the effects of process control agents on mechanical

8/4/2019 The Effects of Process Control Agents on Mechanical

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The effects of process control agents on mechanicalmechanisms in the Ti-AI system

Wonsik Lee. S.I. Kwun

alloying

The mechanical alloying (MA) of mult~~omponcntpowders [l-3] and the production of nano-srykd linemnvd ers bv ball millina 14.51 have attracted muchattention from researchers in rcccnt decades owing totheir several advantages [h.7]. However, some rc-searchers [8.9! hare repoiicd problems related to thedesired final chemical composition. homogeneity andgrain refinement after bal! millmg due to wvere plasticdeforma tion and welding of very ductile f.c.c. powders.There are two method s to prevent the seve re welding:one is to lower the temp erature of the milling v ial[l,lO] and the other is to add PC.4 to the powders[Y.ll]; the latter is more favo red. Orgam c PCAscontaining atoms such as 0, C. H, etc. may affect theprocess of MA [12,13] and sometimes the atoms existas impurities after consolidation o f mecha nically al-loysd powders. For cxa mplc. the ductility of theintermetallic compoun d TiAl decreases as the amountof 0 and/or C increase s [l4]. Therefo re, it IS im-portant to sckcl an appropriate PCA and its amountin order lo avoid complications.

Many researchers [15,16] have also rsparted theformation of the mctastahte f.c.c. phase during MA in

fk Ti-AI \yskzm. Tbc formation mschsnism of thismcta~uble f.c.c. phase has not been prove d clearly. inspire of reports that the f.c.c. formation may be d ue to0 and N conkmunation [IS]. The effect and the roleof PCA on the MA process or mechanism are studiedhere systcmatisally in terms of the amount and speciesof PCA.

Horizontal ball mills with s tainless steci vial (diam-cter I30 X I30 mm ) and balls were run al IO0 revmin and room icmperature for MA in this experi-ment. The nommal composition was chosen as Ti-48at.%Al and tbc high purity elemen tal pov .ders ofYY.Y% Ti (No. - 37.5) and 99.9% Al (Nos. 1 50-25 0)were charged with PCA. To prevent contamination.the halls and inner vial were za ed with nominalcomp osition elematal powder: (TX-48 at.%Al) andmilling was perforRne d alder Ar a tmos phere . In orderto invesligalr the effects of the species of PCA rjn theMA m echa nism, CH,OH (meti:snol) and C,H, (hen-zene) were chosen as PCAs. As mu ch as 0.3 or 3.0wt.% of the PCA CH,OH was adde d to the total


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elemental powders (IOX. g). The amoun t and itsrespecrivc numhcr ol molecules. sod tbc nurr.hcr ol C.0, and H atomi in PCAs. arc given in Tahlc I.Condition B contains the same kind of molecules andatoms hut IO Limrs a, many ar condilion A. Incontrast. condition C contains one third of the numberof molccule~ and no 0 atoms. hut twice the number ofC atoms 8) condition 8; in other words. the number ofC atoms in condition C is equal to the number of Catoms plus 0 atoms in condition R. The powders usedfor investigation of phase translormalions during DTAanalysis wc!c cncapwlatcd in a quartz tube with ahoutIn- Torr pressure. DTA analysis WBS perhrmed attemperatures increasing a: a rate of 10C min usinga Stanton Rrdcroft STA 15(x), and XRD was carriedout with a Co tar&t using a Rigaku DMAX 2200.

3. Results and discussion

The pow der, were milled f or ItXX) h under con-ditions A. 8, and C in Table I. and one without PCAas well. The X-ray diffraciion patterns of each con-dition aflcr various milling times. t o follow the phaserransformations during M A. arc shown in Figs. l-4. Inthe condition without PCA, as shown in Fit. I.elemental powders were transformed to an amorpIhoosohase afler 3 00 h of millinr and to a metastahlc I.c.c.phase [I,21 with a lattice &amctrr II of around 4.20.& after 50 0 h. Under condilion A. which contains lessCH,OH , the met&able disordered TI,AI 11.1 71 wasfor&d after 300 h of milling and an amorpho us phasewas obtained only after IiHKl h (Fin. 21. Under con-dition B, the int&ty oC the main peak of Ti, dif-fracted from the Kill) plane, was reduced rapidly after31x1 h. while the (111 0) peak remained obvious withrelatively strong intensity even after 7 0) h. and thebroadened (011) peak seemed to separate into twopeaks after lOGil h (Fig. 3). Finally, under condition C,which contains no 0 atoms in PCA as shown in Fig. 4.the Ti element peaks. although shifted slightly. stillremained after 70 0 h and a metastable f.c.c. phase


