Materials Science and Engineering A 418 (2006) 25–35

Design of powder metallurgy titanium alloys and composites

Y. Liu a,b,∗, L.F. Chena, H.P. Tanga, C.T. Liua, B. Liu a, B.Y. Huanga

a State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, PR Chinab Department of Materials Science and Engineering, The University of Tennessee, 434 Dougherty Engineering Building, Knoxville, TN 37996, USA

Accepted 31 October 2005

Abstract

Low cost and good performance are two major factors virtually important for Ti alloy development. In this paper, we have studied the effects ofalloying elements, thermo-mechanical treatment and particle reinforcement on microstructures and mechanical properties of powder metallurgy(PM) Ti alloys and their composites. Our results indicate that low cost PM Ti alloys and their composites with attractive properties can be fabricatedthrough a single compaction-sintering process, although secondary treatments are required for high performance applications. Three new PM Tialloys and one TiC/Ti composite of high performance are developed, and new design principles are also proposed. For design of PM Ti alloys,addition of alloying elements has the beneficial effect of enhanced sintering and/or improved mechanical properties. For example, Fe elementaccelerates the sintering process, Mo and Al are good candidates for solution strengthening, and rare earth elements effectively increase the materiald and solids forp©

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uctility by scavenging oxygen from the Ti matrix. For the design of Ti-based composites, in situ formation of strengthening particlesolution hardening of the matrix both should be considered simultaneously for alloy development. Cr3C2 is found to be a very suitable additiverocessing particle reinforced Ti composites.2005 Elsevier B.V. All rights reserved.

eywords: Ti alloys; Powder metallurgy; Ti base composites; Alloy design

. Introduction

Titanium and its alloys have been widely used in aerospacend terrestrial systems. More and more attentions have beenaid for expanding the markets of titanium alloys in automotive

ndustry, owing to both the demand of growth of titanium indus-ry and the decrease of energy consumption of vehicles[1,2].owever, titanium alloys cannot be used in a large scale in auto-obiles unless the cost of titanium alloy parts is lowered ton acceptable level. The high cost of titanium alloy parts comes

rom both alloy materials and metalworking. For example, whenanufacturing a 2.5 cm-thick plate, the former cost accounts

or about 53% and the latter 47%[3]. Therefore, both factorshould be considered in order to reduce the cost of titanium alloyarts.

Powder metallurgy (PM) has been taken into account for low-ring the cost of titanium alloy parts since 1970s[4,5]. Some PM

itanium parts have been used successfully for specific applica-ions[6–8].

PM titanium alloys can be classified, according to the ation of raw powder, into three categories: pre-alloyed PMalloys, rapid solidified PM Ti alloys, and blended elemePM Ti alloys [9]. Use of blended elemental powder is mmore cost-effective, due to cheap Ti and other elementalders. Commonly used titanium powder includes sponge Tiand the hydrogenation and dehydrogenation (HDDH) Ti pow[10]. Recently, significant progress has been made in lowthe cost of processing Ti powder[11,12]. These attempts woulead to further lower cost of PM Ti alloys. Another aspeccost reduction is the feasibility of achieving nearly fully deTi alloys by a single compressing-sintering step. In most cHIPping is always involved in processing PM Ti alloy pausing prealloyed powder, and this makes the materials proing much more costly. Most important of all, the mechanproperties of PM Ti alloys should be comparable to thosingot and wrought Ti alloys, as shown inTable 1 [8,13–18].Even if using blended elemental powders, which may conthigh level of oxygen and other impurities in alloys, the mechical properties are not deteriorated as much as has been ex[19].

∗ Corresponding author. Tel.: +1 865 771 6204; fax: +1 865 974 4115.E-mail address: Yonliu11@yahoo.com.cn (Y. Liu).

