Young Won Chang

Kinetics of deformation induced martensitic transformation in a 304 stainless steel

Deformation induced martensitic transformation (DIMT) phenomena in 304 stainless steel have been investigated in relation to the inelastic deformation theory in this study. A new kinetics equation for DIMT has been formulated as f/fs=1−exp[−β(e−e0)n] with the parameter β characterizing the stability of retained austenite, n denoting a deformation mode, and fs the saturation value of transformed martensite volume fraction.

An internal variable theory of structural superplasticity

Abstract A new approach for structural superplasticity (SSP) is attempted in this study based on the internal deformation variables. The basic mechanisms of SSP are thought here to consist of interface sliding (IS), i.e. grain or phase boundary sliding (GBS/PBS), and a dislocation glide process accommodating the incompatibilities due to IS. For this purpose, a new constitutive framework for inelastic deformation is first developed by a simple consideration of dislocation kinematics to reveal the existence of “internal strain” and “internal spin” tensors. The evolution relationship of internal strain tensor is then shown to lead into a kinematic relationship between the observable deformation variables. To complete the constitutive structures, the constitutive relationships for each deformation variable including the strain rate due to IS are also prescribed by kinetics consideration of each process. The theoretical results are then applied to the experimental results on a fine-grained 7475 Al and a Pb–Sn eutectic alloy obtained from load relaxation and tensile tests.

High temperature deformation behavior of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass

The high temperature deformation behavior of a Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 bulk amorphous alloy has been studied in the temperature range between 351 and 461 °C under compressive as well as tensile loading. Three types of nominal stress-strain curves under compressive loading have been identified depending on the strain rate and test temperature, viz., a linear stress-strain relationship with fracture at the maximum stress, large plastic deformation after stress overshoot, and steady state plastic flow without stress overshoot. The DSC analysis for the compressed specimens was also carried out to determine the change in the crystallization fraction under the various test conditions. Under tensile loading, superplastic-like deformation with a maximum elongation over 500% was observed at 431 °C under a relatively high strain rate of 2 × 10 -2 /s. The tensile test specimens were, however, observed to exhibit a brittle fracture mode at temperatures below the glass transition temperature and above the crystallization temperature under the same initial strain rate. The superplastic-like properties characterized by the low flow stress and large elongation then appear to be very sensitive to test temperature and initial strain rate.

Deformation behavior of Zr-based bulk metallic glass in an undercooled liquid state under compressive loading

High temperature deformation behavior of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk amorphous alloy has been studied in a temperature range between 355 and 460°C under compressive loading after rapid heating. A transition of flow behavior, viz. from, a Newtonian to a non-Newtonian flow, has been reported by many researchers as the temperature is decreased at a given strain rate. In the present study, two different theoretical relations based on a viscous flow model and a transition state theory have been applied to analyze the transition behavior of deformation in terms of viscosity and flow stress. An experimental deformation map was then constructed to specify the boundaries between Newtonian and non-Newtonian flow, based on the relationship between the flow stress and strain rate in an undercooled liquid state. It has further been confirmed that the stress overshoot phenomena can be observed mostly in a non-Newtonian flow regime appearing in an intermediate temperature and strain rate region in this deformation map.

Temperature Effect on Twin Formation Kinetics and Deformation Behavior of Fe–18Mn-0.6C TWIP Steel

Temperature effect on deformation behavior has been investigated in relation to formation kinetics of twins in a Fe-18Mn-0.6C TWIP steel. Total elongation was found to reach a maximum value of 88% at 200 °C and then decreased continuously with the increase in test temperature from 300 °C up to 600 °C. This reversed temperature dependence on ductility could be attributed to the formation kinetics of deformation twins, as was prescribed by an internal variable theory of inelastic deformation. It was found that twins became more difficult to form at higher temperatures due to insufficient internal strain energy accumulated to reduce ductility progressively in this temperature range. Dislocation glide mechanism became, however, dominant at higher temperatures above 600 °C to increase total elongation following the usual temperature dependence. Finally the stacking fault energy was related with the stability parameter, β, used in the transformation kinetics relation.

Dynamic fracture behavior of SiC whisker-reinforced aluminum alloys

This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2, and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,KId, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness.

Mechanical and microstructural analysis on the superplastic deformation behavior of Ti–6Al–4V Alloy

Abstract The present investigation has been made to study the superplastic deformation behavior of Ti–6Al–4V alloy based on the theory of inelastic deformation, and to analyze the boundary sliding characteristics using transmission electron microscopy. Flow characteristics for the microstructures of 2.5–16 μm grain sizes were analyzed by the load relaxation tests at various temperatures ranging from 600 to 927°C. The results showed that at relatively low temperatures such as 600°C the grain matrix deformation was dominant and found to be consistent with the state equation based on the dislocation dynamics. On the contrary, above the temperature of 800°C, the grain boundary sliding became dominant resulting in the change of curvature in the stress–strain rate curves, which was more pronounced in the finer microstructures. However, the deformation mode changes from grain boundary sliding to grain matrix deformation with the increase in grain size as evidenced by transmission electron microscopy.

Effect of deformation induced transformation of ɛ-martensite on ductility enhancement in a Fe-12 Mn steel at cryogenic temperatures

Metastable ɛ-martensite (ɛ-Ms) formed during a prior heat treatment of Fe-12Mn steel has been reported to transform into α-Ms during subsequent inelastic deformation. This deformation induced phase transformation (DIPT) from ɛ-Ms to α-Ms has been investigated in the present study within the framework of kinetics relation proposed based upon an internal variable theory of inelastic deformation. The ɛ-Ms phase was found to become more stable at lower temperatures to provide more prolonged DIPT from ɛ-Ms to α-Ms during an inelastic deformation at cryogenic temperatures, contrary to the case of austenite phase in various transformation induced plasticity steels being more stable at higher temperatures. This reversed stability-temperature relationship in Fe-12Mn steel appeared to provide a significant ductility enhancement at lower temperatures as well as significant strengthening effect.

Temperature and grain size dependence of superplasticity in a Zn−0.3wt.%Al alloy

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt. %Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 °C) to 200 °C. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average agrain size of 1 μm, the IS behavior could be described as a viscous flow process characterized by a power index of Mg=0.5. A large elongation of about 1400% was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 μm) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 °C, however, the contribution from IS was observed at a very low strain rate range. A high elongation of ∼400% was obtained in a specimen of 10-μm-grain-size at 200 °C under a strain rate of 2×10−4/sec.

An internal variable approach to the grain size effect on the superplastic deformation behavior of a 7475 Al alloy

Abstract The effect of grain size on the superplastic deformation behavior of a 7475 Al alloy has been studied within the framework of the recently proposed internal variable theory of structural superplasticity. A simple rheological model, including grain boundary sliding (GBS) and accommodating grain matrix plastic deformation (GMD), has been employed to interpret the plastic flow behavior of superplastic 7475 Al alloy at 516°C. A series of load relaxation and tensile tests have been carried out after obtaining various grain sizes ranging from 13 to 42 μm through proper thermomechanical treatment processes. The flow curves of log σ vs. log e have been found to be composite curves consisting of GBS and accommodating GMD by dislocation. The GBS appears to be a Newtonian viscous flow characterized by the power index value of M g =1.0. A Hall–Petch type relation has been found between the grain size used in this study and an internal strength variable ( σ *). The decrease in grain size appears to lower the friction stress for GBS ( Σ g ) and to enhance the superplasticity due to GBS.