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.     

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.     

Thermodynamic prediction of MS and driving force for martensitic transformation in FeMnC alloys

The M_S temperatures of FeMnC alloys have been calculated by the application of LFG model of ΔG^γ→α with Mogutnov's ΔG^γ→α_Fe, Hsu-(A) model with Orr-Chipman's ΔG^γ→α_Fe and Hsu-(B) model with Orr-Chipman's ΔG^γ→α_Fe and are in good agreement with the experimental values. Through the mathematical treatment, the relationship between M_S and the composition of FeMnC alloys can be obtained as M_S(K) = 817.4 − 7513.4X_C− 4141.9X_Mn− 32083.5X_CX_Mn (LFG with Mogutnov et al.'s ΔG^γ→tα_Fe); M_S(K) = 829.9 − 7580.5X_C− 4146.0X_Mn− 15727.8X_CX_Mn [Hsu-(A) with Orr-Chipman's ΔG^γ→α_Fe]; M_S(K) = 829.2 − 7276.1X_C− 2915.4X_Mn− 43825.7X_CX_Mn [Hsu-(B) with Orr-Chipman's ΔG^γ→α_Fe]. Where X_C = atom fraction of carbon and X_Mn = atom fraction of manganese. The linear correlation coefficients of these relations are larger than 0.992. Both carbon and manganese depress M_S linearly and the effect of carbon is almost two-fold stronger than that of manganese. Introduction of an interaction term (X_CX_Mn) between alloying elements in the present treatment shows that carbon and manganese enhance the effect upon M_S of each other. The driving force for transformation increases monotonically with carbon and manganese content and there is no singularity. The calculated M_S and the driving force largely depend on the ΔG^γ→α model and ΔG^γ→α_Fe values adopted.     

Stress affected bainitic transformation in a FeCSiMn alloy

Isothermal transformation experiments are reported in which the formation of bainitic ferrite occurs under the influences of stresses below the yield strength of the austenite. The response of the transformation was monitored by simultaneously measuring the longitudinal and radial transformation strains. This enabled the dilatational and deviatoric strain components to be deconvoluted from the total transformation strain. The data have been analysed by comparison with a theoretical model for the stress-assisted growth of bainite. The results confirm that the microstructure readily responds to stresses well below the yield strength of the parent phase. Furthermore, those crystallographic variants which are favoured by the stress grow first in the sequence of transformation. Experiments where the stress just exceeds the yield strength are also reported.     

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.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

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.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

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.