Alloy design is another way to lower the cost of Ti alloys.Table 2show various elements are used to lower the cost of raw

921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.10.057

26 Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35

Table 1Mechanical properties of PM Ti–6Al–4V prepared by different processes

Relative density (%) Tensile strengthσb (MPa) Yield strengthσ0.2 (MPa) Elongationδs (%) Reduced area (%)

Conventional BE[13] 95 773 683 6 6CIP + VS BE[14] 95 830 740 6 10MR-9TM BE [13] 99.2 932 849 14 29CHIP BE[15] ∼100 960 882 17 35P&S + HT + HIP[15] ∼100 921 1000 17 40TIARA BE [16] 99.6 926 809 19 31PA [8,17] ∼100 992 930 15 33Ceramic mold PA[18] ∼100 958 889 14 39Wrought[8] ∼100 978 923 16 44

BE: blended elemental powder processing, CIP: cold isostatic pressing, VS: vacuum sintering, CHIP: cold and hot isostatic pressing, P&S: single pressing andsintering, HT: heat treatment, HIP: hot isostatic pressing, PA: prealloy powder processing, MR-9TM: a patented blended elemental powder processing, TIARA: apatented blended elemental powder processing.

Table 2Typical compositions of serials low-cost titanium alloys

Type Component Name Merits Place and time

� Ti–0.8Fe–O–N TIX-80 Use cheap elementsTIX-90 Improve cold-workining properties Japan, 1989[20]

� + � Ti–4.5Al–3V–2Fe–2Mo SP-700 Improve heat-machining properties, perform superplasticprocessing under low temperature

Japan, 1989[21]

Ti–6Al–1.7Fe–0.1Si TIMETAL-62S Use cheap elements Fe and Si to reduce cost about 15–20%. USA, 1987[21]� Ti–4.5Fe–6.8Mo–1.5Al TIMETAL-LCB Use Fe–Mo master alloy to lower the cost USA, 1990[22]

materials and/or increase the workability of alloy ingots[20–22].For PM alloys, alloy design is also necessary for further loweringthe cost, and meeting the requirements of mechanical propertiesat the same time. Several research had been reported on the effectof alloying addtions in PM Ti alloy for accelerating densifica-tion process[23–25]. However, the controlling mechanisms arenot well known at the present time. As a result, the scientificrules for PM Ti alloy design are not well established yet. Inthis study, we reported our recent progress in design of PM Tialloys and composites, including the selection of alloy elementsfor enhanced sintering, increased alloy ductility by adding rareearth elements, improved mechanical properties by forging, andthe addition of secondary particles.

2. Experimental procedure

The physical characteristics of elemental and master alloypowders used in fabrication of Ti alloys and composites areshown inTable 3. The Nd–Al alloy was prepared by arc-melting500 g ingot from high purity Nd (>99.9%) and high purity alu-minum (>99.9%) in a dry argon atmosphere, and the nominalcomposition is 40Nd–60Al (at.%). Then the as-cast Nd–Al alloyingots were crashed into powder in a dry argon atmosphere.According to the X-ray diffraction analysis, the Nd–Al alloy ismainly composed of NdAl2 and NdAl3. Cr2C3 powder used forT ix-t here

comp tiono er ap ed a

temperatures ranging from 1200 to 1350◦C for 3 h in a vacuumof 5× 10−3 Pa, followed by furnace cooling. For some samples,hot forging was conducted at a temperature around 1100◦C witha maximum strain of 50%, and the forged billet was then heattreated according to the following schedule: holding at 810◦Cfor 1 h, water quenching and then annealing at 580◦C for 8 h.

For all the samples, the sintered bulk density was determinedby the Archimedes method. For mechanical property testing,selected samples were cut and machined into round bars. Ten-sile tests were conducted on samples with a gauge size of Ø6 mm× 30 mm using a CSS-2210 Type Mechanical PropertiesTester at both room temperature and at 400◦C in air. Low cyclefatigue test were conducted on round samples of gauge size ofØ 8 mm× 30 mm by using a MTS 810 Type Fatigue PropertyTester. TheR ratio was−1, and the stain rateε was 6× 10−3/s.

