E. I. Anufrieva

Kinetics and the mechanism of the realization of the reverse α-γ transformation in the metastable Fe-Ni-Ti alloys: II. Electron-microscopic examination of the alloy structure

The mechanism of the formation of reverted austenite has been investigated by studying the structure of samples annealed in the temperature interval of the reverse α-γ transformation at consecutively rising temperatures. A hypothesis on the cause for the transition from the isothermal to athermal kinetics in the middle of the interval of the reverse α-γ transformation is suggested. The heat liberation in the middle part of the transformation range and an essential influence of this effect on the structure of reverted austenite in the N25KhT2 alloy has been confirmed experimentally.

Effect of preliminary plastic deformation on the structure and physicomechanical properties of aging invar alloy N30K10T3

The invar alloy N30K10T3 after water quenching from 1150°C (austenite, γ phase) has the temperature of the start of martensitic transformation Ms ≈ −80°C and the Curie temperature TC ≈ 200°C. The effect of aging-induced phase decomposition in a deformed supersaturated solid solution on its hardness HV, electrical conductivity σ, magnetic permeability μ, and linear expansion coefficient β has been studied. It has been shown that cold plastic deformation of the alloy (at 20°C) to 30–50% increases its hardness, virtually does not change the conductivity, and decreases permeability. Aging of the deformed invar results in increasing HV and σ and decreasing μ. At room temperature, the deformed invar has a low linear expansion coefficient; its magnitude grows the faster, the greater the aging temperature Ta. Plastic deformation increases the density of dislocations, which form a banded substructure in austenitic grains. Besides, a metastable martensitic phase has been observed, which undergoes a reverse martensitic transformation into austenite upon heating in the temperature range from 550°C to 650°C. This transformation causes a decrease in the linear expansion coefficient β(T) of the deformed material. In samples aged at Ta = 700°C (after deformation), an athermal aging-induced martensite (αa phase) appears after cooling them to 20°C. The appearance of the αa phase is due to an increase in the temperature of the start of the martensitic transformation to above the room temperature caused by aging. In the samples containing the αa phase, there is observed a decrease in β in the temperature range from 350 to 670°C, which is due to the reverse transformation of the aging-induced martensite into austenite (αa → γ).

Effect of heat and thermomechanical treatments on the structure and physical and mechanical properties of the N30K10T3 invar

Invar alloy N30K10T3, whose austenite is metastable with respect to the martensitic γ → α transformation that occurs upon cooling below the martensitic point (Ms = −80°C), has been studied. The following six ways of the alloy strengthening have been tested: (1) aging (a) in a temperature range of ΔTa = 20–700°C; (2) liquid-nitrogen cooling (lnc) of the material preliminarily hardened by aging under the aforementioned conditions (route 1) (a + lnc); (3) preliminary phase-transformation-induced hardening (ph) (γ → α → γph) and aging in the temperature range of ΔTa (ph + a); (4) liquid-nitrogen cooling of the material preliminary hardened via route 3 (ph + a + lnc); (5) preliminary cold deformation (to 30%) at room temperature and aging in a temperature range of ΔTa (cd + a); and (6) liquid-nitrogen cooling of the material preliminary hardened via route 5 (cd + a + lnc). The six ways of hardening were found to affect the hardness, electrical conductivity, magnetic permeability, and temperature dependence of the thermal expansion coefficient.

Adjusting the linear expansion coefficient Lin Fe-Ni-Co-Ti invars by aging and phase hardening

The N30K10T3 and N40K10T3 invars with the Curie points θC ≈ 200°C and θC ≈ 310°C and the martensite temperatures Ms ≈ −80°C and Ms < −196°C, respectively, have been studied. The two alloys were hardened by quenching in the range of temperatures from 100 to 750°C. In addition, the first alloy was hardened by a combination treatment including phase-transformation-induced hardening and aging. The method of phase hardening consisted in the use of a forward (γ → α) and a reverse (α → γph) martensitic transformations. It has been shown that the temperature dependences of the linear expansion coefficient and the dependences of the hardness on the temperature and time of aging are considerably different for both alloys upon decomposition of the supersaturated solid solution. Both the ordinary and the double aging have been studied.

