Annealing Behavior of a Nanostructured Fe1.5Mo Alloy MATTEO LEONI, PAOLO SCARDI, MIRCO D’INCAU, and GIUSEPPINA LUCIANI In-situ synchrotron radiation X-ray powder diffraction is shown to be complementary to thermal analysis for the study of the annealing of a ball-milled nanocrystalline Fe 1.5 wt pct Mo powder. The evolution of domain size distribution and defects is quantified via whole powder pattern modeling (WPPM) of the diffraction data. A possible annealing mechanism is proposed for the powder. DOI: 10.1007/s11661-011-0762-4 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION THERMAL analysis, thermogravimetry (TG), and dilatometry are probably the most common tools for the study of the annealing or sintering behavior of tradi- tional and nanocrystalline materials. The information they provide, however, is limited to the type and energetics of the observed reactions: the mechanisms involved in the phenomena occurring in the material in temperature can just be inferred from, but not com- pletely unveiled by, thermal analysis alone. Microscopy, and in particular transmission electron microscopy (TEM), could be the ideal complement to thermal analysis, as the evolution of a material could be followed directly and in situ. Both the structural changes and the microstructure evolution, in principle, can be monitored down to atomic resolution by TEM, as hot stages are nowadays available. The technique, however, is quite local and works only on thin specimens or loose nanostructured powders; therefore, following the evo- lution of a bulk is an impossible task. An alternative, albeit one that does not provide visual evidence, is offered by X-ray powder diffraction (XRD). In particular, synchrotron radiation XRD (SRXRD) allows structure and microstructure evolution to be monitored nondestructively and in situ even on small amounts of powder or where fast processes are involved. A large set of isothermal or isochronal data are usually collected to study, e.g., thermal expansion, phase transformation, and hydration/dehydration; the recent trend is in the use of the Rietveld method to analyze multiple patterns simultaneously, constraining the tem- perature-dependent parameters in the so-called para- metric or surface studies. [1,2] To gain some insight in the microstructure evolution, isothermal data are often analyzed with traditional line profile analysis (LPA) techniques such as the Scherrer formula, [3] the Williamson–Hall (WH) plot, [4] and the Warren–Averbach (WA) method. [5] Those techniques are known to be fast and to provide some ‘‘mean crystallite size’’ and ‘‘microstrain’’ values. [6] The true meaning of those results is seldom investigated, leading to possibly erroneous interpretations; in the general case, in fact, the mean crystallite size is not the mean of some domain size distribution, whereas microstrain is a generic term effectively describing the presence of defects in the material, but not providing any hints on their origin. Some modifications to the WH and WA were presented in the literature to improve the physical meaning of the results, considering, e.g., dislocations as a possible source for microstrain: [7,8] the size term as well as most of the underlying simplified hypotheses and operative analysis method, however, are unchanged. Therefore, doubts can arise on the validity of these LPA techniques to follow the evolution of a nanocrys- talline or highly deformed powder in temperature as, e.g., large changes in the size distribution as well as in the distribution and quantity of the defects are expected. This is the case for typical recovery/recrystallization processes, where nucleation of small domains would occur and a bimodal distribution thus would be present in a certain temperature range. Further issues may also be related to isothermal data that often carry incomplete information, as the initial stages of the transformation can be missed while the specimen is heated to temperature. Examples of possible issues can be found even in the simple cases of fcc or bcc metals. Krill et al., [9] for instance, proposed a quite detailed in-situ study of the annealing of ball-milled nanocrystalline Fe using iso- thermal SRXRD data. In their study, a large difference is present between the initial size of the domains (approximately 35 nm) and the size at the beginning of the analysis (approximately 70 nm), clearly demonstrat- ing that a large part of the initial annealing process is lost. This difference might have an impact on the final result, as the annealing kinetics is shown to change with the calculated average domain size (two mecha- nisms are proposed to interpret the data: linear at low and parabolic at high annealing times). Nothing in Reference 9, however, is said about possible recrystal- lization and issues related to secondary nucleation in the powder, even if some physical meaning was attached to MATTEO LEONI, Assistant Professor, PAOLO SCARDI, Pro- fessor, and MIRCO D’INCAU, Laboratory Researcher, are with the Department of Materials Engineering and Industrial Technologies, University of Trento-via Mesiano, 77-38123 Trento, Italy. Contact e-mail: matteo.leoni@unitn.it GIUSEPPINA LUCIANI, Assistant Professor, is with the Department of Materials and Production Engineering, University of Napoli ‘‘Federico II’’-piazzale Tecchio, 80-80125 Napoli, Italy. Manuscript submitted March 17, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A