The microstructural examination of tested samples wasperformed by optical microscopy (OP), scanning electronmicroscopy (SEM) and transmission electron microscopy

Table 3Characterizations of raw elemental powders

Element Average particlesize (�m)

Oxygen content(wt.%)

Preparation

Ti 8.0 0.34 HDDHAl 15.1 0.36 GAFMNC

H ition;R syn-t

i alloy composites was prepared by milling Cr–C powder mures, which were combustion synthesized in an Ar atmosp

These raw powders were blended, according to differentositions, in a high efficient blender for 1 h under the protecf Ar atmosphere, followed by cold isostatic pressing undressure of 200 MPa. The powder compacts were sinter

.-

t

e 4.15 0.54 CDo 4.96 0.20 ROd–Al 8.65 0.66 DMr3C2 20 0.20 CS

DDH: hydride–dehydride; GA: gas atomize; CD: carbonyl decomposO: reduction oxide; DM: disintegration of master alloy; CS: combustion

hesis.

Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35 27

(TEM). After sectioning, metallographic samples weregrounded, polished, and then etched with a solution of 10%HF, 5% HNO3 and 85% H2O (in volume). Scanning electronmicroscopy was done only on polished sections by back-scattered electron (BSE) imaging. Energy dispersive X-rayanalysis (EDX) was also performed. The scanning electronmicroscope used was JSM-6460 coupled to an EDX analyzer.The TEM analyses were carried out by using a JEM-2010microscope operating at 200 kV coupling with a LIN-IncaEDAX apparatus. Thin foil specimens of sintered alloys wereprepared by ion beam thinning because the electro-thinningtechnique induced large holes and a loss of the second-phaseparticles in the alloy foils. X-ray diffraction (XRD) analysis forphase identification was conducted using a RIGAKU-D/max-rAdiffractometer operated at 50 kV and 20 mA. A Ni-filtered CuK� radiation was used for the study.

3. Results

3.1. Effects of alloying elements

Originally we had a broad scope of studying alloying ele-ments by sintering Ti-2Me (Me = Fe, Mo, Nd) powder compactsunder the same condition. After comparisons of primary data ofsintered density and mechanical properties among sintered sam-ples, most elements were excluded, while only Al, Fe and Mow

lloy,a r ofT eg rel-a and9 addi-th ering[ ism inter-d af n ofF er-a ntm f theFbi -t rical� daryfs tss the�

sin-td elyr rd-i ef state

Fig. 1. The expansion/contraction behavior of Ti–xFe compacts sintered at1250◦C for 30 min: linear shrinkage vs. (a) sintering temperature and sinter-ing time (b).

diffusion could be the main densification mechanism of Ti–Mopowder compact. However, the diffusivity of Mo in Ti is muchlower than that of the self-diffusion of Ti, and this would slowdown the inter-diffusivity of Ti–Mo alloys[30]. That is the mainreason why the density of Ti–Mo decreases with Mo. But thelow diffusivity of Mo in Ti would hinder the migration of grainboundary and this is beneficial to the microstructural refinementfor Ti alloys (Fig. 4). With Mo content increasing, the primary�-grain size was refined from 80�m at 1% Mo to 27�m at 4%Mo after sintering at 1350◦C for 3 h. Al is commonly used forstrengthening Ti alloys. However, the effect of Al additions onthe sintered density is even much worse than that of Mo addi-tions. It is well established that partial diffusion of Al in Ti, orknown as the Kirkendall effect, occurs during sintering Ti–Alpowder compacts, and hence leads to formation of residue poresin the compact after the consumption of Al particles[31]. Sothe sintered density drops very fast with Al. It is interesting topoint out that the sintered density of Ti–Al compact decreaseswith sintering temperature. The reason is possibly due to the

ere selected for detailed studies.Fe addition accelerated the densification process of Ti a

s indicated by the in situ examination of sintering behavioi–xFe compacts in Ar (Fig. 1). The relative densities of threen compacts were 86%, and after in situ sintering, thetive densities of pure Ti, Ti–3Fe and Ti–5Fe were 90, 925%, respectively. Previous investigation reported that the

ion of Fe led to the formation of eutectic phase at 1085◦C,ence, increasing the density by transient liquid-phase sint

