Young G. Kim

Low temperature mechanical behavior of microalloyed and controlled-rolled Fe-Mn-Al-C-X alloys

high fractions liquid, as indeed appears to happen. 5 The approximations made in the derivation of [10] make it only semiquantitatively accurate. The percolation model of interdendritic flow implicit in [4] is not accurate at high fractions liquid. 6 Values for the constant n in [10] are difficult to measure. The picture of a defect as an ellipsoidal region of different permeability is certainly an oversimplification. Nevertheless, we believe that the basic conclusions of our analysis are correct: the growth of channel segregates is greatly assisted by the presence of small defects in the dendrite network. Since such defects are unavoidable in a real casting, the condition [10] should provide a more reliable indication of the occurrence of channel segregates than [1]. Another interesting result of the analysis is that channel segregates may form in situations where the interdendritic flow direction is substantially parallel to isotherms. This is because defects oriented out of the plane of the isotherms can divert interdendritic flow to a direction which will lead to channel segregation.

Optimum bolted joints for hybrid composite materials

Abstract The optimum bolted joints for hybrid composite materials composed of glass-epoxy and carbon-epoxy under tensile loading were investigated. The design parameters considered for the bolted joints were ply angle, stacking sequence, the ratio of glass-epoxy to carbon-epoxy, the outer diameters of washers and the clamping pressure. As bearing failure was desirable for bolted joints, the geometry of the bolted joint specimen was designed to undergo bearing failure only. By inspecting the fracture surfaces of the specimens it was found that delamination on the loaded periphery of the holes and extensive damage on the edge region constrained by a washer occurred. To assess the delamination of the hybrid composite materials, three-dimensional stress analysis of the bolted joint was performed using a commercial finite-element software and compared with the experimental results.

Composition and temperature dependence of tensile properties of austenitic FeMnAlC alloys

Abstract The effects of aluminium addition (0–4 wt.%) to Fe-26MnAl alloys and carbon addition (0.1 – 0.3 wt.%) to Fe-30Mn-1AlC alloys on tensile properties, especially tensile elongation, were investigated from room temperature to 4 K. The transformation of austenite to deformation twins during the tests was very beneficial in enhancing tensile elongation at cryogenic temperatures. The amount of deformation twins formed during plastic deformation was not the major factor for maximum elongation, but the optimum work hardening rate by the gradual formation of deformation twins played an important role. The maximum elongation peak shifted to lower temperatures with increased aluminium (5 aluminium wt.%), but moved to higher temperatures with increased carbon content (0.3 wt.%). The new flow equation σ = Kϵ N exp ( Mϵ ) was applied to calculate uniform elongations. The calculated values were in reasonable agreement with the measured values.

Influence of Fabrication Residual Thermal Stresses on Rubber-toughened Adhesive Tubular Single Lap Steel-Steel Joints under Tensile Load

Abstract The tensile load bearing capability of adhesively-bonded tubular single lap joints which is calculated under the assumption of linear mechanical adhesive properties is usually much less than the experimentally-determined because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives. Also, as the adhesive thickness increases, the calculated tensile load bearing capability with the linear mechanical adhesive properties increases, while, on the contrary, the experimentally-determined tensile load bearing capability decreases. In this paper, the stress analysis of adhesively-bonded tubular single lap steel-steel joints under tensile load was performed taking into account the nonlinear mechanical properties and fabrication residual thermal stresses of the adhesive. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ult...

Effect of heat treatment on precipitation behaviour in a Cu-Ni-Si-P alloy

The effects of applying different solution and ageing conditions on the electrical resistivity and precipitation behaviour of a Cu-1.3Ni-0.3Si-0.03P (wt%) alloy were studied. The electrical resistivity of solution-treated material is greatly reduced, by about 50%, by the ageing processes. The reduction in resistivity is due to depletion of solute atoms from the copper matrix by the formation of precipitates. Double ageing peaks appeared during isothermal ageing due to the formation of Ni3P and Ni2Si precipitates. The first maximum, due to the precipitation of Ni3P, appeared at about 1 h of ageing time, while the second peak, due to Ni2Si, appeared at around 10 h of ageing time when aged at 450°C. The precipitate Ni3P forms early and the alloy starts to over-age before Ni2Si precipitates and the alloy reaches maximum hardness. The maximum hardness produced by the precipitations of Ni3P and Ni2Si decreased with increasing ageing temperature from 450 to 550°C. The time to reach the maximum hardness due to Ni3P precipitation became shorter, while that of Ni2Si became longer, as the solution treatment temperature increased from 780 to 1020° C. The apparent activation energy for Ni2Si precipitation was found to be about 80 kJ mol−1 while that for Ni3P precipitation was about 25 kJ mol−1.

