Si-containing transformation induced plasticity (TRIP) steel is noted for good balance of excellent formability and high strength as the advanced high strength steel (AHSS). The advantage of this steel can be attributed to the TRIP effect, which is the transformation of the retained austenite. Furthermore, the local increase in specific volume caused by the TRIP effect can help to close propagating cracks. It is favorable for the automotive structural components based on the high work hardening rate and energy absorption behavior. Low Si-containing can optimize the galvanized performance of the cold rolling TRIP steel, and the ferrite stabilization can be compensated by adding Al. Microalloying with Nb and Ti may provide effective means for further strengthening via grain refinement and precipitation strengthening. The ultra-fast continuous annealing comprised of rapid heating and short austempering is a new-style process for grain refinement and precipitation solidifying. However, the influences of the process on the cold rolling low Si TRIP steel, especially the austenite transformation characteristics and their effects on microstructure and mechanical properties, were rarely reported. Therefore, in this work the microstructures of low Si grade Nb-Ti microalloying TRIP steel under different ultra-fast continuous annealing conditions were observed via EBSD and TEM, and the tensile properties were discussed. The results show that the polygonal ferrite is refined by heating rate of 100 ℃/s and short asutempering procedure. The dispersive and fine microalloyed carbonitrides formed during the hot-rolling stage are reserved. Therefore, the strength and ductility are enhanced simultaneously. The slow cooling procedure can effectively contribute to eliminate the yield point, while the strength is slightly decreased. As the annealing temperature increasing, the strength is enhanced. When the annealing temperature is 830 ℃, the morphology of retained austenite consists of alternated film and bainite-ferrite plates, resulting in optimal combination of strength and ductility: tensile strength 748 MPa, yield strength 408 MPa, uniform elongation 21.3%, work hardening exponent 0.27, balance of strength and ductility is 15932.4 MPa·%.
The microstructure, mechanical properties and precipitation behaviors in a low carbon V-Ti microalloyed steel were investigated using thermal simulation. The microstructural characteristics of tested steel were analyzed using OM and TEM. The results show that the larger volume fraction of ferrite can be obtained for different isothermal temperatures. The ferrite volume fraction is increased and ferrite grain size is reduced as the isothermal temperature is lowered. The planar interphase precipitation can be observed for different isothermal temperatures, and both sheet spacing and precipitates size are refined by lowering isothermal temperature. Moreover, the nanometer-sized carbides have a NaCl-type crystal structure with a lattice parameter of about 0.436 nm and they can obey one variant of Baker-Nutting (B-N) orientation relationship of (100)carbide//(100)ferrite and carbide//ferrite. The precipitation hardening for the specimen treated at 680 ℃ for 30 min can reach 360.6 MPa.
The C content in high strength steel must be controlled at a lower level for the good weldability. However, the lower level of C content will reduce the C partitioning efficiency and influence the stability of retained austenite, which leads to the decrease of the product of tensile strength and elongation of high strength steel. A novel preparation mechanism of high strength steel is to employ some kind of substitutional alloying elements, for example Mn, instead of C to partitioning to enhance the austenitic stability, which would not remarkably reduce the weldability of the steel. One low alloy C-Si-Mn steel was used in present work. The Mn partitioning behavior and its effect on the stability of the retained austenite and the mechanical property were studied by means of intercritical annealing, subsequent austenitizing, then quenching and partitioning process (I&Q&P). The results show that in the process of intercritical annealing at 760 ℃, by extending the annealing time, austenite volume fraction increases gradually until it reaches the saturation, meanwhile the Mn partitioning behavior occurs and Mn content increases gradually from ferrite to austenite until it reaches the chemical potential balance in two phases. The sample is heated to 930 ℃ for 120 s, then rapidly quenching to 220 ℃, the carbon diffuses from martensite to austenite phase in the process of partitioning. After I&Q&P process, the tensile strength of experimental steel is 1310 MPa, elongation up to 12%, the product of strength and elongation up to more than 15000 MPa.%. The steel only contains a small amount of retained austenite by only C partitioning after traditional Q&P process, while the steel contains more Mn-rich retained austenite after I&Q&P process. Hence, the content and stability of retained austenite of steel can be improved significantly, which enhance the formability at room temperature.
