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Stress-introduced α″ martensite and twinning in a multifunctional titanium alloy

Stress-introduced a 00martensite and twinning in a

multifunctional titanium alloy

Y.Yang,G.P.Li,*G.M.Cheng,H.Wang,M.Zhang,F.Xu and K.Yang

Institute of Metal Research,Chinese Academy of Sciences,72Wenhua Road,Shenyang,Liaoning 110016,China

Received 24July 2007;revised 29August 2007;accepted 5September 2007

Available online 4October 2007

Previous investigations have shown that Gum Metal with body-centered cubic crystal structure needs to be free of dislocations,phase transformation and twinning during elastic and plastic deformation in order to achieve its multifunctional properties.This paper reports that such properties can also be exhibited in an alloy containing small amounts of pre-existing a 00martensite produced by cold compression.Both deformation twins and isothermal x phase were observed in the alloy even after a short period of aging at 300°C.

Keywords:Multifunctional titanium alloy;Stress-induced a 00martensite;Twinning

Recently,Saito et al.have developed a new group of multifunctional b titanium alloys (Gum Metal)by con-trolling chemical compositions to match simultaneously the following three magic electronic parameters:valence electron number (e/a)$4.24,bond order (Bo)$2.87and electron orbital energy level (Md)$2.45[1–7].After severe cold swaging these alloys exhibit super-properties such as ultralow elastic modulus,superelasticity,ultra-high strength,superplasticity and greatly reduced thermal expansion.The above unique properties are attributed to the lack of conventional dislocations,deformation twins and stress-induced martensitic transformation.

The above chemical restrictions,however,appear not to be required for the Ti–Nb–Zr–Sn alloy developed by Hao et al.[8–10]to exhibit multifunctional properties.(i)Hao et al.’s alloy has e/a $4.15,which is much less than that of Gum Metal.(ii)The alloy has an oxygen content similar to that of conventional titanium alloys but much less than the restricted oxygen level of Gum Metal.(iii)The alloy also shows very weak work hard-ening during cold deformation.(iv)The alloy exhibits multifunctional properties without restriction of cold deformation.In addition,Hao et al.’s alloy has ultralow elastic modulus and can be easily nanostructured by conventional cold rolling.Previous investigations also showed compression–tension asymmetry for the mar-tensitic transformation.That is,the a 00martensite can be detected by X-ray di?raction after compression but is barely detectable after tensile test,even on the tensile fracture surface.This evidence suggests that the multi-functional properties of Gum Metal may be attainable if the valence electron number e/a is slightly lower than 4.24and the stress-induced martensitic transformation may take place in compression.

In the present study,a Ti–22.4Nb–0.73Ta–2.0Zr 1.34O alloy with e/a $4.231,exhibiting multifunctional properties after cold compression,was studied.In sharp contrast to Gum Metal,stress-induced a 00martensite (SIM a 00)was detected by X-ray di?raction in the com-pressed specimen while both deformation twins and iso-thermal x were observed by transmission electron microscopy (TEM)even in an aged specimen at 300°C for 10min.

An ingot of 1.3kg was melted three times in a non-consumable arc-melting furnace under argon protection.The ingot was forged at 1080and 750°C,and then homogenized at 1010°C for 30min followed by iced water quenching.Uniaxial compression was conducted at room temperature with a compression speed of 0.1mm s à1.

Uniaxial tensile testing was carried out using an AG-5000A machine at à196and 19°C with strain rates of 8.3·10à4and 25·10à4s à1before and after yielding,respectively,and a DCS-25T machine at 250°C with strain rates of 2.5·10à4and 12.5·10à4s à1before and after yielding,respectively.The tensile stress–strain

*Corresponding author.Tel.:+862483978619;fax:+862423902021;e-mail:gpli@https://www.wendangku.net/doc/5817328344.html,

Available online at https://www.wendangku.net/doc/5817328344.html,

Scripta Materialia 58(2008)

9–12

curve was recorded by two strain gauges coupled to an

Instron8872machine at a strain rate of5·10à4sà1.

The linear expansion curve was recorded by a Netzsch DIL402C horizontal thermal expansion setup with a

heating rate of0.083°C sà1.Microstructure observa-

tions were performed by an Axiovert200MAT optical microscope and a Tecnai G220transmission electron

microscope operating at200kV.Phase constitutions

were detected on a Rigaku D/max-2400PC X-ray di?ractometer using Cu K a radiation,at a voltage of

56kV and a current of182mA.TEM foils were

mechanically thinned to about30l m in thickness and

further reduction was carried out using a MTP-1A mag-netic force-driven twin-jet electrolytic polisher in a solu-

tion of20%perchloric acid+30%butyl alcohol+50%

methanol(vol.%)atà30toà40°C and15–20V.

Chemical analysis showed that the composition of

the melted ingot is Ti–22.4Nb–0.73Ta–2.0Zr–1.34O

(at.%).The analyses of the distributions of the elements showed there was no accumulation of elements.The

three electronic parameters were calculated as e/a $4.231,Bo$2.867and Md$2.452.These are quite close to those of gum metal;the former two are lower

and the last is slightly higher.

During the uniaxial compression with height reduc-

tion up to77%,the Vickers hardness remained at about 270.After77%compression,the alloy still exhibits good ductility,in particular there were very large reductions in area at the tested temperature ranges(Fig.1a).The stress–strain curve recorded by strain gauges shows non-linear elasticity up to$2.3%and ultralow elastic modu-lus$55GPa at room temperature(Fig.1b).The thermal expansion is very weak,close to zero below $300°C,and increases dramatically above$350°C (Fig.1c).All the above properties are similar to those of Gum Metal[1].

