We investigated the effects of resolved interstitial oxygen or nitrogen atoms and of quenched-in strain on the hardening of the single crystals grown from molten Ti-14 and 20wt pct Mo alloys. The aging treatment at 623K in atmosphere much more increased in the hardness of specimen surface than that in argon atmosphere. The quenched-in compressive stress enhanced age hardening due to omega formation. We surveyed in detail the hardness changes toward the center of plate-shaped single crystal

Hida, Moritaka

Takemoto, Yoshito

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering,Okayama University,Vol.21,No.2,February 1987Depth Dependency of Hardness Change of Ti-Mo AlloysYoshito TAKEMOTO· and Moritaka HIDA·(Received January 16, 1987)SYNOPSISWe investigated the effects of resolved interstitialoxygen or nitrogen atoms and of quenched-in strain onthe hardening of the single crystals grown from moltenTi-14 and 20wt pct Mo alloys. The aging treatment at623K in atmosphere much more increased in the hardnessof specimen surface than that in argon atmosphere. Thequenched-in compressive stress enhanced age hardeningdue to omega formation. We surveyed in detail thehardness changes toward the center of plate-shapedsingle crystal.1. INTRODUCTIONBeta-Ti alloys are well known as one of excellent structuralmaterials. The omega phase formed during aging in this alloy isundesirable as the cause of embrittlement. Therefore, in engineeringlevel, aging treatment has been achieved without using the agingtemperature range of omega formation. On the other hand~ in academiclevel, especially in reseaching pre-transformation, there has beeninterested in the formation mechanism of omega and the origin ofembrittlement owing to isothermal omega phase. The isothermal omegaphase has been considered to be caused by displacive transformation,unlike G.P.zone formation in Al alloys[Sl. In order to elucidate thetransformation mechanism, we have to overcome the experimental* Department of Industrial Science2 Yoshito TAKEMOTO and Moritaka HIDAdifficulties coming from thermal stress and absorption which causeatomic displacement for the omega formatIon. Many reports inregarding to omega formation include the role of oxygen[4J and ofcompressive stress on omega formation[2-4J, and their remarkableeffects on hardness[l-4J. These factors give the alloys a certaininhomogeneity of omega formation from place to place.Here we report some results of preliminary experiments to revealthe factors causing the inhomogeneity. In the present work, thechange of hardness depending on depth was investigated on two kindsof Ti-Mo alloys prepared for the present measurement. One of them isTi-14wt pct Mo alloy being thermally instable beta phase alloy. Theother is Ti-20wt pct Mo alloy being comparatively stable.2. EXPERIMENTAL PROCEDUREButton shaped ingots of Ti-14 and 20wt pet Mo (abbreviated to 14Moand 20Mo later, respectively) alloys were prepared by a non-consumptive arc-melting practice from same materials[lJ. The ingotswere zone-mel ted in pure argon atmosphere in order to grow largescale grains. Plate-shaped single crystals were sliced off from thezone-melted ingots and the surface normal of plate was <110>direction. The plate-shaped single crystals were mechanicallypolished. A silica capsule(vacuum 1.3mPa) containing plate-shapedsingle crystals was heated for 4.5ks at 1223K, quenched in ice waterand then mechanically fractured immediately after immersion. Thequenched single crystals were divided into comparatively small pieces(about 4x.4XO.8mm). The aging treatment was performed at 623K for10"s in air or pure argon atmosphere. After aging treatments,specimens were ground with emery paper by about 20~m. Micro-Vicker'shardness at 0.3kgf load was measured wi th 8 indentations, and wasdetermined by using the mean values. The measurements toward thecenter were performed by a repetition of the same procedure asgrinding by 20~m just mentioned above.3. RESULTS3.1 14Mo alloyDept/I DePtnden(v oj Hardness Change oj Ti-Mo alloyFigure 1 shows the change of hardness depending on the depth fromsurface of the specimens respectively treated for 14Mo.3The curve marked by X shows the change of the as-quenchedthe other hand, done to a Ii gh t one in argonto aging atmosphere, the hardness level foris higher than the that for argon.by e "rapidl y falls in the region of' oxidespecimen that was ground with emery paper. The hardness rapidlyfalls from 320 on the top to about 270 at 201l.m in depth just underthe oxide layer. The classification of· other marks in this report isshown in Table 1.Table 1.Classification of the marks in figuresmark aging atmosphere aging condition of the surface0 air 20,um in depthe air oxide f i 1mt::. argon 20Jlm in depth• argon oxide f i 1m~ argon 110J,lm in depthCl aged in atmosphere after experiment marked by AThe polished surface of the specimens were changed to a darkyellow in atmosphere, onatmosphere. Wi th regardthe specimen aged in airThe hardness markedlayer (from the top to about 201l.m in depth). Howe~ei its feature issimi lar to the hardness change marked by 0 shown as a mountainouscurve, except for the region of oxide layer.As to the agi ng in argon atmosphere, every change of hardness(marked by A,.,'&') monotonously decreases. The specimen marked by .&.was aged again in argon atmosphere after measuring the hardnesschange marked by X in order to investigate the .effect of residualstress on age hardening. Furthermore the specimen marked by [J wasaged in air atmosphere after carrying out a series of experimentmarked by A, that is, we obtained three resul ts for one specimenunder three conditions. In spite of the aging in air atmosphere, thechange of hardness marked by. [J does no t show any obvi ousincrease like the result marked by 0 or e. The deviation ofhardness was at most within ~Hv=%8 in hardness through all specimensof 14Mo.4 y oshita TAKEMOTO and Moritaka HIDATi -14MoTq=1223KTa= 623K100 150. 