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Discontinuously reinforced titanium-titanium boride whisker (TiB_w) composites are becoming a commercial reality for a number of demanding automotive, aerospace, and other advanced applications. This article presents the general microstructural characteristics and mechanical properties of Ti-TiB_w composites of various matrix compositions and TiB_w volume fractions based on the authors’ research as well as work done elsewhere. The directions for future development in terms of the choice of titanium matrix compositions, microstructure control, and mechanical property optimization, as well as the realistic possibilities of near-term commercial applications in industry are discussed.

INTRODUCTION

Discontinuously reinforced titanium alloys containing titanium boride whiskers (TiB_w) are emerging as candidate materials for advanced applications. A variety of titanium alloys including a, a+ß, and ß alloys are already in use,¹ meeting diverse needs for specific strength, creep, and corrosion resistance. However, there appears to be a prevalent need to make titanium competitive with high-strength steels in specialized applications that demand properties beyond the levels provided by the conventional titanium alloys. This continues to drive the attempts to explore new material concepts such as titanium composites. The possibility that in-situ titanium composites can be far less expensive and easily amenable for net-shape manufacturing relative to continuous-fiber-reinforced titanium metal-matrix composites is a major impetus to developmental efforts in this area. In the aluminum family, Al-SiC composites reinforced with SiC particulates or whiskers have already reached significant use where their increased cost is offset by the performance improvements they offer. Owing to problems caused by the reactivity of titanium, interest in similar discontinuously reinforced titanium composites was not great until recently.

Virtually all of the attractive reinforcements such as SiC, Al₂O₃, Si₃N₄, and B₄C lead to the formation of one or more reaction products at the interface. This is a major barrier in attempts to develop a viable composite concept in the titanium system. Nevertheless, recent research efforts^2–12 seem to have recognized the property advantages that can be achieved by incorporating TiB whiskers as reinforcements in titanium or titanium alloys. This article highlights some interesting aspects of the Ti-TiB_w discontinuously reinforced composite system and outlines some important issues in processing, microstructure, and mechanical properties.

PROCESSING OF IN-SITU Ti-TiB_w COMPOSITES

Why TiB Whiskers Are the Most Suitable

Three boron compounds exist in the Ti-B system: TiB, Ti₃B₄, and TiB₂ at boron concentrations of about 18%, 22%, and 30% by weight. Although one may consider TiB₂ as the first choice for reinforcing titanium due to the relatively high values of melting point (3,054°C), elastic modulus (540 GPa), and hardness (2,200 kg/mm² Vickers hardness), the possibility of formation of Ti₃B₄ and TiB at the Ti-TiB₂ interface seems to have deterred the composite development. However, the use of TiB as reinforcement is attractive because there is no intermediate phase between titanium and TiB and the formation of TiB requires a far lower amount of boron compared to TiB₂. Further, the relatively lower temperatures involved in solid-state composite processing (900–1,100°C) offer manufacturing ease. In addition, TiB forms as long, pristine single-crystal whiskers in the titanium matrix. This means that, on the basis of the theories of stiffening and strengthening by whisker reinforcement, large increases in composite modulus and strength can be obtained with a relatively smaller amount of reinforcement. Such advantages do not seem to exist with other titanium compounds such as TiC, TiN, Ti₃Si₅, and TiB₂, since these compounds do not exist in thermodynamic equilibrium as whiskers as in titanium matrix. Additionally, the compositional stoichiometry of TiB is spread over a very narrow range (18–18.5 wt.%), unlike TiC, where the stoichiometry is spread over a large range (15.5–19 wt.% at 500°C), giving variability in the defect structure and properties in the compound. Table I lists some of the properties of the TiB phase along with those of TiB₂, TiC, and TiN.

Table I. Properties of TiB Reinforcement, Compared with Ti and Other Compounds
Property		Ti	TiB	TiB2	TiC	TiN
Density (g/cc)		4.57	4.56	4.52	4.92	5.43
Elastic modulus (GPa)		110	371*	540	450	390
Coeff. of thermal exp. at RT (°C)		8.6 × 10-6	7.15 × 10-6	6.2 × 10-6	7.95 × 10-6	9.35 × 10-6
Vickers hardness (kg/mm2)		150	1,800↑	2,200	3,200	2,300
Melting/decomposition temp. (°C)		1,668	2,200	2,970	3,054	3,220
* Determined in Reference 10; ↑ measured in Reference 12.