23,26,27]. Our work[28] revealed that the diffusion rate of Feuch faster than that of Ti at the same temperature and theiffusivity of Ti–5Fe is higher than the self-diffusivity of Ti by

actor of two. Therefore, we found almost complete diffusioe in the Ti particles at 1080◦C, just before the eutectic tempture. The high diffusivity of Fe in Ti alloy induced significaicrostructural change with Fe content. With the increase oe content, the�-phase was refined (Fig. 2). Primary�-grainoundaries can be clearly seen at a Fe content of 4.5% (Fig. 2c),

ndicating a primary�-grain size of about 300�m. At a Fe conent of 6.0%, the�-phase is even more refined, and sphe-particles appear. Although no characterized grain boun

eature was found, it can be estimated that the primary�-grainize could be as large as 500�m fromFig. 2d. The above resulhow that Fe is a�-stable element, so the precipitation of-phase is suppressed and the size of the�-phase is refined.

Unlike Fe, addition of Mo and Al separately degraded theered density of Ti alloys, as shown inFig. 3. The relatively lowensification rate of Ti–Mo and Ti–Al compacts is also closelated to the diffusivity of elements in Ti particles. Accong to the Ti–Mo phase diagram[29], there is no liquid phasormation within the sintering temperature range. So solid-

28 Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35

Fig. 2. Optical microstructures of P/M titanium alloys with different Fe contents sintered at 1250◦C for 30 min: (a) Ti–1.5 wt.% Fe, 1250◦C; (b) Ti–3 wt.% Fe,1250◦C; (c) Ti–4.5 wt.% Fe, 1250◦C; (d) Ti–6 wt.% Fe, 1250◦C.

expansion of the entrapped gas in the residue pores where Alparticles were there before. The gas may come from the atom-ization process of Al powder. As most pores are closed up at arelative density above 90%, it would be difficult to eliminate theentrapped gas, which expands with temperature.

Effects of alloying elements on the mechanical properties ofTi alloy are different, and do not obey a linear relationship, asshown inFig. 5 andTable 4. For Ti–Fe alloys, Ti–3Fe% alloyhas the highest yield strength. However, its elongation is as pooras 1.0%, the Ti alloys has a ductility larger than 2% when theFe content is smaller than 2%. When Fe content is above 3%,the strength drops sharply due to poor ductility. The drop oftensile strength and poor elongation are due to the coarse pri-mary�-grain, as Fe is a strong�-stable element. Another reasoncould be the formation of intermetallic phase during cooling. Asindicated in Ti–Fe binary phase diagram[29], a eutectoid reac-tion occurs at 595◦C,�-Ti → �-Ti + TiFe. The existence of TiFeintermetallic phase could deteriorate the ductility and hence thetensile strength. For Ti–Mo alloys, the microstructural refine-

ment effect of Mo leads to the rapid increase of strength of PMTi–Mo alloy and good ductility. Ti–3Mo alloy has the highestelongation, 23%. Another reason for the ductility enhancementcould be the stabilization of�-phase, which is more ductile than�-phase. Unlike addition of Fe, no intermetallic phase based onTi–Mo forms. The drop of elongation of PM Ti–Mo alloys atMo content above 3% could be attributed to their low density,which decreases with the Mo content. The strengthening effectof Al is not as significant as that of Mo, and the elongationof PM Ti–Al alloy is very low, which must be due to the highresidue porosity, as indicated inFig. 3c. Even so, the strength ofTi–4.5Al alloy is still above 100 MPa higher than that of pure Ti.The mechanical behavior of PM Ti alloy depends on the sintereddensity, alloying effect, and microstructural features, so it is thusdifficult to evaluate the alloying effect if the elements inducedifferent density and microstructures. However, considering thenear net shaping and cost-effective aspects of PM Ti alloys,alloying elements are required to improve the densification pro-cess and/or the mechanical properties. Thus, Fe, Mo and Al are