200 250 300 350 400Depth from Surface / )J mFigure 1. Change of hardness depending on depth in Ti-14Mo.450........._ ..Ti -20Mo~400 1~350·~~O(/)! 301>c.··)(·'''.x. K••••• _•• x x Tq=1223K~ Ta=623Ko 50 100 150 200 250Depth from Surface / AJ mFigure 2. Change of hardness depending on depth in Ti-20Mo.Depth Dcpenden(\' of Hardness Change of Ti-Mo alloy3.2 20Mo alloyFigure 2 shows the change of hardness depending on depth from thesurface of specimens of 20Mo alloy under the following experiments.The do t ted line marked by X shows the change of the as-quenchedspecimen. Only the dotted curve shows the change of the specimenaged for very long time of 10 6 s in air atmosphere wi thout removingoxide layer. These curves are referred from the resul ts of Hida etal.[l].The aging conditions marked by 0 and A are the same one mentionedin the section 3.1. Hardness level marked by 0 is higher than thatmarked by A. However the feature of hardness change is similar eachother, and the gap between the two curves marked by 0 and A becomesto be smaller as deeper the measurement position is. In comparisonwith 14Mo alloy, the hardness level of 20Mo is low, and the scattersof hardness values, 6Hv=±15 from place to place are large for thesame aging time.4. DISCUSSION4.1 14Mo alloyIt can be roughly said that oxide film on the surface influencedthe alloys to keep a certain residual stress and to modify thediffusivity of interstitial impurities.First, there is the essential difference for the hardeningbehavior between in atmosphere and in argon gas as pointed out inFig.I, the specimen aged in argon gas (see the curve marked by A)shows a monotonous decrease in the hardness toward the depth of thespecimen, and on the other hand the aging treatment in atmospheregives a broad maximum, besides the monotonic decreasing by the_factor of interstitial impurities (see Fig.3(a»Second, the as-quenched specimens have a certain thermally inducedcompressive stress near the surface and on the contrary a giventensile one in the region near the center of specimen[I]. From thisfacts isothermal omega phase is seemed to be formed easily near the56 y oshita TAKEMOTO and Moritaka HIDATherefore, thisin the interior ofsurface by the retai ned compress i ve stress.resultant effect causes the decrease in hardnessspecimen (see Fig.3(b) )Third, on the other hand, as the formation of omega phase issuppressed by resolved oxygen atoms, it necessarily follows that thecloser to the surface of specimens the measuring position is, thehigher the hardness is. In spite of an additional aging in air, thefact that the remarkable increase in hardness was not observed showsno residual compressive stress on the surface after our grinding outabout 400.lLm or more in thickness (see Fig.3(c) ).When these factors are totally superimosed so as to be shown inthe schematic diagram (see Fig.3(d) ), it is reasonable that theprofile of hardness vs. the depth becomes to the curve marked by 0of the aging treatment in atmosphere in Fig.I.SurfacefromDepth+->U<D..-....-W(f) ca)cb) (C) Cd)~ + r---------, r------........... r-'-----~ r------...,CDL.<0Ic 0 r------H--~.,........-__l t---2fl"C----q-------loFigure 3. Effects on hardness(a);effect of interstitial atoms, (b);effect of omega phaseinduced by residual compressive stress, (c);effect of omegaphase formation suppressed by interstitial oxygen atoms,(d) ;superimposi tion of (a), (b) and (c).4.2 20Mo alloyAs beta structure is considerably stable in 20Mo, the isothermalomega phase formation is difficult and the solution hardening byinterstitial impurities plays relatively an important role for theprofile and the level of hardness curve. However the solubility ofDepth DejJe"de"cy of Hard"ess .cIulIlge of Ti-Mo alloyoxygen in 20Mo alloy seems to be more limited than 14Mo alloy[6J.Therefore we have to take into consideration specific factorsbelonging to 20Mo alloy, besides the ones on the hardness changes of14Mo.It may be explained by the effect of solution hardening that thedifference of hardness between the cu~ves marked by 0 and A becomesto be smaller at deeper site in the specimen. But we can notrecognize the remarkable difference of the effects caused by agingatmosphere between in air and in argon eas on the depth dependency ofhardness change. Aging time of 10 5 s for 20Mo alloy was not enough toreach the maxi mum hardness. I f the time was pro longed, the hardnessprofile might be monotonous such as the dotted curve obtained by Hidaet al. [1]. Because, as the isothermal age-hardening retarded nearsurface is stimulated by the compressive stress owing to disoluvedinterstitial atoms. the hollow of the curve around 20~m from surfacemight be lifted up and the hardening profile must be monotonous.5. CONCLUSIONThe change of hardness depends on composition, quenched-in stressand atmosphere. 14Mo has a sens it i ve dependence 0 f the hardness onboth the ease of omega formation and on the amount of interstitialatoms.6. REFERENCES1) M.Hida, E.Sukedai : J.Japan Inst. Metals, 50 (1986),893.2) M.Hida, E.Sukedai, Y.Yokohari, A,Nagakawa : Proc. of 4th Inst.Conf. on Titanium 80 (Kyoto Japan,1980), pp.1327-1334.3) M.Oka, Y.Taniguchi :J.Japan Inst. Metals, 42 (1978), 814.4) S.Hanada, M.Ozeki, O.Izumi :Metall. Trans. , 16A (1985),789.5) J.M.Silcok :Acta. Metall, fi (1958), 481.6) T.Khaled, G.H.Narayanan, S.M.Copley :Metall. Trans. , 9A (1978), 1883.7

Depth Dependency of Hardness Change of Ti-Mo Alloys

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