Strength analysis of adhesively-bonded tubular single lap steel-steel joints under axial loads considering residual thermal stresses

Abstract The static tensile load bearing capability of adhesively-bonded tubular single lap joints calculated using linear mechanical adhesive properties is usually far less than the experimentally-determined one because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of the rubber-toughened epoxy adhesive In this paper, both the nonlinear mechanical properties and the residual thermal stresses in the adhesive resulting from joint fabrication were included in the stress calculation of adhesively-bonded joints. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive. From the tensile tests and the stress analyses of adhesively-bonded joints, a failure model for adhesively-bonded tubular single lap joints under axial loads was proposed.

Effect of aluminum content on low temperature tensile properties in cryogenic Fe/Mn/AI/Nb/C steels

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Effects of ordering type and degree on monoclinic distortion of 18R-type martensite in Cu-Zn-Al alloys

In order to clarify the relationship between the extent of the distortion from the ideal structure and the nature of ordering (type, degree), a rigid hard sphere model for the M18R structure is developed by introducing the modified Bragg-Williams-Gorski-type order parameters. The ide- ally close-packed 9R or 18R structures are produced from the disordered or D03 ordered parent phases, while the modified 9R and 18R structures are produced from the B2 ordered and L21 ordered parent phases, respectively. The extent of the distortion in the close-packed plane in- creases with increasing degree of B2 order or with decreasing degree of L21 order. Related experiments based on the variation of X-ray diffraction pattern for the stabilization of Cu- Zn-Al shape memory alloy have confirmed the validity of the theoretical development.

A transformation kinetic model and its application to CuZnAl shape memory alloys—II. Non-isothermal conditions

It has been demonstrated that the activation energies of the solid state transformations obeying the modified Johnson-Mehl-Avrami/Austin-Rickett type kinetic equation can be determined from both the Kissinger method and the Ozawa method QRTm is lager than 5. The errors involved in the activation energy determined from the two methods have been presented as a function of Q/RTm is larger than 5. The errors involved in the effects of the kinetic exponents, c and n, and the heating rate θ and ym and Tm have been analyzed It has shown that the fraction ym is nearly independent of the heating rate while the peak temperature Tm is a very weak function of the exponents n and c. The effects of the kinetic parameters and the heating rate on the (yvs T) and (dy/dt vs T) curves are also discussed. Moreover, procedures for the simultaneous determination of the exponents c and n from a single DSC trace are provided. This kinetic model has been applied to the analyses of the non-isothermal aging kinetics of three CuZnAl shape memory alloys. The comparison between isothermal and non-isothermal results further supports the validity of this kinetic equation for the description of heterogeneous solid state transformations.

Low Cycle Fatigue Behaviour of A Cryogenic Fe-30Mn-5Al-0.1Nb-0.3C Steel

Abstract A new austenitic cryogenic Fe-30Mn-5Al-0.1Nb-0.3C steel (where the composition is in approximate weight per cent) exhibited inverse ductility behaviour with decreasing temperature; the elongation at room temperature was 43% compared with 69% at 77 K in the annealed condition. The effect of the test temperatures (room temperature and 77 K) on strain-controlled low cycle fatigue behaviour in the annealed steel was investigated. The degree of cyclic hardening increased with decreasing test temperature from room temperature to 77 K. Total-strain-controlled fatigue tests showed that the fatigue resistance at 77 K was superior to that at room temperature in the entire fatigue life range tested (102-105 cycles). The absolute values of fatigue strength and ductility exponents increased somewhat with decreasing temperature from room temperature to 77 K. However, a pronounced increase in the fatigue ductility coefficient at 77 K occurred as the result of the inverse ductility behaviour in the alloy. The superior low cycle fatigue resistance at 77 K was attributed to the significant increase in the fatigue ductility coefficient. The inverse ductility behaviour was very beneficial in increasing the strain-controlled fatigue resistance at 77 K.