Table 4Tensile property of PM Ti–xAl (wt.%) alloys

Relative density (%) Tensile strengthσb (MPa) Yield strengthσ0.2 (MPa) Elongationδs (%) Reduction of cross sectionψ (%)

Ti–4.5Al 96.3 867 861 1.2 2.3Ti–6.0Al 94.9 804 802 1.2 3T

i 99 745 700 4.0 7.5

Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35 29

Fig. 3. The relative density of (a) Ti–xFe (b) Ti–xMo (c) Ti–xAl alloys sintered at different temperatures for 3 h.

selected as alloying elements for alloy design of low cost PM Tialloys.

3.2. Effect of rare earth element Nd

Except for the Ti–Mo alloys, the ductility of pure PM Ti,Ti–Fe alloys and Ti–Al alloys is generally poor. Microstructureand density are the key factors affecting the ductility. However,another important factor affecting the brittleness of PM Ti alloyis the high content of oxygen introduced by the raw elementalpowders. Rare earths elements have a high affinity to oxygen,and have been used widely to scavenge oxygen in PM alloys.Also, rare earth oxide particles formed have improved the hightemperature properties of Ti-based alloys[32].

In this study, rare earth element Nd was chosen to add to PMTi alloys, using Al–Nd master alloy particles. For each com-position, the content of Al is about 1/3 that of Nd in weightpercent. Additions of Al–Nd particles lead to near fully densityof sintered PM Ti alloys (Fig. 6). Due to the depletion of sur-face oxygen, the sintering activity of Ti particles is improved,and the growth rate of sintering neck is accelerated. Meanwhile,Al–Nd particles have a low melting point than the sinteringtemperature[29], thus they form transient liquid phase during

sintering. Both of these factors account for the high density ofPM Nd-containing Ti alloys. The addition of Nd significantlyimproves the tensile elongation of PM Ti alloy, but it does notincrease the tensile strength (Fig. 6). Depletion of oxygen in Tialloys increases the dislocation mobility and also induces twin-ning[33], both of which should improve the ductility. Twinningin sintered Ti–1.2Nd–0.4Al (wt.%) alloy is much more signifi-cant than twinning in Ti–0.6Nd–0.2Al (wt.%) alloy. Oxygen isknown to be an�-strengthening element for Ti alloys[34], andthe depletion of oxygen causes a decrease of the strength.

3.3. New PM Ti alloys

Based on the above results, we have designed a seriesof PM Ti alloys, and their compositions are given as:Ti–1.5Fe–6.8Mo–4.8Al–1.2Nd (wt.%) (T12LCC), Ti–1Fe–1Mo–6.3Al–1.2Nd (wt.%) (T8LCC) and Ti–1.5Fe–2.25Mo–1.2Nd–0.3Al (wt.%) (TN). All the three alloys have beensintered at 1300◦C for 3 h, then hot forged and heat treatedunder conditions as above-mentioned. Results indicate that allthree alloys high tensile strength (∼1000 MPa) and excellentductility (4–20%). Among the three alloys, T12LCC has thebest strength and TN has the best ductility. The mechanical

30 Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35

Fig. 4. Optical microstructures of P/M Ti alloys with different Mo contents sintered at 1350◦C for 3 h: (a) Ti–1 wt.% Mo; (b) Ti–2 wt.% Mo; (c) Ti–3 wt.% Mo; (d)Ti–4 wt.% Mo.

properties of these alloys are summarized inTable 5. Themicrostructures of all of the three sintered alloys varied withcomposition. Nd exists in the form of oxide particles, in whichTi, Fe and Al elements are also found. Note that no alloyingeffect of Nd is detected in the Ti matrix.

