ti-62S-0.8B 6.1 – – 2.2 0.07 – – 0.80 0.01 0.13 50 180(700◦C/2h/AC)

Ti-6Al-4V-0.5B 6.5 4.2V – – – – – 0.48 0.02 0.13 30 –(700◦C/2h/AC)

Ti-62S-0.6B 5.75 1.62 0.08 0.62 0.09 0.09 60 –(700◦C/2h/AC)

Ti-8-1-1-0.6B 7.9 1.0V 0.9 – – – – 0.60 0.01 0.09 40 –(700◦C/2h/AC)

AC: Air Cooled.

approximately 90 mm in diameter. Elemental boron was generally added duringcasting of all of the alloys. However, the Ti-6Al-2Fe-0.6B alloy was cast atTIMET (Henderson, Nevada) and subsequently forged to form 87 mm (3.5 inch)diameter billets. The boron in this composite was added as TiB2. The billetwas extruded at 1150◦C to form ‘T’ shaped rods. The extrusion operation wasperformed at Extrusion Technologies, Inc., at Oakwood Village, Ohio.

3. Experimental Techniques

3.1. INITIAL MICROSTRUCTURE EVALUATION

Metallographic samples were prepared from the as-received TMCs by mount-ing in Bakelite and wet grinding with silicon carbide (SiC) impregnated emerypaper using water as the lubricant. The mounted samples were subsequentlymechanically polished using an alumina-based polishing compound. Reinforce-ment morphology, size and distribution in the titanium alloy metal matrix, andother intrinsic microstructural features were examined with an optical micro-scope, and photographed using a standard bright-field illumination technique.Quantitative microscopy techniques were also used to characterize the microstruc-tural properties of the in-situ titanium composites.

3.2. SPECIMEN DESCRIPTION AND PREPARATION

Tensile test specimens were precision machined from cylindrical blanks of theas-received composites using a diamond-tipped cutting tool. The specimens weremachined with the stress axis parallel to the extrusion direction and conformed to

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standards specified in ASTM E-8 [32]. To minimize the effects of surface irregu-larities and finish, the machined surfaces of the test specimens were mechanicallypolished to remove all circumferential scratches and surface machining marks.Single-edge notch (SEN) specimens were prepared from the cylindrical rods forfracture toughness testing. The specimens were 50 mm long, 13 mm wide and6.35 mm thick. Notches were machined into the specimen using electro-dischargemachining (EDM) and a 0.1 mm diamond wire. Fracture toughness values wereobtained by monotonically loading the SEN specimens to failure under three-point bend loading. Prior to testing all of the TMCs were annealed in vacuum at700◦C for 2 h and then allowed to cool in laboratory air.

4. Results and Discussion

4.1. INITIAL MICROSTRUCTURE

The microstructures of the as-received composites are shown in Figure 1 (a–g).All of the composites have Widmanstatten (α + β) titanium matrix microstruc-tures. The Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.5B and Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.8B(700◦C/2h/AC) composites have very fine α lath widths of size 0.3–2 µm, whencompared to the Ti-6Al-2Fe-0.5B; Ti-6Al-2Fe-0.6B (700◦C/2h/AC) and Ti-6Al-2Fe-0.8B (700◦C/2h/AC) composites in which the width of the α laths vary from2.4–8.0 µm (Table II). The Ti-6Al-4V-0.5B (700◦C/2h/AC) composite also hasα laths which vary in width from 1.0 to 1.3 µm. In all of the titanium-matrixcomposites, the lengths of the laths range from 5.0 µm to 40.0 µm.

The volume fraction of the β phase in Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.5B (Fig-ure 1a) and Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.8B (700◦C/2h/AC) (Figure 1c) is about30–34% compared to values between 11 and 14% for Ti-6Al-2Fe-0.5B (Fig-ure 1b) and Ti-6Al-2Fe-0.8B (700◦C/2h/AC) (Figure 1d). This is attributed tothe higher level of β stabilizers in the first two alloys (Table I). The Ti-6Al-4V-0.5B (Figure 1g) also had a high volume fraction of the β phase (about 36%).

