Dynamic compression tests on high manganese TRIP steel, pure copper, IF steel and armor steel were conducted on Hopkinson bar at the strain rate of 10³~10⁴ s^-1 to make comparisons of impact resistance and microstructural features. Results show that under dynamic compression, adiabatic shear bands (ASBs) do not occur easily on pure copper and IF steel. In addition, both pure copper and IF steel show a weak resistance to impact loading due to the poor work hardening capability. The ASB occurs quickly in armor steel containing martensite and the steel shows higher residual strength, which renders it suitable application in the condition of high speed deformation. TRIP steel consisting mainly of austenite has the highest work hardening rate and the α′-M induced by deformation can delay the ASBs formation and prevent the crack extension, manifesting that it is suitable for the use at high speed deformation. Elongated subgrains and low angle grain boundaries are found within the shear bands in pure copper and IF steel with weak microtextures, whereas the ASBs in both TRIP steel and armor steel demonstrate small equiaxed grains and high angle grain boundaries. Strong fcc shearing-type microtexture of {111}-{112}<110> and weak bcc shearing-type microtexture of {110}<111> are formed within ASBs of TRIP steel and armor steel respectively.

In order to reveal the solute existing form and its effect on the microstructure and mechanical properties in the supersaturated solid solute films, TiB_x/Al composite film with 7.1%TiB_x (atomic fraction) was synthesized by magnetron co-sputtering using Al and TiB₂ targets. The microstructure and mechanical properties of the film were characterized by 3D atom probe cambined with X-ray diffraction, transmission electron microscopy and nanoindentation. The results reveal that TiB_x compounds were largely dissolved in the Al lattice due to atomic mixing and sufficiently low mobility of the atoms during growth. The film grain was refined to nanoscale and amorphous gradually result from the severe lattice distortion. The film consisted of a two-phase structure in which lower solute content (about 2%TiB_x) nanocrystals dispersed in amorphous matrix with a higher solute content and gained high hardness. Combined with the experimental results, the microstructure formation of the composite films was discussed from thermodynamic and kinetic aspects. This study provided the direct experimental evidence of compound dissolved in highly supersaturated solid solution.

Medium carbon steel is widely used in structural steels because of its favorable strength, but lack of toughness is a limitation in industrial applications. Among the different strengthening mechanism, grain refinement is the only method to improve both strength and toughness simultaneously. The toughness of steels can be affected by micro-alloying elements and microstructures, for medium carbon wheel steel, the fracture toughness is proportional to the square root of ferrite fraction and inversely proportional to the cube root of prior austenitic grain size. In this work, Nb micro-alloying is used to improve mechanical properties of medium carbon steel. Microstructures and mechanical properties of Nb-bearing medium carbon steel were studied in contrast with traditional Nb-free steel. The continuous cooling transformation (CCT) curves of investigated steels were drawn by adopting dilatometry and metallographic method. The typical microstructures were observed by OM and SEM with EDS. The morphologies of precipitates were obtained by TEM. The effects of cooling rates on microstructure and hardness of the steel were studied with the above experimental methods. The results showed that the typical microstructure of medium carbon steel was ferrite and pearlite and the volume fraction of ferrite was increased from 4% to 24% by adding 0.06%Nb with refined microstructure. The yield strength of Nb-bearing steel was improved from 385 to 455 MPa and the Charpy V-notch energy at -20 ℃ was increased from 7 to 19 J in the condition of almost no reduction in tensile strength. It is because of Nb addition, which makes the transformation products of medium carbon steel be composed of ferrite and pearlite in a wider region of cooling rates (≤10 ℃/s) and a broader temperature range (530~690 ℃), with the hardness lower than 300 HV. With the calculation of Thermal-Calc software and solid solubility formula, Nb exists in medium carbon steel in the form of precipitate. The result of observation by TEM indicates the size of Nb precipitates was distributed in 20~50 nm. To sum up, the grain refining and precipitation strengthening are the main mechanism of Nb to promote the ferrite-pearlite transformation and improve toughness in medium carbon steel.

