Hua Ding

Formability of TRIP/TWIP Steel Containing Manganese of 18. 8%

The mechanical property and forming limit experiments were carried out on a high manganese TRIP/TWIP steels with manganese of 18.8%. And the forming limit diagram was obtained. This high manganese steel shows outstanding mechanical properties combining high strength with good formability. Its limit plane strain corresponding to the lowest point in the plain strain condition is about 38 %. The effects of conventional mechanical property on the limit plane strain were analyzed. With increasing the content of Mn, the limit plane strain increases, indicating that TWIP effect is advantageous to the increase of formability. Combining with three classical models and volume invariable principle, a FLD model was built, which is suitable for high manganese steels very well.

Effect of welding heat input and post-welded heat treatment on hardness of stir zone for friction stir-welded 2024-T3 aluminum alloy

Abstract The microstructure and hardness of the stir zone (SZ) with different welding heat inputs were investigated for friction stir-welded 2024-T3 aluminum by transmission electron microscopy, differential scanning calorimeter and Vickers micro-hardness test. The results show that welding heat input has a significant effect on the hardness of the SZ. Under high welding heat input condition, a higher welding speed is beneficial for improving the hardness of the SZ. However, when the welding heat input is low, the hardness of the SZ elevates with increasing the rotation speed. The hardness of the SZ decreases after post-welded heat treatment due to overaging. The joints welded at 500 r/min and 100 mm/min show a high resistance to overaging. The reduction of hardness in the SZ is only 3.8%, while in other joints, the reduction is more than 10%. The morphology of strengthening precipitates plays important roles for the improvement of hardness.

Influence of hydrogenation on microstructures and microhardness of Ti6Al4V alloy

The microstructures of Ti6Al4V alloy after hydrogenation were investigated by optical microscopy(OM), X-ray diffraction(XRD) and transmission electron microscopy(TEM). The influence of hydrogenation on the hardness of α and β phases was analysed by microhardness testing. The influence of hydrogenation on alloying elements diffusion was studied by electron probe microanalysis(EPMA). The microstructural observation reveals that hydride δ (FCC structure) as well as large number of dislocations precipitate in the specimens with 0.278% and 0.514% hydrogen, and a lot of twins are found in the specimen with 0.514% hydrogen, simultaneously. The result of microhardness testing shows that the hardness of α and β phases increases synchronously with the increase of hydrogen and the hardness increment of β is larger than that of α. According to analysis of EPMA, the diffusion ability of alloy elements Al and V increases after hydrogenation. It is considered that hydrogen solution strengthening and V element diffusion are the main factors causing the hardness of α phase increase with the increase of hydrogen, and the formation of δ hydrides, lattice defects, hydrogen solution strengthening and Al element diffusion jointly cause the hardness of β phase increase with the increasing hydrogen.

Effects of temperature and strain rate on flow behavior and microstructural evolution of super duplex stainless steel under hot deformation

Hot compression tests were carried out in the temperature range of 1223—1473 K and strain rate range of 0.01—30 s–1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507 (SDSS2507). It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization (DRX) and the flow stress increases with the decrease of temperature and the increase of strain rate. The apparent activation energy Q of SDSS2507 with varying true strain and strain rate is determined. As the strain increases, the value of Q declines in different ways with varying strain rate. The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter (Z). At a lower InZ, dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX) of the ferrite dominate the softening mechanism during the compression. At this time, steady state deformation takes place at the last stage of deformation. In contrast, a higher InZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization (DDRX) of the austenite, which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.

Intermetallics and phase relations of Mg-Zn-Ce alloys at 400 °C

Abstract The crystal structures, compositions and phase relations of the intermetallics of Mg-Zn-Ce system in the Mg-rich corner at 400 °C were identified through equilibrium alloy method. For Mg-Zn-Ce system, there is a linear ternary compound ( T phase), whose chemical formula is (Mg 1- x Zn x ) 11 Ce. The range of Zn content in T phase is from 9.6% to 43.6% (molar fraction). The crystal structure of T phase is C -centered orthorhombic lattice with lattice parameters of a =0.96-1.029 nm, b =1.115-1.204 nm, c =0.940-1.015 nm. And the lattice parameters of T phase are decreasing a little with increasing Zn content. According to the results of composition and crystal structure, the maximal solubility of Zn in Mg 12 Ce is about 7.8% (molar fraction), and the chemical formula of the solid solution can be identified as (Mg 1- x Zn x ) 12 Ce. The isothermal section of Mg-Zn-Ce system in Mg-rich corner at 400 °C was constructed.

