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Materials Science and Engineering A 438–440 (2006) 315–319

Phase transformation of quenched �′′ martensite by aging in Ti–Nb alloys

Y. Mantani ∗, M. TajimaDepartment of Mechanical Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan

Received 13 April 2005; received in revised form 5 December 2005; accepted 10 February 2006

bstract

The purpose of this study is to investigate the phase transformation behavior of quenched orthorhombic �′′ martensite during aging initanium–niobium binary alloys. Four kinds of alloys with niobium concentrations of 25, 30, 35, and 40 (mass%) were used. Differential thermalnalysis, optical microscopy, X-ray diffraction, and a Vickers hardness test were performed to investigate the phase transformation behavior ofach alloy under different aging conditions. The phase transformations observed in this study were the reverse transformation of the �′′ martensite

o �, � phase precipitation in the � matrix, and � phase precipitation. Based on these results, the phase transformation temperature ranges wereiscussed. It was pointed out that there existed a possibility of a difference in the aging transformation and hardening behavior between the �′′

artensite structure and the �′′ + � duplex phase structure.2006 Elsevier B.V. All rights reserved.

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eywords: Titanium–niobium alloy; Differential thermal analysis; Aging; Orth

. Introduction

Molybdenum, vanadium, and niobium are representativesf � stabilizer elements of titanium alloys. Recently, detailedtudies have been conducted on the biocompatibility of tita-ium alloys with niobium [1–3]. These studies also examine theunctional characteristics of titanium alloys such as their lowoung’s modulus or shape memory effect. It is well known thatetastable phases such as hexagonal �′ martensite, orthorhom-

ic �′′ martensite, and the � phase are formed by quenching,ging, or deformation. The metallurgical aspects of the phaseonstitutions of quenched structures [4–7], the reverse transfor-ation of the �′′ martensite to the � phase [5], and the � phase

recipitation due to aging [8] have been reported with regard toi–Nb binary alloys.

It is necessary to investigate the relationships between theicrostructure and mechanical property because the latter varies

ue to these structural changes. We have reported that a quenched′′ martensite of Ti–Mo alloy exhibited a change in the innertructure and very high hardness after aging treatment; however,

he change in the phase constitution was not identified by X-rayiffraction (XRD) [9]. It is interesting to observe whether theuenched �′′ martensite can lead to a similar effect in Ti–Nb

∗ Corresponding author. Tel.: +81 45 481 5661; fax: +81 45 481 5122.E-mail address: mantani@mech.kanagawa-u.ac.jp (Y. Mantani).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.02.180

bic martensite; Omega phase

lloys; further, it is also important to recognize the reverseransformation behavior of the �′′ martensite or the � phaserecipitation behavior with aging treatment.

The purpose of this study is to investigate the change in thehase constitutions in quenched �′′ martensite and the variationn the hardness due to aging treatment. The temperature rangesn which the phase constitution changed were predicted by dif-erential thermal analysis. A Vickers hardness test and X-rayiffraction were applied after these aging treatments.

. Experimental procedures

Ti–Nb binary alloy sheets of 2 mm thickness were usedn this study. The niobium and oxygen concentrations ofach alloy are shown in Table 1; these were labeled as25N, T30N, T35N, and T40N, respectively. These speci-ens were separately capsulated in evacuated quartz tubes,

nd solution treated at 1223 K for 3.6 ks by breaking theseuartz tubes in iced water. They were then mechanicallyround and electrolytically polished by using a solution ofClO4(6% vol.) + butanol(35% vol.) + CH3OH(bal.) at 223 K.he optical microstructures were revealed by using the etchant of

NO3(5% vol.) + HF(10% vol.) + water(bal.). Differential scan-ing calorimetry (DSC) was performed at a constant heating ratef 5 K/min in an Ar atmosphere to predict the phase transforma-ion behavior. The load applied in the Vickers hardness test was

316 Y. Mantani, M. Tajima / Materials Science and Engineering A 438–440 (2006) 315–319

Table 1The niobium and oxygen contents of Ti–Nb alloys used in this study (mass%)

T25N T30N T35N T40N

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iobium 25.17 30.23 35.37 40.54xygen 0.232 0.293 0.214 0.220

kgf and the holding time was 15 s. The specimen was held in thevacuated quartz tube when the aging treatment was performedn an electric furnace. X-ray diffraction using Cu K� was per-ormed at 40 kV–100 mA to determine the phase constitutionsf these specimens.

