Yoshihiro Takahara

Irreversible structural relaxation in Fe–B–Si amorphous alloys

Abstract Structural relaxation in Fe 78 B 7 Si 15 and Fe 75 B 10 Si 15 amorphous alloys with annealing has been studied by means of the measurements of electrical resistance and Mossbauer spectroscopy. The electrical resistance of as-prepared alloys decreased abruptly in the initial stages of annealing and then attained a quasi-equilibrium value that depends on annealing temperature. A model, combining the Ziman theory for resistance and free-volume model for viscous flow, was used to analyze the irreversible decrease of resistance. The observed resistance changes were well reproduced with the model. The activation energy for annihilation of free volume was evaluated to be 154 kJ mol −1 for the Fe 78 B 7 Si 15 alloy and 222 kJ mol −1 for the Fe 75 B 10 Si 15 alloy from resistance changes. On the other hand, we observed a significant difference in the annealing behavior of the average isomer shift and hyperfine field with time between these two alloys. This indicates that the local structural change during relaxation process differs much in both amorphous alloys. The observed difference in relaxation behavior between the two alloys can be explained in terms of Dubois and Le Caers structural model of Fe–B–Si amorphous alloys.

EXAFS and Mössbauer studies on local atomic structure in an amorphous Fe–B–Si alloy

Abstract Local atomic structures of an amorphous Fe79B16Si5 alloy and the structural changes by thermal annealing (structural relaxation) have been studied by extended X-ray absorption fine structure (EXAFS) and Mossbauer measurements. The structural analyses indicate that the interatomic distance of Fe–Si is larger than that of Fe–B and is close to the Fe–Fe distance. Such geometry becomes more obvious after structural relaxation. Discussions based on the structural model composed of capped trigonal prisms suggest that Si atoms do not locate at the central site of trigonal prisms, but substitutionally occupy the sites of Fe atoms at the vertices. During structural relaxation, the distance of Fe–B decreases and that of Fe–Fe is slightly shortened, while the Fe–Si distance remains unchanged. The coordination number of B atoms around Fe decreases but that of Fe atoms increases. The coordination number of Si around Fe is almost constant. The increase in coordination number of Fe around Fe during the structural relaxation is also confirmed by the Mossbauer effect study.

Brittle-to-ductile transition and slip band structure in ionic crystals

Abstract The interrelation between the brittle-to-ductile transition (BDT) and slip structure has been examined using NaCl and KCl crystals. In pre-cracked crystals of NaCl and KCl having a ⟨100⟩ axis, the fracture toughness K*ic increases above 300 and 220 K, and the BDT occurs at 510 and 430 K respectively. Scale-like wavy lines are revealed ahead of a pre-crack on etched fracture faces of NaCl tested above 300 K. As the temperature increases, the step height and density of the lines are increased but the lines are more restricted near the pre-crack front. In NaCl and KCl crystals without pre-cracking, the critical resolved shear stress for {110} main slip is very low above 77 K, while that for {010} slip markedly increases as temperature decreases below 500 and 300 K respectively. In ⟨100⟩-uniaxial tests for non-pre-cracked NaCl crystals, the BDT takes place at around 380 and 480 K at strain rates of 5.5 × 10−6 and 5.5 × 10−5s−1 respectively. Above the BDT temperature, wavy slip lines are abundantly produced by the marked operation of cross-slip. A mechanism based on the crack tip shielding through the cross-slip of dislocations along a crack front is proposed to explain observed crack face morphology as well as the BDT in ionic crystals of NaCl structure.

Reversible structural relaxation in FeBSi amorphous alloys

Abstract Reversible structural relaxation in two FeBSi amorphous alloys, Fe 78 B 7 Si 15 and Fe 75 B 10 Si 15 , has been studied through measurements of the electrical resistance and relaxation enthalpy using a differential scanning calorimeter. Both alloys exhibited reversible changes of these physical properties due to the reversible relaxation. Larger reversible changes were observed in Fe 78 B 7 Si 15 than in Fe 75 B 10 Si 15 . This result can be well explained by the idea that the reversible relaxation is caused by changes in chemical short-range order among iron and silicon atoms according to Dubois and Le Caers structural model of amorphous alloys. The activation energy spectrum characterizing the reversible relaxation process was obtained from reversible enthalpy changes during isothermal annealing at 500 K. It results in an asymmetric gaugassian-like distribution with a peak maximum at 1.7 eV.