Pre-strain Rate Research Articles

Long-time strain aging responses of commercial dual-phase steels at room temperature

ABSTRACT In this study, the effects of room temperature static strain aging period on the tensile properties of DP600 dual-phase steel were investigated. 6% and 12% of pre-strained DP600 steels were carried out static strain aging treatment at room temperature for up to 4 months. It was observed that with increasing both pre-strain rate and room temperature static aging period, the tensile strength values increased and ductility values decreased significantly. Also, dual-phase steels maintain the continuous yield behaviour which is general characteristic. The fracture surfaces exhibited a lower depth dimple type mode with increasing room temperature static strain aging period.

A novel approach to analyse combined pre and post strain rate dependent phase transformation in uniaxially deformed AISI 304 stainless steel using XRD and EBSD

ABSTRACT The effect of pre-strain, pre-strain rate and post-strain rates on the deformation-induced austenite to martensite phase transformation have been studied in AISI 304 stainless steel. The martensite content obtained from the X-ray diffraction analysis and complemented by the magnetisation measurements showed that the phase transformation is enhanced for a unique combination of pre-strain, pre-strain rate and post-strain rate. The microstructural characterisation using detailed X-ray diffraction line profile analysis in terms of the size of the coherently scattering region and the dislocations that cause the microstrain within this region could give evidence of the enhanced/reduced martensite formation. The results of the electron backscattered diffraction in terms of the angular distribution of the lattice correlation boundaries and the orientation relationship boundaries of the austenite-martensite phase substantiated this finding. Considering that the multiple combinations of deformation parameters control the mechanism of martensitic transformation, this study will also contribute to the understanding of deformation-induced phase transformation of other materials with improved plasticity.

Twinning behavior and hydrogen embrittlement of a pre-strained twinning-induced plasticity (TWIP) steel

The twinning behavior and hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel were investigated for different pre-strains at different strain rates. The lower pre-strain rate led to thinner twins and higher twin volume fraction. The influence of the twin characteristics on HE sensitivity was analyzed from statistical data on the evolution of surface crack characteristics during secondary straining period after cathodic hydrogen charging. Based on the ratio of crack initiation sites caused by the intersection of twins and grain boundaries, it was found that thinner twins in twin bundles of similar thickness promoted the generation of hydrogen-induced cracks (HICs).

Strain effect on microstructural properties of SUS 304L joint by PAW+GTAW

JIS SUS 304L stainless steel was joined by a combination welding process of plasma arc welding and gas tungsten arc welding (PAW+GTAW). Then pre-strain treatment on 304L welded joint of 9 % was carried out using uniaxial quasi-static tensile at room temperature. Effect of strain on microstructure evolution in joint was analyzed by field emission scanning electron microscope (FESEM) and on mechanical properties was also studied. The results indicated that the pre-strain rate of 304L joint showed inhomogeneity including 3 % in the weld metal and 13 % in the heat-affected zone (HAZ), which induced martensitic transformation occurring in HAZ. Tensile strength of the joint increased from 700 MPa as welded to 804 MPa after pre-strain treatment at room temperature, and it reached 1700 MPa from 1480 MPa as welded at low temperature of −196 °C. Impact energy in the HAZ was the least among the whole 304L weld, but it was still 94 J at −196 °C after pre-strain. The fracture morphology showed large numbered of cleavage steps with elongated parabolic dimples.

Uniaxial high strain rate tension of a TiNi alloy provided by the magnetic pulse method

This study makes use of the magnetic pulse method for providing the uniaxial tension of TiNi shape memory alloy specimens. Finite element simulations demonstrate good agreement between the evaluated residual strains and experimental values. The evaluated average strain rates are ~ 4000–5000 s−1 and in local areas, they reach 10,000–12,000 s−1. The functional properties of the alloy after magnetic pulse tension are shown and compared with the results after quasistatic tension. The values of the shape memory effect after magnetic pulse tension decrease by 15–20%. Magnetic field simulation shows that induced currents are negligible and do not lead to heating in the working part of the specimens. It is concluded that the reason for the decrease in the shape memory effect is the high pre-strain rate. Reorientation processes must be sensitive to the strain rate, so the proportion of the oriented martensite decreases with increasing strain rate.

