Lattice Parameter Of Austenite Research Articles

Microstructure and mechanical properties of Fe–Mn–Ge Alloys

The microstructure and mechanical properties of Fe–Mn–Ge alloys have been investigated using optical microscopy, transmission electron microscopy, room temperature and high temperature X-ray diffraction (XRD) and tensile testing. It has been found that increasing Ge content in Fe–24Mn–Ge alloys considerably decrease the amount and width of ε martensite plates, which verifies the role of Ge in depressing the γ→ ε martensitic transformation in Fe–24Mn–Ge alloys. It has also been found that increasing Ge content decreases yield and tensile strength, and furthermore, increases the lattice parameter of austenite and the ductility in Fe–Mn–Ge alloys. High-temperature XRD shows that there is an anomalous expansion of the lattice parameter for the untransformed austenite below martensitic transformation temperature M s, compared with the linear relationship of lattice parameter vs. temperature between the Néel temperature T N, and the M s. Comparisons of the lattice parameter of austenite and ε martensite for several Fe–Mn–Ge and Fe–Mn–C alloys show that the alloy's contraction during the γ→ ε martensitic transformation mainly occurs at the direction being perpendicular to ε martensite plates. Therefore, it is believed that an internal stress, caused by the contraction of alloy being perpendicular to ε martensite plate, is responsible for the anomalous expansion of the untransformed austenite below the M s, and, the formation of the triangular grid microstructure of ε martensites. While the internal stress, which originated from the inhomogeneous dilation in the alloy may play important role in the thermally induced γ→ ε martensitic transformation.

Characteristics of the γ→α transformation in Fe–Mn alloys

The relationships between the lattice parameters of austenite and ferrite in Fe–Mn alloys and Mn content were established by means of X-ray diffraction and revised literature data. These results were used to calculate the dilatation due to the isothermal γ→α transformation. The dilatometric experimental results are in very good agreement with the calculations. It was found that the increase of hardness of the α phase is a linear function of Mn content and is mainly determined by the lattice strain of this phase. The Curie temperature of the alloy was established as a function of Mn composition. The austenite fraction and enthalpy difference between ferrite and austenite accompanying the γ→α transformation were also determined as a function of temperature and Mn content by means of isochronal DTA measurements.

The anomalous expansion of lattice parameter as a function of temperature for an Fe-24Mn alloy during γ→ϵ martensitic transformation

In the present letter, the authors will report their experiment on an Fe-24Mn alloy using high temperature XRD and dilation. A relation between the lattice parameter of austenite and temperature during {gamma}{yields}{epsilon} martensitic transformation was determined. It is expected to be helpful in the understanding of {gamma}{yields}{epsilon} martensitic transformation in Fe-Mn based alloys.

Study of the paramagnetic-antiferromagnetic transition and the gamma ->epsilon martensitic transformation in Fe-Mn alloys

The paramagnetic-antiferromagnetic transition and the gamma --> epsilon martensitic transformation of Fe-Mn (Mn 15-32 wt%) alloys have been investigated by resistivity, dilatometry and X-ray diffraction (XRD). The results show that paramagnetic-antiferromagnetic transition increases the resistivity and the volume of alloys, whereas the gamma --> epsilon martensitic transformation reduces the resistivity and volume of alloys. The A(f)( that was determined by the dilatometric method is not the temperature that epsilon martensites in the Fe-Mn alloys have reverse transformed to austenites completely. Mn additions reduce M-s, increase T-N and the lattice parameter of austenite in the Fe-Mn alloys. Both the antiferromagnetic transition and the gamma --> epsilon martensitic transformation lead to an increase in the lattice parameter of austenite. The lattice parameters both above T-N and below T-N decrease linearly with temperature. The lattice parameter below M-s increases first and then decreases. Moreover, the alpha (110) and epsilon (002) atomic planes in the Fe-15Mn-0.15C alloy are separated into two peaks: 2 theta for epsilon>(*) over bar * (002) is 44.16 degrees, 2 theta for alpha>(*) over bar * (110) is 44.47 degrees.

