Plastic deformation of FeNi invar alloys

We report on the deformation behaviour of FeNi single crystals oriented for single slip in the temperature region between 77 and 610 K. Below the Curie temperature the critical shear stress of the Invar alloys increases much more rapidly than that of any f.c.c. substitutional alloy investigated till now. Simultaneously the activation volume decreases down to 10–20 b³. Below the Curie temperature the stress-strain curves of the Invar crystals differ from those of other f.c.c. alloys. All results taken together indicate a dependence of the dislocation mobility on the magnetic properties of FeNi Invar. It is shown that the experiments cannot be understood with a simple magnetic friction stress arising from the interaction between the hydrostatic stress field of the edge dislocations and the stress dependence of the magnetization [3]. However, they can be described with an empirical friction law which had been introduced originally [18,19] for the case of strongly localized thermal activation, e.g. kink motion.     

The martensite transformation in FeNiC alloys

The effect of austenite defect structure upon the sub-zero martensite burst transformation temperature in FeNiC has been investigated using a combination of optical and electron microscopy, differential scanning calorimetry and microhardness testing. In the absence of a change in composition or dislocation density, the martensite start transformation temperature (M_s) was found to be determined by the grain size of the austenite. Above a grain size of 150 μm, M_s was found to be independent of grain size, but below 150 μm, the transformation temperature was strongly depressed by up to approximately 50 K at a grain size of 10 μm. For any given grain size, an increase in the dislocation density from that typical of a fully recrystallised specimen, i.e. approximately 10¹⁰ lines m⁻², to that of approximately 10¹⁵ lines m⁻² raised M_s by approximately 15 K. The depression of M_s and reduction in the initial burst size of the transformation with decreasing grain size was found to be related to the observation that a fine grain size results in a heterogeneous transformation restricted to a few small pockets of grains. The depression of M_s in the fine grained alloy is consistent with a segregation of active martensite nuclei into a few small grains, a suppression of the autocatalytic stimulation of martensite plates between adjacent grains, and a possible reduction in the number of martensite nuclei.     

Morphology transition of deformation-induced lenticular martensite in FeNiC alloys

The morphology and habit planes of deformation-induced lenticular martensite were investigated by optical and transmission electron microscopy in Fe30Ni and Fe30Ni0.11C alloys. Transitions in morphology were observed with progressive deformation levels going from lenticular to butterfly and to compact martensite for the Fe30Ni alloy and lenticular to butterfly and to small butterfly martensite for the Fe30Ni0.11C alloy. The habit planes changed from {225}_f or {259}_f for the thermal lenticular martensite to {111}_f for the strain-induced martensite. The morphology and crystallography of the small butterfly martensites was also investigated. A change in the orientation relationships from K-S to N-W relations was also observed. These changes were attributed to the contribution of mobile dislocations which modified the shear mode from twinning to slip, and to a plastic accomodation of transformation strains.     

On the abnormally large tetragonality of martensite in FeNiC alloys

In order to elucidate the origin of abnormally large tetragonality (c/a) of martensite in the FeNiC alloy system, a large number of alloys of which the Ni content is varied for a constant carbon content have been examined, using low temperature X-ray diffraction, optical and electron microscopy. The results show that the tetragonality of martensite in this alloy system is strongly dependent on martensite morphology. Namely, it is very large for a plate martensite, while it is normal or not so large for a lenticular martensite. In more physical words, the martensite tetragonality is dependent on the mode of the lattice invariant deformation in martensitic transformation. If the lattice invariant deformation is twinning, the resulting c/a is large, while in the case of slip it is small.     

A study of phase decomposition in CuNiFe alloys

A study of phase decomposition in two CuNiFe alloys was realized by AP-FIM. It was possible to confirm that phase decomposition takes place via spinodal decomposition in this alloy system, as the amplitude of the composition modulation increased with ageing time without practically any change in the wavelength of the modulation. It was also observed that the morphology of the decomposed phases is related to the coherency-strain energy as predicted by Cahn's theory of spinodal decomposition. By analysis of the data of the modulation wavelength, it was possible to estimate the coherent spinodal temperature to be 800 ± 25 and 900 ± 25 K in Cu46 at.% Ni4 at.% Fe and Cu48 at.% Ni8 at.% Fe alloys, respectively. In the early stage of decomposition, the change in modulation wavelength showed a time exponent as small as about 0.07. On the other hand, in the coarsening stage the change in the modulation wavelength agreed well with the LSW theory of thermally activated growth. The activation energy for this coarsening process was determined to be 216 ± 10 and 232 ± 10 kJ/mol in the Cu46 at.% Ni4 at.% Fe and Cu8 at.% Fe alloys, respectively. The compositions of the decomposed phases are consistent with the miscibility gap in the calculatedequilibrium CuNiFe phase diagram.     