Methods for Creating TiB_w In-Situ in Ti-TiB Composites

TiB_w can be formed in titanium by solid-state reaction. Figure 1 illustrates the free energy of formation of several titanium compounds calculated from standard thermodynamic data. Although the DG of formation of TiB₂ is most negative, titanium and TiB₂ can react to form TiB due to the small negativity of the free energy of this reaction. This means that one can form TiB whiskers in titanium by reacting titanium and TiB₂ particles as long as the average composition of boron is less than that required to form TiB₂. Thus, solid-state fabrication techniques based on powder metallurgy and sintering can be applied⁹ to manufacture these composites over the complete range of volume fractions of TiB_w. Table II presents the different composites that have been reported in literature, along with some of the authors’ recent work. It can be seen that the most common technique used is powder metallurgy that either uses commercial powders^{2,6,7,9,12,14} or prealloyed gas-atomized powders,⁵ or mechanically mixed or alloyed powders.^3,4,11 Following initial alloying and/or cold pressing, the composites are usually free-sintered or hot isostatically pressed. This enables the densification and completes the reaction between titanium and TiB₂ powders to form the TiB whiskers inside a continuous titanium alloy matrix.

Table II. Room-Temperature Mechanical Properties of Discontinuously Reinforced Ti Alloys Having Different Vol.% of TiB Reinforcements
Matrix Composition (wt. %)		Vol. % Tib	E (GPa)	sy (MPa)	sUTS (MPa)	% Elong.	Method of Manufacture	Ref.
Ti (ASTM Grade-4)		0	110	480	550	15.0	Wrought	1
Ti-6Al-4V		10	136.6	540	1,000	0.25	MA + HIPing	11
Ti-24Al-10Nb (at. %)		10		*	695	0.0	PM + HIPing	11
Ti-6Al-4V		11	144	1,315	1,470	3.1	GA + HIPing/ extrusion	5
Ti		15	139	842	903	0.4	VAR + hot swaging	8
Ti-5Al-2.5Fe		15	151	*	1,092	0.0	PM + HIPing	2
Ti		20	148	*	673	0.0	PM	7
Ti-6Al-4V		20	154	1,170		2.5	MA + HIPing	3
Ti-4.3Fe-7Mo-1.4Al-1.4V		0	110		1,080	17.5	MM + CIP + sintering + hot swaging	13
Ti-4.3Fe-7Mo-1.4Al-1.4V		10	134		1,380	~7	Same	13
Ti-4.3Fe-7Mo-1.4Al-1.4V		20	156		1,640	~3	Same	13
Ti-4.3Fe-7Mo-1.4Al-1.4V		30	180		1,820	~1	Same	13
Ti-6.4Fe-10.3Mo		34	163.2	*	737	0.49	PM	Authors
Ti-24.3Mo		34	171.2	*	1,105	0.85	PM	Authors
Ti-53Nb		34	122.12	710	724	1.65	PM	Authors
Ti-6Al-4V		20	170	1,181	1,251	0.5	PM + E	14
Ti-6Al-4V		40	210		864	0	PM + E	14
MA: mechanical alloying; MM: mechanical mixing; PM: powder metallurgy processing; GA: argon gas atomization; VAR: vacuum arc melting; CIP: cold isostatic pressing; E: extrusion. * Specimen failed before yielding; data determined by extrapolation.

