Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys

Jan Dutkiewicz1;2;*, Wojciech Maziarz1, Lucyna Jaworska2 and Kinga Zapaa1

1Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 30-059 Krakow, Reymonta 25, Poland2Pedagogical Academy, 30-059 Krakow, Podchorazych 2, Poland

The eect of increased Ta and Nb additions on the structure of high energy ball milled Ti alloys was studied using X-ray diraction andhigh resolution TEM. Ball milled powders were consolidated using ultra high pressure from 47GPa at temperatures of 650700C. Themicrostructure of the compacts consisted of ultra ne grains in the range of 20 nm of and phases. Micro-hardness measurements showed veryhigh hardness of ball milled and compacted powders close to 7GPa (slightly decreasing with the increase of alloying additions) and a decrease inthe Youngs modulus with the increase of Nb and Ta content i.e. the amount of phase. [doi:10.2320/matertrans.48.909]

(Received October 26, 2006; Accepted December 19, 2006; Published April 25, 2007)

Keywords: mechanical alloying, nanocrystalline TiNbTa alloys, ultra high pressure consolidation, structure, mechanical properties

1. Introduction

Nanocrystalline titanium base alloys exhibit attractiveproperties such as unusually high strength combined withreasonable ductility and toughness.16) Nanocrystalline struc-ture can be obtained in titanium alloys using rapid solid-ication,6) severe plastic deformation (SPD),15) ball milling(BM) and compacting79) or hydrostatic extrusion.10) Equalchannel angular pressing (ECAP) in the temperature range400450C improves the workability of Ti15) and allowssignicant renement of the grain size, increasing thestrength by 6090%.13) Hydrostatic extrusion has beenrecently used for grain renement of commercially puretitanium down to nano-metric scale.10) The preliminaryexperiments have shown the ability to decrease the grain sizedown to 100 nm. Structural evolution of titanium powderduring ball milling (BM) under dierent atmospheres wasstudied in several papers.79) Ball milling of the commercialpurity titanium enables a decrease in crystal size down to10 nm after 40 hours of milling in a high energy planetarymill.10) Especially interesting are additions of bio-neutralelements like Ta or Nb increasing ductility and decreasingYoung Modulus of titanium alloys9) due to increase of thefraction of the phase. In the present paper high pressureconsolidation of MA powders was used to prepare nano-crystalline TiNbTa alloys.

2. Experimental

Powders of titanium (110 mm size and of purity > 99:9%),tantalum (150 mm size and of purity 99.98%) and niobium(10 mm size and of purity > 99:8%) were used as startingmaterials. The powders were initially blended to the desiredcompositions of Ti-5Ta5Nb and Ti-10Ta10Nb (numbersindicate at%) in a glove-box under argon atmosphere andsubjected to ball milling up to 80 hrs in high energy planetarymill (Fritsch Pulverisette P5/4). Subsequent ultra highpressure consolidation of MA powders at 7GPa and 650Cusing Bridgeman method was applied. The structural changesduring milling as well as of the consolidated samples were

studied in a Philips PW 1830 diractometer using Cu Kradiation and a Philips CM 20 transmission electron micro-scope (TEM) equipped with a Phoenix energy-dispersive X-ray analysis system or Technai G20 FEG for high resolution.Thin foils from hot pressed samples were prepared bydimpling and ion milling using Gatan equipment, whereas themilled powders were embedded in resin and cut by usingLeica microtome. The dynamic microhardness test wasperformed on CSEM Mikro-Combi-Tester with load of100mN and compression tests using Instron machine.

3. Results and Discussion

Figure 1 shows X-Ray diraction patterns from theelementary powders of composition TiTa5Nb5 ball milledfor 5, 10, 20, 40 and 80 hours. One can see broadening of thepeaks due to gradual decrease of the grain size with a millingtime. After 80 hours of milling only one broad peak existsdue to structure changes approaching the amorphous state.Figure 2 shows bright and dark eld micrographs taken fromthe Ti5Ta5Nb powder ball milled for 80 h. One can seeclearly bright areas of size 520 nm indicating the existenceof nanocrystals within the powder. The average crystal sizeapproaches 10 nm. The electron diraction pattern placed as

30 50 70 90 1102

Inte

nsity

[a.u.

]

0h

5h

10h

20h

40h

80h

0 0 00 0

Fig. 1 X-ray diraction pattern taken from the TiTa5Nb5 composition

elemental powders ball milled for 0, 5, 10, 20, 40 and 80 hours.*Corresponding author: nmdutkie@imim-pan.krakow.pl

Materials Transactions, Vol. 48, No. 5 (2007) pp. 909 to 914Special Issue on New Developments and Analysis for Fabrication of Functional Nanostructures#2007 The Japan Institute of Metals

an insert shows diused ring conrming the nanocrystallinestate. The high resolution microscopy of milled powders hasshown that depending on composition of powders thestructure was composed of a mixture amorphous and or phases. Figure 3 shows HREM image of Ti-10Ta10Nb alloymilled 80 h and corresponding Fast Fourier Transform (FFT)and Inverse Fast Fourier Transform (IFFT) images. One cansee in the IFFT image that two areas as the crystalline (c) oramorphous (a) of size of few nanometers exists in MA alloy.Between these areas the highly deformed close to amorphousstructure can be seen, often characterized by a short rangeordered regions. The FFT made from the crystalline areaindicates that two most intense rings corresponding to0.135 nm and 0.165 nm lattice plane spacings are in goodagreement with (101) and (002) lattice planes of -Tistructure. The UHP consolidation at temperatures below700C allowed the retention of the nanocrystalline structure.In Fig. 4 are shown two X-Ray diraction curves fromTi5Ta5Nb and Ti10Ta10Nb consolidated samples. One cansee that TiTa5Nb5 alloy consists mostly only of the phase,whereas in the TiTa10Nb10 alloy a two phase structureforms. The broadening and relatively small intensities ofdiracted peaks indicates, that the structure is of nanometricscale. This assumption is conrmed by the TEM observation.Figure 5 shows TEM bright and dark eld images taken fromthe ultra high pressure compacted samples. One can see slight