Research in liquid phase sintering of aluminium-transition metal system: G.N.Romanov, N.S.Timofeev (Yakut University, Russia.)

The expanded austenite γN can be produced in austenitic stainless steels by plasma nitriding, carburizing or nitrocarburizing at low temperatures. This metastable phase presents higher hardness and toughness if compared with traditional nitride layers whilst also maintaining the corrosion resistance. However, the application of plasmas composed by both nitrogen and carbon is technologically recent and the effect of such process on the microstructure and properties of the nitrocarburized layers is still under investigation. In this study, samples of UNS S31254 superaustenitic stainless steel were produced by plasma nitriding and nitrocarburizing at 400 °C, 450 °C and 500 °C for 5 h. The plasma treated samples were observed by optical and transmission electron microscopy and also analyzed by X-ray diffraction. The thickness of the layers increased with temperature and the nitrocarburized layers were thicker than nitrided at a given temperature. The presence of expanded austenite was confirmed by X-ray diffraction through its characteristic anomalous shift on the diffracted peaks related to the austenite. Nitride formation on samples produced at 400 °C was only identified by transmission electron microscopy where fine rounded particles with 10–15 nm size revealed reflections consistent with the CrN cubic chromium nitride. The estimated lattice parameter from the expanded austenite ranged from 0.38 to 0.41 nm depending on the employed {h k l} reflection which was found to be 6–11% larger than the untreated austenite lattice parameter.

The lattice parameters of austenite and ferrite in FeC alloys as functions of carbon concentration and temperature

Despite its relevance for various calculations involving phase transformations in Fe-C alloys, little information is available on the lattice parameter of austenite at elevated temperatures and its dependence on the carbon concentration. Furthermore, severe scatter exists in the literature for the lattice parameter at high temperature. Most literature data were acquired using X-ray diffraction, although neutron diffraction seems to be a more suitable technique penetrate and probe a large volume of material, the advantages of neutron diffraction over X-ray diffraction are its reduced sensitivity to surface decarbonization and improved crystal statistics (both decarbonization and grain growth can occur during diffraction experiments at high temperatures requiring long exposure times). In this work in-situ neutron diffraction experiments on Fe-C alloys were performed to determine the lattice parameter of the austenitic and ferritic phases in a temperature region from just below to just above the bi-phasic austenite/ferrite region.

On the change in the austenite lattice parameter due to the martensitic transformation in an Fe-32Ni alloy

For the purpose of clarifying the mechanism of the often reported reduction in the austenite lattice parameter due to the martensitic transformation, the lattice parameters of an Fe-32Ni alloy prepared by different cooling rates from the austenitization temperature have been measured in the states before and after the martensitic transformation. From the measurements made, the stress levels have also been calculated for both states. In the slowly cooled stress-free specimen no significant change in the lattice parameter was found after the martensitic transformation, which is in agreement with the theoretical prediction, whereas in the quenched specimens, in which quenching stress is present, an appreciable reduction in the parameter was found. The latter is accounted for by stress relaxation due to the martensitic transformation.

The Effect of Order on the Martensitic Transformation in Fe3Pt

ABSTRACTThe lattice parameter and Curie temperature of austenite in Fe3Pt increase sharply on ordering for times up to 2 hr at 650° C. These effects, together with a rapid decrease in the martensitic transformation temperatures, are interpreted in terms of increased short-range order. The onset of thermoelastic reversibility of the martensitic transformation is associated with the development of a significant volume fraction of long-range ordered domains.

Austenite Formation from Fe-13Ni-1.1C Martensite

Experimental measurements of hardness, optical microstructure, and austenite lattice parameter were carried out on samples cooled to — 196 °C to form martensite and reheated for 1 h at 100 to 1200 °C. Reheating first results in depletion of carbon from the martensitic matrix and in formation of a small amount of austenite and for temperatures above 400 °C it results in secondary hardening presumably due to the formation of low-nickel cementite. Above 600 °C the low-carbon matrix reverses to austenite whose carbon content is determined by the metastable carbon and nickel equilibration of austenite and cementite. Between 700 °C and 800 °C graphite forms and cementite disappears; concurrently the austenite recrystallizes. Above 900 °C the austenite carbon level follows the stable austenite graphite equilibrium.

