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Transcript of Effect of Deformation-Induced ω Phase on the of Deformation-Induced ½ Phase on the Mechanical...

Effect of Deformation-Induced Phase on the Mechanical Propertiesof Metastable -Type TiVAlloys

Xingfeng Zhao1,+1, Mitsuo Niinomi2,+2, Masaaki Nakai2 and Junko Hieda2

1Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

A series of metastable -type binary Ti(1822)V alloys were prepared to investigate the effect of deformation-induced products(deformation-induced phase transformation and mechanical twinning) on the mechanical properties of metastable -type titanium alloys. Themicrostructures, Youngs moduli, and tensile properties of the alloys were systemically examined.

Ti(1820)V alloys subjected to solution treatment comprise a phase and a small amount of athermal phase, while Ti22V alloysubjected to solution treatment consists of a single phase. Ti(1820)V alloys subjected to solution treatment exhibit relatively low Youngsmoduli and low tensile strengths as compared to cold-rolled specimens. Both deformation-induced phase transformation and {332}h113imechanical twinning occur in all of the alloys during cold rolling. The occurrences of {332}h113i mechanical twinning and deformation-induced phase transformation are dependent on the stability of the alloys. After cold rolling, all of the alloys comprise a phase and an phase. The Youngs moduli of Ti(1822)V alloys increase because of the formation of a deformation-induced phase during cold rolling. Thesignificant increase in tensile strength is attributed to the combined effect of the deformation-induced phase transformation and work-hardening during cold rolling. [doi:10.2320/matertrans.M2012116]

(Received March 26, 2012; Accepted May 16, 2012; Published July 11, 2012)

Keywords: beta-type alloys, deformation-induced omega phase transformation, mechanical twinning, Youngs modulus, tensile properties

1. Introduction

Non-equilibrium phases, such as A (hcp-structuredmartensite), AA (orthorhombic-structured martensite), or (simple hexagonal structure phase), can appear in certainmetastable -type titanium alloys during deformation.1,2) Itwould appear that the deformation-induced products arerelated to the amount of stabilizing element that is present,i.e., the stability of the phase. As the amount of stabilizerincreases, the stability of the phase increases so that suchproducts change from deformation-induced martensites (Aand AA) to a deformation-induced phase and mechanicaltwin.3) Over the past few decades, deformation-inducedmartensites and their effect on the mechanical properties oftitanium alloys have been extensively investigated.410) Zhaoet al. reported that deformation-induced A transformationoccurs in Ti30Zr4Cr alloy and result in decreasing ofYoungs modulus.5) Kim et al. reported that stress-induced AAtransformation can take place in the metastable region inTiNbSi alloys, and result in pseudoelasticity.6) Matsumotoet al. released that stress-induced AA phase is formed by coldrolling in Ti35Nb4Sn alloy, and the Youngs modulusof the alloy decreases after cold rolling, while the tensilestrength increases.8) Additionally, there are some reports thathave reported on the phenomenon of deformation-induced phase transformation in metastable -type alloys. Kuanet al.11) observed that the phase was stress-induced duringtensile deformation of quenched Ti16 and Ti20mass% Vsingle crystals. In addition, deformation-induced phasetransformation was also found in metastable -type TiCr,TiFe and TiMo alloys.12,13) Nevertheless, there is verylittle literature available that has focused on the effectof deformation-induced phase transformation on the

mechanical properties, such as Youngs modulus and tensileproperties, for example.

As mentioned above, the type of deformation-inducedproducts are related to the stability of the phase.3) There hasa possibility that both of deformation-induced martensitictransformation and deformation-induced phase transforma-tion occur simultaneously because they are competitive ina certain range of chemical composition.5) In this case,it is difficult to clarify the effects of deformation-inducedmartensitic transformation and deformation-induced phasetransformation on the mechanical properties separately. It isnecessary to eliminate the effect of the deformation-inducedmartensite on the mechanical properties to clarify the effectof the deformation-induced phase on the mechanicalproperties. Therefore, a relatively large amount of stabilizershould be added to obtain a relatively high stability in the phase to avoid deformation-induced martensitic transforma-tion. Vanadium (V) is an effective stabilizer for designing-type alloys, and TiValloys have a large composition rangecapable of producing deformation-induced phase trans-formation.11,13) According to the previous reports, a compo-sition range of 18 to 22mass% V is expected to producedeformation-induced phase transformation for the binaryTiV alloys.11,13) Thus, in the present work, the effect ofdeformation-induced phase on Youngs modulus andtensile properties of -type titanium alloys were systemati-cally investigated using the binary TiV alloys.

