Ti-Al Alloys Prepared by Ball Milling and Hot Isostatic Pressing
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TI-AL ALLOYS PREPARED BY BALL MILLING AND HOT ISOSTATIC PRESSING M. OEHRING, T. KLASSEN AND R. BORMANN GKSS-Research Center, Institute for Materials Research, Max-Planck-Str., D-2054 Geesthacht, Germany
ABSTRACT For the powder metallurgical production of Ti-Al alloys, elemental powder blends and prealloyed powders were mechanically alloyed in a planetary ball mill. Thereby metastable solid solutions or amorphous phases are formed depending on the overall composition and the milling conditions. The crystalline phases exhibit an extremely small crystallite size of about 10 - 30 nm. This favors the consolidation of the powders which was performed by hot isostatic pressing in the temperature range between 500 °C and 800 *C. The results show that fully dense Ti-48 at.% Al alloys can be obtained only for a temperature of 800 'C. The crystallite sizes increase during compaction, but remain below about 0.15 gm. This allows the preparation of test specimens to investigate the mechanical properties of Ti-Al alloys with submicron crystallite sizes. INTRODUCTION The powder metallurgical production of alloy components can exhibit several advantages with respect to ingot metallurgy. For example, the coarse-grained microstructures and chemical inhomogeneities caused by solidification can be avoided. Since powders can be compacted in one step to complex parts with close tolerances, material consumption is reduced and machining can be saved. In addition, metal powders are required for advanced powder metallurgical techniques such as injection molding and plasma spraying. Therefore, the powder metallurgical processing has been applied for the production of NbAl'- 3 , Ti-A14 -6 and Ti-Si3' 4' 7 alloys. In particular, mechanical alloying of elemental powder blends and of intermetallic compounds have been employed in order to prepare homogeneous alloy powders with a submicron grain size. Due to the non-equilibrium processing, mechanically alloyed Nb-Al, Ti-Al, and Ti-Si powders consist of metastable solid solutions or amorphous phases, depending on the composition of the alloy. Details of the preparation and the mechanisms of phase formation have been described in previous publications'1 .5 7 ,8 . In this article we will report on first consolidation experiments by hot isostatic pressing performed on mechanically alloyed Ti-Al powders. EXPERIMENTAL Powders of the pure elements Ti and Al were blended in the desired composition for mechanical alloying. The milling of the powder blends and of powders of intermetallic phases was performed with a planetary ball mill of type Fritsch pulverisette 5 using Cr steel vials and balls. The milling intensity (an arbitrary rotation speed scale) was adjusted to 5 or 7 corresponding to rotation speeds of 150 min-' and 230 min-', respectively. To avoid atmospheric impurities all handling of the powders including the milling was carried out in glove boxes supplied with Ar via a gas purification system which maintains the oxygen and moisture content of the atmosphere below 1 ppm. After milling, the powde
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