C. M. Wan

The influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system

Four Fe-30Mn-0.9C-XAl alloys are employed to investigate the influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system. The range of aluminium content is zero to 8.47 wt%. Based on the thermodynamic model, the stacking fault energy can be obtained through calculation. Increasing the aluminium content will make the stacking fault energy of Fe-30Mn-0.9C based alloys increase at 300 K.

The study of work hardening in Fe-Mn-Al-C alloys

Three austenitic Fe-Mn-Al-C alloys with different aluminium content from 0, 5.1 and to 8.5wt% are chosen for the present work hardening study. Serrated stress-strain curves with pronounced work hardening were observed during tensile testing, also the serration of stress-strain curve is found to be decreasing as the increase of aluminium content. The serration can, however, still be observed even if the aluminium content is increased to as much as 8.5wt%. According to morphology studies and electron microscopic investigations, it is found that strain-induced deformation twins are closely related to the work hardening in the present alloys. Therefore, deformation twinning is strongly suggested as a major cause of work hardening in Fe-Mn-Al-C alloys, and also plays an important role on the serration of stress-strain curve.

The determination of tensile properties from hardness measurements for Al-Zn-Mg alloys

AbstractThe correlations between tensile properties (yield strength, ultimate tensile strength and uniform strain) and indentation hardness are studied for two types of Al-Zn-Mg alloys. The reasons why Tabors equations do not well fit the experimental data when the strain-hardening coefficient is larger than 0.3 are discussed. New equations for the determination of tensile properties from hardness measurements are theoretically derived and found to be in excellent agreement with the experimental data for Al-Zn-Mg alloys. The equations areTu=(Hv/c2)[4.6(m−2)]m−2 and σy=(Hv/C2)1-(3−m>n) +25 (m−2), whereTu andσy are ultimate tensile strength and yield strength,Hv is Vickers hardness number,m is Meyers hardness coefficient,E is Youngs modulus,c2 is a constant about 2.9 in magnitude. In these equationsTu,σy,Hv andE are all expressed in kg mm−2.

Effect of temperature on the oxidation of Fe-7.5A1-o.65C alloy

An alloy with the chemical composition Fe-7.5A1-0.65C was employed to investigate the effect of temperature on oxidation between 600 and 900° C in dry air. Kinetic curves were determined by thermogravimetry analyses (TGA). Optical metallography and electron probe microanalysis (EPMA) were used to examine the oxide scales. At 600°C, the initial stage of oxidation followed a parabolic rate law, and oxidation subsequently, increased dramatically. Internal oxidation occurred beneath the nodule formed on the present alloy at 600°C. In contrast, no internal oxidation could be found in specimens of the Fe-7.5A1-0.65C alloy after oxidizing at 700, 800 and 900°C. The kinetic results have two distinct parabolic rates for the present alloy.

Fatigue studies on dual-phase low carbon steel

Low cycle fatigue studies have been carried out on 2 wt% Mn, 2 wt% Si and 0.1 wt% C steels with dual-phase and tempered martensitic structures. Fatigue crack initiation and propagation were investigated using scanning electron microscopy as well as optical microscopy. In addition, taper-section and cross-section techniques were also performed for more detail studies on the correlation of crack initiation with the internal microstructures of the testing samples. Internal microstructures were also investigated on the dual-phase steel sample before and after fatigue fracture by transmission electron microscopy.

Stability and work hardening of an Fe-12Cr-23Mn austenitic steel

The effect of pre-strain on the strain-induced martensitic transformation of an Fe-12Cr-23Mn austenitic steel has been investigated through transmission electron microscopy and X-ray analysis. Pre-strain was performed either at room temperature or at 200° C. Final strain was carried out at liquid-nitrogen temperature. Theε phase was shown to form on {1 1 1} planes of the austenite matrix predominantly by overlapping of stacking faults. The martensite transformation sequence wasγ →ε →α. Nucleation of theα phase mainly occurred at intersections ofε bands. Austenite stability was shown to increase by pre-strain at 25° C or 200° C. Pre-strain at 200° C has a greater effect on austenite stability than does pre-strain at 25° C. The mechanism was discussed in terms of martensite transformation rate and various substructures introduced during straining. Work hardening was shown to depend on the degree of pre-strain and final strain. The correlation between work hardening and substructures introduced during straining was examined.

Effect of carbon on the oxidation of Fe-5.5 Al-0.55 C alloy

An alloy with the chemical composition Fe-5.5 wt% Al-0.55 wt% C is employed to investigate the effect of carbon on the oxidation at 600, 800 and 1000° C in dry air. Kinetic curves were determined by thermogravimetry analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales formed on the alloy surfaces. The kinetic curves observed at 600, 800 and 1000° C had simple, three-stage and two-stage parabolic rate laws, respectively. No carbide-free layer could be observed in the alloy which was oxidized at 600° C. In contrast, a carbide-free zone was found in specimens of the alloy after oxidizing at 800 and 1000°C.

Effect of manganese on the oxidation of Fe-Mn-Al-C alloys

The effects of manganese on the oxidation of alloys with the chemical composition (wt%) Fe-5AAl-1.5Mn-0.58C and Fe-5.3Al-3.5Mn-0.53C at 600, 800 and 1000° C in dry air were investigated. Kinetic curves were determined by thermogravimetric analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales. The kinetic curves of Fe-5.4Al-1.5Mn-0.58C alloy oxidized at 600, 800 and 1000° C had simple, three- and two-stage parabolic rate laws, respectively. On the other hand, two stages of linear rate law were observed in Fe-53Al-3ZMn-0.53C alloy when oxidized at 600° C, while two distinct parabolic rate laws were found in the same alloy oxidized at 800 and 1000° C. Oxidation behaviours and the oxide formation mechanisms of the alloys at different temperatures are discussed in this paper.

Oxidation and decarburization of an Fe-AI-C alloy

The oxidation behaviour of the Fe-5.5A1-0.55C (wt%) alloy was studied in air at 500 to 950° C. Specimens of two types of preparation were used: (i) homogenized for 4 h at 1100° C under an argon protective atmosphere to form carbide particles in the ferrite matrix (referred to Type 1 alloy); and (ii) homogenized for 96 h at 1100° C under an air atmosphere to form a ferrite matrix with full decarburization (Type 2 alloy). Type 1 alloy with or without a decarburized zone after oxidation had a higher oxidation rate than Type 2 alloy. The oxidation kinetics of Type 1 alloy are controlled by the stability of carbide particles. The interference of CO or CO2 formation influences vitally the formation of protective aluminium oxide. During the oxidation of Type 2 alloy, two oxidation periods appear with slightly different parabolic constant values. The oxidation rate increased with temperature to a maximum at 600° C and then decreased to a minimum at 800° C.

High temperature studies of Fe-Mn-Al-C alloys with different manganese concentrations in air and nitrogen

The formation of AIN was studied using a series of Fe-Mn-Al-C alloys with manganese contents from 20 to 40 wt%. All investigations were carried out in air and nitrogen at 1000° C. The permeability (DNNN) of nitrogen in Fe-Mn-Al-C alloys was found to be a major factor influencing the formation of AIN precipitates which are always observed underneath the oxide scales of the specimen. The formation of AIN is related to the lower formation rate of Al2O3 and high permeability of nitrogen; therefore AIN can only be formed in those alloys with a higher manganese concentration because the specimens are treated in air and in pure nitrogen at 1000 C.