Morphology of TiB Whiskers

Figure 2a shows the distribution of TiB whiskers of the titanium matrix in the Ti-30TiB_w composite. When the volume fraction is significant, some of the TiB_w generally form interconnected morphologies as shown in Figure 2b. Otherwise, mostly independent TiB_w occurs. The interconnections between TiB whiskers are due to the formation of more than one relatively short TiB_w from one parent TiB₂ particle as a result of the spatial diffusion limitations of boron atoms and interceptions with other TiB_w. The mechanism of multiple whisker formation from one parent TiB₂ is presented in detail elsewhere.⁹ The crystal structure of TiB is B27 class orthorhombic, and the cross sections of whiskers formed are often hexagonal in shape (Figure 2c). This is because the transverse growth of TiB_w occurs at a much slower rate than that in the axial direction and the surface is bound by (100), (), (101), and () planes, giving more or less a hexagonal cross section. One or more of the bounding surfaces may be further decomposed into a stepped surface bound by () and () planes (Figure 2c). This can produce transverse TiB_w surfaces with a stepped morphology (Figure 2c and 2d), yet the cross-sectional shape more or less conforms to the hexagon.

Figure 2. The morphologies of TiB whiskers: (a) the general distribution, (b) the interconnectedness, (c) the cross section, and (d) the transverse surface of TiBw in Ti-30%TiBw composite.
Figure 3. Micrographs of TiB whisker distributions in various titanium alloy matrices. (a) and (b) are for Ti-6.4Fe-10.3Mo+34%TiBw, (c) is for Ti-24.3Mo+34%TiBw, and (d) is for Ti-53Nb+34%TiBw.

Microstructures of Ti-TiB Composites

The authors’ previous work^7,9 used the reaction-sintering technique, where commercially available powders of suitable sizes were reacted and densified in a one-step process. It was shown that a complete range of Ti-TiB_w composites with any TiB_w volume fraction can be fabricated by one-step reaction sintering. The microstructures were fully dense with a uniform distribution of TiB_w throughout the matrix. The authors also attempted,¹² using trimodal powder mixtures, to alloy the titanium-matrix such that some ß phase could be retained to increase the room-temperature ductility of these composites. Figure 3a through d illustrates the microstructures of some of the resulting composites. Micrographs 3a, 3c, and 3d are backscattered electron images taken with the scanning electron microscope. The TiB is seen as the dark phase in these micrographs. Micrograph 3b is a secondary electron image of deeply etched surfaces, revealing the morphological characteristics of TiB whiskers. It is interesting to see the presence of extremely fine TiB whiskers in the Ti-6.4Fe-10.3Mo composition. It is evident that a simple reaction in hot pressing alone can lead to the formation of TiB whiskers in the ß-Ti matrix throughout the composite.

MECHANICAL PROPERTIES OF Ti-TiB_w COMPOSITES

Figure 4. The elastic moduli of Ti-TiBw composites showing the fitting of the experimental data points by the predictions from Halpin-Tsai equations to determine an approximate modulus for 100% TiB phase.

Elastic Modulus of Ti-TiB_w Composites

Table II lists the compositions and tensile properties of Ti-TiB composites. The stiffness of Ti-TiB_w composites increases significantly when reinforced with TiB whiskers. Table II shows that with 10 vol.% TiB whiskers in the matrix, an increase of 20–25% in stiffness can be obtained. Even higher values of stiffness were obtained at higher volume fractions of the reinforcements. For example, the Ti-24.3Mo composite containing 34 vol.% TiB_w had a stiffness of 171 GPa. Atri et al.¹⁰ measured elastic modulus of Ti-TiB_w composites with volume fractions up to about 86% and showed that the stiffening effect can be well represented by Halpin-Tsai equations that express the composite stiffness as a function of aspect ratio and the volume fraction of TiB whiskers (Figure 4).

Elastic Modulus of TiB Whisker

The elastic modulus of a Ti-TiB_w composite depends on the modulus of the TiB whisker. The continuous boron chain along the length of the whisker suggests that TiB may have significant elastic anisotropy, perhaps more than that of TiB₂. Table III compiles the elastic modulus of TiB, either estimated, indirectly measured, or assumed, and reported in different studies.

