It is known that one of the methods for ductilization of refractory metals, e.g. W and Mo, and for decreasing the temperature of ductile-to-brittle transition is grain-size refinement. During the recent decade, methods for grain size refinement employing severe plastic deformation were being developed intensively. It has been shown that grain size reduction of Mo can be effectively achieved by high pressure torsion (HPT). HPT-treatment results in the formation of submicrocrystalline structure (average grain size ~0.2 μm) in molybdenum and in residual porosity removal. However, HPT-samples have very limited size, which renders impossible their practical application. In view of the above, we studied the feasibility to form submicrocrystalline structure in molybdenum using severe plastic deformation by multiple all-round pressing (so-called abc-pressing) which offers a number of advantages over the other methods of severe plastic deformation. For example, abc-pressing allows one to obtain submicrocrystalline structure in bulk samples, it is less labour-intensive and is superior to equal channel angular pressing in that it requires no specific equipment and setup, which is a very attractive feature in view of possible commercial applications.

It is established that molybdenum in the as-received state has the columnar structure with cavities between the columns. Grain size in the cross-section is about 5 μm. After abc-pressing at elevated temperature the cavities on the metallographic section are not observed. The electron microscopic investigations reveal submicrocrystalline structure (d~0.5 μm). As a result of the abc-pressing the microhardness value rises by 1.5 times relative to the as-received state. It is found that thermal stability of the submicrocrystalline structure produced by abc-pressing is significantly lower relative to that produced by high-pressure torsion. The physical reasons for the above unusual difference are discussed.

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