Tae Kwon Ha

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.

Powder injection molding of a 17-4 PH stainless steel and the effect of sintering temperature on its microstructure and mechanical properties

Abstract The 17-4 PH stainless steel powders with average diameter of 10xa0μm were injection-molded into plate-type tensile specimens. Sintering of the compacts was carried out at the various temperatures ranging from 900 to 1350xa0°C after solution extraction and thermal decomposition. Sintering behavior of the powder injection-molded specimens and room temperature tensile properties of sintered specimens were investigated. With increase in the sintering temperature, relative density of the specimens increased from 61% at 900xa0°C to 99% at 1350xa0°C. The pores of sintered specimens appeared to become more rounded in shape and isolated in distribution as the sintering temperature was increased. The tensile strength of sintered specimens was found to increase almost linearly with the sintering temperature, which could not be explained with any model reported so far. The elongation was enhanced with the closure of pores with proceeding of sintering into the intermediate and the final stages.

Superplastic deformation behavior of 8090 aluminum-lithium alloy

The superplastic deformation behavior of crystalline materials has generally been described phenomenologically by a power law relation between the two external variables, stress ({sigma}) and strain rate ({dot {epsilon}}). A new approach for structural superplasticity has been made in this study by taking the dislocating glide mechanism as the major accommodation process for grain boundary sliding (GBS) instead of the generally accepted high temperature diffusion process. For the purpose of generating a proper relationship between {sigma} and {dot {epsilon}}, a series of load relaxation tests has been carried out at temperatures ranging from 470 C to 530 C after obtaining the required grain sizes through a thermomechanical treatment. The load relaxation test provides the flow data in a much wider range of strain rate with minimal microstructural change during the test. The flow curves of 8090 Al-Li alloy have consequently been analyzed systematically based on the internal deformation theory described. From the analysis of load relaxation test results for 8090 Al-Li alloy with d = 10{mu}m, the following important results are obtained. (1) The load relaxation curves at high temperatures consists of the plastic strain rate ({dot {alpha}}) and the grain boundary sliding rate ({dot g}). (2) The plastic flow morexa0» curves of log {sigma}{sup 1} vs. log {dot {alpha}} can precisely be described by Eq. (3) for 8090 Al-Li alloy. (3) The grain boundary sliding appears to be a Newtonian viscous flow process characterized by Eq. (5) with M{sub g} = 1.0. (4) The condition {Sigma}{sub g} {le} {sigma} {le} {sigma}* must be satisfied for the superplastic deformation of crystalline materials. «xa0less

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.

Fabrication of spray-formed hypereutectic Al–25Si alloy and its deformation behavior

Abstract This article reports the fabrication of hypereutectic Al–25Si alloy, which is expected to be applied to the cylinder liner part of the engine block of an automobile due to its excellent wear resistance, low density and low thermal expansion coefficient, through a spray casting process and the characterization of the microstructural and the mechanical properties of this alloy. The OSPREY process used in this study is one of spray forming techniques, which can produce semi-finished products such as billet, tube, and plate with rapid solidification (RS) structure and density in a single operation from molten metal. The obtained microstructure of the hypereutectic Al–25Si alloy appeared to consist of Al matrix and equiaxed Si particles with average diameter of 5–7xa0μm. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to 500xa0°C. The strain rate sensitivity parameter (m) of this alloy has been found to be very low (≤0.1) below 300xa0°C and reached maximum value of about 0.2 at 500xa0°C. During the deformation above 300xa0°C in compression, remarkable strain softening has been observed. The extrusion has been successfully conducted at the temperatures of 300xa0°C and above with the ratio of area reduction of 28 and 40 in this study.

Superplastic deformation of a fine-grained Zn-0.3wt.%Al alloy at room temperature

Abstract The aims of this study are firstly to obtain a fine and stable microstructure in a quasi-single phase Zn–Al alloy and secondly to characterize the superplastic deformation behavior of this alloy performing load relaxation and tensile tests, as well as a microstructural study using a scanning and a transmission electron microscopy. A very fine and stable microstructure with an average grain size of about 1 μm was produced by modifying the conventional thermomechanical treatment process for commercial Al alloys to obtain a remarkable elongation of 1400% at room temperature, which is the largest ever reported for this alloy. The flow curves of logxa0σ versus logxa0 e constructed from load relaxation test and a series of tensile tests conducted under various initial strain rates appeared to have a sigmoidal shape typical for superplastic materials. Even after the specimen was elongated up to 100% at room temperature, no evidence for grain growth was noticed. From the observations of microstructural changes and the scratch offsets on the surface of a deformed specimen, the dominant deformation mechanism could be identified as the grain boundary sliding (GBS), possibly accommodated by plastic flow due to dislocation glide near grain boundaries. At a still higher strain rate, the accommodation mechanism of GBS appears to change from dislocation activity near grain boundaries to deformation twinning at stress concentrations such as triple junctions and grain boundary ledges. When the grain size was increased by an order, predominant deformation mechanism appeared to be dislocation activity evidenced by the cell structure reducing the tensile elongation substantially to around 100%.

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.

An internal variable approach to deformation behavior of a Pb-Sn eutectic alloy

A new approach for inelastic deformation has been proposed by utilizing internal variables derived directly from a simple consideration of dislocation dynamics. An extension of the theory to a structural superplasticity has also been made recently by taking the dislocation glide process as the major accommodation mechanism for the boundary sliding process instead of the generally accepted diffusional accommodation. The internal variable theory of structural superplasticity has been successfully applied to the cases of quasi-single phase Al alloys. In this paper, the inelastic deformation behavior of a Pb-Sn eutectic alloy as well as the constituent pure metals has been examined specifically in connection with the internal variable theory. A special attention was focused on the deformation characteristics of the superplastic eutectic alloy, well known to reveal a pronounced {alpha}/{beta} phase boundary sliding. For this purpose, a series of load relaxation tests has been conducted to obtain the flow curves under the condition of a constant structure and the results have consequently been analyzed based on the internal variable theory of structural superplasticity. A method to determine the optimum strain rate for a superplastic forming process has been proposed to this end by the quantitative consideration of PBS and its accommodationmorexa0» process. Additional tensile tests were also performed to verify the results of analysis.«xa0less