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IV LCC. s. , Ka~m, i I, ,I,,, 1u1,0, n,,


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Ti(Ol I) was coincident with that calculated by Ti(010)and Ti(OO2) (Table 2). Tbcsc results indicate that thelattice parameter of the c-axis increases under con-ditions B and C during milling. while it decreasesunder condition A.

The variations in latficc parameter with millinn timeunder each PC A conditionare shown in Fig. 6. Undercondition A. both the (I- and c-axis decreased rapidlyuntil 300 h of milling, when a metastable disorderedTi,AI phase was formed (Fig. 2): they then remainedalmost constant until 700 h. This implies that MA

200 100 WnJ BW oolMilling Time (hr)


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proceeds by substitutional difiusion of Al atoms in Ti.and this diffusion is completed before XXI h.

Condition B. which contains relatively many molc-cules and 0 atoms. showed B very broad main peak ofTi after 3fll1 h of milling (Fig. 3). and only the c-axisincreawd with liltle change in the u-axis until 500 II ofmilling (Fig. 6). This means that interstitial penutra-lion. instead of substilullonal diffusion. prevails at themt~:tace bctwecn powders under lhn condition. Theinterplanar spacing d of t/&l) planes m Ihc b.c.p.structure is cxprcrsed as

(1)The random pcnctration of Al atoms into Ti intcr-

stitial sites will caose the random ddatcd d~rtortwn otthe lattice along the c-axis. resulting in an incrcaw inc-axis and a decrease in intensities for the planes 1 f 0.for cxample (01 I) and (iH12). The reason for the smallchange in u-axis lattice parameter of the peaks fromplan& l-0. for cxampl; (011)). IS rhc o&t hcrwccnthe decrease bv substitutional dilfusinn and the in-crease by inters&ial pcnctration. which rccuIt\ from Bsmall change of o-axis by interstitial penetration. Incontrast, lbe increase in c-axis by intcrstilial pcnetra-tion is too large 10 be offset by the small decrease dueto substitutional diffusion.

Under condition C, which contains one third of thenumber of molecules in condillon B and the most Catoms but no 0 atom s. the Ti peaks still rsmainedafter 7tM) h of milling (Fig. 4) with a slight dccrcasc inn-axis and a large increase m L-axis (Fig, 61. This resultsuggests that the substitutional diffusion of Al in Tiand the interstitial penetration occur simuttaecouslyunder this condition. and that interstitial penetration isrelatively more prevailing in condition B. However. itis not yet clear whether the overall smaller drcrease inn-axts under condition B compared with condition Ccomes from mote restraint of the wbstltutional dilfu-sion due to the number of molecules and/or thespecies. or due to 0 atoms in the former.

In our previous investigation 1341 . when 9 .1 ml ofC,H, wai added so that-the n&b& of H atoms isalmost three tim es that in condition C. the DTA curveshowed a different shape fro m that for condition B.similar to condition A instead. This result suggests thatH atoms at least do cot affect the MA m echanismHowever. the role of H atoms in PCAs is not clear atpresent.