During solidification, peritectic reaction governs the formation of TiB from titanium-rich liquid and TiB₂ particles. Hence, it has not yet been possible to make a single-crystal TiB phase to allow unambiguous measurements of nine independent anisotropic elastic constants of the B27 orthorhombic TiB phase using ultrasonic techniques. Recognizing this, Atri et al.¹⁰ back-calculated the polycrystalline elastic modulus of TiB from elastic modulus data of Ti-TiB_w composites with TiB_w ranging from 30–83 vol.% on the basis of Tsai-Halpin theory (Figure 4). The calculations yielded elastic modulus values of 325–435 GPa. An average of 371 GPa from these measurements was suggested as a first estimated value of the elastic modulus of TiB because this value fit the Ti-TiB_w composite data well, as shown in Figure 4. Later, other works,¹⁴ following the same Halpin-Tsai method, estimated the polycrystalline modulus of TiB to be 482 GPa. This estimate was based on the longitudinal and transverse elastic moduli of TiB whiskers estimated individually from the application of the Halpin-Tsai equation to the longitudinal and transverse elastic modulus measurements on an extruded Ti-TiB_w composite having a high degree of TiB whisker alignment. The estimated longitudinal and transverse-modulus values of TiB whiskers were 450 GPa and 514 GPa, respectively. The polycrystalline modulus value of 482 GPa was determined from these data. There could be two reasons for the relatively high value of 482 GPa. First, since the extruded Ti-6Al-4V matrix is likely to be somewhat textured, the modulus value of the Ti-6Al-4V matrix that was used in the Halpin-Tsai-equation-based back-calculation might be too low to account for the matrix texture and resulting high stiffness of the matrix in the longitudinal direction. In this context, it may be noted that high-purity titanium has elastic modulus values of 145 GPa and 100 GPa in the axis and the transverse directions of the hexagonal close packed unit cell.¹⁶ Therefore, a possible consequence of TiB elastic modulus estimations in oriented Ti-TiB_w microstructures is, quite likely, a higher value of TiB in the estimates. Nevertheless, further studies are clearly needed not only to determine the polycrystalline modulus of TiB, but also the nine independent elastic constants¹⁷ of the orthorhombic TiB such that a full picture of the elastic anisotropy of TiB can be constructed.

Table III. Elastic Modulus of TiB as Estimated in Different Investigations
Manufacturing Method		Estimation Method	Matrix Texture	Estimated Elastic Modulus of TiB (GPa)	Ref.
Blended element + reaction sintering		Halpin-Tsai equation	No	371	10
Blended element + extrusion		Halpin-Tsai equation	Possible	482	14
Blended element + extrusion		Longitudinal TiB modulus	Possible	450 ± 48	14
Blended element + extrusion		Transverse TiB modulus	Possible	514 ± 135	14
Assumption based on ZrB2		Unknown	No	550	15
N/A		Unknown	No	550	13

Thermal Expansion of TiB

Figure 4. The elastic moduli of Ti-TiBw composites showing the fitting of the experimental data points by the predictions from Halpin-Tsai equations to determine an approximate modulus for 100% TiB phase.

Because of the peritectic reaction leading to the formation of TiB from liquid and TiB₂, it is difficult to grow a single crystal and measure the thermal expansion characteristics in the principal crystallographic directions. Even synthesizing polycrystalline TiB in solid state is not easy and the compacts, although fully dense, often contain⁹ residual TiB₂ due to the directionally limiting nature of boron diffusion in TiB. Thus, we are limited to using a TiB-rich compact containing some residual TiB₂ in the matrix in order to measure the thermal expansion characteristics. The thermal expansion data and thethermal expansion coefficient (CTE) measured using TiB-16TiB₂ compact as a function of temperature are presented in Figure 5. The CTE of this material is 7.15 × 10^-6/K at room temperature and it increases to 11.32 × 10^-6/K at 1,625 K. The CTE values of polycrystalline TiB₂ are 5.6 × 10^-6/K and 10 × 10^-6/K at room temperature and at 1,625 K, respectively. Thus, the CTE values of TiB can be slightly larger than these values reported previously. The application of rule of mixtures is a reasonable approximation to the CTE of composites, especially when the difference in CTEs of components is not too large. Based on rule-of-mixtures, the estimates of CTE value of 100% TiB are 7.45 × 10^-6/K and 11.6 × 10^-6/K at room temperature and at 1,625 K, respectively. These estimates for the CTE of 100% TiB are different from the measured data of TiB-16TiB₂ only by 4% and 2% at room temperature and 1,625 K, respectively. Thus, one may take the CTE of Ti-16TiB₂ compact as that of the TiB itself and such an approximation may be sufficient for most practical purposes.

