Mössbauer investigation of intermixing during ball milling ofFe0.3Cr0.7

Abstract

The intermixing of Fe and T ͑TϭCr,W͒ during ball milling of elemental powder mixtures Fe 1Ϫx T x , with xϭ0.70 for TϭCr and xϭ0.50 for TϭW, has been followed by 57 Fe Mössbauer spectroscopy at room temperature ͑RT͒ and by magnetization measurements for TϭW. The chemical compositions have been chosen to yield final alloys or compounds which are nonmagnetic at RT to better follow the evolution of magnetic phases with milling times. For a long period of milling time t m before reaching the final stationary state, the hyperfine magnetic field distributions remain stationary in shape for both TϭCr and TϭW. Only the relative weight of the magnetic contribution decreases with t m . For TϭW, the average moment of magnetic Fe atoms is further shown to remain constant with t m . Stationary hyperfine field distribution shapes are found to be similar not only for TϭCr and W but also for TϭSi ͑xϭ0.50͒ while published spectra suggest to add TϭAl, Ti, V, Ta, Re to the latter nonexhaustive list. The stationary shape is characterized by a narrow peak located at a field close to the field of alpha iron at RT ͑330 kG͒ and by a broad, almost featureless, band from ϳ50-100 kG to ϳ300-320 kG. The broad band represents about 2/3 of the normalized field distribution. We deduce that the interpretation which consists in attributing the x-ray diffraction peaks of Fe-based bcc solid solutions to a single Fe-rich homogeneous solid solution must be done with care for intermediate milling times. We cannot infer from such hyperfine measurements a detailed description of the regions of the powders which are responsible for such magnetic features. We argue however that irregular interfaces between nanometer-sized Fe-rich zones and T-rich zones may play a role to explain the observed shape of the hyperfine field distributions. The general conditions ͑process and materials͒ in which such phenomena may occur remain to be clarified. ͓S0163-1829͑96͒07941-6͔

Comment on “Grain-boundary structure and magnetic behavior in nanocrystalline ball-milled iron”

“…The small peak in the center of the spectrum which was not fitted by the binomial distribution most probably belongs to a high Cr content alloy phase. 7 Recrystallization heat treatments were performed in a vacuum of 10 Ϫ4 Pa, and similar to the results of Del Bianco et al, 1 the Mössbauer spectra of the heat treated samples agreed well with those of unmilled iron in spite of the measured Cr content. It is well known 8 from corrosion studies that Cr has a very high affinity to oxygen and heat treatment at low oxygen pressure is used to form a protective Cr 2 O 3 layer on the surface.…”

Metastable Metallic Alloys

“…Its shape remains unchanged during the progress of alloying, only decreasing in intensity. This decrease is matched by that of the saturation magnetization per Fe atom [36]. The interesting observation was made that this broad HMF distribution has a ªquasiuniversalº character 820 R. Manaila In the Fe 1Àx Cr x system with 0X50`x`0X72, alloying finally resulted in an amorphous phase and a homogeneous b.c.c.…”

“…solution [47,48]. The Mo È ssbauer spectra [36] recorded for x 0X7 at 80 and 300 K showed the same type of broadened HMF distribution as for Fe±W alloys. This typical Mo È ssbauer pattern was present, with constant shape, for milling times over 0.4 to 0.6 of those necessary for attaining the final (meta)equilibrium structure.…”

“…solutions Fe(W) can be obtained up to 14 at% W. For higher W contents, an amorphous Fe±W phase becomes more stable [45]. Mo È ssbauer spectrometry data showed [36] that mechanical alloying of Fe 0X5 W 0X5 powders beyond 16 to 40 h resulted in a non-magnetic, apparently homogeneous phase, yielding a paramagnetic doublet with QS 0.38 mm/s and IS À0.15 mm/s. X-ray diffraction showed a mixture of an amorphous phase and a W-rich b.c.c.…”

“…Mechanical alloying was reported to result in metastable solid solutions and amorphous phases even in systems with positive heat of mixing, such as: Fe±W [35,36], Cu±Ag [37], Cu±Ta [38], Cu±Cr [39], Cu±W [40], Cu±V [41], Fe±Cu [42,43] and Cu±Co [44]. The elastic energy stored in the numerous defects and grain boundaries is apparently able to overcome the thermodynamic tendencies, forcing the mixtures of elements into supersaturated solid solutions.…”