The above results imply that the predominant MAmechanism chanecs from substitutional diffusion toatomic penetrat& into in:erstitial sites as the amoun tof PCA increases, even though the reason for this hasnot been explained clearly yet. The reason for ealierformation of amorph ous phase in the condition with-

out PCA com pared wth condition A is that thecompos mon for amorph ous phase can be obtained~ooer due to higher energy transfer fro,,, halls topowders durmg milling. Our previous investigation13 4 I ohscrvcd that the amo t of powders recoveredfrom the vial altsr m illing under the condition withoutPCA was about one third of that under condition A.lh,r means that the amount of powders trappedbctwecn balls will hc Icss. and the cncreics that thepowders cxpcrience will thus be higher-in the con-dit~on without PCA. Eckerl et al. 135 1 also reoorled. .tba! it took ICC\ time to ahtain the amorph ous phase asthe mtcnsity of the balls increased. In older to conlirmthn analysis in another way. half of the elementalpowdcru and PCA under c ondition A wa5 charged intbc milling vial so that the powders in this conditioncxpctaeu~cd mwc energy than those under conditionA. In this experiment. an amorpho us phase wasalready obtamcd after 4(K) h, compared with IWO hunder condition A. and the lattice parameters ofpowders after XX) h of milling were a =X9.3 and( =4.X3 A (Table 2). indicating that the c-axis wasmcreascd by mdline in this condition. while decreasedunder cond~tlon A: It is clear to say from the abovecnpenmetal rewlts that the time to form the amor-pbnu~ phase dccrcascs as the energy transferred fromhdlls to pow& r\ increases. The results also suggestthat Ihc predominant mechanism for the conditionwithout PCA IS atomic penentration into interstitial\itc\. and the prcdom& nt mechanism for MAchanger f rom subaitutional diffusion to atomic pene-

Al system w;n obtainedin orde; to investigate how theamoun t of PCA. number of impurity atoms, affects thecrystallizatwn behavior of the amorpho us phase. TheDTA curve of each condition displays two peaks, asshown in Fig. 7. The first peaks due to crystallizationto Ti,AI under conditions-A and B are sharp, whilethose due to ordering of the f.c.c. phace under thecondition without PCA and condition C are verybroad and slo\*lv varvinr. However. the second peaksdue to cryrt;tlliz~tion~to~iAl are very sharp under allconditions. Therefore. the srcond peaks in the DTAcurve were used to detcrminc lbe activation cncrgy forcrystallization to TiAl using Kissingers method [36].The result given in Fit. 8 shows the trend that. as thenumber oi impurity &ms. especially 0 atoms, in PCAincreases. the activation energy increases. The lowestvalue of 281 kJ maI_ was oblained under the con-dilion without PCA and the highest, 411 kl mol ,under the condition B containing most 0 atoms. Thistendency is similar to that rcportcd [37] for the Ni-50


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at.%Zr system, in whic h the ac tivation ene rgy forcrystallization increased from 240 to 290 kJ mole byad da ioo of 2.3 at.%0

The metastable f.c.c. pha se was formed after longtime milling only under tbe co ndi:ian without PCAand co ndition C, in which a tomic penetration was theprevailing mec hanism of MA. Work on MA in the

T-AI system b y bigh energy ep ex milling [2,16.38]also reported the formation of a mc tastable f.c.c.pha se. It is thus thought that atomic pe netratiun playsan important role in formation of the f.c.c. pha se.However, the reason for not forming an f.c.c. pha seunde r mndition B. in which atomic pe netration is alsoa prevailing mecha nism. seems to be acc ounted for by0 atoms.

4. Conclusions

By investigating the effec ts of PCA during MA o fthe T-48 at.%Al system. several co nc lusions wereobtained.

1. As the energy transferred from ba lls to pow dersmc reases, it takes less time to form an amo rphousphase.

2. As the intensity of the energy b etween balls andthe amo unt 01 PCA increa ses. the prevailing MAmec hanism cha nge s from substitutional diffusion toatomic pe netra tion into intersUtia1 sites.

3. The meta s!able f.c.c. pha ss with lattice p arame tera of around 4.20 A is formed after SC0 and IOil hunde r the co ndition without PCA and co ndition Crespec tively. Atomic pe netration into interstitial sitesmay he responsible for the formation of the meta -stable f.c.c. pha se.

4. The ac tivation energy for crystallization be co meslarger BS the numb cr of impurity atoms. espcc iatly 0atoms. in PCA inc reases.


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the effects of process control agents on mechanical

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