In-situ observations and modeling of metadynamic recrystallization in 300M steel

The metadynamic recrystallization (MDRX) behaviors and microstructure evolution of 300 M steel are studied by hot compression experiments and high temperature laser scanning confocal microscopy (HTLSCM) experiments. The effects of insulation temperature, pre-strain rate and holding time on the MDRX behaviors are discussed in detail. Based on the statistical results of the microstructure evolution, the MDRX kinetics model and recrystallization (RX) grain size model of 300 M steel are established, respectively. It can be confirmed from the in-situ observations that strain-induced boundary migration mechanism and curvature-driven grain boundary migration mechanism are the main MDRX mechanisms of 300 M steel. The MDRX develops rapidly under the high temperature or low pre-strain rate. The insulation temperature has a more important effect on the MDRX rate. The RX grain size increases with the increased insulation temperature, pre-strain rate and holding time. Compared to the pre-strain rate, the effects of insulation temperature on the maximum MDRX fraction and RX grain size are negligible. Comparisons between the experimental and predicted results verify that the proposed MDRX kinetics models and RX grain size model can quantificationally reveal the MDRX behaviors of 300 M steel well.

In-situ observations and modeling of static recrystallization in 300 M steel

The study of static recrystallization (SRX) has been widely concerned for its importance in regulating the microstructure and mechanical properties of deformed materials. In the paper, the effects of SRX on the flow behaviors of 300 M steel are investigated by thermal compression experiments. The SRX mechanism of 300 M steel is investigated by high temperature laser scanning confocal microscopy (HTLSCM) setup. The variations in the flow behaviors and microstructure under different experimental conditions are discussed in detail. Also, the relationships between microstructure evolution and mechanical properties are analyzed. The SRX mechanism is detected by the analysis of microstructure. According to the statistical results of microstructure, the SRX kinetics models and the average recrystallization (RX) grain size model are proposed. It can be found that the flow stress increases with the increased strain rate and reduced deformation temperature. The yield stress is affected by both the softening mechanisms of static recovery and SRX. The main SRX mechanism of 300 M steel is strain-induced boundary migration (SIBM). Compared to the pre-strain rate, the insulation temperature has a more significant effect on the SRX rate, maximum SRX volume fraction and average RX grain size. And the effects of pre-strain rate on the RX grain size are negligible at low pre-strain rate. The comparison between the calculated and experimental results is consistent, which implies that the established models can describe the SRX process accurately. Besides, compared to the softening volume fraction calculated based on the stress-strain curves, the SRX volume fraction calculated based on the statistic results of microstructure can reflect the SRX process more precisely, which indicates the effectiveness of the research method used in the paper.

Prediction of Post-deformation Recrystallization Kinetics in AISI 321 Austenitic Stainless Steel Using Double-Stage Hot Compression

In the present research, a series of double-stage hot compression tests were conducted in the temperature range of 950-1150 °C, strain rates in the range of 0.01-1 s−1, strain in the range of 0.1-0.3, and inter-pass time range of 3-100 s to evaluate the kinetics of the post-deformation recrystallization in AISI 321 austenitic stainless steel. An Avrami-type kinetics equation has been determined based on the obtained experimental data. Results show that the extent of post-deformation recrystallization increases with increase in inter-pass time, deformation temperature, pre-strain, and strain rate. The calculated activation energy for recrystallization (174 kJ/mol) is lower than the activation energy for static recrystallization of austenitic stainless steels. It is concluded that the both static and metadynamic recrystallization occurred during the inter-pass period. Also, the predicted softening fraction using the proposed kinetics equation agreed well with those obtained from experimental results.

