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Session Chairpersons: T.H. Sanders, Georgia Institute of Technology, Atlanta, GA 30332-0245; G. Das, Pratt and Whitney, P O Box 109600, West Palm Beach, FL-33410-9600
STABILITY OF NANOCRYSTALLINE MICROSTRUCTURES IN A Ti-47.5A1-3Cr ALLOY PRODUCED BY MECHANICAL ALLOYING AND HOT ISOSTATIC PRESSING: N. Srisukhumbowornchai, M.L. Ovecoglu*, O.N. Senkov, and F.H. Froes, Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83844-3026, USA; *Department of Metallurgical Engineering, Faculty of Chemistry & Metallurgy, Istanbul Technical University, Maslak, Istanbul 80626, Turkey
Fully dense nanocrystalline Ti-47.5A1-3Cr intermetallic compacts were produced by mechanical alloying and hot isostatic pressing. Grain growth behaviour of these compacts was studied in the temperature range of 725 to 1100°C for annealing times of 0 to 500 hours, using analytical transmission electron microscopy techniques. The temperature and time dependencies of the grain sizes and the grain size distributions were determined, allowing the evaluation of the prevailing transport mechanism responsible for the growth of nano-sized grains to be defined.
PHASE FORMATION AND PROPERTIES OF MECHANICALLY ALLOYED Mg-BASED MULTICOMPONENT LIGHTWEIGHT ALLOYS: J. Eckert, N. Schlorke, C.A.R.T. Miranda1, and L. Schultz; Institute fur Festk"rper-und Werkstofforschung Dresden, Institute fur Metallische Werkstoffe, D-01171 Dresden, Germany; 1Permanent Address: LIJÓ-Barcelos, Portugal
Multicomponent Mg-Y-Cu lightweight alloys were prepared by mechanical alloying of elemental powder mixtures. The progress of alloying, the resulting phases and their thermal stability were characterised by x-ray diffraction, differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA). Coexistent amorphous and nanoscale crystalline phases are found for a variety of alloys with different composition. The thermal stability data of the materials as obtained by constant-rate heating DSC and TMA measurements and isothermal annealing experiments are discussed with respect to phase separation in the undercooled liquid state and nucleation and growth of nanocrystalline phases. Some characteristic properties of the glassy phase are presented and discussed with respect to the composition dependence of the glass transition and the crystalline temperature. The results are compared with data for melt-quenched samples. Finally, the influence of processing conditions and contamination effects during milling on the phase formation and the thermal stability of the mechanically attrited powders is critically assessed.
HIGH TEMPERATURE DEFORMATION BEHAVIOUR OF MECHANICALLY ALLOYED Al-10Ti-XSI ALLOY: C.J. Choi, W.W. Park, Department of Materials Processing, Korea Institute of Machinery and Materials, 66 Sangnam-Dong, Changwon, Kyungnam 641-010, S.Korea ; J.K. Park, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-Dong, Yusong-Gu, Taejon 305-701, S. Korea
The creep property of mechanically alloyed A1-10Ti-xSi (in wt%, x=0,2 4) alloy was investigated in a wide range of stress (60-310MPa) and temperature (300-450°C). Creep mechanisms were analyzed in some details and compared with each other. Two distinctive creep mechanisms have been observed depending on the testing stress and temperature, ie, Coble creep at low stresses and temperatures and dislocation creep at high stresses and temperatures. True creep activation energies in dislocation creep regime were calculated using modified semi-emperical creep equation. The calculated values were in good agreement with that for self lattice diffusion in pure aluminium. The diffusional (Coble) creep regime significantly enlarged to high stress and temperature region with an increase of Si content due to an increasing volume fraction of dispersiods.
