Investigation on the martensitic transformation in γ-Fe(n) nanoparticles

γ-Fe(5.90–10.9 at.% N) nanoparticles ranging in diameters from 30 to 100 nm were prepared by laser-induced pyrolysis of mixtures of Fe(CO)₅ and NH₃. During the quenching from high temperature to room temperature and even down to 4.2 K, no martensitic transformation occurred in these particles. Based on the effects of surface tension and refinement of grain size on the yield strength of the nanoparticles, the dependencies of nucleation driving force and the martensite-start temperature on the particle size was revealed, from a viewpoint of thermodynamics of martensitic transformation in the γ-Fe(N) nanoparticles. It is suggested that the yield strength increment due to the surface tension and to the very fine crystallite grains is probably the predominant aspect responsible for inhibiting the nucleation of martensite and drastic decrease of the martensite-start temperature of the nanoparticles. High pressure of 0.5–4.0 GPa were adopted to induce the martensitic transition of the γ-Fe nanoparticles. It is found that the extent of martensitic transformation increased with the increasing pressures, most individual γ-Fe particles were partially transformed to martensite after pressurization.     

The effect of deformation on γ-ε martensitic transformation in an Fe-Mn-Mo alloy

Effect of prior plastic deformation of austenite on the martensite start temperature, volume fraction and strength of martensite have been studied in an Fe-14.3%Mn-3.7%Mo alloy. Mo was chosen to examine the possible effect of the third alloying element in an Fe-Mn based alloy and the obtained results were compared with those of the Fe-Mn binary alloys given in the literature. Predeformation of austenite created considerable changes on the formation characteristics and also the strength of the martensitic phase and the obtained results were discussed in terms of the dislocations formed during the deformation process.     

Thermal conductivity of γ-irradiated LiF single crystals

The thermal conductivity of γ-irradiated lithium fluoride (LiF) single crystals has been studied using the method of stationary longitudinal heat flux in a temperature range of 50–300 K. An increase in the irradiation dose to 2 × 10⁹ rad is accompanied by a monotonic decrease in the thermal conductivity by 10% at 300 K and by a factor of 10 at 50 K. This increase in the irradiation dose also leads to an increase in the microhardness of LiF crystals from 140 to 222.5 kgf/mm².     

Orientation and temperature dependence of superelasticity caused by reversible γ-α′ martensitic transformations in FeNiCoAlTa single crystals

Reversible thermoelastic γ-α′ martensite transformations (MTs) have been studied in Fe-28%Ni-17%Co-11.5%Al-2.5%Ta (at. %) single crystals under cooling/heating and loading conditions. It is established that the precipitation of dispersed γ′ phase particles with an average size of d ≤ 5 nm leads to thermoelastic γ-α′ MTs with a small temperature hysteresis (ΔT = 20 K). The [001] oriented crystals favor the attaining of maximum superelasticity (SE) ɛ_SE = 6.8%, minimum mechanical hysteresis Δσ = 130 MPa, and large SE temperature interval ΔT _SE = 130 K. In contrast, in the [111] oriented crystals, small values of ɛ_SE = 2.0%, high values of Δσ = 350–400 MPa and narrow SE interval ΔT _SE = 55 K are found.     

The effect of DO3 ordering on the parent ⇄ martensitic transformation in the CuZnAl shape-memory alloy

The character of D03 ordering resulting from different quenching rates and its effect on the characteristic martensitic transformation temperatures and shape-memory effect in the Cu-21.5 at % Zn-12.5 at % Al alloy has been analysed. ln room temperature water-quenched samples with a cooling rate 1000° C sec^–1 showing B2 long-range order and DO₃ Short-range order, e significant stabilization of martensite in the reverse transformation was observed. This stabilization was eliminated in the air-cooled samples with a cooling rate 20° C sec^–1 showing DO₃ long-range order. Mechanical tests revealed a more complete shape recovery in the air-cooled samples, when compared to room temperature waterquenched ones.     

Effect of martensitic transformation in Ti–15 at %V β-phase particles on lamellar boundary decohesion in γ-TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers     

Effect of martensitic transformation in Ti–15 at % V β-phase particles on lamellar boundary decohesion in γ-TiAl Part I Derivation of interface decohesion potentials

Molecular statics atomistic simulations of the Ti–15 at% V body-centred cubic (bcc) -phase–-TiAl interface and – lamellar boundary decohesion processes have been carried out to determine the corresponding decohesion potentials. The potentials are subsequently related to the dislocation structure of the interface–boundary. Atomic interactions have been represented using the appropriate embedded atom method (EAM) interatomic potential functions. The results obtained show that the decohesion potential functions are quite complex because they have to account for the instabilities that occur under some modes of interface decohesion and for the periodic character of the interfacial shear. Lastly, the use of decohesion potentials to derive constitutive relations for continuum-type interfacial elements and their implementation in the finite element method are presented. © 1998 Kluwer Academic Publishers     

Effect of zinc on the magnetic properties of acicular cobalt-modified γ-Fe2O3 particles

Acicular -FeOOH particles with a particle length of about 0.35 m and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl₂-NaOH. The (Co, Zn)-modified -F₂O₃ particles were produced by absorbing Co²⁺ and Zn²⁺ ions on the surfaces of -FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)--Fe₂O₃ particles were all higher than those of Co--F₂O₃ particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)--Fe₂O₃ particles increased with increasing zinc content but the coercivity decreased.     