The alloys Ti-6Al-2Fe-0.6B (700◦C/2h/AC) (Figure 1f) and Ti-8Al-1V-1Mo-0.6B (700◦C/2h/AC) (Figure 1g) also have lower amounts of the β phase (10–14%). The α laths in Ti-8Al-1V-1Mo-0.6B (700◦C/2h/AC) are also much finer.Boron levels (0.5%, 0.6% or 0.8%) appeared to have no significant effect onmatrix microstructure. This was, however, strongly influenced by alloy compo-sition and the secondary processing (extrusion) temperature.

Optical micrographs revealed an inhomogeneous distribution of the TiB whisk-er reinforcements in the titanium alloy metal matrix, resulting in whisker-richand whisker-depleted regions. An agglomerated or clustered site consisted ofthe smaller TiB rod-shaped reinforcements, of varying size, intermingled withfew larger rod-shaped reinforcements. The majority of the TiB whisker rein-forcements were aligned along the direction of extrusion. The volume fractionof TiBw increases with increasing boron content. The as-extruded Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.5B and Ti-6Al-2Fe-0.5B composites have reinforcements that

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Figure 1. Microstructure of in situ titanium matrix composites. (a) Ti-62222-0.5B (asextruded); (b) Ti-62S-0.5B (as extruded); (c) Ti-62222-0.8B (700◦C/2h/AC); (d) Ti-62S-0.8B (700◦C/2h/AC); (e) Ti-8-1-1-0.6B (700◦C/2h/AC); (f) Ti-62S-0.6B (700◦C/2h/AC);(g) Ti-6Al-4V-0.5B (700◦C/2h/AC).

are between 0.5 and 30 µm in length with mean aspect ratios (ratio of lengthto diameter) of about 5 : 1. The widths of the TiB whisker reinforcements weregenerally between 2.3 and 5.0 µm. The reinforcements were found to be largerin the in-situ composites with higher boron content.

The TiB whiskers in the heat treated composites (Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.8B and Ti-6Al-2Fe-0.8B) were 10–40 µm in length, and their diameters wereabout three times greater than those in the alloys with low levels of boron

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Table II. Summary of microstructural parameters.

Alloy TiB whisker a lath β

Width Length Aspect Ratio Length Width Volume(µm) (µm) (µm) (µm) fraction

Ti-62222-0.5B 0.26–5.5 0.3–23 Mean 5 : 1 8.0–27.0 0.3–2.5 34%Mean 2.3 Mean 12 Mean 18 Mean 2.0

Ti-62S-0.5B 0.30–9.7 8.7–32.0 Mean 5 : 1 6.3–18.3 2.3–4.8 14%Mean 5.0 Mean 15 Mean 12 Mean 3.2

Ti-62222-0.8B 5.0–14.0 11.0–43 Mean 4 : 1 6.2–33.0 0.3–1.8 30%(700◦C/2h/AC) Mean 7.7 Mean 24 Mean 17 Mean 1.5

Ti-62S-0.8B 3.2–22.0 6.8–30.0 Mean 4 : 1 9.0–27.0 2.4–5.0 11%(700◦C/2h/AC) Mean 8.5 Mean 16 Mean 15 Mean 3.8

Ti-6Al-4V-0.5B 0.60–6.0 7.0–23.0 Mean 8 : 1 5.0–24.0 1.0–1.3 36%(700◦C/2h/AC) Mean 2.3 Mean 13 Mean 12 Mean 1.1

Ti-62S-0.6B 0.50–25.0 11.0–55.0 Mean 3 : 1 10.0–42.0 3.1–8.0 10%(700◦C/2h/AC) Mean 3.0 Mean 14.0 Mean 17 Mean 4.3

Ti-811-0.6B 0.50–5.0 6.0–100.0 Mean 9 : 1 10.0–43.0 1.0–6.0 9%(700◦C/2h/AC) Mean 1.0 Mean 24 Mean 20 Mean 1.4