. Results and discussion

Fig. 1 shows the optical microstructures of Ti–Nb alloysuenched from 1223 K. Fine needle-like traces of martensiteere observed at the prior � grain boundary in the image of25N(a).Very clear needle-like traces of martensite in the �rain were observed in the image of T30N(b), and similar tracesere observed in the T35N(c) image; however, the amount of

races was less than that of T30N. Only the � grain boundaryas observed in the image of T40N(d).In order to predict the phase transformation behavior of these

uenched Ti–Nb alloys, DSC heating measurements were per-ormed. Fig. 2 shows the DSC heating curves at a constant heat-

ng rate of 5 K/min in quenched Ti–Nb alloys. The exothermicnd endothermic peaks were in the upper and lower directions,espectively. T25N(a) exhibits an endothermic peak near 695 Knd an exothermic peak near 715 K followed by the endothermic

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Fig. 1. The optical microstructures of Ti–

Fig. 2. The DSC heating curves of Ti–Nb alloys quenched from 1223 K.

eak. T30N(b) exhibited an endothermic peak near 565 K, whichad gradually increased after 450 K, and an exothermic peakear 580 K followed by the endothermic peak. A big endother-ic peak was also observed near 790 K. T35N(c) exhibited an

ndothermic peak near 460 K, which had gradually increasedfter 390 K. An exothermic peak near 650 K extending broad

emperature range was considered to exist. In addition to theseeaks, a big endothermic peak was also observed near 765 K.o clear peaks could be observed for T40N(d). Although the

Nb alloys quenched from 1223 K.

Y. Mantani, M. Tajima / Materials Science and

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tatXrTmphardness up to about Hv 400 occurred 240 s after aging at 573 K.This was apparently affected by the � precipitation in the �matrix that was recognized in the XRD profile. Therefore, itwas considered that a gradual increase in the temperature cor-

ig. 3. The variation in the hardness (a) during the aging treatment and the XRDrofiles (b) after each aging treatment in T25N.

eaks of T30N(b) near 790 K and those of T35N(c) near 765 Kere considered to indicate the � transus, the other peaks were

onsidered to be related to the change in the metastable phasesuch as phase decomposition of martensite, precipitation of the �hase, or precipitation of the � phase. Although the DSC resultseveal dynamic phase transformations, they do not always corre-pond to the aging phase transformation behavior. Therefore, theickers hardness tests and XRD measurements were performedfter the aging treatments in order to investigate the change inhe metastable phases. The aging temperatures, which are indi-ated by dotted lines in this figure, were determined from theSC measurements at 573 and 723 K for all specimens, andarticularly at 423 K for T30N and T35N.

Fig. 3 shows the variation in the Vickers hardness (a) duringhe aging treatment and the XRD profiles (b) after each agingreatment in T25N. In the as quenched state (AQ) of T25N,nly the �′′ martensite peaks were observed in the XRD pro-le. On aging at 573 K, no remarkable increase in the hardnessas observed, and a slight change in the XRD profile caused by

he diffraction peak of the � phase was recognized. Therefore,remarkable phase transformation did not occur at this aging

emperature. This is confirmed by the DSC result (Fig. 2(a))n which peaks were not observed still in 573 K. At 723 K, the

lloy was hardened up to about Hv 310 until 480 s and graduallyoftened with increasing aging time. It was considered that theardening was related to the structural change of the �′′ marten-

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Engineering A 438–440 (2006) 315–319 317

ite or the � precipitation. In the DSC heating curve (Fig. 2(a)),he exothermic peak near 715 K indicated phase decompositionrom the �′′ martensite to the � and � phases; therefore, theRD profile obtained after aging at 723 K indicates diffractioneaks of these phases. We have already reported that the �′′artensite was drastically hardened due to aging in Ti–8 mass%o alloy at a temperature before the phase decomposition peak

n the DSC heating curve [9]. A similar effect was consideredo have occurred before phase decomposition. The aging tem-erature was higher than the phase decomposition temperature;hus, it was considered that the hardness temporarily increasedue to this effect and then decreased gradually with the progressf phase decomposition.

Fig. 4 shows the variation in the Vickers hardness (a) duringhe aging treatment and the XRD profiles (b) obtained after eachging treatment in T30N. In the as quenched state (AQ) of T30N,he �′′ martensite and the � phase peaks were observed in theRD profile; this, the �′′ and � duplex phase structures were

evealed. When aged at 423 K, the hardness increased slightly.his was considered to be related to the � precipitation in the �atrix. The � phase peak was slightly recognized in the XRD

rofile obtained after aging at 473 K. A drastic increase in the

ig. 4. The variation in the hardness (a) during the aging treatment and the XRDrofiles (b) after each aging treatment in T30N.

318 Y. Mantani, M. Tajima / Materials Science and Engineering A 438–440 (2006) 315–319

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ig. 5. The variation in the hardness (a) during the aging treatment and the XRDrofiles (b) after each aging treatment in T35N.

esponding to the endothermic peak from 450 K was related tohe reverse transformation of the �′′ martensite to the � phase;urther, the temperature near the top of the peak was also closelyelated to the � precipitation temperature. On the other hand, at23 K, the alloy was hardened up to about Hv 360 until 480 snd gradually softened with increasing aging time. The � and �hases were identified in the XRD profile obtained after agingt 723 K; as a result, it was considered that the increase in theardness at the beginning of aging was temporarily affected byhe � precipitation, and it decreased with the progress of phaseecomposition to the � and � phases.