Strength

Table II presents a compilation of tensile properties of Ti-TiB_w composites. The listing is by no means exhaustive and is presented here merely to discuss the strength levels found in these composites. The ultimate strength levels vary between 673 MPa and 1,820 MPa, depending on the matrix composition and the reinforcement level in the composite. These levels are significantly higher than that of the wrought commercially pure titanium, which is about 550 MPa. The data for titanium alloy reinforced with 10%, 20% and 30% TiB_w, taken from Reference 13, serve to illustrate the general trend. The ultimate strength increases from 1,080 MPa to 1,820 MPa when the alloy is reinforced with 30% TiB_w. The ductility decreases from 17.5% for the unreinforced matrix to about 1% at 30% TiB_w reinforcement level. However, it is difficult at this time to systematize the strength of all the composites in Table II as a function of TiB_w volume fraction, since the matrix-strengthening levels and the aspect ratios of whiskers are not known and are varied among the composites reported in Table II. Further, the authors have observed¹² the occurrence of extremely fine whiskers, the diameters of which are in the nanometer range (Figure 3b) and are unaccounted for, in the stiffness and strength calculations. The matter is further complicated by the fact that the different composites have different amounts of interstitials (C, O, N) as well as a-stabilizing (Al) and ß-stabilizing (V, Fe, Mo, Nb) elements. These components lead to different proportions of a/ß phases in the matrix and the phases contain unknown amounts of interstitials (and some boron) in solution, partitioned selectively between the phases. A careful and systematic study is needed to determine unambiguously the relative contributions of TiB_w of different sizes, phase-level strengthening, and solid-solution strengthening in individual phases, to the total composite strength.

Ductility

Many of the composites, particularly those with an unalloyed titanium matrix and a high TiB_w volume fraction, show very low ductility levels, some approaching zero ductility. This is understandable, due to the brittleness of TiB_w as well as the connectivity between the reinforcements that exists even at TiB_w content of 30 vol.% (Figure 2b). The ductility of the a phase in unalloyed titanium is sensitive to the interstitial elements such as carbon, nitrogen, and oxygen, that can be absorbed during the processing steps. These elements are known to affect the ductility of titanium. However, the data in Table II suggest that by increasing the amount of ß phase in the matrix by alloying titanium with ß-stabilizing elements such as iron, molybdenum, and niobium, some ductility can be restored, even in composites having TiB_w volume fractions in excess of 30%. It is encouraging to note that the Ti-24.3Mo composites with 34% TiB showed a measurable ductility of about 0.9% at an ultimate strength level of about 1,100 MPa. The ductility in the Ti-53Nb composite is even higher, possibly due to a greater amount of ß phase in the matrix. However, the stiffness and strength is lower, relative to the Ti-24.3Mo composite, because of a reduction in the amount of high-aspect-ratio TiB whiskers in this composite. It is to be noted that this composite was tested without any post-processing treatments to achieve microstructure control. Obviously, considerable potential exists to combine composition adjustment and thermo-mechanical processing to increase the ductility of composites in this class.

High-Temperature Deformation Behavior

Reinforcement of a plastically deforming metal matrix normally increases the resistance to plastic deformation. Therefore, the introduction of TiB_w should increase the deformation resistance at high temperature, increasing high-temperature strength. Figure 6 illustrates the variation of tensile strength of Ti-30TiB_w composites as a function of temperature. The tests were conducted in a back-filled inert atmosphere using samples of 25.4 mm gage length and 3 × 3 mm² cross section, mechanically polished prior to testing. At room temperature, both the titanium and Ti-30TiB_w composites, fabricated similarly, failed in a nearly brittle manner. Thus, any benefit expected out of TiB_w reinforcement is not reflected in the strength. However, at 400°C, an increase of tensile strength from about 170 MPa for the titanium to 470 MPa for Ti-30TiB_w is seen, indicating the strengthening potential. The strength increase is significantly decreased at higher temperatures. The increases in strength are 45 MPa and 37 MPa, at 700°C and 1,000°C, respectively, for the Ti-30TiB_w composite, relative to titanium.

