• Journal of the Korean Society for Heat Treatment
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• v.24 no.6
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• pp.311-317
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• 2011

The effect of strain rate on the yield stress of an Al-Li alloy has been investigated at temperatures between 77 and 523 K and over the strain rate range from $1.77{\times}10^{-4}s^{-1}$ to $1.77{\times}10^{-2}s^{-1}$. At testing temperatures below 373 K, the yield stress is almost independent of strain rate at any aging stage. At testing temperatures above 373 K, the yield stress increases linearly with the logarithm of strain rate, and the strain rate dependence increases with increasing testing temperature. The yield stresses of under-aged alloy at temperatures between 373 and 473 K at high strain rates are greater than the yield stress at 77 K. For the alloy under-aged or aged nearly to its peak strength, the temperature range within which the positive temperature dependence of yield stress appears expands to the higher temperature side with increasing strain rate. The strain rate dependence of the yield stress is slightly negative at this aging stage. The yield stress of the over-aged alloy decreases monotonically with decreasing strain rate and with increasing testing temperature above 373 K. The modulus normalized yield stress is nearly constant at testing temperatures below 373 K at any strain rate investigated. And, strength depends largely both on the aging conditions and on the testing temperature. The peak positions in strength vs. aging time curves shift to the side of shorter aging time with increasing testing temperature. For the specimens aged nearly to the peak strength, the positive temperature dependence of yield stress is observed in the temperature range. The shift of peak positions in the aging curves are explained in terms of the positive temperature dependence of cutting stress and the negative temperature dependence of by-passing stress.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.29-32
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• 2000

The static and metadynamic recrystallization of non-heat treated medium carbon steel(Fe - 0.45wt.$\%C\;-\;0.6wt.\%Si\;-\;1.2wt.\%Mn\;-\;-0.12wt.\%Cr \;-\;0.1wt.\%V \;-\;0.017wt\%$.Ti) were studied by the torsion test in the strain rate range of 0.05 - 5 $sec^{-1}$, and in the temperature range of $900\;-\;1100\;^{\circ}C$. Interrupted deformation was performed with 2 pass deformation in the pass strain range of $0.25 {\varepsilon}_p(peak strain)\;and\;{\varepsilon}_p$, and in the interpass time range or 0.5 - 100 sec. The dependence or pass strain(${\varepsilon}_i$), strain rate( $\dot{\varepsilon}$ ), temperature(T), and interpass time($t_i$) on static recrystallization (SRX) and metadynamic recrystallization (MDRX) were predicted from the modified Avrami's equations respectively. Comparison of the softening kinetics between SRX and MDRX was indicated that the rate of MDRX was more rapid than that of SRX under the same deformation variables.

• Korean Journal of Metals and Materials
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• v.49 no.7
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• pp.541-548
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• 2011

Thermomechanical fatigue (TMF) behavior of heat resistant austenitic stainless steel was evaluated in the temperature range from 100$^{\circ}C$ to peak temperatures of 600 to 800$^{\circ}C$; The fatigue lives under TMF conditions were plotted against the plastic strain range and the dissipated energy per cycle. In the expression of the inelastic strain range versus fatigue life, the TMF data obtained at different temperature ranges were located close to a single line with a small deviation; however, when the dissipated energy per cycle, calculated from the area of the stress-strain hysteresis loops at the half of the fatigue life, was plotted against the fatigue life, the data showed greater scattering than the TMF life against the inelastic strain range. A noticeable stress relaxation in the stress-strain hysteresis curve took place at the peak temperatures higher than 700$^{\circ}C$, but all specimens in this study exhibited cyclic hardening behavior with TMF cycles. Recrystallization occurred during the TMF cycle concurrent with the formation of fine subgrains in the recrystallized region, which is considered to cause the cyclic hardening of the steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.04a
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• pp.183-186
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• 2000

The high temperature deformation behavior of spray-formed Al-19wt%Si-1.87wt%Mg-0.085wt.%Fe alloy was studied by torsion testing in the strain rate range of 0.001-1 sec-1 and in the temperature range of 300-500 $^{\circ}C$. The relationship between stress temperature and strain rate is expressed using the Power law. the behavior of dynamic recrystallization is showed in 300-35$0^{\circ}C$, 1-0.1sec-1 and the behavior of dynamic recovery is showed in 450-50$0^{\circ}C$, 0.01-0.001sec-1 The size of Si particles is mall when the temperature is low and the strain rate is high. The strain rate sensitivity(m) and the apparent activation energy(Q) indicate the dependence on strain rate and temperature for flow stress respectively. The hot ductility is high when m is high and Q is low. The maps of strain rate sensitivity and apparent activation energy suggest the optimum processing conditions.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.139-141
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• 2007

Compression tests were carried out to investigate morphologies of compressed specimen, deformation microstructure and stress-strain relation in high-nitrogen austenite stainless steel. Tests were performed under a wide range of temperature and, with true strain rates up to $\dot{\varepsilon}$ =0.05, 0.1, 0.5 and $1.0s^{-1}$. The activation energy of loading force was equal to plastic deformation energy within the temperature range of $900^{\circ}C$ to $1250^{\circ}C$. Dynamically recrystallized grain size decreased with an increasing strain rate and temperature. Flow stresses and deformation microstructures, were used to quantify the critical strain rate and recrystallized grain size. The grain size versus strain rate-temperature map obtained in the study was in good agreement with the deformation microstructures of compressed specimens.