MECHANICAL ALLOYING OF Ti-Ni BASED MATERIALS USING THE HORIZONTAL ATTRITOR: H. Zoz1, D. Ernst2, I.S. Ahn3, W.H. Kwon;1Zoz GmbH, D-57482 Wenden, Germany; 2University of Siegen, D-57068 Siegen, Germany; 3Gyeonsang National University, Chinju, Gyeongnam, 660-701 Korea
The production of large quantities of contamination free mechanically alloyed powders from titanium and nickle based materials has proven to be a major challenge. Feasibility of such a goal can be carried out, at laboratory level, by any milling device like the very common planetary ball mill. In this case however, the possibility of a subsequent scaling up for larger production is hindered by the intrinsic limits of a planetary ball mill design. On the contrary the horizontal Zoz attritor can be experimented at laboratory level using small volume chamber-units (0.25, 0.5, and 2.1) and, for industrial production, using the large volume units (up to 400 1) based on the same conceptual design. Therefore, experiments have been conducted on blended elemental Ti-Ni compositions in the proportions Ti-51.5Ni, Ti-50Ni, Ti-49.5Ni and Ti-48.5Ni (at %) using a Zoz attritor with a small unit-chamber (0.51). Due to the inherent ductility of the powder, the material has the tendency to adhere to the grinding unit and the grinding media. Further, in order to avoid high contamination and to make the process realistic from an economical point of view, the milling time has to be reduced to a minimum. The above points identify a Critical Milling Behaviour (CMB) of the system under investigation that must be kept under control to achieve the wanted goal. It will be shown by the present paper that by adopting a suitable milling and discharging procedure (Cycle Operation by Operation Cycle and Discharging Cycle) low contamination and good yield have been substantially achieved. This is investigated by chemical analysis, by scanning electron microscopy and X-ray diffraction.
MICROSTRUCTURAL EVOLUTION OF MECHANICALLY ALLOYED AND HOT ISOSTATICALLY PRESSED NANOCRYSTALLINE Ti-47.5A1-3Cr ALLOY DURING ANNEALING AT 120°C AND AIR COOLING: M.L.Övecoglu, O.N.Senkov, N.Srisukhumbowornchai, N. Hoo, C.M. Ward-Close, P. Goodwin, P.Tsakiropoulos, F.H. Froes, Institute for Materials and Advanced Processes University of Idaho, Moscow, ID 83844-3026; Department of Metallurgical Engineering, Istanbul Technical University, Maslak, Istanbul 80626, Turkey; DRA Farnborough, Hampshire, UK; University of Surrey, Guildford, Surrey, UK
Mechanical alloying and hot isostatic pressing techniques were employed to synthesise a fully dense Ti-47.5AJ-3Cr alloy containing equipped nanocrystalline TiA1 grains. Isothermal annealing was conducted at 1200°C (above the eutectoid transformation temperature) at holding times of 5, 10, 25 and 35 hours. Analytical transmission electron microscopy investigations revealed the presence of a dual structure comprising 2/ laths and equipped g grains some of which were twinned. The grains coarsened during annealing and the 2/ laths were formed during cooling as a result of the eutecroid transformation. At the maximum annealing time studied (35 hours), the average grain size of grains was about 2 mm and the width of the 2/ laths was -0.3mm.
10:10 am BREAK
CHARACTERISATION OF A Ti-48AJ-2Nb-2Cr ALLOY SYNTHESIZED FROM MIXED GAS ATOMIZED AND MECHANICALLY ALLOYED POWDERS: O.N. Senkov, N. Srisukhumbowarnchai, M.L Ovecoglu*, N. Hoo, C.C. Ward-Close**, P. Goodwin**, P. Tsakiropoulos*, F.H. Froes, Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83844-3026; *Department of Metallurgical Engineering, Istanbul Technical University, Maslak, Istanbul 80626, Turkey; **DRA Farnborough, Hampshire, UK; University of Surrey, Guildford, Surrey, UK
Gamma TiA1 based compacts were synthesized by hot isostatic pressing (HIP'ing) a mixture of pre-alloyed Ti-48A1-2Nb-2Cr gas atomized and mechanically alloyed powders. Mechanical alloying of the initial powder was performed in a SPEX mill for 15 hours. HIP'ing was conducted at 850°C under 206 MPa pressure. Two different volume proportions of the gas atomized and mechanically alloyed powders were utilized in the HIP'ing process. This allowed production of microstructures in HIP'd compacts: a homogeneous nanocrystalline structures, when 100% mechanically alloyed powder was used, and a bimodal structure consisted of the mixture of micron-sized and nono-sized grains, when the compacts were HIP'd from a blend of the gas atomized and mechanically alloyed powders. Evolution of the microstructure during mechanical alloying and HIP'ing was studied by XRD, SEM and TEM techniques.