Effect of strain rate on bake hardening response of BH220 steel

This study aims at understanding the bake hardening ability of ultra low carbon BH220 steel at different strain rates. The as-received material has been pre-strained to four different levels and then deformed in tension under (a) as pre-strained state and (b) after baking at 170 ∘ C for 20 minutes, at three different strain rates of 0.001, 0.1 and 100/s. In both the conditions, yield stress increased with pre-strain and strain rate, but bake hardening ability was found to decrease when strain rate was increased. The strain rate sensitivity of the material was also found to decrease with bake hardening. Generation of dislocation forests and their subsequent immobility during baking treatment enables them to act as long range obstacles during further deformation. At higher strain rates, less amount of dislocations are produced which can interact with themselves and produce hardening, because of which bake hardening ability and the strain rate drops. A dislocation based strengthening model, as proposed by Larour et al. 2011 [7], was used to predict the yield stress values obtained at different conditions. The equation produced excellent co-relation with the experimental data.

Influence of Boron on the Precipitation Kinetics in Advanced Ultra-High Strength Steels

ABSTRACTIn the present work, the stress relaxation method was employed to determine the influence of B addition on the kinetics of strain-induced precipitation and its interaction with the static austenite recrystallization. For this purpose, the behavior of two low carbon advanced ultra-high strength steels was analyzed during stress relaxation tests at different temperatures and constant pre-strain rate. The precipitation start (Ps) and finish (Pf) times were determined from the relaxation curves and then the corresponding precipitation-time-temperature diagrams were constructed for each steel. Transmission Electron Microscopy was used to determine the chemical nature and evolution of precipitation. In general, the results show that the addition of B retards the austenite recrystallization, tends to accelerate the precipitation kinetics of carbonitrides and leads to a finer and denser distribution of precipitates. These results are discussed in terms of the driving force for the nucleation of precipitation, which in turn is controlled by the degree of supersaturation of microalloying element and as a function of B segregation and B-vacancy complexes to dislocations and grain boundaries.

The effects of pre-strain, recovery temperature, and bending deformation on shape memory effect in an Fe–Mn–Si–Cr–Ni alloy

In this study, effects of pre-strain, recovery temperature, and number of bending test cycles on the microstructure and shape memory behavior of Fe-12.5%Mn-5.5%Si-9%Cr-3.5%Ni alloy have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-Ray diffraction method, and bending test. Experiments show that the shape memory effect of the alloy can be enhanced by convenient recovery temperature and pre-strain rate. The shape recovery ratio increases with increasing pre-strain and recovery temperature, while it decreases with an increase in the number of bending test cycles. The changes in the shape memory are attributed to the variation of amount of ɛ martensite and crystal defects. The amount of ɛ martensite increases with increasing the pre-stain ratio. Crystal defects decrease with an increase in the recovery temperature, while they increase with increasing the number of the bending test cycles.

Influence of large pre-straining of aluminium alloys on their residual fatigue resistance

The aim of this experimental study is to quantify the influence of a tensile pre-strain on the residual fatigue life of aluminium alloys. Two alloys with different chemical compositions and hardening modes have been investigated: 2017A-T3 used in the aircraft industry and 5454-O used in automotive applications. The pre-loadings were carried out with two strain rates (quasi-static and impact) and for several controlled elongations. The residual fatigue life was estimated under a level which corresponds to about 2 × 10 5 cycles for the undamaged material. From this study, two conclusions have been highlighted. The pre-strain rate does not have any influence on the residual material behaviour, whereas each material has a different strain rate sensitivity. The material aspect seems to be fundamental: the Al–Mg alloy (5454-O) which is a solid solution hardening alloy remains insensitive to all pre-loading whereas the Al–Cu alloy (2017A-T3) which can be age hardened, undergoes a large decrease in fatigue performance after pre-strain.

Role of imposed cyclic straining on the stress relaxation behavior of eutectic Sn-3.5Ag solder joints

Solder joints experience repeated reverse straining during thermal excursions encountered in service, as a consequence of stresses that arise due to coefficient of thermal expansion (CTE) mismatches between entities present in the joint. They also undergo stress relaxation under fixed strain during dwell times at temperature extremes encountered during service. In order to understand the fundamental processes involved under such conditions, cyclic shear straining with associated stress relaxation at the shear strain extremes were imposed during stress relaxation of pre-strained solder joints at various temperatures. Results of such studies were compared with previously reported findings from monotonic shear stressing and stress relaxation tests. Residual stress during stress relaxation under repeated reverse straining exhibited significant decrease for specimens deformed to a higher pre-strain at a higher pre-strain rate, at lower temperature. Stress relaxation during subsequent cycles of straining was found to be strongly dependant on the test temperature and the imposed pre-strain amplitude and pre-strain rate.