Figure 6. The tensile strength of Ti-30TiBw as a function of temperature, compared with similarly processed titanium.		Figure 7. The compressive flow stress of Ti-30TiBw as a function of temperature (strain rate 0.001/s), compared with similarly processed titanium.		Figure 8. The steady-state creep behavior of Ti-TiBw composites at T = 823 K. (From Reference 18 as well as present work.)

The high-temperature deformation data for Ti-30TiB_w, taken from the work of S. Kumari et al in this issue, is presented in Figure 7. The flow stresses were determined using constant-displacement rate (average strain rate of 10^–3/s) compression tests on 6.25 mm diameter and 12.5 mm long specimens. At 700°C, the increase in flow stress from 43 MPa for titanium to 103 MPa for Ti-30TiB_w is of the order seen in tensile tests (45 MPa).

The high-temperature strength at 600°C for Ti-30TiB_w is relatively lower compared to one of the standard high-temperature alloys, Ti-Al-2.7Sn-4Zr-0.4Mo-0.45Si, known as Ti-1100.¹ For example, the tensile strength of Ti-30TiB_w is 226 MPa by interpolation of data at 600°C, compared to about 575 MPa for Ti-1100. However, this comparison does not present a fair perspective because the matrix microstructure of the composite is not optimized to the degree of Ti-1100. It should be noted that the matrix in Ti-30TiB_w is a relatively coarse microstructure that resulted from hot pressing at 1,350°C. Such a hot-processing step can cause excessive beta grain growth, leading to a coarse grain structure in the matrix. Further, except perhaps for the presence of a very small unknown amount of B as interstitial, the titanium matrix in Ti-30TiB_w is devoid of solid-solution strengtheners such as Al, Sn, Zr, and Mo. It can be expected that with a greater degree of microstructural refinement as well as matrix alloying, the strength of Ti-30TiB_w can be significantly increased, quite possibly beyond 600 MPa.

Creep of Ti+TiB_w Composites

A general alloy design goal in the Ti-TiB_w composite development is a significant increase in creep resistance relative to titanium at temperatures where pure titanium is hopelessly soft. As there are only a few titanium alloys that have respectable creep resistance around 873 K, it is of interest to examine the creep-resistance capabilities of the Ti-TiB_w composites near this temperature. Figure 8 compiles the steady-state creep data at 823 K available to date. The data for titanium containing 5%, 10%, and 15% TiB_w were obtained by Tsang et al.¹⁸ by testing cylindrical tensile specimens of 4 mm diameter and 20 mm gage length at 823 K. The composites were made by vacuum-arc-melting of pellets made from titanium and boron powder mixture and appeared to have a fully dense and homogeneous microstructure. The data for Ti-30TiB_w were generated in the laboratory using tensile specimens 25.4 mm in gage length and 3 × 3 mm² in cross section. The unreinforced titanium specimens used for comparison in both studies had a relatively coarse matrix structure when compared to the common wrought-titanium microstructures. This was done to make a fair assessment of the enhancement of creep performance by TiB_w reinforcement. Figure 8 indicates that a significant reduction in creep rates can be achieved by TiB reinforcement relative to similarly processed titanium. Figures 9a and 9b illustrate the microstructures of the Ti-30TiB_w composite before and after creep testing. It can be seen that the TiB_w underwent significant rotation in the creeping matrix and tended to align in the direction of applied stress. It is believed that the large improvement in creep resistance comes mainly as a result of the work done in the mechanical rotation of the TiB_w as well as the degree of interconnectedness of TiB_w observed in the microstructure (Figure 2b).

Figure 9. The microstructures of the Ti- 30TiBw composite (a) before and (b) after creep deformation.

