Large Uniform Elongation Research Articles

Enhancement of uniform elongation in high strength Ti–Mo based alloys by combination of deformation modes

A combination of different deformation modes, namely, dislocation slip and {3 3 2}〈1 1 3〉 twinning was found to be effective for achieving high yield strength and large uniform elongation in the β type Ti–15Mo–5Zr and Ti–10Mo–2Fe alloys in the as-solution treated condition, where the Mo equivalency was designed to be between 15.3 and 18.7 mass%. The high yield strength was caused mainly by the slip, and the large uniform elongation was caused by the twinning through significant work hardening. The change in the work hardening rate with strain correlated well with the formation of mechanical twins. The deformation was heterogeneous among the grains and the twins were not seen in some of the grains even after the tensile fracture. This heterogeneity was discussed based on the effects of the grain orientation and the segregation of alloying elements.

Excellent room-temperature ductility and formability of rolled Mg–Gd–Zn alloy sheets

In order to develop new magnesium alloy sheets with high formability at room temperature, the microstructure, texture, ductility, and stretch formability of rolled Mg–2%Gd–1%Zn and Mg–3%Gd–1%Zn sheets were investigated. The microstructures of these rolled sheets consist of fine recrystallized grains with a large amount of homogeneously distributed tiny particles in the matrix. The basal plane texture intensity is quite low and the basal pole is tilted by about 30° from the normal direction toward both the rolling direction and the transverse direction. The sheets exhibit an excellent ultimate elongation of ∼50% and a uniform elongation greater than 30%, and the Erichsen values reach ∼8 at room temperature. The flow curves of the two Mg–Gd–Zn alloys sheets display a remarkable linear hardening after an obvious yield point. The majority of the grains in the tilted texture have an orientation favorable for both basal slip and tensile twinning because of a high Schmid factor. The excellent stretch formability at room temperature can be attributed to the non-basal texture and low texture intensity, which led to the following characteristics: a lower 0.2% proof stress, a larger uniform elongation, a smaller Lankford value and a larger strain hardening exponent.

Work hardening mechanism in high nitrogen austenitic steel studied by in situ neutron diffraction and in situ electron backscattering diffraction

With a focus on microstructural hierarchy, work hardening behaviour in high nitrogen-bearing austenitic steel (HNS) was investigated mainly by a combined technique of in situ neutron diffraction and in situ electron backscattering diffraction (EBSD). Stress partitioning due to difference in deformability among grains is enhanced in HNS. The larger stress partitioning among [ h k l]-oriented family grains seems to realize high work hardening at a small strain. At a larger strain, dislocation density is higher in HNS than in low nitrogen austenitic steel (LNS), which is a possible reason for high work hardening after straining proceeds, resulting in large uniform elongation.

Improvement of formability of Mg–Al–Zn alloy sheet at low temperatures using differential speed rolling

The differential speed rolling (DSR) with a roll speed ratio of 1.167 was carried out on an AZ31B magnesium alloy in order to investigate its effects on the formability. Compared with the normal rolled sheet exhibiting approximately the same average grain size, the Erichsen values of the DSR processed sheet with an inclination of basal pole in the rolling direction significantly increased by about 1.5 and 1.9 times at room temperature and at 423 K, respectively. The deep-drawing temperature limit for a drawing ratio of 1.5 was also lowered from 443 K to 423 K. The improvement of the press formability at low temperatures can be attributed to the texture modifications, which led to a lower 0.2% proof stress, a larger uniform elongation, a smaller Lankford value and a larger strain hardening exponent.