Over the past years, TiSiN coatings have gained increasing importance in the field of cutting tool coatings due to its enhanced hardness and superior oxidation resistance properties produced by the nanocomposite microstructure of TiN nanocrystals embedded in an amorphous Si3N4 matrix. Many methods have been developed to prepare TiSiN coatings, typically named by the DC magnetron sputtering (DCMS) technique and cathodic arc ion plating (AIP), whereas limited studies have been carried out on the deposition of nanocomposite coatings using the high power impulse magnetron sputtering (HIPIMS) approach. The TiSiN coatings were reactively magnetron sputtered in mixed Ar/N2 precursor gases in a new HIPIMS system with different flow rate of N2 in this work. The deposition rate, crystal structure, composition, surface morphology, microstructure and mechanical properties were investigated systematically by surface profilometer, XRD, XPS, SPM, SEM, HRTEM and nano-indentation and the plasma discharge also was studied. The results show that increasing the flow rate of N2 caused the decrease of deposition rate as expected, accompanying with the change of preferred orientation from (200) orientation to (220) orientation and the decreased compactness, discharge degree and ionization rate. Contrary to the changes of Ti content, Si content gradually increased with increasing the flow rate of N2, but their changing scale were small. Combined with XRD and XPS analysis, the results indicated that the coatings were composed of crystalline TiN, amorphous Si3N4 and free Si. Besides, free Si disappeared with further increasing the flow rate of N2. This nanocomposite structure can ultimately be assessed by HRTEM where individual grains and the amorphous regions can be distinguished. In addition, the grain size increased gradually with increasing the flow rate of N2. Furthermore, both the hardness and elastic modulus linearly decreased with increasing the flow rate of N2 .
Coaxial powder feeding laser alloying of the Stellite 12-B4C mixed powders on an aviation material TA15-2 titanium alloy substrate can form a wear resistance composite coating. Investigation indicated that the Cu addition promoted a great quantity of the ultrafine nanoscale polycrystals and amorphous phases to be produced in such coating, leading to an improvement of wear resistance. The nanocrystallization process of Cu on laser clad coatings, i.e. and the process of the productions of the nanoscale polycrystals, and also the AlCu2Ti ultrafine nanocrystals which were produced through in situ chemical reaction in laser molten pool retarded greatly growth of the particles. The coating with Cu mainly consisted of γ-Co, M12C, M23C6, W-C, Ti-B, AlCu2Ti and also the amorphous phases. AlCu2Ti ultrafine nanocrystals owned the high diffusibility in such high temperature molten pool, causing the lattice distortions, which also played an important amorphization effect on such coating.
FeCoCrAlCu laser high entropy alloying coating has been synthetized by high power semiconductor laser alloying of equimolar ratio of Co, Cr, Al, Cu four elements mixture powder on the surface of single-element Fe base alloy Q235 steel. The microstructure, constituent phases, composition distribution and mechanical properties of FeCoCrAlCu laser high entropy alloying coating were investigated by SEM, XRD, EDS and microhardness tester. Experimental results show that the principal element of Fe in the single-element base alloy Q235 substrate participates surface alloying process during the laser irradiation, forming FeCoCrAlCu five principal high entropy alloy coating. The alloying coating is composed of simple bcc solid solution and the microstructure is mainly composed of typical dendritic structure. Intermediate phase σ with tetragonal structure merely appears near the interface between laser alloying coating and substrate. From the surface of high entropy alloying coating to substrate, it presents the gradual distribution of the mixing entropy from high entropy, medium entropy to low entropy. the microhardness of FeCoCrAlCu laser high entropy alloying coating reaches 8.3 GPa, which is three times as much as that of the Q235 substrate.