Thermodynamics of the massive,bainitic and martensitic transformations in FeC,FeNi,FeCr and FeCu alloys

The γ → α transformation in pure iron and iron alloys with dilute alloying elements is a five-staged process, each stage has its own transformation-start temperature and transformation product: the first and second stages are a massive-type and a bainitic-type transformation respectively, while the subsequent stages (3, 4 and 5) are martensitic transformations. Thermodynamic calculation reveals that the driving force varies linearly with the transformation-start temperature for each stage of the transformation in FeC, FeNi, FeCr and FeCu alloys. Based on the linear relations, the calculated M_a, B_s and M_s are in good agreement with experimental points.     

A study of work hardening in austenitic FeMnC and FeMnAlC alloys

Two austenitic FeMnAlC alloys with aluminium contents of 0 and 2.7 wt% were strained in tension between 193 and 823 K. Serrated stress-strain curves, inverse strain-rate dependence of flow stress, and high work hardening exhibited in particular temperature ranges for both alloys were characteristic of dynamic strain aging. The apparent activation energy for the onset of serration increased from 14.4 to 22.3 kcal/mol due to the addition of 2.7 wt% Al. It was found that the high work-hardening rate cannot be attributed to strain-induced deformation twinning when serrated stress-strain curves occurred. From the evidence of the present study and the known effect of aluminium on the diffusivity and activity of carbon in austenitic high-manganese steel, it is suggested that dynamic strain aging is the major cause of work hardening within the intermediate temperature range from 298 to 493 K for 0 wt% Al and 393 to 593 K for 2.7 wt% Al in the present austenitic FeMnAlC alloys.     

Effects of austenitizing temperature on the kinetics of bainite reaction at constant austenite grain size in FeC and FeNiC alloys

The influence of austenitizing temperature on a subsequent diffusional transformation has been investigated. The overall bainite transformation rate in a slightly hypo-eutectoid Fe-7.6Ni-0.48C alloy is increased by a double austenitizing treatment. Optical microscope observations have revealed that this acceleration of bainite transformation is caused mostly by the acceleration of nucleation rate at austenite grain boundaries. Intergranular fracture surface observations have shown that a second austenitizing treatment enhances cementite precipitation on grain boundaries. It is considered that the enrichment of sulfur at grain boundaries by a second austenitizing treatment enhances the precipitation of cementite during the subsequent cooling to the transformation temperature. When the second austenitizing temperature is 750°C or below, disc shaped Fe-rich FeS precipitates are formed together with the cementites. The initial stage of bainite transformation has shown that the cementites and Fe-rich FeS precipitates act as preferential nucleation sites for bainite. The precipitation of cementite and Fe-rich FeS by a double austenitizing treatment is also observed in a Ni free Fe-0.61C alloy.     

Shape memory effect and related transformation behavior in FeNiC alloys

Shape memory effect in two representative FeNiC alloys, 31% Ni-0.4% C and 27% Ni-0.8% C, with low Ms temperatures has been studied in detail. The shape memory test was conducted not only on as-austenitized specimens but also on ausformed specimens. The effect of an external load during reverse transformation was also examined. The reverse martensitic transformation behavior related with the shape memory effect was observed in situ, using a high temperature optical microscope. The results are (1) about 50% shape recovery is observed for up to 5% initial tensile strain, while 75–95% shape recovery is obtained for 1–2% initial bending strain. (2) An ausformed specimen shows a better shape memory effect probably owing to the increased austenite strength. (3) The austenite-martensite interface moves backward on heating not only for a plate-type martensite but also for a curved or irregular shaped martensite in a specimen strengthened by ausforming. (4) The shape recovery decreases with increasing an applied load, but even in this case the reversible interface movement occurs.     

C-C interaction energy in FeC alloys

A new approach has been suggested for the determination of the C-C interaction energy, the partial molar enthalpy and nonconfigurational entropy of carbon in FeC austenite and ferrite from available activity data. By application of the values obtained through the approach to the calculation of the FeC phase diagram, the results in the equilibrium region are in very good agreement with experiment. From the scattered and limited experimental activity data, the C-C interaction energy obtained through the present approach should be more reasonable than that through previous efforts. Further analysis indicates, however, that up to now the activity data on carbon in ferrite are not accurate enough for obtaining the C-C interaction energy in ferrite with clear physical significance.     

