Атомное разупорядочение и ОЦК -> ГЦК превращение в сплаве Гейслера Ni-=SUB=-54-=/SUB=-Mn-=SUB=-20-=/SUB=-Fe-=SUB=-1-=/SUB=-Ga-=SUB=-25-=/SUB=-, подвергнутом мегапластической деформации кручением под высоким давлением

Heusler L21 alloy Ni54Mn20Fe1Ga25 subjected to megaplastic deformation by high pressure torsion was first systematically investigated by in situ phase x-ray diffraction, transmission and scanning electron microscopy. It is established that shear deformation by torsion at room temperature grinds the polycrystalline structure of the alloy to a nanocrystalline and partially amorphous state. It is found that as the pressure value (from 3 to 5 GPa) and the degree of deformation (from 2 to 5 revolutions) increases, total atomic disordering and stepwise structural-phase transformation according to the scheme B2(BCC)→A2(BCC)→A1(FCC) occur. It is shown that annealing at temperatures below 570 K entails devitrification of the amorphous phase, and at 620 K and above – the restoration of the L21 structure. The dimensional effect of suppressing thermoelastic martensitic transformation in a nanostructured austenitic L21 alloy with a grain size less than 80 nm while its cooling down to 120 K. The ability to thermoelastic martensitic transformation and shape memory effect in submicrocrystalline ultrafine alloy after recrystallization annealing at temperatures exceeding 600 K is restored.

Atomic Order and Submicrostructure in Iron Alloys at Megaplastic Deformation

The subject of the present review consists of summing up our previous results on the study of the relaxation of structure along the way (i) of atomic redistribution—in the form of short-range clustering in binary iron alloys—induced by megaplastic deformation (i.e., of super large value), and (ii) of the dissolution and precipitation of disperse nitrides and carbides in steels and intermetallics in ageing alloys. Within the capacity of the main method of executing megaplastic deformation, along with the practically important milling in ball mills and friction-providing external action, we employed high pressure torsion (HPT) in Bridgman anvils, which permitted the control of the degree, rate, and temperature of deformation action. At the local level of two nearest neighbors (one or two coordination shells in relation to an iron atom) we studied atomic mass transfer, stipulated by generation of a large number of point defects of deformation origin, and conducted a comparison with a case of irradiation by high-energy electrons. We established a change in the direction of phase transformations, as well as anomalous acceleration of the ordering and precipitation of disperse phases upon altering the temperature (T < 0.3Tmelt) and rate of deformation (from 2 × 10−2 to 8 × 10−2 s−1). We also demonstrated the possibility of regulating the ultra-fine-grained structure with solid–solution strengthening and dispersion hardening.