(whiskers were observed to have widths between 1 and 20 µm and a mean aspectratio of 4 : 1). The Ti-6Al-4V-0.5B (700◦C/2h/AC) revealed a relatively uniformdistribution of the TiB whiskers with their lengths ranging from 7–23 µm andwidths between 1 and 1.3 µm. The reinforcements had an average aspect ratio of8 : 1. The reinforcements were observed to have different mean dimensions inthe two composites investigated. The Ti-8Al-1V-1Mo-0.6B (700◦C/2h/AC) hasTiB whiskers that are about 1 µm in diameter, whereas the Ti-6Al-2Fe-0.6B(700◦C/2h/AC) has coarse reinforcements of the order of 3 µm in diameter.The differences in the reinforcement morphologies of the two composites areattributed to the distinctly different methods used for the addition of boron. Themicrostructural parameters obtained for the different TMCs are summarized inTable II.

4.2. TENSILE AND TOUGHNESS PROPERTIES

4.2.1. Elastic Modulus

The as-extruded Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.5B composite has an elastic modu-lus of 126 GPa, while the Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.8B (700◦C/2h/AC) com-posite has a modulus of 129 GPa. The Young’s modulus of the Ti-6Al-2Fe-0.5Bcomposite in the as-extruded condition is 135 GPa, while that of the Ti-6Al-2Fe-0.8B (700◦C/2h/AC) is 136 GPa (see Table III).

The influence of the TiBw reinforcements on composite modulus is rational-ized using the shear-lag model developed originally by Cox [34], and modified in

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Table III. Summary of basic mechanical properties.

Alloy Heat Plastic Yield Ultimate Young’s KIc

treatment/ elongation stress tensile moduluscondition to failure (%) (MPa) strength (GPa) (MPa(m))

(MPa)

Ti-62222-0.5B AE 8.9 1078 1191 126 50.1

Ti-62S-0.5B AE 5.4 1096 1164 135 40.7

Ti-62222-0.8B 700◦C/2h/AC 3.2 1200 1282 129 50.2

Ti-62S-0.8B 700◦C/2h/AC 7.2 1071 1129 136 45.4

Ti-62S-0.6B 700◦C/2h/AC 10.1 950 1043 137 44.6

Ti-8-1-1-0.6B 700◦C/2h/AC 10.9 959 1060 136 39.6

subsequent work by Nardone and Prewo [35]. The shear-lag model was formu-lated to compute the stiffness and yield strength of composites reinforced withshort fibers. The stiffness of short fiber-reinforced metal matrix is expressed as[36]:

Ec/EM = (1− Vw) + Vw(Ew/EM)[1− tanh(X)/X] (1)

where:

X = 1/d[(1 + νM)(Ew/EM)ln(Vw)−1/2]−1/2 (2)

where EM, Ew and Ec are the respective moduli of the matrix, whisker and thecomposite; Vw is the whisker volume fraction; νM is the Poisson’s ratio of thematrix; and 1/d is the whisker aspect (length-to-diameter) ratio. The presenceof 0.5 wt.-% boron corresponds to 3.1 vol.-% (VM) of TiB reinforcements [37].Similarly, 0.6 wt.-% B corresponds to 3.72 vol.-% and 0.8 wt.-% corresponds to4.96 vol.-% TiB whisker reinforcements. The predicted values for elastic modulusand strength is comparable to the measured value (Table IV). Small differencesin the measured and predicted values are attributed to the fact that the aspectratios of the TiB whiskers (TiBw) used for calculation are approximate meanvalues based on a determination using the image analysis technique.

4.2.2. Strength and Ductility

The yield strength (defined as the stress required at a plastic strain of 0.2%) ofthe Ti/TiBw composites increased from 1078 MPa for the as-extruded Ti-6Al-

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Table IV. Prediction of strength and modulus using the shear-lag and rule-of-mixturesmodels.