3.4. Secondary treatment

The mechanical properties of the PM Ti alloys obtained so faressentially meet the requirements for most applications. How-ever, when considering applications in power train systems andadvanced engine systems, the high risk is still a concern becauseof the residual porosity and inhomogenuous grain size. Like PMferrous gears or connecting rods, PM Ti alloys are required tohave a secondary processing operation. Hot forging and subse-quent heat treatment are necessary to obtain more homogeneousand suitable microstructures. For T12LCC, all the primary�-grains were disintegrated after hot forging, and much finer andhomogeneous microstructure was obtained after heat treatment

(Fig. 7). Compared with alloys of the as-sintered state, the ten-sile strength of both T12LCC and TN alloys has been increasedby about 100 MPa. The ductility is also improved, in particularthe reduction of area is even doubled for T12LCC alloy. Mostimportant of all, after 105 cycles in low cycle fatigue test, themaximum plastic strain for hot-forged and heat-treated T12LCCalloy is much lower than that of the same alloy in the as-sinteredcondition (Fig. 8).

3.5. Particle reinforced Ti base composites

For application in advanced engine systems, Ti-basedcomposites are usually used as structural components. Forexample, Saito[35] developed TiB-Ti MMC alloy (5 vol.%TiB/Ti–6Al–4Sn–4Zr–1Nb–1Mo–0.2Si) for exhaust valves.The high temperature strength of Ti alloys can be furtherimproved by secondary particles. TiC and TiB have been con-sidered as two promising particle phases for strengthening Tialloys, due to their high thermal stability. On the processing of

Table 5Mechanical properties of new PM Ti–Fe–Mo–Al alloys tested at room temperature

Relative density (%) Tensile strengthσb (MPa) Yield strengthσ0.2 (MPa) Elongationδs (%)

Ti8LCC (as-sintered) 97 885 858 4Ti8LCC (as-forged and heat-treated) 100 964 890 18T 082T 1014T 910T 1072

i12LCC (as-sintered) 97.5 1i12LCC (as-forged and heat treated) 100N (as-sintered) 97N (as-forged and heat treated) 100

1016 6998 19

850 17983 20

Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35 31

Fig. 5. Mechanical properties of binary PM Ti alloys at room temperature: (a)effect of Fe content on the mechanical properties and (b) effect of Mo contenton the mechanical properties.

TiC/Ti composites, raw TiC powder was added in most cases.In this study, we introduce a new in situ reaction for formationof TiC phase:

Ti + Cr3C2 → TiC + Ti(Cr),

where Ti(Cr) represents the solid solution of Cr in the Ti matrix.The addition of Cr3C2, instead of Cr and C elemental powders

is to avoid the segregation of C when preparing elemental powdermixtures, because of the large density difference between C andother metallic elements.

X-ray diffraction analysis of sintered TN–5% Cr3C2 (vol.%)compacts confirmed the feasibility of the above reaction, that is,the products from the reaction are TiC and�-Ti. The sinteredmicrostructures also show that the Ti matrix is of single�-phase

Fig. 6. Effect of Nd on (a) sintering density and (b) mechanical properties ofP/M ternary Ti–Al–Nd alloys.

due to the dissolution of Cr, a strong�-stable element (Fig. 9). Itis interesting to note that the grain size of TN–TiC composite isabout 40�m, much finer than that of TN alloy. Mechanical prop-erties of TN alloy composite with an addition of 5 vol.% Cr3C2are shown inTable 6. In as-sintered state, TN–TiC compositeshows a RT tensile strength as high as 1330 MPa, i.e.,∼400 MPahigher than that of as-sintered TN alloy (Tables 5 and 6), butthe ductility is quiet poor. After forging and heat-treating, theRT tensile strength of TN–TiC composite is still as high as1179 MPa, and the ductility is also improved significantly. Thebenefit of Cr3C2 addition is to significantly improve the hightemperature strength. The high strength of Ti-based compositesin this study is attributed to three effects: particulate strengthen-ing, solution strengthening and grain refining. The former twoeffects should be more effective in improving the high temper-