Effects of hot deformation and subsequent austempering on mechanical properties of high silicon and low silicon TRIP steel

In the present paper, effects of hot deformation and subsequent austempering on the mechanical properties of high silicon and low silicon transformation induced plasticity (TRIP) steel were investigated. The results have shown that polygonal ferrite, granular bainite and a large amount of stabilised retained austenite can be obtained by thermomechanical processing (TMP). Mechanical properties increase with increasing finish rolling reduction and decreasing finish rolling temperature because of the stabilisation of retained austenite. The tensile strength and the total elongation of both steels reach the maximum (791, 794 MPa and 36, 37% respectively). Isothermal holding for 20 min at 400°C after hot deformation is the optimal process. Low silicon TRIP steel shows a larger uniform elongation owing to a stable n value, and it exhibits better mechanical properties than conventional high silicon TRIP steel.

Managing Both Strength and Ductility in Ultrafine Grained Steels

Ultrafine grained (UFG) steels provide surprisingly high strength but sometimes show limited tensile ductility. In the present paper, systematic experimental results on mechanical properties of UFG steel with ferrite single phase are shown first. The limited tensile ductility of the UFG ferritic steel was due to very small uniform elongation, which was attributed to the early plastic instability in the UFG microstructures. This basic understanding suggested a way to overcome the low tensile ductility: if the strain-hardening of the matrix is enhanced by any means, both high strength and adequate ductility can be managed even in UFG structures. Actual examples of the UFG steels that could achieve good strength-ductility balance are also presented. Dispersing fine carbides within the UFG ferrite matrix, and making the UFG dual-phase structure composed of ferrite and martensite were both effective to manage high strength and large uniform elongation. It was clearly shown that the future studies on the UFG steels from practical viewpoint should be directed to make the UFG structures multi-phased.

Effect of Deformation Route on the Mechanical Properties of CP-Ti Deformed by Equal Channel Angular Pressing (ECAP)

The effect of deformation route on the mechanical properties of grade-1 CP-Ti deformed by equal-channel angular pressing (ECAP) was studied using tensile testing, TEM observation, Xray pole figure measurement, and ODF calculation. Route Bc showed high yield stress and comparatively large uniform elongation, thus high ultimate tensile strength, because of its fine grain structure with high angle grain boundary. The reason for this was because of the occurrence of prismatic slip in route Bc, in addition to the commonly occurring basal slip. Route C showed surprisingly high yield stress despite of its unfavorable grain structure because of the significant contribution of texture hardening.

Physically Based Model for Static and Dynamic Behaviour of TRIP Steel

Low alloy multiphase TRansformation Induced Plasticity (TRIP) steels offer an excellent combination of a large uniform elongation and a high strength. This results from the composite behaviour of the different phases that are present in these steels: polygonal ferrite, bainitic ferrite and a martensite/austenite constituent. In order to obtain a clear understanding of the behaviour of the different constituents within the multiphase steel, they were prepared separately. The stress-strain relationship of the different single phase and multiphase steels were simulated with physically based micromechanical models. The model used to describe the stress-strain curves of the separate phases is based on the Mecking-Kocks and Seeger-Kocks theories and uses physical properties such as the microstructural properties and the chemical composition of the different phases. Strain-induced transformation kinetics, based on a generalized form of the Olson-Cohen law, were used to include the influence of the transformation of the metastable austenite. Static stress-strain properties of multiphase steels were modelled by the successive application of a Gladman type mixture law for two-phase steels. The model yields detailed information of stress and strain partitioning between the different phases during a static tensile test. A model for the dynamic stress-strain properties of ferritic steels is also proposed.

Microstructure-based model for the static mechanical behaviour of multiphase steels

Low-alloy multiphase transformation-induced plasticity (TRIP) steels offer an excellent combination of a large uniform elongation and high strength. This results from the composite behaviour of the different constituent phases that are present in these steels: polygonal ferrite, bainitic ferrite, and martensite/austenite. The different constituents were prepared separately in order to obtain a clear understanding of their individual behaviour within the multiphase steel. The stress–strain relationships of these different types of single- and multiphase steels were simulated by physically based micromechanical models. The model used for the simulations of the stress–strain curves of the separate phases is based on the Mecking–Kocks theory and utilizes physical properties such as the microstructural parameters, the dislocation density, and the chemical composition of the different phases. Strain-induced transformation kinetics, based on a generalized form of the Olson–Cohen law, are utilized in order to include the influence of the transformation of the metastable austenite on the mechanical properties of the TRIP steels. Static stress–strain properties of multiphase steels were modelled by the successive application of a Gladman-type mixture law for two-phase steels. The model yields detailed information of stress and strain partitioning between the different phases during a static tensile test.