Martensitic transformation of γ-Fe precipitates in a Cu 1.5 at % Fe Alloy

In the present work, the mechanism of martensitic transformation, the influence of -Fe particle size on the martensitic transformation induced by cold working, and the transformation of -Fe into -Fe by thermal treatment alone in a Cu-1.5 at% Fe alloy, was studied using field ion microscopy (FIM) and transmission electron microscopy (TEM). It has been found that -Fe precipitates smaller than about 10 nm did not transform martensitically to -Fe by cold working. Precipitates larger than 10 nm adopted a Kurdjumov-Sachs orientation relationship with the copper matrix; and the martensitically transformed -Fe precipitates were ellipsoidal in shape, with their major axes being oriented parallel to the 1 1 0 direction in the matrix. Dislocations were found in the matrix near the vicinity of transformed -Fe precipitates, giving support to the dislocation cutting mechanism proposed by other workers for the transformation. In thermally aged alloys, no transformation of -Fe to -Fe was observed during the coarsening of -Fe precipitates up to sizes as large as about 50 nm. These precipitates still remained coherent or semi-coherent with the copper matrix.     

The effect of quenching on the martensitic transformation in ß1 phase of Au-Cd alloys

The effect of quenching on the martensitic transformation mechanism in ₁ Au-Cd alloys has been investigated by measurements of the electrical resistivity and X-ray diffraction. In the case of the Au-47.5 at%Cd alloy, the ₂-martensite is the characteristic product under quenching conditions, but it always exists with the equilibrium ₂-martensite phase. Consequently, the _{1 2} and ₂transformations occur simultaneously during the heating and cooling cycles. The corresponding resistivity behaviour is very complicated and extremely sensitive to thermal treatments such as quenching temperature and thermal cycling. On the other hand, in the case of the Au-49.0 at%Cd alloy, only the _{1 2} transformation occurs even when quenched, and the transformation is unaffected structurally by quenching. A distinct resistivity anomaly, which is considered to be due to the disappearance of quenched-in vacancies, is observed in quenched alloys. Some important characteristics of this anomaly are determined. In particular, the quenching effect disappears when the specimen is heated above the temperature at which the resisitivity anomaly begins. This result suggests that the quenched-in vacancies play an essential role in the martensitic transformation process under quenching conditions.     

Anisotropy in the electrical properties of zirconium doped α -Fe2O3 single crystals

The electrical properties of zirconium doped α -Fe₂O₃ single crystals have been investigated from 470 up to 1300 K.As for the pure crystal, an anisotropy appears for the conductivity between the [001] direction and the (001) plane, although lowered by the dopant.Combining the results of electrical conductivity, thermoelectric power and complex impedance measurements, it was established that there was no significant kinetic contribution of the electrons and that their mobility was lower than a few tenths of cm²V⁻¹s⁻¹.     

Experimental and theoretical investigations on morphological changes of γ′ precipitates in Ni -Al single crystals during uniaxial stress-annealing

Influences of the external tension and compression on the morphological changes of precipitates during coarsening in an Ni-15 at. % Al single crystal were experimentally investigated, and theoretical evalutions on the energetically favourable shape and its orientation were also derived based upon the anisotropic elasticity theory. The shape transformation with growth in size is experimentally found to follow the sequence: cuboidrod plate, under all annealing conditions, i.e. compression-annealing, tension-annealing and external stress-free annealing. However, the external stresses give preferential orientation to such microstructures. The tension of the [001] direction aligns the rods and plates along the tension axis, while the compression aligns them on a plane perpendicular to the compression axis. Theoretical evaluations based upon the anisotropic elasticity for the energetically favourable shapes and orientations of are consistent with experimental results except in the appearance of rods after compression-annealing. This discrepancy is understood by a conception that the shape transformation is influenced not only by energetics in the elastic strain and interfacial energies, but also by elastic interaction energy.Formerly at the Graduate School of Nagoya Institute of Technology.     

Study on martensitic transformation of mechanically alloyed nanocrystalline Fe–Ni

Phase transformation of Fe–Ni powders with different nickel content during mechanical alloying was studied, as well as reverse transformation of mechanically alloyed nanocrystalline Fe–Ni upon heating. Results show that nickel content plays an important role in the phase transformation tendency during mechanical alloying. When heated at 300 °C, neither grain size nor phase changes in Fe–30 wt.% Ni milled for 80 h, indicating the nanometer-sized martensite is very stable below 300 °C. When the temperature increases to 350 °C, concurrently with grain growth reverse transformation takes place. The reverse transformation temperature of mechanically alloyed nanocrystalline Fe–Ni is higher than that of bulk alloys.     

Work performance in TiNi alloy during heat cycling in the temperature range of B2 ↔ R martensitic transformation

The possibility of useful work performance in a TiNi alloy during heat cycling in the temperature range of the B2 ? R martensitic transformation has been studied. In the regime of work performance during heat cycling, the alloy sample was cooled under a constant applied stress of 0, 50, 100, or 150 MPa, while heating was performed under a constant stress of 200 MPa. The results show that a TiNi alloy is capable of performing useful work during heat cycling in the temperature range of B2 ? R transformation. A maximum work output of 0.45 MJ/m³ was observed in the case of cooling at an applied stress of 100MPa. The efficiency of work performance in this case amounted to 1.3%, which was about half of the value for heat cycling in the temperature range of the B2 ? B19’ martensitic transformation.