Alloys Measured Calculated

Shear-lag Rule-of-mixtures

E (GPa) σys (MPa) E (GPa) σys (MPa) E (GPa) σys (MPa)

Ti-6Al-4V 112 863

Ti-6Al-4V-0.5B 112.2 862 118 967 123 942

Ti-62222 114 904

Ti-62222-0.5B 126 1088 117 972 125 982

Ti-62222-0.8B 129 1200 117 994 132 1034

Ti-62S 117 938

Ti-62S-0.5B 135 1073 120 1011 128 1015

Ti-62S-0.6B 137 950 118 999 132 1040

Ti-62S-0.8B 136 1071 120 1031 135 1066

Ti-811 117 938

Ti-811-0.6B 136 959 125 1107 132 1040

2Cr-2Mo-2Sn-2Zr-0.5B to 1200 MPa for the Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.8B(700◦C/2h/AC): it decreased from 1096 MPa for the as-extruded Ti-6Al-2Fe-0.5B to 1071 MPa for the Ti-6Al-2Fe-0.8B (700◦C/2h/AC) and to 950 MPa forthe Ti-6Al-2Fe-0.6B (700◦C/2h/AC). The ultimate tensile strength (UTS) of theas-extruded Ti-62222-0.5B composite is 1191 MPa which increases to 1282 MPafor the Ti-62222-0.8B (700◦C/2h/AC) composite. The ultimate tensile strength ofthe as-extruded Ti-6Al-2Fe-0.5B composite reduces from 1164 MPa to 1129 MPafor the Ti-6Al-2Fe-0.8B (700◦C/2h/AC) and 1043 MPa for the Ti-6Al-2Fe-0.6B(700◦C/2h/AC) composite (Table III).

The theoretical yields strength of the composite is expressed as [36, 37]:

σyc/σym = 0.5Vw(2 + 1/d) + (1− Vw) (3)

where σyc and σym are respective yield stresses of the composite and the matrixmaterial.

A small difference is observed between the calculated tensile yield strengthand the experimentally measured value. The difference is attributed to the fact thatthe aspect ratios of the TiB whiskers (TiBw) used for calculation are an approx-imate mean value based on a determination using the image analysis technique.The ultimate tensile strength is only marginally higher than the yield strength,indicating that the rate of work hardening past yield is low. The ultimate tensilestrength follows the same trend as the yield strength of the in-situ compositematerial.

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Ductility, expressed by plastic elongation to failure, of the as-extruded Ti-6Al-2Cr-2Mo-2Sn-2Zr-0.5B composite is 8.9% and reduces to 3.2% for theTi-6Al-2Cr-2Mo-2Sn-2Zr-0.8B (700◦C/2h/AC). The plastic elongation to fail-ure increases from 5.4% for the as-extruded Ti-6Al-2Fe-0.5B to 7.2% for theTi-6Al-2Fe-0.8B (700◦C/2h/AC) (Table III). The Ti-6Al-2Mo-2Cr-2Sn-2Zr-B-based composites lose ductility with an icrease in boron content, whereas theTi-6Al-2Fe-B-based composites have improved ductility as the boron contentincreases. The decreased strength and a concurrent increase in ductility, as thereinforcements become coarse, suggests the dependence of both strength and duc-tility on reinforcement geometry, which in turn is dependent on the synergisticinfluences of boron content and heat treatment.

4.2.3. Fracture Toughness

The Ti-6Al-2Mo-2Cr-2Sn-2Zr-base composites have the maximum fracture tough-ness [50.2 MPa(m)0.5]. The fracture toughness values of the as-extruded Ti-6Al-2Fe-0.5B composite is 40.7 MPa(m)0.5. However, the fracture toughness of theheat treated counterparts Ti-6Al-2Fe-0.6B (700◦C/2h/AC) and Ti-6Al-2Fe-0.8B(700◦C/2h/AC) was 44.6 MPa(m)0.5 and 45.4 MPa(m)0.5 respectively. The frac-ture toughness of the Ti-8Al-1V-1Mo-0.6B (700◦C/2h/AC) composite was thelowest, being only 39.6 MPa(m)0.5 (Table III). These values reveal a strongdependence between composition and boron content and fracture toughness ofthe in-situ composites examined in this study. Heat treatment was found to haveno influence on fracture toughness of the Ti-6Al-2Mo-2Cr-2Sn-2Zr-base com-posites, while the effect was observable for the Ti-6Al-2Fe-base composites.The observed improvements in fracture toughness is ascribed to the change inreinforcement morphology and number that occurs during heat treatment.