Ultrafine grained steels processed by equal channel angular pressing

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

Effect of Austempering Process on the Properties of TRIP-Steel

Transformation-induced-plasticity (TRIP) –aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The influences of the austempering temperature and time on the microstructure mechanical properties and the transformation behavior of a Si-Mn TRIP steel are investigated in this paper.

メカニカル・ミリング法によって作製したAl-1.1mol%Mg-1.2mol%Cu合金の高温変形中における転位の熱回復と均一変形特性

Tensile deformation was carried out for a mechanically milled Al-1.1 mol%Mg-1.2 mol%Cu alloy at temperatures of 523-823 K and a true strain rate of 1×100 s-1. The largest uniform elongation prior to local necking occurred at an intermediate temperature (748 K). This temperature dependence of the uniform elongation was analyzed from a strain hardening viewpoint in which the balance of the stored dislocations and the thermal recovery of them is responsible for maintaining a high mobile dislocation density that is exclusively temperature-dependent to sustain a large uniform elongation. It was found that thermal activation process of liberating unlocked immobile dislocations from solute atmosphere is responsible for the temperature dependence of thermal recovery of stored dislocations. The same process applies for the deformation parameters such as the re-mobilization probability of immobile dislocations and the mobile dislocation density that were previously obtained from the analysis of stress-strain behavior on a basis of dislocation dynamics. An activation energy of approximately 32 kJ/mol is equivalent to the vacancy migration energy in aluminum minus the binding energy between a vacancy and a solute atom of magnesium or copper.

Galvanisability of silicon free CMnAl TRIP steels

Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si–Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved significantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high affinity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.

Thermal Recovery of Dislocations and Nature of Uniform Elongation during High Temperature Deformation of a Mechanically Milled Al-1.1 mol%Mg-1.2 mol%Cu Alloy

Tensile deformation was carried out for a mechanically milled Al-1.1 mol%Mg-1.2 mol%Cu alloy at temperatures of 523-823 K and a true strain rate of 1 x 10 0 s -1 . The largest uniform elongation prior to local necking occurred at an intermediate temperature (748 K). This temperature dependence of the uniform elongation was analyzed from a strain hardening viewpoint in which the balance of the stored dislocations and the thermal recovery of them is responsible for maintaining a high mobile disclocation density that is exclusively temperature-dependent to sustain a large uniform elongation. It was found that themal activation process of liberating tunlocked immobile disclocations from solute atmosphere is responsible for the temperature dependence of thermal recovery of stored dislocations. The same process applies for the deformation parameters such as the re-mobilization probability of immobile dislocations and the mobile dislocation density that were previously obtained from the analysis of stress-strain behavior on a basis of dislocation dynamics. An activation energy of approximately 32 kJ/mol is equivalent to the vacancy migration energy in alutminum minus the binding energy between a vacancy and a solute atom of magnesium or copper.

Phase transformation and mechanical properties of si-free CMnAl transformation-induced plasticity-aided steel

Conventional CMnSi transformation-induced plasticity (TRIP)-aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The CMnSi TRIP-aided steels, with more than 1 mass pct Si, have the drawback of poor galvanizability due to the presence of complex Si-Mn oxides on the surface. The full substitution of the Si by Al in cold-rolled and intercritically annealed TRIP-aided steels, therefore, was evaluated in detail. The phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. The allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite. The results imply that short isothermal bainitic transformation times are sufficient to obtain the TRIP microstructure and that the processing of CMnAl TRIP-aided steels in a continuous annealing line not equipped for overaging is possible. The mechanical properties were evaluated for CMnAl TRIP-aided steels obtained using an industrial thermal cycle: the properties matched those of the conventional CMnSi TRIP-aided steels, where it was found that the high-Al CMnAl TRIP-aided steel had a high strain-hardening coefficient of 0.25, which was stable up to a true strain of 0.25.