When a homogeneous, single-phase liquid of monotectic alloy is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Many efforts have been made to use the demixing phenomenon for the production of well dispersed composite materials. It has been demonstrated that the rapid directional solidification technique is an effective method to prevent the formation of the phase segregated microstructure in immiscible alloys. Directional solidification experiments were carried out to study the influence of the addition of Sn on the solidification process of Al-Pb alloys. The experimental results show that the addition of a small amount of Sn causes a decrease in the interface energy between the matrix and the minority phase liquids and, thus, an increase in the nucleation rate of the minority phase droplets during the liquid-liquid phase transformation or a decrease in the average size of the minority phase particles. With the increase of the Sn content, both the volume fraction of the minority phase droplets and the temperature range of the liquid-liquid phase zone and the liquid-liquid-solid tertiary phase zone of the phase diagram increase. These are favorable for the coarsening of the minority phase droplets. The addition of Sn leads to the formation of a dendrite solid/liquid interface. This may promote the formation of a well dispersed microstructure and shows great effect on the distribution of the minority phase particles.
Semi-solid billet of ZCuSn10 alloy is prepared by strain induced melt activation (SIMA) method which included the rolling and remelting process. Firstly, ZCuSn10 alloy is cast, and samples are cut from ingot casting. Secondly, the samples are rolled with 2~4 passes after holding at 450 ℃ for 15 min, then the new samples are cut from deformed alloy. Lastly, the new samples are reheated up to 850 ℃ or 875 ℃ for 15 min, then water quenching. Semi-solid microstructure is observed and compared with microstructure of ZCuSn10 alloy directly reheated after casting. The distribution of Sn element in microstructure under different conditions is measured by using EDS function of SEM, and the microstructure changes during the SIMA process are observed by means of OM and TEM. Based on the experiments, the microstructure evolution is synthetically analyzed and explained during the course of semi-solid billet of ZCuSn10 alloy prepared by SIMA method. The results indicate that semi-solid microstructure of ZCuSn10 alloy by rolling- remelting SIMA process is equal-fine grain, and spheroidization of solid particle is well. The optimum semi-solid microstructure is obtained when alloy deformed 19.7% is remelted at 875 ℃ for 15 min, the average grain diameter is 75.8 μm, shape factor is 1.62, and volume fraction of liquid phase is 17.28%. Deformation process plays a crucial role in grain refinement and spheroidization during SIMA process for preparing the semi-solid billet of ZCuSn10 alloy, as deformation and remelting temperature increases, the size and shape of solid phase in semi solid microstructure are smaller and more round, volume fraction of liquid phase increases. The main mechanism of SIMA process preparing semi-solid billet of ZCuSn10 alloy is that predeformation breaks dendrites and stores energy of deformation into dendrites, and promotes dendrites melting through remelting process. Meanwhile, liquid phase occupies sharp corners of solid particles by Sn element diffusing from liquid phase into α solid phase, so that fine, uniform and roundness α solid particles are gained.
As a type of structure functional high temperature alloy, the ignition resistance performance of fireproof titanium alloy is an important basis for the safety in the application. In this work, the relationship between the friction contact pressure P and oxygen concentration c0 of mixed airflow was established to describe the ignition resistance of fireproof titanium alloys. The ignition resistance of the traditional titanium alloys and typical Ti-V-Cr type titanium alloys was investigated and compared. Based on the principle of friction-induced heat and the thermal explosion theory of ignition, the mechanism of the ignition resistance of fireproof titanium alloys was modeled, calculated and analyzed. The results showed that Ti40 was ignited immediately at room temperature as c0≥70%. The ignition resistance of Ti40 was 2.5% lower than that of Alloy C+ and 40% higher than that of TC4. The ignition originated from the micro-tip formed during friction and the chemical adsorption of oxygen on the micro-tip was the key step for the interaction. With increasing of equivalent pressure Peq, the critical temperature T * ignited by friction decreasd. When Peq varied from 0.1 to 0.5 MPa, T * of Ti40 ranged from 1073 to 1323 K. The surface under friction was 2~5 μm and composed of the fusion of the oxides including TiO2, V2O5 and Cr2O3. The lubrication condition between the contacting surfaces was improved by the fused layer and resulted in great temperature decrease in the friction area. Consequently, the ignition resistance of fireproof titanium alloys was improved.