• Journal of the Korean Society for Precision Engineering
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• v.11 no.4
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• pp.114-125
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• 1994

Fatigue behavior and life prediction method were presented for themal-mechanical and isothermal low cycle fatigue of 12 Cr forged steel used for high temperature applications. In-phase and out-of-phase thermal-mechanical fatigue test from 350 .deg. C to 600 .deg. C and isothermal low cycle fatigue test at 600 .deg. C, 475 .deg. C, 350 .deg. C were conducted using smooth cylindrical hollow specimen under strain-control with total strain ranges from 0.006 to 0.015. The phase difference between temperature and strain in thermal-mechanical fatigue resulted in significantly shorter fatigue life for out-of-phase than for in-phase. Thermal-mechanical fatigue life predication was made by partitioning the strain ranges of the hysteresis loops and the results of isothermal low cycle fatigue tests which were performed under the combination of slow and fast strain rates. Predicted fatigue lives for out-of-phase using the strain range partitioning method showed an excellent agreement with the actual out-of-phase thermal-mechanical fatigue lives within a factor of 1.5. Conventional strain range partitioning method exhibited a poor accuracy in the prediction of in-phase range partitioning method in a conservative way. By the way life prediction of thermal-mechanical fatigue by Taira's equivalent temperature method and spanning fartor method showed good agreement within out-of-phase thermal-mechanical fatigue.

• Korean Journal of Materials Research
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• v.24 no.11
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• pp.625-631
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• 2014

To investigate the deformation properties of TiC-(5-20) mol% Mo solid solution single crystals at high temperature by compression testing, single crystals of various compositions were grown by the radio frequency floating zone technique and were deformed by compression at temperature from 1250K to 2270K at strain rates from $5.1{\times}10^{-5}$ to $5.9{\times}10^{-3}/s$. The plastic flow property of solid solution single crystals was found to be clearly different among a three-temperature range (low, intermediate and high temperature ranges) whose boundaries were dependent on the strain rate. From the observed property, we conclude that the deformation in the low temperature range is controlled by the Peierls mechanism, in the intermediate temperature range by the dynamic strain aging and in the high temperature range by the solute atmosphere dragging mechanism. The work softening tends to become less evident with an increasing experimental temperature and with a decreasing strain rate. The temperature and strain rate dependence of the critical resolved shear stress is the strongest in the high temperature range. The curves are divided into three parts with different slopes by a transition temperature. The critical resolved shear stress (${\tau}_{0.2}$) at the high temperature range showed that Mo content dependence of ${\tau}_{0.2}$ with temperature and the dependence is very marked at lower temperature. In the higher temperature range, ${\tau}_{0.2}$ increases monotonously with an increasing Mo content.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.174-177
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• 2004

The nickel-based alloy Nimonic 80A possesses strength, and corrosion, creep and oxidation resistance at high temperature. These products are used for aerospace, marine engineering and power generation, etc. The control of forging parameters such as strain, strain rate, temperature and holding time is important because the microstructure change in hot working affects the mechanical properties. It is necessary to understand the microstructure variation evolution. The microstructure change evolution occurs by recovery, recrystallization and grain growth phenomena. The dynamic recrystallization evolution has been studied in the temperature range $950-1250^{\circ}C$ and strain rate range $0.05-5s^{-1}$ using hot compression tests. The metadynamic recrystallization and grain growth evolution has been studied in the temperature range $950-1250^{\circ}C$ and strain rate range 0.05, $5s^{-1}$, holding time range 5, 10, 100, 600 sec using hot compression tests. Modeling equations are developed to represent the flow curve, recrystallized grain size, recrystallized fraction and grain growth phenomena by various tests. Parameters of modeling equation are expressed as a function of the Zener-Hollomon parameter. The modeling equation for grain growth is expressed as a function of initial grain size and holding time.

• Journal of the Korean Society for Heat Treatment
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• v.9 no.2
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• pp.112-120
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• 1996

The effects of ${\varepsilon}$ martensite content and strain amplitude on damping capacity of an Fe-17%Mn alloy have been studied to establish damping mechanism of Fe-Mn system corresponding to the magnitude of strain amplitude. In a range of $1{\times}10^{-4}{\sim}3{\times}10^{-4}$ strain amplitude, the damping capacity is linearly proportional to the ${\varepsilon}$ martensite content, which suggests that stacking faults and ${\varepsilon}$ martensite variant boundaries are the principal damping sources. In the range of $4{\times}10^{-4}{\sim}6{\times}10^{-4}$ strain amplitude, however, a maximum damping capacity is observed around 68 vol.% ${\varepsilon}$. This behavior is very similar to dependence of relative area of ${\gamma}/{\varepsilon}$ interface on ${\varepsilon}$ martensite content. This means that in this strain range, ${\gamma}/{\varepsilon}$ interface acts as damping source in addition to the stacking faults and variant boundaries in Fe-17%Mn alloy.

• Geotechnical Engineering
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• v.10 no.1
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• pp.27-46
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• 1994

It has been demonstrated in plane strain compression tests performed on dense Toyoura sand and Silver Leighton Buzzard sand, that the newly developed instrumentation for small strain measurements was capable of measuring the altering stiffness of sands for a wide range of shear strain from ($10^{-6}$to $10^{-2}$. It was found that for the range of shear strain($\gamma$) from $10^{-5}$ to those at peak, the Rowe's stressiilatancy relation seemed to be a good approximation for Toyoura sand and Silver Leighton Buzzard sand. However, the value of K and Poisson's ratio(at elastic range:${\nu}_{psc}^e$) varied with sand types. It was also found that the value of ${\nu}_{psc}^e$ and stress -dilatancy relation was irrespective of overconsolidation ratio(OCR).