The study of phase transformations in metallic systems is of wide interst for the tailoring of new materials, with improved properties with respect to traditional ones. In order to describe the transformation mechanisms, it is necessary to correlate the structural and microstructural characterization to a kinetic analysis of the processes. This goal can be obtained, in the case of metallic materials, by means of X-ray diffraction techniques. A qualitative analysis of the phase mixture can give information on the type of phase transformation, whereas a quantitative analysis allows one to follow phase evolution after thermal treatments. The line broadening analysis of diffraction peaks gives information on the microstructure of the various phases, which can be related to the transformation mechanism. In the present paper, some examples of the application of X-ray diffraction techniques to the study of phase transformations, induced in metallic materials by suitable thermal and mechanical treatments, is presented. A fine grained phase of bcc-Fe, with interesting magnetic properties, has been obtained by annealing of the Fe73.5Cu1Nb3Si13.5B9 amorphous alloy. From the analysis of the lattice parameter as a function of the transformed fraction, a Si enrichment in the nanocrystalline phase during the crystallization has been inferred. An average grain size of 12 nm has been estimated from a Sherrer analysis of diffraction peaks. From a Warren-Averbach analysis of (110) and (220) diffraction peaks of the bcc phase, the grain size distribution of the nanocrystalline phase has been obtained and it has been compared with the results of TEM analysis. On the basis of the microstructural characterization, the mechanism of nanocrystalline phase formation is put in evidence. The immiscibility of Cu in Fe promotes a copious nucleation of a crystalline phase, with a composition close to pure Fe. The growth of this phase is strongly limited by the presence of Nb in the surrounding amorphous phase, so that a nanocrystalline phase is formed. In the last years it has been established that intermetallic compounds can be prepared by means of syntesis in the solid state. The reaction between powder of pure elements can be induced by severe mechanical deformation, obtained by means of ball milling. The phase transformations in Ni50Al50 during mechanical alloying and subsequent thermal treatments has been followed by X-ray diffraction. The formation of intermetallic Ni-Al compounds takes place according to different routes. After 1 hour of milling, a line broadening of diffraction peaks of parent elements has been observed and a grain size of about 25 nm has been estimated. With prolonged mechanical treatment, the formation of ordered NiAl is directly induced. By thermal treatment of the prealloyed powders, activated by ball milling, the sequence of NiAl3 → Ni2Al3 → NiAl intermetallics has been evidenced. The thermodynamic and kinetic factors involved in the different pathways are discussed. In policrystalline materials, the preferred orientation of grains may affect significantly some properties which depend on the crystallographic direction. For instance, plastic deformation processes induce significant anysotropy in the mechanical properties (yield and fracture strenght) in sheet, plate and extruded products. The preferred orientation of grains can be determined by X-ray diffraction with the Schulz reflection technique. It has been used to determine the texture gradient between the surface and the core of a plate in an Al-Li (A.A.8090-T8) alloy. A texture gradient has been observed, corresponding to a (110)[011] (cube texture) at the surface and a (110)[112] (brass texture) in the core. It can be assumed that near the surface of the plate, recrystallization processes occurred, as evidenced by the presence of the cube texture. The strong contribution of the brass texture in the core of the sample suggests, on the contrary, the presence of a still deformed structure. It is concluded that the texture gradient may be a source of toughness and strength variability in the alloy.
X-ray diffraction techniques for the study of phase tranformations in metallic materials [Applicazione di tecniche di diffrazione di raggi X allo studio di trasformazioni di fase in materiali metallici]
BARICCO, Marcello
1997-01-01
Abstract
The study of phase transformations in metallic systems is of wide interst for the tailoring of new materials, with improved properties with respect to traditional ones. In order to describe the transformation mechanisms, it is necessary to correlate the structural and microstructural characterization to a kinetic analysis of the processes. This goal can be obtained, in the case of metallic materials, by means of X-ray diffraction techniques. A qualitative analysis of the phase mixture can give information on the type of phase transformation, whereas a quantitative analysis allows one to follow phase evolution after thermal treatments. The line broadening analysis of diffraction peaks gives information on the microstructure of the various phases, which can be related to the transformation mechanism. In the present paper, some examples of the application of X-ray diffraction techniques to the study of phase transformations, induced in metallic materials by suitable thermal and mechanical treatments, is presented. A fine grained phase of bcc-Fe, with interesting magnetic properties, has been obtained by annealing of the Fe73.5Cu1Nb3Si13.5B9 amorphous alloy. From the analysis of the lattice parameter as a function of the transformed fraction, a Si enrichment in the nanocrystalline phase during the crystallization has been inferred. An average grain size of 12 nm has been estimated from a Sherrer analysis of diffraction peaks. From a Warren-Averbach analysis of (110) and (220) diffraction peaks of the bcc phase, the grain size distribution of the nanocrystalline phase has been obtained and it has been compared with the results of TEM analysis. On the basis of the microstructural characterization, the mechanism of nanocrystalline phase formation is put in evidence. The immiscibility of Cu in Fe promotes a copious nucleation of a crystalline phase, with a composition close to pure Fe. The growth of this phase is strongly limited by the presence of Nb in the surrounding amorphous phase, so that a nanocrystalline phase is formed. In the last years it has been established that intermetallic compounds can be prepared by means of syntesis in the solid state. The reaction between powder of pure elements can be induced by severe mechanical deformation, obtained by means of ball milling. The phase transformations in Ni50Al50 during mechanical alloying and subsequent thermal treatments has been followed by X-ray diffraction. The formation of intermetallic Ni-Al compounds takes place according to different routes. After 1 hour of milling, a line broadening of diffraction peaks of parent elements has been observed and a grain size of about 25 nm has been estimated. With prolonged mechanical treatment, the formation of ordered NiAl is directly induced. By thermal treatment of the prealloyed powders, activated by ball milling, the sequence of NiAl3 → Ni2Al3 → NiAl intermetallics has been evidenced. The thermodynamic and kinetic factors involved in the different pathways are discussed. In policrystalline materials, the preferred orientation of grains may affect significantly some properties which depend on the crystallographic direction. For instance, plastic deformation processes induce significant anysotropy in the mechanical properties (yield and fracture strenght) in sheet, plate and extruded products. The preferred orientation of grains can be determined by X-ray diffraction with the Schulz reflection technique. It has been used to determine the texture gradient between the surface and the core of a plate in an Al-Li (A.A.8090-T8) alloy. A texture gradient has been observed, corresponding to a (110)[011] (cube texture) at the surface and a (110)[112] (brass texture) in the core. It can be assumed that near the surface of the plate, recrystallization processes occurred, as evidenced by the presence of the cube texture. The strong contribution of the brass texture in the core of the sample suggests, on the contrary, the presence of a still deformed structure. It is concluded that the texture gradient may be a source of toughness and strength variability in the alloy.
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