Scanning electron microscopy revealed the presence of broken TiB reinforce-ments in the Ti-6Al-2Mo-2Cr-2Sn-2Zr, Ti-6Al-2Fe and the Ti-8Al-1V-1Mo-basecomposites. The TiB whiskers either debond or fracture in the plastic zone beforesignificant bridging can occur in these in-situ composites. Elastic bridging wasnot observed in either the Ti-6Al-2Mo-2Cr-2Sn-2Zr or Ti-6Al-2Fe base compos-ites. Small scale bridging was evident in the Ti-6Al-4V-0.5B composites. TheTiB whiskers behind the crack tip and within the plastic wake were intact. Tough-ening of the in-situ Ti-6Al-4V-0.5B composite can at least partly be ascribed tothe elastic bridging of the TiB whisker reinforcements, details of which can befound elsewhere [38].

4.3. STRENGTH AND DEFORMATION MECHANISMS

Several strengthening or hardening mechanisms have been proposed which, eitherindependently or concurrently, are considered responsible for the enhanced strengthof discontinuously-reinforced metal matrices as compared to unreinforced metal

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matrices. The plausible mechanisms for the enhanced strength of this discontinu-ous TiBw-reinforced Ti alloy matrix, over the unreinforced counterpart, include:(a) An overall strengthening resulting from the strength of the individual con-

stituents of the composite, as explained by the classical rule-of-mixtures[ROM] theory [33].

(b) Classical composite strengthening through load transfer between the titaniumalloy metal matrix and the discontinuous whisker reinforcements [39, 40].Load transfer is largely dependent on the bond integrity at the interfaces ofthe whisker reinforcement and the metal matrix.

(c) High dislocation densities generated in the composite matrix as a direct resultof mechanical deformation (extrusion) of the in-situ Ti/TiBw composite [41–43]. Evidence in support of this view is based on: (i) electron microscopicobservations of heavily deformed in-situ composite containing the niobiumphase [44]; and (ii) resistivity measurements [45].

(d) Enhanced strength of the heavily deformed in-situ composite (HDISC) aris-ing from inherent strain incompatibility in the composite matrix as com-pared to the unreinforced metal matrix [46, 47]. The strain incompatibilityis accommodated by the generation of dislocations [48]. In these titaniummatrix composites (Ti-6Al-2Cr-2Mo-2Sn-2Zr/TiBw and Ti-6Al-2Fe/TiBw),a two-phase material, the strain incompatibility results from the conjointand mutually interactive influences of crystal orientation differences and theinherently different flow characteristics of the composite constituents: (a) thetitanium alloy matrix; and (b) the TiB whisker reinforcement. The result isthat a greater number of dislocations are generated during deformation ofthe two-phase material. The substructure that evolves from the presence ofadditional dislocations contributes to strength [47].

(e) Strengthening of the titanium alloy metal matrix by barriers to dislocationflow provided by boundaries of the discontinuous whisker reinforcements[49, 50]. This strength can be related to spacing between the discontinuouswhisker reinforcements. The intrinsic influence of the TiB whisker rein-forcements is accounted for by its unfluence on both matrix deformationcharacteristics and modulus.

(f) Strengthening arising from constrained plastic flow and triaxiality in thetitanium alloy matrix due to the presence of the discontinuous whisker rein-forcement phase [51]. As a direct result of the TiB whisker reinforcementsresisting plastic flow of the composite matrix an average internal stress, saya back stress [σb], is generated.

(g) Contributions from intrinsic differences in texture between the compositematrix and the unreinforced matrix material [51].