Table 6Mechanical properties of TN–TiC composite at different states

Room temperature 400◦C

Tensile strengthσb (MPa)

Yield strengthσ0.2 (MPa)

Elongationδs (%)

Tensile strengthσb (MPa)

Yield strengthσ0.2 (MPa)

Elongationδs (%)

TN as-forged and heat-treated 1072 983 20 583 505 8TN–TiC as-sintered 1330 1280 1.6 675 581 8.5TN–TiC as-forged and heat-treated 1179 1089 4 987 946 6.0

32 Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35

Fig. 7. Microstructure of P/M T12LCC alloy before and after hot forging and heat treatment: (a) Ti12LCC + 1.2 wt.% Nd (sintered at 1300◦C for 3 h); (b)Ti12LCC + 1.2 wt.% Nd (forged 50% at 1150◦C for 30 min); (c) Ti12LCC + 1.2 wt.% Nd (heat treated at 810◦C for 1 h and annealed at 580◦C for 8 h afterforging).

Fig. 8. Fatigue properties of as-sintered and as-forged and heat-treated P/MT12LCC alloy.

ature strength. Thus, Cr3C2 should be considered as a suitableadditive for processing particle reinforced Ti composite.

4. Discussions

4.1. Design of PM Ti alloys

Low cost and mechanical properties comparable to thoseof wrought materials are two essential factors for structural

use of PM Ti alloys. Besides the low cost of Ti powder, theprocessing cost can also be reduced by adding cheap alloyingelements for shortening the sintering-time and for achieving full-density products. The processing effect is generally referred toas strengthened sintering, which means to accelerate the masstransfer (or filling the pores in densification process) by provid-ing rapid diffusion channels between particles. Three rules ofthumbs[36–38]have been proposed to explain the strengthenedsintering: (1) solubility standard—the base element should havea high solubility in the additive elements, while the contraryprocess is difficult; (2) segregation standard—the additives canbe segregated on particle boundaries during the whole sinter-ing process; (3) diffusion standard—Di /Ds� 1, whereDi is theinter-diffusion coefficient of the alloy, andDs is the self-diffusioncoefficient of the base element. The solubility standards and theprecipitation standards are related to the formation of diffusionchannel, while the diffusion standards is related to the acceler-ation of mass transfer rate. However, our results show that theformation of diffusion channel is not necessary for the sinter-ing of PM Ti alloys, while the diffusion standards is much moreimportant. According to Ti–Fe, Ti–Al and Ti–Nd phase diagrams[29], the solubility of the above alloying elements in Ti at sinter-ing temperatures is much higher than that of Ti in these alloyingelements, and continuous solid solution forms in Ti–Mo systemat elevated temperature. At the same time, we can’t find pre-cipitates formed on particle boundaries except for Nd oxides.T ring

herefore, no diffusion channel is developed during sinte

Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35 33

Fig. 9. Microstructures of (a) TN alloy and (b) Ti–TiC composites after being sintered at 1300◦C for 2 h; (c) SEM SEI image show the TiC particles in TiN–TiCcomposite.

of these binary Ti alloys. The fast migration and bonding ofparticle boundaries can only be attributed to improved inter-diffusivity by Fe and elimination of oxides on particle surfaceby Nd. So mass transfer rate can be accelerated directly throughparticle–particle contacts. Any factors that enhance mass trans-fer are helpful for strengthened sintering of PM Ti alloys.

For high performance, alloying effect is also very impor-tant. In this study, Al for solution strengthening, and Mo forboth solution strengthening and grain refining, are beneficial ele-ments for PM Ti alloys. Nd is effective in improving the roomtemperature ductility. However, for high performance applica-tions, thermo-mechanical treatment is necessary to homogenizemicrostructures and eliminate residue porosity, eventually toimprove the fatigue strength.

In summary, the design of low cost and high performanceP/M Ti alloy involves the following scientific principles:

(1) Adding elements with high diffusivity to accelerate masstransfer during densification process.