Nitrogen solubility and nitride formation in FeCrNi alloys

A series of FeCrMnNi alloys was melted under high nitrogen gas pressure. The nitrogen concentration in the solidified metal was found to be experimentally related to the melt pressures that ranged from 0.1 to 200 MPa, to the alloy composition, and to alloy concentration. The nitrogen solubility followed Sievert's law. The activity coefficients determined at these higher pressures for the alloying elements Cr, Mn and Ni were similar to those previously obtained at lower pressures. At the higher nitrogen melt-pressures, the nitrogen concentration exceeded the interstitial nitrogen solubility resulting in the formation of metal-nitrides.     

Anomalous strain rate dependence of the serrated flow in NiH and NiCH alloys

Serrated flow in NiH, NiCH, and NiC alloys was studied over a wide range of temperature and strain rates. The results for C related serrated flow in NiC or NiCH alloys were in excellent agreement with previous results and were consistent with dislocation pinning by C solutes at the dislocation cores. Hydrogen related serrated yielding was observed in NiH and NiHC alloys. Solute C had only a small effect on the temperature range of this H related serrated flow. The results could be interpreted on the basis of hydride formation at the dislocation cores and diffusion of H in these hydrides.     

Grain coarsening in FeNiCr alloys and the influence of second phase particles

The effects of second phase particles, e.g. M₂₃C₆, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M₂₃C₆ carbides varied, since Fe, Ni and Mo entered the M₂₃C₆ unit cell; and complex carbides such as (Cr₁₅Fe₄Ni₂Mo₂)C₆ were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.     

A theoretical analysis of the chilling susceptibility of hypoeutectic FeC alloys

In this work an analytical solution of general validity is proposed to explain the chill tendency of cast iron of different compositions. A balance of the various heat contributions during the solidification process is employed in arriving at an expression for the chilling equivalent E. It is found that E can be directly related to the modulus of the casting and it includes the effects of (a) nucleation and growth of pro-eutectic as well as eutectic phases, (b) the range of metastability, and (c) the pouring temperatures. Moreover, the predictions of the present analysis are in good agreement with the experimental data of chill penetrations found in wedge shaped castings. Quantitatively, this analysis predicts that small sulphur additions impede the growth of graphite eutectic cells while raising the degree of undercooling in cast iron.     

Low-temperature aging of nitrided FeTi alloys

In nitrided and quenched FeTi alloys containing <0.5 wt% Ti, the nitrogen occupies four types of site in a distribution that differs from that at the nitriding temperature and which depends on nitriding temperature, rate of quench, titanium content and the nitrogen potential. Electron microscopy, lattice parameter and internal friction measurements are used to determine the distribution of nitrogen and the transformations that occur on aging. Aging observations are explained by the heterogeneous clustering and then precipitation of “excess” nitrogen in random solid solution upon the existing homogeneous distribution of titanium-nitrogen atom clusters (GP zones) formed at the nitriding temperature and which retain 1Ti: 1N ratio throughout the transformation sequence. The transformations are analogous to those in quench-aged nitrogen-ferrite except that the stabilities of the “FeN” clusters and the α-Fe₁₆N₂ are enhanced by the titanium-nitrogen atom clusters on which they form.     

Thermal stability of amorphous Fe1−xTix,alloys produced by vapor quenching

Amorphous Fe_l−xTi_x alloys with x = 0.2–0.75 have been investigated by differential scanning calorimetry, differential thermal analysis, X-ray diffraction and Mössbauer effect measurements. The amorphous phase is thermally the most stable for x = 0.3–0.4, where we have the Laves phase, TiFe₂ (C14), in the equilibrium phase diagram. After annealing at 670 K for few hours, the amorphous phase is retained for 0.25 < x < 0.45, while it transforms to an α Fe-type b.c.c. phase for x < 0.25, to a B2-type b.c.c. phase for 0.5 < x < 0.6, to an intermediate phase of Ti₂Fe (E9₃) for 0.6 < x < 0.7 and to a β Ti-type b.c.c. phase for 0.7 < x < 0.75, respectively. As being annealed above 870 K, the amorphous phase with x = 0.3–0.4 transforms to the Laves phase of TiFe₂ (C14).