Partial Molar Volumes from High-Temperature Lattice Parameters of Iron–Carbon Austenites

AbstractThe lattice parameters of a number of iron–carbon alloys have been measured in the austenite field for temperatures up to 1200° C (1473 K). By extrapolation and interpolation of these data the variation with carbon concentration of the lattice parameter of austenite has been determined for a wide range of temperatures, and used to calculate partial molar volumes of iron and carbon in austenite. Both partial molar quantities show little variation with composition but V c, γ decreases with increasing temperature, while V Fe, γ increases. The values obtained for the partial molar volume of carbon in austenite are appreciably lower than those cited earlier. It is probable that the differences previously reported between the measured and calculated effect of pressure on the composition of austenite in equilibrium with cementite at 727° C (1000 K), are due primarily to the use in the calculations of too high a value for the partial molar volume of carbon in austenite.

Lattice Parameters of Martensite and Austenite in Fe–Ni Alloys

The lattice parameters of the body-centered cubic phase (martensite) and the face-centered cubic phase (austenite) were determined for a series of Fe–Ni alloys. Compositions ranged from pure Fe to Fe-35 at.% Ni. Careful film and diffractometer Debye-Scherrer techniques on powder and bulk samples and computerized calculations and error analyses were employed. Results indicate that the martensite lattice parameter reaches a maximum between 10 and 20 at.% Ni and the room-temperature lattice parameter of the face-centered cubic phase has a maximum in alloys having greater than 35 at.% Ni.

All-round compression and phase hardening of residual austenite

l. At the beginning of martensitic transformation there occurs a “cold” plastic deformation of austenite, which is replaced by the compression of residual austenite after a considerable hardening as the quantity of martensite begins to increase. Damping of the plastic deformation and the compression of residual austenite are followed by an intensive growth of the microscopic and submicroscopic distortions. 2. The diminution of the lattice parameter in the process of martensitic transformation, and its increase at the first stage of tempering, occur without diffusion. 3. The lattice parameter of residual austenite in steels is always smaller than that of pure austenite. 4. The total value by which the lattice parameter of residual austenite is reduced in the process of martensitic transformation may be determined at the initial tempering stage. 5. The presence of residual austenite in hardened steels is due to submicroscopic distortions in austenite and the stresses of all-round compression.

Changes in the fine structure of the austenite retained when high-speed steel is tempered

1. At tempering temperatures of 200–600° C, the diffusionless increase in the lattice parameter of retained austenite is replaced by a diffusional decrease in this parameter. 2. The maximum displacement of the iron atoms from their equilibrium positions caused by three-dimensional compression of the retained austenite amounts to 0.008 A. 3. The decrease in hardness in the first stage of tempering is accompanied by a diffusionless increase in the retained austenite lattice parameter, a breakdown of the mosaic blocks and a reduction in the 2nd-order distortions. 4. During tempering, until the steel reaches maximum hardness, there is a continuous process of simultaneous hardening and softening of the retained austenite, due to precipitation-hardening, distortionless strain hardening by the martensite, and recrystallization. This is accompanied by an increase in the 2nd- and 3rd- order distortions and mosaic block size. The degree of strain hardening depends on the relative magnitudes of these three factors. 5. The reduction in the lattice parameter of the retained austenite at temperatures above 540° C is due to loss of carbon, and partially to loss of alloying elements. These details confirm the theory about the aging of retained austenite, which causes a secondary martensite transformation during cooling. 6. The temperature at which the martensite transformation begins during multiple tempering depends on the magnitudes of the 3rd-order static distortions in the retained austenite, and does not depend on the 2nd-order distortions or the unit cell dimensions; the greater the distortions, the more does the retained austenite become stabilized.

Influence of Low Temperatures on the Mechanism of the Austenite – Martensite Transformation

SYSTEMATIC investigations have been carried out on the influence of low temperatures on the lattice parameter of austenite and martensite and on the axial ratio c/a in martensite.