The constraints on deformation caused by the presence of TiB whisker rein-forcements in the ductile titanium alloy metal matrix and the concomitant devel-opment of a triaxial stress state aids in limiting flow stress of the compositematrix and favors:

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(i) void initiation and limited growth; and(ii) cracking and debonding at the whisker-matrix interfaces.As a direct consequence of the deformation constraints induced by the presenceof discontinuous TiB whisker reinforcements, a higher applied stress is requiredto initiate plastic deformation in the composite matrix. This translates to a higherelastic constant and higher yield strength of the Ti/TiBw composite over theunreinforced monolithic counterpart.

Failure of the discontinuous TiBw reinforcements during uniaxial deformationis governed by competing and mutually interactive influences of local plasticconstraints, whisker size, and distribution. The local plastic constraints are par-ticularly important for the larger-sized whiskers and for whisker clusters duringdeformation of the Ti/TiBw composite material. The composite microstructuresuggests that the damage may be highly localized at the TiB whisker reinforce-ments through the conjoint action of whisker separation and interfacial failure.This offers evidence of the plastic strain becoming localized during tensile defor-mation. The localized deformation results either in whisker separation from thematrix or the occurrence of debonding at the whisker–matrix interfaces. Thedeformation behavior of the composite matrix is therefore significantly alteredby the presence of discontinuous TiB whisker reinforcements.

The overall ‘damage’ occuring due to uniaxial straining of the Ti/TiBw com-posite can be ascribed to the conjoint action of damage associated with thediscontinuous TiB whisker reinforcements such as whisker separation from thecomposite matrix with possible decohesion at the whisker–matrix interfaces.

5. Conclusions

Based on the results of this investigation on the tensile deformation and fracturetoughness properties of Ti alloy discontinuously-reinforced with TiB whiskers(TiBw), the following are the key observations:

1. The matrix microstructure of the in-situ composites showed a marked depen-dence on alloy composition and less dependence on boron content andheat treatment. The dimensions of the TiB whisker reinforcements showedmarked dependence on the mutually interactive and competing influencesof: (i) boron content in the composite; (ii) the manner in which boron wasadded; and (iii) the extrusion temperature.

2. The Ti-6Al-2Mo-2Cr-2Sn-2Zr-base composites have high strength and elas-tic modulus and reasonably good ductility (plastic elongation to failure). TheTi-6Al-2Fe base composites have lower strength than the Ti-6Al-2Mo-2Cr-2Sn-2Zr-base composites but higher elastic modulus. The Ti-6Al-2Fe-0.6Bcomposite had lower strength compared to other composites based on thealloy Ti-6Al-2Fe. This is attributed to the formation and presence of coarseTiB whiskers resulting from using a different method for the addition ofboron. The Ti-8Al-1V-1Mo-0.6B composite shows excellent combinations

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of elastic modulus and density, but relatively lower strength (yield and ulti-mate tensile).

3. The Ti-6Al-2Mo-2Cr-2Sn-2Zr-base composites had the highest fracture tough-ness at about 52 MPa(m)0.5. The Ti-6Al-2Fe composites had fracture tough-ness values ranging from 40.7 MPa(m)0.5 to 45.4 MPa(m)0.5. The Ti-6Al-4V-0.5B has an intermediate fracture toughness value of 39.9 MPa(m)0.5, whichis an improvement over the powder-metallurgy processed Ti-6Al-4V matrix(KIC = 37 MPa(m)0.5) value obtained in an earlier study. The Ti-8Al-1V-1Mo-0.6B composite has a low fracture toughness value of 39.6 MPa(m)0.5.

Acknowledgements

This research was jointly supported by the McDonnell Douglas Corporationand the Division of Materials Research of The National Science Foundation(NSF). The authors are grateful to Dr. B. MacDonald (Program Monitor: NSF)and Mr. R. J. Lederich (Program Monitor: McDonnell Douglas Corporation) fortheir encouragement, support and criticism of this research.

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deformation and fracture properties of damage tolerant in-situ titanium matrix composites

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