Mechanically induced phase transformations of the sigma phase of nanograined and of coarse-grained near-equiatomic FeCr alloys

Abstract

A coarse-grained tetragonal sigma phase -Fe 0.54 Cr 0.46 , prepared from an as-cast alloy, and a nanocrystalline -Fe 0.52 Cr 0.48 phase were ballmilled in argon in a Fritsch P0 ball-mill at different vibration amplitudes. X-ray diffraction evidences solely a gradual transformation of the sigma phases into bcc phases. Room-temperature Mössbauer spectroscopy shows further the presence of a non-magnetic phase which remains non-magnetic at least down to 35 K. It is attributed to an amorphous phase whose formation is favoured at grain boundaries by oxygen in such milling conditions. Possible explanations of the different final structures reported in the literature in high-energy ball-milled near-equiatomic Fe-Cr alloys are discussed.

Phase transformations of σ-FeCr induced by ball milling

“…As discussed in more detail in [16], the formation of an amorphous phase at grain boundaries favoured by the presence of oxygen, or even that of an amorphous-bcc composite with amorphous layers surrounding bcc grains, appears to be a reasonable assumption both consistent with the Xray diffraction pattern of a bcc phase and the change of magnetic properties. In a dynamical equilibrium phase diagram of near-equiatomic Fe-Cr alloys milled under vacuum, the domain of amorphization is expected to be shifted downwards to smaller injected powers with respect to that of Fe-Cr alloys milled in the presence of residual gases which stabilize the amorphous phase [16]. An amorphous Fe-Cr phase can indeed be formed in experimental conditions in which oxygen does not play a significant role [5,12].…”

“…A DSC trace of the latter sample is shown in Figure 3. The two exothermic peaks, located at about 600°C and 680°C, are attributed to a two-step crystallization of an amorphous phase as advocated in [16].…”

“…Mössbauer spectroscopy shows that the formation of the bcc phase occurs together with a partial amorphization of the ball-milled sample. Amorphization, which is known to be favoured by the presence of oxygen, might however be largely of intrinsic origin as discussed further in [16].…”

“…Fe. The amorphous phase is thus not fully crystallized as suggested by the small contribution seen in the central part of the spectrum of Figure 4a and reflected by a peak at low field in the corresponding P (B) distribution (see also [16]). …”

Debye temperature of nanocrystalline Fe–Cr alloys obtained by mechanical alloying

“…In particular, in equiatomic or near-equiatomic FeCr alloys milled in vacuum or argon atmosphere, this effect was observed both in alpha and sigma phases, e.g. [29][30][31][32]. …”