2. Materials and Methods

2.1 Material preparationA series of binary TixV (x = 18, 20 and 22mass%) alloys

were prepared by the following procedure. Appropriateamounts of high-purity sponge Ti (99.7%) and lumps of V(99.7%) were mixed together. The mixtures were then arcmelted with a non-consumable tungsten electrode under a

+1Graduate Student, Tohoku University+2Corresponding author, E-mail: niinomi@imr.tohoku.ac.jp

Materials Transactions, Vol. 53, No. 8 (2012) pp. 1379 to 13842012 The Japan Institute of Metals


high-purity argon (Ar) atmosphere. The ingots were invertedand remelted at least six times to ensure compositionalhomogeneity. The arc-melted ingots of the TixValloys werehomogenized at 1373K for 21.6 ks in an Ar atmosphere, afterwhich they were subjected to water quenching. The alloyswere then hot rolled into plates with a reduction ratio of70% at 1273K in the same atmosphere and subjected toair cooling. Then, the hot-rolled specimens were subjectedto solution treatment at 1123K for 3.6 ks under vacuumconditions, followed by water quenching. Finally, tointroduce deformation-induced products into the Ti(1822)V alloys, the solution-treated samples were cold rolledwith a reduction ratio of 10% at room temperature. Thesolution-treated and cold-rolled specimens are labeled ST andCR, respectively. More particularly, from here on they arereferred to as Ti18VST, Ti20VST and Ti22VST, andTi18VCR, Ti20VCR and Ti22VCR, respectively.

2.2 Microstructural analysisThe phase constitutions were identified by X-ray diffrac-

tion (XRD) analysis using a Bruker D8 Discover two-dimensional X-ray diffractometer with Cu-K radiation at anaccelerating voltage of 40 kV and a current of 40mA. Themicrostructures were examined by optical microscopy (OM;Olympus BX51), electron backscatter diffraction (EBSD;Quanta 200 3D SEM-TSL), and transmission electronmicroscopy (TEM; JEOL JEM-2000EX). For OM and EBSDobservations, the specimens were mechanically polishedusing SiC waterproof papers of up to #2400 grit and a col-loidal SiO2 suspension. The mirror-polished specimens werethen etched with mixed aqueous solutions of hydrofluoricacid and nitric acid. The specimens for TEM were preparedby mechanical polishing and ion milling. Specifically, thesamples were first mechanically polished to a thickness ofapproximately 50 m, after which they were dimpled with aphosphor bronze ring to a thickness of around 15 m. Finally,the specimens were ion milled to a thin foil. TEM observa-tions were conducted at an accelerating voltage of 200 kV.

2.3 Evaluation of mechanical propertiesThe mechanical properties of the prepared alloys were

evaluated by performing Youngs modulus measurementsand tensile tests.

Specimens, of size 40mm 10mm 1.5mm, were cutfrom ST and CR plates for Youngs modulus measurements,with their longitudinal direction parallel to the rollingdirection. These specimens were polished using SiC water-proof papers of up to #2400 grit. The Youngs moduli of thealloys were measured at room temperature in air using a freeresonance method (Nippon Techno-Plus Co., Ltd. JE-RT3).

For the tensile tests, specimens with a thickness of 1.5mm,width of 3mm, and gage length of 13mm were cut from theST and CR plates. The longitudinal directions of thesespecimens were parallel to the rolling direction. The tensiletest specimens were polished using SiC waterproof papers ofup to #2400 grit. The tensile tests were performed at acrosshead speed of 8.33 106m s1, at room temperature inair, using an Instron-type machine (Shimadzu AGS-20kNG).The Youngs modulus measurements and tensile tests wereperformed in triplicate to minimize the experimental errors.

3. Results and Discussion

3.1 MicrostructuresFigure 1 shows the XRD profiles of the Ti(1822)Valloys

subjected to solution treatment and cold rolling. In Ti18VST, small peaks corresponding to the phase are detected incombination with the phase. Only peaks corresponding tothe phase are detected in Ti20VST and Ti22VST, andno peaks corresponding to the phase are detected for thesespecimens. In contrast, after cold rolling, sharp peaks corre-sponding to the phase are detected in Ti18VCR. Inaddition, a weak peak of the phase is also found in Ti20VCR, while only a single phase is detected in Ti22VCR.

Typical optical micrographs for Ti(1822)V alloyssubjected to solution treatment and cold rolling are shownin Fig. 2. After solution treatment, all of the alloys consist ofequiaxial grains, with an average grain diameter of around300 m and without any pre