(2) Rare earth elements are necessary for cleaning particlesurface and improving room temperature ductility. Theimprovement in lowering the process cost (for example, thedifficulty in machining can be worked out by the increasedductility) can compensate for increasing the material costby small amount of rare earth elements.

( y for

(4) Thermo-mechanical treatment is necessary for high perfor-mance.

4.2. Design of PM Ti base composites

Ti base composites have been widely studied for struc-tural applications. Borides, carbides and nitrides are com-monly used for particle strengthening. Among particles, TiBand TiC are considered as the most promising ones due totheir good thermal and chemical compatibility with the Timatrix [39–41]. Most Ti base composites are fabricated bypowder metallurgy method and subsequent thermo-mechanicaltreatment. However, up to now, the ductility of PM Ti basecomposites is quite poor although the strength is rather high.For example, Saito[24] developed a Ti-based composite ofthe composition Ti–4.3Fe–7.0Mo–1.4Al–1.4V–5.4B. The roomtemperature tensile strength is as high as 2025 MPa, whilethe ductility is only 1.4%. Another composite developed bythem, Ti–2.0Co–2.0Mo–1.4Al–1.4V–1.8B, has a good balancebetween the strength (1350 MPa) and the ductility (5.2%)[24].For TiC-strengthened Ti composites, their ductility is also lessthan 5%[42]. The high strength and low ductility would makemetalworking of Ti base composites more difficult, and thiswould increase the fabrication cost.

The poor ductility of Ti composites is presumably due tot theb is

3) Cheap elements with good solubility in Ti are necessarsolution strengthening and grain refinement.

he interfacial debonding during plastic deformation andrittle nature of the PM Ti alloy matrix. In situ reaction

34 Y. Liu et al. / Materials Science and Engineering A 418 (2006) 25–35

helpful for the formation of clean interface and improvementof bonding between particles and the matrix. Therefore, in situreaction should be used preferentially for formation of strength-ening phases, instead of adding particles directly in the Timatrix. Another important way to improve the ductility and othermechanical properties (for example, high temperature strength)is to modify Ti matrix simultaneously, which has been seldomlyconsidered in most studies. Cr has been used extensively toimprove the ductility of Ti alloys for its effect on stabilizing�-phase, which is more ductile than the�-phase[43]. The highstrength at elevated temperature is attributed to not only thestrengthening of TiC particles, but also the solution of Cr inthe�-phase. Although no intermetallic phases (TiCr2 or TiCr3)were detected based on the XRD result of TN–TiC composite,it is probable that these phases might exist or precipitate outwhen tested at 400◦C, because the equilibrium phases below667◦C in the Ti–Cr system are�-phase and�-TiCr3 accordingto the Ti–Cr binary phase diagram[29]. The intermetallic phasewould hinder the migration of grain boundaries during elevatedtemperature deformation, and hence improve the strength of Ticomposites.

The success of addition of Cr2C3 in the Ti matrix to formin situ TiC/Ti composites with improved material performancessuggests a new way for the design of Ti base composites. Twoconclusions can be drawn from the current study:

( ce inces.

( con-e Ti

T

w thep anst llicp

5

pli-c andc twoe sitesF utet meno e Tm sult-i ion,s urthei thea igno par-t uldbb d Ti

composites, because it not only induces in situ formation of TiCparticles, but also brings about solid solution strengthening ofCr in the Ti matrix.

Acknowledgement

This work is supported by National Advanced MaterialsCommittee of PR China under the grant no. 2001AA332010.

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1) In situ reaction is highly recommended for its convenienthe formation of clean and strong particle–matrix interfa

2) Selection of additive phases for in situ reaction shouldsider their positive effect on the performances of thmatrix. The in situ reaction can be expressed as:

i + MeX → TiX + Ti(Me),

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. Conclusion

In summary, in order to meet the requirements of civil apations, such as automotive industry, sporting industryhemical industry, low cost and high performances aressential attributes in the design of PM Ti alloys and compoor design of PM Ti alloys, alloying elements should contrib

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