Figure2shows typical microstructures of Ti–22.4

Nb–0.73Ta–2.0Zr–1.34O alloy before(Fig.2a and b)and after cold compression(Fig.2c–f).After homog-enization at1010°C for30min followed by water quenching,the microstructure of the alloy was a typical equiaxed structure with an average grain size of about 200l m(Fig.2a).Both TEM(Fig.2b)and X-ray di?rac-tion(XRD)analysis(Fig.3a)showed that the homoge-nized alloy contains only single b phase with body-centered cubic(bcc)crystal structure.This indicates that in the process of quenching,oxygen and zirconium should work as the b-stabilizers in Ti–22.4Nb–0.73 Ta–2.0Zr–1.34O alloy,as reported in other papers [1,3,7,11–13],because the sum of Nb and Ta contents in the present alloy is not high enough to suppress the phase transformation in the rapid cooling from b phase ?eld[7,14–16].

The compression led to a non-uniform plastic defor-mation with morphology di?erent to the so-called‘‘mar-ble-like’’microstructure[1,2].It is conceivable that the equiaxed b grain was changed into thallus,like a discus (Fig.2c and d).TEM observations showed the discon-tinuous elastic strain?elds in most areas in the alloy (Fig.2e)and very?ne lamella10–60nm in width (Fig.2f).This kind of lamellar structure is characteristic of SIM a00or deformed twins in the metastable b alloys and b alloys[13,17].However,the microdi?raction pat-tern from the?ne lamella failed to detect any di?raction spot contributed by SIM a00,and only a small part of them were twins as described in the following context. XRD analysis showed that the compressed specimen formed a small amount of SIM a00(Fig.3b).This sug-gests that a part of the lamellar structure(as shown in Fig.2f)should be debris from the SIM a00.The lack of di?raction patterns is due to the thin foil relaxation ef-fect[18–20]which causes reversible phase transforma-tion,back to the b phase,during the preparation of thin TEM specimens.This is consistent with experimen-tal results of Ti–Nb–Zr–Sn multifunctional alloy[8].

Many works have studied the phase stability of b alloys,metastable or otherwise[11,13–15,21].The present paper is focused particularly on the Ti–Nb–Ta–Zr–O system.Hwang et al.[21]have reported that the compo-sition of Gum Metal(Ti–23Nb–0.7Ta–2Zr–1.2O, at.%)with e/a$4.24,Bo$2.87and Md$2.45was the phase boundary between a00+b phases and b single phase.For small e/a and Bo,quenched a00martensite and SIM a00are generated in the alloy.Furuta et al. [2,3]reported that SIM a00transformation also occurred in Ti–23Nb–0.7Ta–2Zr–0.3O and Ti–21Nb–0.7Ta–2 Zr–1.2O alloy without the unique properties,and there was no phase transformation in Gum Metal with the composition of Ti–23Nb–0.7Ta–2Zr–1.2O[1–5].This

10Y.Yang et al./Scripta Materialia58(2008)9–12

indicates that oxygen had the same e?ect on the stability of b phase as niobium in quenching and cold working,and that slight di?erences in the composition may cause the phase transformation to destroy the unique proper-ties of Gum Metal.However,in the present alloy,with slightly lower niobium and higher oxygen contents,the quenched a 00martensite does not exist and SIM a 00trans-formation occurred in cold compression;most impor-tantly,the present alloy showed properties similar to those of Gum Metal.

In the metastable b titanium alloys,two kinds of deformed twins may exist,i.e.f 332g b h 113i b twins and f 112g b h 111i b twins [13,22],which is related to the sta-bility of quenched b phase.In Gum Metal,previous reports indicated that there was no twinning in the plastic deformation [1,5].However,as mentioned above,slightly deformed twins were found in cold-compressed Ti–22.4Nb–0.73Ta–2.0Zr–1.34O alloy aged at 300°C for 10min.The twins was identi?ed as f 112g b h 111i b (Fig.4).Previous reports [5,6]showed that h 111if 112g was one of the directions of elastic

softening in Gum Metal,and cold working caused the localized crystal to shear along this direction,forming nanodisturbances.We consider that when the shear takes place over a large area continuously,the deformed twins that formed in the present alloy will also be able to form in the Gum Metal.Furthermore,in such a short aging time,isothermal x phase has already precipitated from the cold-compressed b matrix (Fig.4a).The phase transformations in this alloy will be reported in more de-tail elsewhere.It is not yet clear why SIM a 00and twins do not destroy the unique properties in cold-compressed alloy like cold-swaged Gum Metal and whether the phase transformations in aging treatment have some in?uence on super-properties,and these areas need fur-ther investigation.

With e/a $4.231,Bo $2.867and Md $2.452,which are quite close to the parameters of Gum Metal,Ti–22.4Nb–0.73Ta–2.0Zr–1.34O alloy prepared by non-consumable arc-melting followed by homogeniza-tion and uniaxial cold compression exhibits multifunc-tional properties quite similar to those of Gum Metal.The SIM a 00and f 112g b h 111i b twins were found in the present alloy,but were found not to damage its un-ique properties.x phase precipitated from the cold-com-pressed b matrix even in a specimen aged at 300°C for 10min.

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Figure 2.Microstructures of homogenized (a,b)and cold-compressed (c–f)specimens,in which (a),(c)and (d)are optical images,and (b),(e)and (f)are TEM bright-?eld images.The insets in (b)and (f)are the [111]b zone axis selected-area di?raction pattern from the equiaxed b grain and [100]b zone axis microdi?raction pattern from the ?ne lamella,respectively.

100nm 100nm

Twin axis

::1:

:twin {112} <111> 112

000

110

222

c d

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