During friction stir welding, the nugget zone (NZ) underwent severe plastic deformation and high temperature. This process resulted in high density of dislocations and dissolving of the precipitation. In this study, the stored energy of the NZ in friction stir welded Al-Mg-Si joint was quantitatively analyzed by means of differential scanning calorimetry (DSC). The microstructure of the NZ was investigated by electron back scattering diffraction (EBSD) and transmission electron microscope (TEM). DSC analysis showed that the energy stored in the NZ was about 8.565 J/g. Microstructure investigation showed that the NZ was composed of low-angle grain boundary (42%) and high-angle grain boundary (58%). Meanwhile, there were high density dislocations in the NZ. The stored energy was quantitatively analyzed based on EBSD data and dislocation density. The results showed that the stored energy resulting from the grain boundary and dislocations was about 0.0247 J/g and 0.0712 J/g, respectively. These results proved that the precipitation played dominant role in stored energy while the contribution of grain boundary and dislocations are negligible.
By the investigation of the microstructures of Mg82.13Zn13.85Y4.02 (mass fraction, %) alloy solidified under different pressure, it is found that the solidification microstructure of the alloy is consisted of a-Mg matrix, W-Mg3Y2Zn3 phase and I-Mg3YZn6 phase. In the microstructure of the alloy solidified under ambient pressure, the networks of the secondary phases of eutectic-like and rod-like shape are distributed in the a-Mg interdendritic space. As the solidification pressure increases, eutectic network is disconnected gradually and the amount of eutectic is diminished and the solubility of Zn in a-Mg rises gradually. The results of the measurement of the mechanical properties show that the compression strength, the yield strength and the compressibility of the alloy sample solidified under ambient pressure is 344 MPa, 331 MPa and 16% respectively. However, the compression strength, the yield strength and the compressibility of the alloy sample solidified from 1250 ℃ under 6 GPa is 455 MPa, 426 MPa and 25%, respectively. The observation of fracture surfaces shows that, in the alloy solidified under high pressure, the cleavage surface of the compressed fracture is decreased, and the tear ridge and tearing dimple can be found. The degree of cleavage fracture is decreased.
The solidification behavior and microstructure evolution of sand cast Mg-6Al-xZn alloy (named as AZ6x alloys, x=0, 2, 4, 6, mass fraction, %) were characterized by two-thermocouple thermal analysis technology and SEM. The grain sizes of the alloys were quantitatively determined by EBSD technology. Thermodynamic calculations were applied in Pandat software for phase diagram calculation, Scheil model solidification simulation and growth restriction factor values (GRF or values). The results show that solidification of AZ6x alloys follows non-equilibrium solidification paths. Besides the γ-Mg17Al12 phase, which is the only secondary phase in AZ60 alloy, another Φ-Mg21(Al, Zn)17 phase appears in the as-cast microstructure of AZ62 to AZ66 alloys. With the increase of the Zn content, the amount of γ-Mg17Al12 phase decreases and while increase the amount of Φ-Mg21(Al, Zn)17 phase. Calculated equilibrium phase diagram shows that in the AZ60~AZ64 alloys both γ-Mg17Al12 phase and Φ-Mg21(Al, Zn)17 phase can be dissolved into α-Mg under proper heat treatment conditions. However, Φ-Mg21(Al, Zn)17 phase in AZ66 alloy can not be completely dissolved into a-Mg for any temperature. The results also indicate that higher Zn content alloys have higher values and smaller grain size, and lower solid fraction at dendrite coherency point (?sDCP). The relationship of values, grain size and ?sDCP has been also discussed.
Effects of rare earth (RE) on the morphologies of the recalescence interface, the growing primary Si during the solidification process and the structure after solidification of Al-80%Si alloy were investigated by means of high speed camera and SEM. The critical undercooling ?T1 and ?T2 for the morphology transition of the recalescence interface, the growing primary Si and the structure have been obtained. When the undercooling is lower than ?T1, the morphology of the growing crystal during the solidification process is flake-like; and the structure after the solidification process is composed of large flake grains with pronounced edges and faces. When the undercooling is greater than ?T2, the recalescence interface is a parallel one, and the structure after solidification is composed of homogenous and fine grains, and there exist several smooth spherical bulges on the surface of each grain. In the undercooling region from ?T1 to ?T2 , the recalescence interface and the growing crystal show dendritic features, but some of the dendrites are distributed regularly; after solidification, the structure is composed of refined equiaxed grains and flake grains. For Al-80%Si alloy, ?T1 and ?T2 are equal to 132 and 250 K, respectively. RE can reduce the values of ?T1 and ?T2 . When 1%RE is added into the alloy,?T1 and ?T2 are changed to 60 and 199 K, respectively.