Transformation behavior of low carbon steels containing two different Si contents

Continuous cooling transformation behaviors of low carbon steels with two Si contents (0. 50% and 1. 35%) were investigated under undeformed and deformed conditions. Effects of Si contents, deformation, and cooling rates on y transformation start temperature (Ar3 ), phase microstructures, and hardness were studied. The results show that, in the case of the deformation with the true strain of 0. 4, the length of bainitic ferrite laths is significantly decreased in low Si steel, whereas, the M/A constituent becomes more uniform in high Si steel. An increase in cooling rates lowers the Ar3 greatly. The steel with higher level of Si exhibits higher Ar3, and higher hardness both under undeformed and deformed conditions compared with the steel with a lower Si content. Especially, the influence of Si on Ar3 is dependent on deformation. Such effects are more significant under the undeformed condition. The hardness of both steels increases with the increase of cooling rates, whereas, the deformation involved in both steels reduces the hardness.

Effect of baking process on microstructures and mechanical properties of low silicon TRIP steel sheet with niobium

After 2% predeformation, the baking treatment with different schedule was carried out for low silicon TRIP steel sheet with niobium. The effects of baking temperature and time on microstructures and mechanical properties were investigated. The results showed that with increasing the baking temperature and time, the volume fraction of retained austenite decreases, and the volume fraction of tempered martensite increases; as baking temperature ranges from 80 °C to 170 °C, the bake-hardening (BH) value increases obviously, while from 170 °C to 230 °C, the variation of BH value is very slight; as baking time ranges from 2 min to 20 min, the BH value increases significantly, while the BH value decreases when baking time exceeds 20 min. So that when the baking temperature is 170 °C and the baking time is 20 min, the low silicon TRIP steel sheet exhibits good bake-hardening behavior, and the highest BH value is above 70 MPa.

Influence of thermo hydrogen treatment on hot deformation behavior of Ti600 alloy

Abstract Hot compressive deformation of Ti600 alloy after thermo hydrogen treatment (THT) was carried out within hydrogen content range of 0–0.5%, temperature range of 760–920 °C and strain rate range of 0.01–10 s−1. The flow stress of Ti600 alloy after THT was obtained under hot deformation condition, and the influence of hydrogen on work-hardening rate (S*), strain energy density (U*), and deformation activation energy (Q) was analysed. The results show that the flow stress of Ti600 alloy decreases remarkably with the increase of hydrogen when the hydrogen content is less than 0.3%. Both S* and U* decrease with the increase of hydrogen when the hydrogen content is less than 0.3%, and when the hydrogen content is more than 0.3%, S* and U* increase with hydrogen addition. The value of Q decreases with the increase of strain at the same hydrogen content. The addition of small quantity of hydrogen leads to an increase of Q at small strain values, and when the strain reaches 0.6, the value of Q decreases gradually with the increase of hydrogen. When the hydrogen content is within the range of 0.1%–0.3%, the flow stress of Ti600 alloy is decreased when being deformed at the temperature range of 760–920 °C.

Effect of Thermomechanical Processing on Microstructures of TRIP Steel

In order to control retained austenite, the effect of hot deformation in the intercritical region on the microstructure of hot-rolled transformation-induced plasticity (TRIP) steel was studied on a Gleeble 1500 hot simulator. Compressive strains varying in amounts from 0 to 60% were imposed in.the intercritical region, and effects on the formation of polygonal ferrite, carbidefree bainite and retained austenite were determined. With increasing the hot deformation amount and the ferrite content and decreasing the carbide-free bainite content, the volume fraction of retained austenite decreases. Increased dislocation density, grain refinement of ferrite and carbon enrichment are the main factors which control retained austenite stability.

Superplastic behavior of a β-forged Ti3Al–Nb alloy

Abstract Superplastic behavior of a β-forged Ti 3 Al-based alloy was investigated by compression tests and tensile tests and the microstructural evolution during the deformation process was studied. The results show that the true strain of the material reached 2.0 without surface cracking at 980°C and 2×10 −3 s −1 in a compression test, while the elongation was only about 100% at 980°C and 10 −4 s −1 . In the primary stage of superplastic deformation, the separation of lath-like O grains occurred. Newly formed equiaxed O grains slid and rotated in B2 phases and dislocation movements in B2 phase accommodated the sliding process.