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.     

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.     

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.     

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.     

The role of deformation twinning on mechanical properties of an austenitic Fe30Mn1.2Al0.3C alloy

An austenitic Fe30Mn1.2Al0.3C alloy has been investigated under tensile and total-strain controlled fatigue tests between room temperature and 77K. The alloy exhibits an elongation peak between room temperature and 77K. It was found that tensile elongation is not controlled by the total amount of deformation twinning, but rather the rate and temperature sensitivity of deformation twin formation. Total-strain-controlled fatigue tests show that the fatigue resistance of the Fe30Mn1.2Al0.3C alloy at 77K is superior to that at room temperature in the 10²–10⁴ cycles fatigue life range, although the monotonic tensile elongation at 77K is lower than that at room temperature. The enhancement of fatigue resistance at 77K is due both to an increased fatigue ductility coefficient, because of strain-induced deformation twins, and to increased strength. Increased formation of deformation twins during the fatigue test is responsible for the increased fatigue resistance at 77K.     

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.     

Micromechanisms of failure in CMn weld metals

Observations on the influence of reheating and preheat temperature on the microstructure and toughness of CMn weld metals are collated, and implications for failure micromechanisms are assessed. Both microvoid coalescence and cleavage micromechanisms are strongly influenced by the weld-metal non-metallic inclusion population. In particular a model for cleavage fracture in CMn weld metals is further developed and discussed. This involves initial plasticity in grain boundary ferrite; strain induced inclusion cracking; propagation of the crack into the ferrite matrix under a critical tensile stress; and preferential, though not exclusive crack propagation through grain boundary ferrite.     

Effects of hydrogen on micro-twinning in a FeTiC alloy

In this study, it was observed that concurrent straining in the presence of a hydrogen concentration above a threshold value, or by high fugacity cathodic charging in the absence of external stress enhances twinning in the FeTiC alloy system. The twins observed have lamellar or prismatic shapes, with a crystallography consistent with that expected in b.c.c. crystals. The prismatic twins are bounded by {112} coherent interfaces with a co-zonal axis along a 〈111〉 twinning direction and incoherent boundaries at the ends. This study further supports the occurrence of hydrogen-induced plastic instability in metals. Micro auto-radiography technique has shown that the twin/matrix interfaces and TiC particles are strong traps for hydrogen.     

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.