The partially transient liquid phase-diffusion bonding (PTLP-DB) on Ti(C, N)-Al2O3 ceramic matrix composites (CMC) was studied using the Zr foil/Cu foil/Zr foil sanwich as an interlayer. Effect of holding time during PTLP-DB on the element diffusion and reacted products at the interface was analyzed and the affected factors on the joint strength during PTLP-DB were explored. The effect of auxiliary pulse current during PTLP-DB between CMC on element diffusion at the interface and joint strength and its mechanism were also studied. The results showed that the optimum holding times were 15~30 min during PTLP-DB on CMC at 950 ℃. With a shorter holding time, the joint strength decreased due to the existance of unreacted Zr elements at the interface, while with a longer holding time, the joint strength decreased due to the overgrowth of CuZr intermetallics at the interface. With the auxiliary pulse current during PTLP-DB, the residual stress at the interface was greatly decreased, which inhibited the propagation of crack into the base materials. While, the auxiliary pulse current can promote the reaction between Cu and Zr and the formation of Zr-Cu intermetallics at the interface, which produced a weaked interface in the joints.
Austenite stainless steels such as SUS304, owing to their good combination of mechanical properties, corrosion resistance and weldability, are widely used in a variety of industries. In the simulation of welding residual stress of an austenite stainless steel joint, because of the high strain hardening rate and the heating-cooling thermal cycles, both the work hardening phenomenon and the annealing effect have to be taken into account in the material constitutive relations. Though a number of numerical models have included the work hardening by using isotropic rule, kinematic rule or mixed rule, limited models have dealt with the annealing effect. For the steels or alloys with high strain hardening coefficient, neglecting the annealing effect will overestimate the welding residual stresses to a large extent. In this study, the thermal elastic plastic finite element method (T-E-P FEM) was used to simulate welding temperature and residual stresses in a SUS304 steel bead-on joint. In the computational approach based on the T-E-P FEM, a moving heat source with uniform density distribution was used to model the heat input, and a simple model was proposed to consider the annealing effect. Using the developed computational approach, the influences of work hardening and annealing effect on the welding residual stress were clarified. In addition, the effect of annealing temperature on the distribution and magnitude of welding residual stress in the weld zone and its vicinity was examined. The simulated results show that annealing effect has a significant influence on the longitudinal residual stress, and the peak value of longitudinal tensile stress increases with annealing temperature. The longitudinal tensile stresses in the fusion zone and its vicinity also increase with annealing temperature. It seems that the annealing temperature has insignificant influence on the transverse residual stresses. Comparing the simulated results and the measured data, it was found that when the annealing temperature was assumed to be 1000 ℃ for SUS304 steel, the longitudinal residual stresses predicted by the T-E-P FEM generally match the measurements. The present work is helpful for developing more advanced materials model to calculate welding residual stress with high accuracy.
Recycling of cemented carbide scraps is drawing more and more attention to companies and countries all over the world. However, the recycling method has always been a problem where there are many factors involved. The feasibility, recycling rate, energy consumption and the environment conservation are all significant factors for the recycling method that need to be considered. In this work, using the cemented carbides scraps as the raw material, the recycled WC-16%Co (mass fraction) composite powder was synthesized by oxidation, reduction and carbonization reactions. Then the recycled composite powder was sintered to prepare the hard metal bulks by sinter-HIP (hot isostatic pressing). The results indicate that with the carbon addition increases, the content of Co6W6C in the composite powders decreases while the total carbon and free carbon increase. When the carbon addition is 16.60%, the high-performance hard metal bulks can be obtained, with a fracture toughness of 23.05 MPa·m1/2 and a transverse rupture strength of 4020 MPa. Moreover, the Co phase distributes more homogeneously in the recycled hard metals. The larger mean free path of the Co phase and the lower contiguity degree of the WC grains lead to the high performance of the recycled hard metal materials.