The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 °C to 25 °C of M s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.

Analysis of creep deformation behaviors of type 2205 duplex stainless steel under continuous annealing conditions

The objective of this study is to investigate the effects of temperature and line tension on the creep deformation behavior of duplex stainless steel strip under continuous annealing conditions. For the creep tests, the stresses were calculated from the line tension applied to hot-rolled and cold-rolled duplex stainless steel (Type 2205) strip during continuous annealing. It was found that Type 2205 steel exhibited the superplastic-like deformation behaviors with a high strain rate sensitivity value ( m) and a large uniform elongation. The cold-rolled Type 2205 steel showed more prominent superplasticity as compared to the hot-rolled steel, which could be attributed to the fine grain size of the cold-rolled steel. It is suggested that the rate controlling mechanisms for the creep deformation of hot-rolled and cold-rolled Type 2205 steels are seem to be power law creep and grain boundary diffusion in γ phase, respectively.

Improvement in post-irradiation ductility of neutron irradiated V–Ti–Cr–Si–Al–Y alloy and the role of interstitial impurities

An attempt to maintain the uniform elongation (UE) of vanadium alloys after irradiation is described. Neutron irradiation was carried out at Advanced Test Reactor (ATR) and at Experimental Breeder Reactor II (EBR-II) to the fluence of 11 dpa. The UE was maintained about 6% after irradiation at 388°C. The UE became negligible after irradiation below 300°C for the alloy annealed at 1000°C and 1100°C. The specimens annealed at 900°C and 700°C still showed relatively large UE after such low-temperature irradiation. Comparing with the results in literature, it is proposed that oxygen concentration should be kept below 200 ppm to maintain the UE after irradiation below 400°C. The control of interstitial impurities, especially oxygen in solution, was important to maintain the UE of the alloy.

Tensile elongation of off-stoichiometric Fe3Si single crystals at high temperatures

Off-stoichiometric Fe3Si single crystals with compositions of Fe–14at%Si and Fe–18at%Si were tensile tested at 973–1223 K and at various strain rates in order to investigate a mechanism for large elongation in coarse-grained Fe3Si polycrystals at elevated temperatures. Maximum elongation over 150% is attained for both the single crystals. 〈111〉 slip is dominantly operative in Fe–14at%Si with B2-ordered structure, while 〈001〉 is in Fe–18at%Si with D03-ordered structure. The m values indicating strain rate sensitivity are between 0.33 and 0.23 depending on orientation and temperature. Dynamic recrystallization is not occurring on large uniform elongation. The large elongation in the single crystals is attributable to a large m value around 0.3, which is not necessarily related to a solute-drag controlled creep mechanism, but to dislocation interactions accompanied with subgrain formation.

Tensile and impact properties of mesoscopic-grained α+β-type titanium alloys obtained through hydrogen treatments

The aim of this investigation was to study the effect of hydrogen treatment on the tensile and impact properties of α+β-type titanium alloys. The results were as follows: it was found that mesoscopic-grained Ti–6Al–4V alloys with grain size 1–3 μm had high ductility together with high strength. In particular, it showed an extremely large uniform elongation. The yield stress of these alloys with grain sizes 1–3 μm increased linearly in proportion to the −1/2 power of grain size. Very fine voids were observed directly below the fractured surface of tensile specimens with mesoscopic structures. The microstructures of highly deformed alloys with mesoscopic structures indicated that a fine β(bcc)-phase was dispersed along the boundaries of equiaxed α(hcp) grains which were preferentially deformed, similar to the grain-boundary-sliding phenomenon of superplasticity. The impact test indicated that the toughness of the mesoscopic-grained alloys was comparatively low.