Effect of the Structure of Aging Invars with a Metastable Austenite on the Frequency Dependence of Their Magnetic Permeability

The effect of the generator current frequency f on the magnetic permeability μ of the N30K10T3 invar is studied for its various structural states formed upon the following treatments: phase naklep (i.e., the phase-transformation-induced hardening of austenite), cold plastic deformation, and cooling in liquid nitrogen. The ferromagnetic austenite of the alloy is metastable with respect to the γ → α martensite transformation upon cooling to temperatures below the temperature of the onset of the martensite transformation (Ms ≈ −80°C) and represents a supersaturated solid solution ageable during heating. The types of treatment are shown not to change the linear character of the μ(f) dependence in the frequency range under study (15–50 kHz) and to decrease μ to a certain extent.

Kinetics and the mechanism of the realization of the reverse α-γ transformation in metastable Fe-Ni-Ti alloys: I. Dilatometric, X-ray diffraction, magnetometric, and metallographic investigations of the reverse transformation

Kinetics of the development of the reverse α → γ transformation has been studied in detail by various methods. It has been established that in the middle part of the temperature interval of the reverse α-γ transformation the rate of the development of transformation is maximum, which indicates its athermal nature. Both in the beginning and at the completing stage of the development, the α-γ transformation is realized according to the isothermal kinetics. An assumption is made that it is precisely the difference in the kinetics that is critical for the observed anomalous behavior of the physical characteristics of the alloy discussed in the work.

A study of cellular decay in austenite alloys using an applied eddy-current transducer

The eddy-current parameter f0 of the N36K10T3 invar has been studied in the range of aging temperatures from 600 to 900°C. The maximal drop in f0 has been observed at the temperature Tag = 800°C, and the drop in this parameter was the larger, the longer the aging process. The drop in this parameter is caused by the cellular decay process in the solid solution, which depletes the austenite of nickel and titanium. The parameter f0 increases notably (from 4 to 46 kHz) when crystals of lowtemperature martensite (α-phase) are generated in samples of the N26T3 steel with 100% cellular decay. This high value (f0 = 46 kHz) persists at Tag < 400°C and drops by a factor of 4.5 over the interval 400 < Tag < 600°C because the ferromagnetic α-phase transforms to the paramagnetic phase-hardened austenite (α → γph). Aging of the phase-hardened austenite in the steel with cellular decay at Tag = 700°C increases the parameter f0 by a factor of two (from 10 to 20 kHz) because the ferromagnetic α-phase is generated when the aged phase-hardened austenite transforms to the martensite (γph → α) as a result of cooling the steel from the aging to room temperature.

Development of new methods for studying the decomposition in aging invars

N30K10T3 and N28K10T3 invar alloys are studied. After water quenching from 1150°C, they consist of supersaturated solid solutions, which can decompose in aging in the temperature range 500–700°C with the precipitation of intermetallic Ni3Ti nanoparticles. It is shown that the decomposition can be controlled by measuring the magnetic (first harmonic amplitude, phase angle φ shift, magnetic susceptibility μ) and electric (electrical conductivity σ) parameters as functions of the isothermal holding time at various aging temperatures. The alloys are studied in the following three initial states: after quenching, phase transformation-induced hardening (γ → α → γpt), and cold (20°C) plastic deformation by 30%.

Effect of thermomechanical treatments on hardness and coercive force of aged austenitic alloy of invar composition

The effect of thermal (aging at temperatures of 500–800°C) and thermomechanical (30% deformation at 20°C and subsequent aging) treatments on the hardness and coercive force of the quenched 60N30K10F alloy has been studied. The alloy subjected to water quenching from 1150°C is shown to be a supersaturated solid solution that can decompose upon subsequent heating. The dependences of the hardness and coercive force on the time and temperature of aging have been obtained, discussed and explained.

Effect of Heat Treatment on Conductivity of Metastable Alloys with Ferromagnetic and Paramagnetic Austenite

Invars N30K10T3 and N28K10T3 and nonmagnetic austenitic alloy N25Kh2T3 are studied after different kinds of treatment, i.e., quenching, mechanical phase hardening, and deformation followed by aging. The structure and the conductivity of the alloys are determined. An optimum treatment for providing high electric conductivity is suggested.