Study on Fine Structure of Gas Atomized LaNi5-based Alloys

J Mater. Sci. Technol., Vol 19 No, 5. 200Study on Fine Structure of Gas Atomized LaNis-based AlloysHai JING, Hong GUO, Shuguang ZHANG, Zili MA and Shaoning ZHANGNational Engineering Research Center for Non-ferrous Metals Composites, General Research Institute for Non-ferrous MetalsBeijing10008。 ChinaManuseript received June 17, 2002, in revised form August. 15, 2002The fine structure of hydrogen storage alloy powders MINi4-3--Co-Mnometal) prepared by rapidly solidifying gas atomization was investigatedof CaCus-type crystal constants were observed in the studied alloys and one set was i analysis method. Two setsdecreasing powder radius the solidification rate of powder increased, and so did the percentage of a particle partith larger crystal constants. The reason why there were two sets of crystal constants might be the difference ofsolidification rate between the outside and inside of a particleKEY WORDS: Hydrogen storage alloys, Gas atomization, Rietveld analysis, Crystal structureand the 74 45 um and the 30 um were chosen as XRD and Ri-etveld samples. The comps107 La, 29.34%Ce, 5.42%Prosition of the La-rich misch metalRapid solidification(RS), including gas atomization andquench, is able to reduce the level of inhomogene- 16.14% Nd. The purity of the other elements was higher thand to refine solidification microstructure in some alloys. 99.5%uilibrium phases are also possible produced by thisThe XRD profiles were observed with M18XHF Xlogy. From early 1990s, this technology was used to diffractometer which have enough power to investigate theprepare hydrogen storage alloys,2. It was found that hy- studied powder. The measurement conditions were as follodrogen storage alloys by this technology have more homoge- ing: CuKa radiation with voltage 50 kV and current 200 mAby the traditional route. Since then, it has been widely re- 1 deg, antiscattering slit(As)I deg, receiving slit(RS)searched because the cyclic stability and the voltage property 0.15 mm, step scanning model, step size 0.02 deg / step, timeof negative electrode could be improved greatly if these RS constant 4 s/step, 20 from 10 deg to 100 demmm and the sub-stitution elements are assumed to occupy the sites indicatedapidly quenched alloys, the atomized alloy powders are of in the table, according to neutron diffraction studies(6,7 ,Aspherical shape. Furthermore, The mechanism for improving crystal structural model using two sets of CaCus-type crys-cyclic stability of MH/Ni battery by RS was attributed by tal constants was used to analyze the phase structure of theresearchers/4l to that RS made the crystal cell volume of hydrogen storage alloys bigger and the crystal clearance largerthan traditional technology and the ratio of latticeTable 1 Crystal structural model for Rietveldsince the formation of hexagonal hydrides was reducedanalysis of alloys MINi43-Co, Mno. 4Alo3Rietveld analysis first used by Rietveld, is a structure(x=0.75,0.45,0.10)profile refinerment for neutron or X-ray powder diffractiondata collected as a function of the scattering variable T(20or TOF). The goodness of refinement can be decided by sueR-factors as RB, Rp, Rwp and Rexp, where the Rb is theBragg R-factor which should be below 12, the Rp is the pro-file R-factor which should be below 10, the Rwp is the mostimportant weighted profile R-factor which should be below 11/2and Rexp is the expected R-factor which should be below 51/21/2to. In this study, the fine structure of the hydrogen storage al- Note: M=Ni, Co, Mn, Al; space group: P6/mmm(No 191)misch metal)prepared by gas atomization were investigatedusing Rietveld analysis of X-ray powder diffraction profile3. Resultsbetter understanding of the phase composition of rapidlysolidified LaNis-based hydrogen storage alloys was achievedRietveld analnd the principle of the fornation during the process ofsis of X-ray powder diffractionof the atomizednidification was delved further intoMINi4.3-Co, Mno, 4ALo3(==0.75, 0.45, 0.10) powders withdiameter