Analytical modal analysis of thin-film flat lenses

The free vibrations of circular and elliptical thin-film lens are investigated. In particular, linear closed-form solutions for free vibrations of these structures were achieved and modal analysis was performed. The vibration response of the thin-film membranes were mathematically modelled using the Mathieu equation. Numerical results for various nodal diameters were computed. For the limited case, when an elliptical lens becomes circular, an excellent comparison was established with the available analytical solution. Experimental analyses were conducted to determine the effects of various parameters, such as material properties, membrane pre-strain rate, and the geometry, on natural frequency and mode shapes of these structures. The comparison verified the adequacy of linear solutions to predict the dynamic response of thin-film lenses.

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 102 to 5 × 103 s-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σy). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 103 s-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 102 to 5 × 103 s-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 102 s-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσy and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.

Possible ways of enhancing high temperature ductility in CuO doped cubic zirconia (8Y–CSZ)

The effect of initial density and rapid prestraining on superplastic ductility of 1 wt-%CuO doped cubic zirconia (8Y–CSZ) was investigated. To obtain a range of initial densities, the tensile test specimens were slip cast to net shape and pressureless sintered over a range of temperatures in air. The specimens were then superplastically tested at a temperature of 1500 K and at a constant strain rate of 1×10-4 s-1. The results showed that specimens with low initial densities had lower flow stresses and higher superplastic elongations to failure than higher density specimens. The reasons for the ductility change were discussed with reference to the presence of porosity and grain growth. For the prestraining test, a specimen with an initial density of 95% was prestrained to 30% at a temperature of 1550 K and at a prestrain rate ɛ ·1 of 1×10-3 s-1, followed by elongation to failure at a slower test strain rate ɛ ·2 of 1×10-4 s-1. It was seen that prestraining at the above test conditions considerably improved superplastic ductility. The reasons for this ductility enhancement were explained in terms of suppression of grain growth.

826 S45C 材のねじり疲労特性に及ぼす予ひずみの影響

Fatigue test has been performed to investigate the torsional fatigue properties of pre-strained carbon steel (S45C). Main results summarized in this study are as follows : (1) In the case of plastic pre-strain rate ε_p≦5%, the fatigue limit is 10% lower than the non-pre-strained specimen. Even if the pre-srtain rate increases, the fatigue limit is almost the same one. (2) In the case of ε_p>5%, the fatigue limit increases with increasing pre-strain rate. For the ε_p=8%, the fatigue limit equals to fatigue limit of non-pre-strained specimen. (3) The non-propagatinig cracks are observed in all specimen's sufaces after suffered by 1×(10)^7 cycles under the stress amplitude of fatigue limit. The length of non-propagating crack of pre-strained specimen has no obvious difference with that of non-pre-strained specimen. On the other hand, the higher the pre-strain rate, the more number of non-propagating cracks are observed in the specimen's suface.

Improvement of tensile ductility in 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) by prestraining

There have been numerous investigations of the effect of material testing variables such as strain rate, temperature and grain size on the elongation to failure of superplastic ceramics. This paper presents information on the effect of rapid prestraining on superplastic ductility in a fine-grained 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP), using two testing programmes: (i) prestraining up to 130% at a prestrain rate of 1×10-3 s-1 followed by elongation to failure at a test strain rate of 1×10-4 s-1 and (ii) prestraining to 60% at prestrain rates of 1×10-3 s-1 and 2.5×10-4 s-1 followed by elongation to failure at a slower test strain rate of 1×10-4 s-1. The results showed that prestraining at the above conditions considerably improved superplastic ductility as well as reducing the time required to achieve a given elongation. The reason for this ductility enhancement is explained in terms of suppression of grain growth. © 1998 Chapman & Hall

Microstructural evolution and superplastic deformation behavior of fine grain 5083Al

The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 µm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.