growth of the crystals (mostly ones) up to about 20 nm inthe TiTa10Nb10 sample whereas the TiTa5Nb5 alloy hasmostly only small crystals. The high resolution observationof TiTa10Nb10 hot pressed sample allowed to conrm thenanometric level of grain size. Figure 6 shows a set of highresolution images where one irregular -Ti grain of size ofabout 20 nm with [311] orientation attached to [100] oriented

BF DF

Fig. 2 TEM micrographs of a powder particle of the TiTa5Nb5 alloy milled for 80 hours showing an average crystal size of 10 nm.

5 nm

c

a

2.34

1.65

HREM IFFT

FFT

Fig. 3 HREM image of Ti-10Ta10Nb alloy milled 80 hours and corresponding FFT and IFFT images.

30 50 70 90 1102

Inte

sity

[a.u.

] alfa Ti beta Ti

0 0 0 0 0

Fig. 4 X-ray diraction patterns of the TiTa5Nb5 (bottom) and Ti-

Ta10Nb10 (top) alloys consolidated by ultra high pressure.

910 J. Dutkiewicz, W. Maziarz, L. Jaworska and K. Zapaa

-Ti grains i.e. forming high angle boundary. HREMobservations allowed to identify the !-phase particles at[001] zone axis orientation (Fig. 7) observed in the grain of

[111] zone axis orientation in TiTa10Nb10 hot pressed alloy.The same phase was described in11) as formed during the! transformation in metastable -Ti alloys. The

-111 Ti002 Ti, 101 Ti

112 Ti, 0-13 Ti 2-10 Ti

004 Ti310 Ti

002 Ti

SAPD

BF DF

-111 Ti

002 Ti

0-12 Ti2-10 Ti-113 Ti1-22 Ti

BF DF

SAPD

TiTa5Nb5

TiTa10Nb10

Fig. 5 TEM images of the TiTa5Nb5 and TiTa10Nb10 alloys ball milled and ultra high pressure compacted at 7GPa and 650C.

Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys 911

mechanical properties of UHP alloys were determined usingboth, microhardness and a compression tests. Figure 8 showsthe Depth-Load curves recorded during dynamic microhard-ness test of investigated alloys. One can see that inTiTa10Nb10 alloy the penetration depth is slightly largerthan in TiTa5Nb5. This suggests that alloy with dominant structure is of higher plasticity. The Young Modulusmeasured during unloading process was calculated at113MPa and 104MPa for TiTa10Nb10 and TiTa5Nb5 alloysrespectively, corresponding to microhardness of about 6GPaand 7GPa. In Fig. 9 the results of compression test ofinvestigated alloys are shown. They are in good agreementwith the microhardness test. The TiTa10Nb10 alloy has ahigher plasticity and reaches about 8% elongation, i.e. morethan TiTa5Nb5 with maximum tensile deformation of about6%. The compression strength for the Ti10Ta10Nb alloycontaining structure is about 1420MPa, higher than of theTi5Ta5Nb alloy consisting only of the phase which reachedUTS=1270MPa.

4. Conclusions

(1) Ball milling of elemental Ti powders containing 510 at% of Ta and Nb lead to nano-crystalline structuresgiving only a broad peak in X-ray diraction. TEMallows the resolution of and crystals with an averagesize of 10 nm and microhardness above 10GPa.

(2) After ultra high pressure consolidation at 7GPa and650C only minor grain growth is observed up to 20 nm.It is connected with a small hardness decrease. Increaseof Ta and Nb content up to 10% causes small increaseof hardness, tensile elongation and a tensile strength butcauses a decrease of the Young modulus.

(3) The phase composition of UHP compacted alloyTiTa5Nb5 consists mainly of the phase, while inthe alloy TiTa10Nb10 forms signicant fraction of the phase in addition to the phase. The ! phase wasidentied using high resolution electron microscopy.

-Ti [100]-Ti [311]

HREM IFFT

FFT

Fig. 6 HREM image of Ti-10Ta10Nb alloy ultra high pressure compacted at 7GPa and 650C corresponding FFT and IFFT images.

912 J. Dutkiewicz, W. Maziarz, L. Jaworska and K. Zapaa

Acknowledgements

Financial support of the Research Project PBZ-KBN-096/T08/2003 is gratefully acknowledged.

HREM IFFT of

IFFT of -Ti[111] -Ti

[001]

Fig. 7 Set of HREM image of Ti-10Ta10Nb alloy ultra high pressure compacted at 7GPa and 650C showing ! phase precipitates.

0

20

40

60

80

100

120

0 200 400 600 800 1000D/nm

Ti10Ta10NbTi5Ta5Nb

L/m

N

Fig. 8 Depth-Load curves recorded during dynamic microhardness test of

investigated alloys.

0.00 2.00 4.00 6.00 8.00e [%]

0.00

400.00

800.00

1200.00

1600.00

S [M

Pa]

10TTN

5TTN

15TTN

Fig. 9 " curves recorded during dynamic compression test of inves-

tigated alloys.

Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys 913

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