size from 74 um to 45 m2. ExperimentalFig 1, it can be observed that when the value of 20 exceedsFirstly, all of the alloy ingots were prepared by middle- 44 deg. all XRD peaks branch around the maxima or areequency induction melting in Ar atmosphere and solidified skewed towards larger angles. There are no XRD peaks froma copper mould with water cooling. Then the ingots were other phases except the CaCus-Lype. Thus all of the alloysuperheated to liquid again. When the temperature came to keethe desired level the alloy liquid was atomized to powder by中国煤化工 the observed profiles wellcool high-speed Ar gas and the diameter of the powders was andfrom 10 /m to 100 Am. Finally, the powders were sieveCN MH Gnd other r factors are alreasonable as shown in Table 2. It can be concluded thereforet Engineer, Master, to whom correspondence should be addressed, that the above Rietveld results are correct, which prove thatJ. Mater. Sci. Technol, Vol 19 No 5. 2003Table 2 Results of Rietveld analysis for hexagonal alloysMINi4s-2 Coz Mno4Alo.3(x=0.75,0.45,0.10)No T Crystal constants a, /om 0,301 A550.501 63 0.5001231/nm0.40500704038930.402641Weight49723.18No. 2 Crystal constants a2/nm 0.504 541 0.504 018 0.502 659c2/mm0.40733704057770.405Weight percentage/%312850.289.2194112.01.42No. 1: first crystal set with smaller constants, No 2tcrystal set withTable 3 Results of Rietveld analysis for 30 um alloy MINi3ss Coo. a5 Mno 4Al.MINCrystal constantsa1/nmn0.502889C2/mm0.405578eight percentage/%c24.43RB]69682611.33.39No. 2 Crystal constantsc2/nm0.406670Weight percentage/%,A一nd crystal set wither constantsx0.75Fig 2 Illustration of solidification process of an alloy drop人Aanalyzed by the rietveld analysis as above and the results areshown in Table 3. The weight percentage of the alloy partshaving larger crystal constants reached to 75.57%The reason why the atomized alloys have two sets of crys-tal constants may be the difference of crystal morphology onthe different stress states between outside and inside parts ofa particle caused by the difference of their solidification rateduring the solidification process of the atomized powder=0.104. 1 Solidification process of the atomized alloy dropsThe alloy melt emerging from the upper delivery tube isbroken into little drops by high-speed gas current and then thesurfaces of the drops begin to be cooled after contacting withthe cool gas. The whole solidificat ion process of any droystarts therefore from surface with a thin layer of solid film气AAformed on its surface which can be seen in Fig. 2 where R in-dicates the radius of a drop and r is the thickness of solid. Atthis moment the thickness of the solid film is almost zero andhe solidification rate is the largest, With the movement to-wards inside of the boundary between liquid and solid phaseFig 1 XRD Profiles of Rietveld analysis for hydrogen stor- the thickness (r)of the solid surfa e film increases and theage alloys MINi43-_Co, Mno. 4Alo 3(az=0.75, 0.45, thermawill rise as a result. 'The rate of the latent0.0,(a)x=0.75,(b)r=0.45,(c)x=0.10)heat tr中国煤化工 perature gradientthe atomized LaNis-based hydrogen storage alloys have two any alldCNMHGdy!! on as-cast andsetsgas atomized LaNis-based alloys with a result that atomizedThe atomized 30 Am alloy MINi3 8sCoo 45Mno 4Alo 3 was alloys had larger crystal constants than that of the as-castJ Mater. Sci. Technol, Vol 19 No5,2003the crystalof the outside part of La.Nis-based alloy LaNis-based alloy particles prepared by gas atomizationhan those of the inside part because theoutside solidification rate is more rapid than the inside rate(2) The crystal constants of gas atomized LaNis-based al-But using classic stress analysis, the outside crystal constantsders are related to upon its solidification rate3)The smaller the atomized particle, the larger theof the alloy should be smaller than the inside ones. So, there weight percentage of the outside part which having greateris further work to do in the distribution of the two sets of crystal const.antscrystal constants in