POTENTIAL APPLICATIONS OF Ti-TiB COMPOSITES

Because of the high stiffness, high hardness, and low density of TiB whiskers, Ti-TiB_w composites can be effective contenders for high-strength and lightweight applications. Table IV compiles properties of Ti-20TiB_w composites, collected from different studies, to present a preliminary outlook on the properties achievable in this class. The purpose of the table is to provide an approximate assessment of property levels that have already been achieved in Ti-20TiB_w composites. Assuming that improved Ti-20TiB_w composites can be made carefully in the near future by controlling composition, processing, and microstructure, it is not unrealistic to expect to attain at least 170 GPa in modulus, 1,200 MPa in yield strength, 1,800 MPa in ultimate strength, and about 2% in ductility. Further, the wear test data indicate that improvements in wear performance anywhere from 3–30 times that of the matrix can be obtained depending on the composition and processing of the composite. The reader is cautioned here that this is simply a projection that appears to be achievable on the basis of disparate data collected from the literature. The variability in wear data is a concern and it appears that this is due to widely different compositions and test arrangements employed in these studies.^13,19,20

The near-term possibility is the replacement of high-strength steels in some applications that can save about 50% of the weight of the structure, at the same time providing significant increases in oxidation, wear, and corrosion resistance. This is promising, considering the early stage of development of these materials. The relatively more successful Al-SiC system has gone through a development time of over two decades in the past. If the past is a guide, then there is room to hope that significant property enhancement can be achieved in Ti-TiB_w composites by alloy design and development. Presently, some notable applications include golf-club heads made by Toyota as well as components in Toyota Altezza vehicles. The latter application is discussed in detail in the article by T. Saito of Toyota Central Research and Development Laboratories, Japan, in this issue.

Table IV. A Compilation of Mechanical Properties of Ti-TiBw Composites with Various Alloy Matrices and a Reinforcement Volume Fraction of about 20%
Composite Matrix	Texture	E (GPa)	YS (MPa)	UTS (MPa)	% El.	Wear Loss	Wear Index-	Ref.
Ti-6Al-4V	Likely	170	1,181	1,251	0.5	—	—	14
Ti-7Mo-4Fe-2Al-2V	Likely	160	—	1,800	—	<1 mg/m*	30	13
Ti-6Al-4V	—	—	1,200	—	2	<1 mg/m*	30	13
Ti-16.3Fe	—	—	—	—	—	0.4 × 10-4 mm/Nm§	12	19
Ti	—	—	—	—	—	9.3 µm/m†	3.2	10
Wear test conditions: * Pin on disc with a sliding speed of 0.5 m/s for 2,000 m at a pressure of 0.24 MPa; § pin on disc test with the composite as a disk, tested at a sliding speed of 0.1 m/s for 1,000 m with a normal force of 2 N on a steel ball abrading against the disk; † pin on drum abrasive wear test against 150 grit garnet abrasive cloth with the pin, under a pressure of 2.1 MPa, moving at 0.36 mm/s and drum surface velocity 0.045 m/s; — The number of times the composite wear rate is smaller than that of the matrix.

RESEARCH NEEDS FOR FURTHER IMPROVEMENTS

The preliminary properties of the Ti-TiB composite compiled here demonstrate the promise shown by these composites for applications and the potential for further improvement. For example, the last three composites in Table II are made by one-step hot pressing without any subsequent processing. Naturally, the size, distribution, and phases in the matrix microstructure are not at optimum levels in these compositions. Therefore, with further optimization of initial powder sizes, composition, and processing conditions, and employing secondary processing steps, there are several avenues for property improvement in this class of composites. Traditional alloy design and development approaches based on physical metallurgy principles and intuition can have the greatest impact in this respect. Research is also needed in understanding the phase relations in the Ti-B-X (where X = Fe, Mo, Nb, etc.) systems, so that the relative proportions of a and ß phases and TiB in equilibrium for a given composition can be predicted. Computational phase diagram determination, high-temperature x-ray diffractometry, and electron microscopy are all critically needed to establish the phase equilibria, kinetics, and microstructural characteristics in this composite system.

ACKNOWLEDGEMENTS

Parts of this research were supported by grants, DAAL19-99-1-0281 from the Army Research Laboratory, Aberdeen Proving Ground, MD, and from the Army Research Office, Research Triangle Park, NC, respectively.

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For more information, contact K.S. Ravi Chandran, University of Utah, Department of Metallurgical Engineering, 135 South 1460 East, Room 412, Salt Lake City, UT 84112; e-mail ravi@mines.utah.edu.