FLOW AND COOLING CHARACTERISTICS OF AL-SI DROPLETS/PARTICLES DURING GAS ATOMIZATION: Seong-Yeon Yoo, Sang-Yoon Lee; Rapidly Solidified Materials Research Center, Department of Mechanical Design Engineering, Chungnam National University, Taejon, 305-764, KOREA
There are two important parameters in gas atomization - particle size and cooling rate. These two parameters are mainly controlled by gas flow and heat transfer between gas and droplets/particles. The purpose of this research is to investigate flow structure, cooling rate and trajectory of Al-Si droplets/particles by numerical simulation. Gas flow is simulated using finite volume method, and particle trajectory is simulated using Eulerian method. Rosin-Rammler formula is used for the particle size distribution. Gas field influences particle trajectory, reversely particles alter the gas field. So momentum and heat exchange between gas and particles are calculated by iterative method. Particle trajectory is found to depend on particle size, the location of particle formation, and turbulent motion of gas. Cooling rate of particles is calculated by taking into account latent heat of melt and local heat transfer between particle and gas. Small particle cools down rapidly, while large diameter particles solidifies very slowly.
PREPARATION OF Ti POWDER BY A MECHANICAL MILLING INDUCED REDUCTION REACTION: E.G. Baburaj, Kevin T. Hubert, Carl Powell, C. Suryanarayana, and F.H. Froes, IMAP, University of Idaho, Moscow, ID 83844-3026
A displacement reaction between TiC14and Mg, induced by mechanical milling, has been employed for preparation of fine Ti powder. The present work shows the possibility of considerable reduction in milling time by the use of pre-milled Mg which enhances the reactivity due to an increase in surface area and surface activity. The milled product has been leached repeatedly to obtaining pure Ti. The reaction products before and after leaching have been examined by XRD, SEM, and TEM. The product after leaching, as determined by XRD is pure Ti. The particle size, as observed by TEM is in the range of 5-200nm. The EDX analysis of individual crystals or agglomerates using TEM, does not show the presence of chloride. Small amount of oxygen and nitrogen as impurities have been detected. Further work on the analysis of the powder is in progress.
MICROSTRUCTURE-PROPERTY STUDIES IN AL-8Fe-4Ni-2 5Zr ALLOYS: P. Keramidas, P. Tsakiropoulos; Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU2 5XH, England, UK
The microstructures of shill cast ingots, gas atomised powders and extruded bars of Al-8Fe-4Ni-2.5Zr (wt%) were studied. Phase selection in the wedge shaped ingots and in the gas atomised powder particles will be discussed. The microstructures of the as extruded and heat treated alloy will be presented and related to the hardness, tensile properties and texture of the alloy. The latter will be compared to the properties of Al-8Fe-4Ni and the effects of Zr addition on the properties of the alloy will be discussed.
STRUCTURE AND PROPERTIES OF RAPIDLY SOLIDIFIED Al-Li-Cu-Mg ALLOYS CONTAINING Zr AND Ag: V.G. Pushin, L.I. Kaigorodova, E.I. Selnikhina, Institute of Metal Physics, Ural Division of Russian Academy of Sciences, +1620219, +0+1 Ekaterinburg, Russia
Rapidly solidified Al-Li-Cu-Mg alloys prepared by argon atomization process were investigated. Using cold compaction and canning, the powder was hot extruded into a bar of diameter 17 mm. Compared with traditional ingot metallurgy alloys, the microstructure and properties of rapidly solidified powder metallurgy alloys has been significantly improved. The effects of adding minor Zr, Ag or rare earth (RE) Nd and La on the Al-Li-Cu-Mg alloys are different. Minor Zr and Ag can efficiently accelerate the ageing process and increase the strength and hardness of alloys. Minor RE raises the ductility. The microstructure of alloys forms due to precipitation process of metastable binary phase Al(3)Li, ternary Tl-phase Al(2)CuLi with hexagonal crystal structure, and also tetragonal metastable phases on base of Al(2)Cu. These three coexistent channels of phase transformations are responsible for strengthening of alloys during subsequent ageing. The particle size of atomized powder for alloys was in the range of 10-150nm, but after optimal thermo-mechanical treatments the microstructure of alloys has been significantly dispersed and more refined. The grains are more homogeneous and have a nano-scale size on the average. In this state particles of this phase has a globula form and preferably heterogenuons nucleation at consequent aging.
MODELLING AND CONSOLIDATION OF NANOCRYSTALLINE ALUMINIUM: J.S. Idasetima, R.B. Bhagat, M.F. Amateau, The Pennsylvania State University Applied Laboratory, Box 30, New ARL Building, State College, PA
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