LaNis-based alloy powder4.2 Formation of two sets of crystal constantsAcknowledgementIt is necessary to consider the process of directional solid-was supported by 863 Project of Chinafication. It has been proved by theory and experiment(@ that (No. 2002AA3230with the solidification rate rising, there are two zones wheresuperfine columnar crystal appears. One is in the area wherehe solidification rate is very low with the columnar crystalREFERENCESgrowing in cellular fashion. The other is in the unbalanced [1]TSakai, H.YoshinagN Kuriyama andpears. Zhou's study/aol on directional solidification of hydro. (2)WZ Tang and G.E. Sun: J.Alloys. Cagen storage alloys showed that the directional solidification [3 Hong GUO, Shaoming ZHANGof LaNis-based alloys followed the same pattern as the otherZHANG, Lei WANG and Likai SHI: Battery, 2000, 30(13)Considering the solidification process of the atomized al- 14] Yufen ZHANG, Yong HOU, Jian WANG and Chuntnao HONGloy drops in the atomization vessel, a state similar to that in (5] H.M. Rietveld: Acta. Cryst.1967.22, 1the directional solidification might occur with the decrease of [6] A Percheron-Guegan, C Lartigue and JCAchard: J.Less-the solidification rate, resulting in two extreme crystal mor-phologies. This may be a way to understand the appearance [7)3.Lamloumi, A Pmi,A Percheron-Guegan, C Lartigue, JCAchardof two sets of crystal constants in LaNis-based alloys. It needs [8] Hai JING, Shuguang ZHANG, Shaoming ZHANG, Hong guo,Zili MA, Caitao LU and Likai SHI: Rare Metals, 2001, 25(4),247.(in Chinese5 Conclusions91 J.G. Li, S.J. Chu and H.Z. Fu: Actat Aeronautica ET Astronau-tica Sinica, 1992, 13, A117(1)There are two sets of CaCus-type crystal constants in [10] Yu ZHOU: Ph.D. Thesis, Zheijiang University, 1996.(in Chi-Crystal Nucleation and Growth of Al-based Alloys Producedby electrolysisZhiyong LIU, Mingxing WANG, Yonggang WENG, Tianfu SONG and Yuping HUOyingpei XIEDepartment of Materials Engineering, Henan University of Science and Technology, Luoyang 471039, ChinaI Manuscript received June 3, 2002, in revised form July 25, 2D02The nucleation and growth of grains in a series of Al-based alloys produced by electrolysis are observed under SEM. The atomiAlof the nuclei and the distribution of Ti at certain points are analyzed by point EDS. The particles in differentatomic Ti/Al ratios might act as the nuclei of a-Al. At the early stage of growth, the spherical Ti-enriched regions might formaround these particles within very limited temperature ranges in which the reactions such as the peritectic reactions etc occurAt the latter stage of growthin the radial orientations and the concentration of ti decreaseslinearly along the dendrite armand dispersion of primary spherical areas in the melts, and thleads to the free growth of dendrite, which is necessary for the formation of equiaxial grainsKEY WORDS: Al based alloys, Nucleation, Spherical growth, Free dendrite growthelectrolyzer. This new efficient niethod has a favoriting effect of transforming the coarse columnar grainsMaster alloys of Al-Ti-(B)are widely used as the grain formed in pure Al into the equiaxial grains. But therefiners in Al based alloys to improve the mechanical proper- nism of grain refinement has not been made clear enough forties, feeding and surface finish, reduce hot-tearing, and dihis method to be put into industrial applicationtribute porosities evenly/ These master alloys are usuallyThe mechanism of grain refine ment is closely related withpreliminarily produced in the induction heating furnaces by the nucleation of Al and the distribution of Ti, thus manymelting pure Ti with Al melts, or by reducing Ti fluorides(or modchloride TiCl4)in AI melts). Liu et中国煤化工 nterface might cause theof Al-based alloys containing low Ti contents by electrolyz- consh restriction 51. The unng Ti dioxide in cryolite-aluminum melts in the industrial dercCNMHGmColko-1)/ko tol, thegrowth restricting factor GRF or Q is defined as m Co(ko1)7, and the relative grain size RGsIB is defined as the solid