Fig. 5 shows the variation in the Vickers hardness (a) dur-ng the aging treatment and the XRD profiles (b) obtained afterach aging treatment in T35N. In the as quenched state (AQ) of35N, the �′′ martensite and the � phase peaks were observed

n the XRD profile; thus, a �′′ + � duplex phase structure wasdentified. When aged at 423 K, the hardness was increased after60 s. In the XRD profile obtained at 423 K, the peaks of the �′′artensite disappeared and transformed into the � phase. This

ging temperature corresponded to a point between the start andop of the endothermic peak (Fig. 2(c)); as a result, only theeverse transformation to the � phase occurred. However, tak-ng into consideration the increase in the hardness with aging at

23 K, the � phase might be gradually precipitated with increas-ng aging time, although this was not recognized in the XRDrofile. At 573 K, a drastic increase in the hardness up to aboutv 360 began after 240 s. This was apparently affected by the �

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ig. 6. The variation in the hardness (a) during the aging treatment and the XRDrofiles (b) after each aging treatment in T40N.

recipitation that was recognized in the XRD profile. At 723 K,his hardness was increased slightly at the beginning of agingnd was maintained at an almost constant level. The � and �hases were recognized in the XRD profile obtained after agingt 723 K.

Fig. 6 shows the variation in the Vickers hardness (a) duringhe aging treatment and the XRD profiles (b) obtained after eachging treatment in T40N. In the as quenched state (AQ) of T40N,nly the � phase peaks were observed in the XRD profile. Theardness did not exhibit a remarkable variation for aging at 573 Knd that at 723 K. Therefore, it was considered that either littler no � precipitation occurred. In the XRD profile, no changeccurred during aging at 573 K; however, � phase precipitationas slightly recognized during aging at 723 K. Although theSC result did not reveal remarkable peaks (Fig. 2(d)), the pre-

ipitation of the � phase could not be recognized by this DSCeasurement condition in T40N.Based on the above mentioned results, we have discussed

he phase transformation temperature ranges. In the alloys thatnclude quenched �′′ martensite in their phase composition,hase transformation peaks were observed in the DSC heatingurves (Fig. 2). In T25N, which exhibits only �′′ martensiteeaks in the XRD profile, reverse transformation of the �′′

artensite to � and the precipitation of the � phase were not

ecognized in this aging condition. Therefore, age hardening at23 K was considered to be related to the change in the inner

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Y. Mantani, M. Tajima / Materials Science

tructure of the �′′ martensite; this structure was similar to thei–8 mass% Mo alloy [9]. On the other hand, in T30N and T35N

hat have the �′′ + � duplex phase structure, the temperatureange between the start and top of the endothermic peak cor-esponded to the reverse transformation of the �′′ martensite tohe � phase. Therefore, it was pointed out that there exists a pos-ibility of a difference in the age hardening behavior betweennly the �′′ martensite structure and the �′′ + � duplex phasetructure. In T30N and T35N, the precipitation of the � phaseccurred in the temperature range between a point close to theop of the endothermic peak and the next exothermic peak. Therecipitation of the � phase was considered to occur after thendothermic peak in each alloy.

It is interesting to note the change in the microstructure withncreasing temperature because the � reverse transformationemperature of the �′′ martensite and the � precipitation tem-erature are very close to each other; this phenomenon shoulde studied further.

. Conclusion

In Ti–Nb binary alloys, the phase transformation of quenched′′ martensite caused by aging has been investigated by means ofptical microscopy, differential scanning calorimetry, a Vickersardness test, and X-ray diffraction. The following conclusionsave been obtained:

1) In the alloys including quenched �′′ martensite, that is,T25N T30N, and T35N, phase transformation peaks were

observed in the DSC heating curves. Based on the resultsof the Vickers hardness test and X-ray diffraction, theseDSC peaks were considered to indicate the following phasetransformations. The endothermic peaks were related to the

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Engineering A 438–440 (2006) 315–319 319

reverse transformation of the �′′ martensite to the � phase,the precipitation of the � phase, or the � transus. On theother hand, the exothermic peaks were related to the pre-cipitation of the � phase. It was pointed out that the reversetransformation of the �′′ martensite to the � phase occurredbetween the start and top of the endothermic peak, and theprecipitation of the � phase occurred between a point closeto the top of the endothermic peak and the next exothermicpeak.

2) Age hardening in T25N at 723 K was considered to berelated to the structural change in the inner structure of the�′′ martensite. Remarkable age hardening corresponding tothe � precipitation was observed in T30N and T35N aged at573 K. Therefore, it was pointed out that there is a possibilityof a difference in the phase transformation behavior betweenonly the �′′ martensite structure and the �′′ + � duplex phasestructure. A remarkable variation in the hardness in T40Nwas not observed.

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664–671.6] A.T. Balcerzak, S.L. Sass, Metall. Trans. 3 (1972) 1601–1605.

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