M. T. Jahn

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> ) +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.

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.

Effects of subgrain structures and stress orientation on fatigue of brass

Subgrain structures of 70 wt% copper — 30 wt% zinc brass were obtained by cold rolling and annealing. Effects of subgrain structure and orientation on fatigue properties were studied. It is found that the fatigue properties of the specimens with subgrains are superior to those of fully annealed specimens. The fatigue properties of specimens tested in the transverse direction (FT) are superior to those of specimens tested in the longitudinal direction (FL) for short annealing time. This result can be rationalized by the textured structre of the specimens in which both the resolved shear and normal stresses on the shear bands formed in FT specimens are smaller than those in FL specimens.

Effect of pre-existing substructures on the fatigue properties of pure aluminium

Pure aluminium was thermomechanically treated (cold rolled to 80% reduction of thickness and annealed at 200° C for various lengths of time) in order to obtain different kinds of substructures. These aluminium specimens were then fatigued with a stress amplitude of 9×103 psi or 90% of their ultimate tensile strength. The substructures of the specimens before and after fatigue were compared with each other and were then correlated to their fatigue property. The size and shape of substructure introduced by fatigue depend on the pre-existing substructure formed by thermomechanical treatment. The sub-boundaries of the fatigued specimen are not as well-defined as the pre-existing subgrain. The substructure size of the fatigued specimen increases initially with annealing time and then reaches a saturation value (2.32 μm) when the annealing time is longer than 2 h. A peak value of fatigue life against the annealing time was found if the stress amplitude of fatigue is 90% of the ultimate tensile strength. The peak value also occurs at an annealing time of 2 h. The reason why the specimen annealed for 2 h possesses the optimum fatigue and property is discussed in terms of the substructures before and after fatigue.

Age Strengthening of Fe-Mn-Al Stainless Steels

ABSTRACT The effects of Mn content on the age hardening of Fe-Mn-Al stainless steels were examined. The Mn content varied from 23% to 36% such that after solution treatment the alloys were full austenite. After solution treatment the specimens were aged at various temperature ranging from 400°C to 900°C for one hour. It was found that peak hardness occurred at 550°C. Other specimens were then aged at 550°C for time periods ranging from 15 min. to 90 hours for the investigation of aging process. It is proposed that the strengthening effect is due to the precipitation of coherent Fe3AlCx particles if the Mn content is not too high. If the Mn content is too high (36% in this case) the precipitate was β-Mn which caused some hardening but with drastic decrease in ductility. The precipitates were examined and identified by X-ray analysis and transmission electron microscopy. The age hardening property is explained in terms of TEM microstructures.

Strain Hardening and Strain Aging in Fe-Al-Mn Alloys

ABSTRACT Four Fe-Al-Mn alloys were selected for this study of strain aging and post-quench aging. Various temperatures and aging times were used in the experiments. Specimens were subjected to different pre-strain cold working treatments during the strain aging process. Microstructural examinations were mostly done by optical means, results were compared and related to the mechanical properties of the materials which had been subjected to different treatments. The appearance of a β-Mn phase is suggested to be the main reason for the hardening phenomenon.

The Fatigue Studies of Steel with Bainitic Structure

ABSTRACT High and low cycle fatigue studies have been conducted on a 2.5 wt% Si, 1.9wt% Mn and 0.08 wt% C steel with mixtures of upper and granular bainite structure. Slip bands, phase and grain boundaries acting as a fatigue crack initiation were observed in low cycle fatigue fractured sample; while the initiation sites in high cycle fatigue tested sample were found mostly on the phase boundaries. Many dislocation cells exist within the ferritic regions underneath the free surface of the low cycle fatigue tested specimen; however, there is no evidence of existing of dislocation cell in that after high cycle fatigue fracture.

An Investigation of Mechanical Thermal Treated Al-Al3Ni Eutectic Alloy

ABSTRACT A strengthening mechanism investigation has been conducted on a Al+6.1wt%Ni eutectic alloy. Samples with different shape of second phase, i.e. spheroidized Al 3 Ni particles as well as prefer orientated Al 3 Ni fibers, were prepared and compared with each other. 20% to 60% cold rolling followed by various annealing process were performed to study both mechanical properties and microstructure changes. A good correlation between the improvement of tensile properties with the existence of both Al 3 Ni fibers and subgrain structures after mechanical thermal treatment process was observed.

Studies on the stress-strain relations of dual-phase steels

The correlations of the work-hardening exponent,n, with quenching temperature, martensite volume-fraction (MVF) and solute concentration in ferrite are discussed and derived for dual-phase steel. The flow stress of dual-phase steel at low strain is suggested to be expressed by the combination of the terms due to plastic deformation in ferrite and elastic deformation of martensite. Previous experimental results are compared with the behaviour suggested by this theoretical work. In addition, an expression for the work hardening exponents at moderate strains and at the onset of necking are also theoretically suggested.