Low temperature gas carburizing has been established as a surface hardening process to improve the wear and fatigue resistance of austenitic stainless steel. In the gas carburizing treatment of stainless steel at low temperature, carbon diffuses into the steel to a depth up to dozens of or even hundreds of micrometers. In industrial practice, it would be useful to establish a model that can predict the carburizing depth from carburizing condition. However, to date a satisfactory model does not exist, and the classic Fick's law has been proved to be inaccurate to describe the carbon diffusion during carburation. It has been observed that the insertion of carbon can lead to the evolution of very high compressive stresses in the surface layer of stainless steel. Since it has well established that the stresses induced by diffusion of atoms can in turn affect the diffusion behavior, the high compressive stresses due to carbon diffusion during carburation are supposed to play a role in the further diffusion of carbon. In this work, 316L stainless steel block specimens were gas carburized at low temperature, and the carbon concentration, diffusion-induced stress along the depth direction of specimens were measured. Based on the coupled stress-diffusion theory, a model was built to calculate the carbon concentration and diffusion-induced stress in the specimens after carburizing. Then the calculated carbon concentration and stress distribution were compared to that obtained by above measurement. The results show as follow: (1) After carburizing, a diffusion layer containing high amounts of carbon and compressive stress was formed near the surface of specimens. The concentration and compressive stress were the maximum at the surface, and decreased with increasing depth. There was a linear relationship between the stress and carbon concentration. With increasing carburizing time, the depth and concentration level of the carbon diffusion layer increased. (2) To describe the carbon diffusion during carburation of stainless steel at low temperature, the model based on coupled stress-diffusion theory is more appropriate than the classical Fick's second law. The diffusion-induced compressive stress played an important role in diffusion of carbon. (3) The compressive stress highly increased the apparent diffusion rate of carbon. This explained the phenomenon that measured diffusion depth of carbon is much higher than that excepted from Fick's law. The interaction between diffusion and diffusion-induced stress should be considered when studying diffusion mechanism of carbon or nitride in carburizing or nitriding similar to the gas carburizing at low temperature.

Twinning-induced plasticity (TWIP) steel, having a great potential in applications in the automotive industry as a new generation of advanced steels, has attracted much attention in recent years because of the excellent combinations of strength and ductility resulting from deformation twinning. The monotonic tension behavior of TWIP steels has been extensively investigated; however, the serration behavior and low-cycle fatigue (LCF) properties have not been well understood. In order to obtain a good understanding of the mechanisms of room temperature serrated flows and the cyclic deformation behavior, the monotonic tensile deformation and fully reversed tension-compression LCF behaviors along with the deformed microstructures of an annealed TWIP steel were investigated in the present work. Both monotonic and fatigue tests were performed at room temperature with a strain rate of 6×10^-3 s^-1. The fatigue tests were conducted under total strain amplitude control with strain amplitudes ranging from 0.002 to 0.01. The tensile results show that the serrated plastic flows of stress-strain curves, presenting distinct characteristics at various strain levels, exhibit strong strain-level sensitivity. With increasing strain, the type A serrations featured by fine step-like flow are gradually replaced by the largely increased amplitude of type A serrations and their oscillation frequency decreases apparently; however, the frequency of type B serrations increases and the amplitude reduces slightly. The LCF fatigue results show that high cyclic hardening capacity is exhibited at all strain levels. At low strain amplitudes, the steel exhibits a very small initial cyclic hardening followed by a long saturation untill fracture. At medium strain amplitudes, a moderate initial cyclic hardening is followed by different degrees of cyclic softening depending on the applied strain amplitude, and then saturation untill fracture. At high strain amplitudes, the steel shows a rapid cyclic hardening quickly followed by softening till final fracture, almost without a saturation stage. Furthermore, at higher strain amplitudes, cyclic loading is found to lead to the generation of fine deformation twins in addition to high density of dislocation substructures, including dislocation walls and cell-like structures.

In the last decade, transformation induced plasticity (TRIP) aided steels and twinning induced plasticity (TWIP) aided steels have attracted a great deal of attention due to their high strength and exceptional ductility at room temperature. In the present work, a TRIP/TWIP austenite steel with the composition as Fe-20Mn-3Al-3Si has been investigated by means of surface mechanical grinding treatment (SMGT) since it is an alloy with low stacking fault energy. The e-M and/or a-M transformations can easily occur in the steel when deformed at room temperature. It is shown that a gradient phase-transformation-strengthened (PTS) surface layer can be formed on bulk Fe-20Mn-3Al-3Si steel by SMGT at room temperature, due to the martensitic transformation of γ→ε→α on its surface. The formation of martensitic phases is dependent on the strain applied and the orientation of grains. The more the {111} or the {110} planes of grains are parallel to the specimen surface plane, the easier the martensitic laths are to be formed in the gains. The total thickness of the PTS surface layer, which formed in the process of turning at room temperature, can be more than 400 μm, but its microstructure and hardness change with depth. The top PTS layer consists of nanometer sized grains while the sublayer contains a great many of martensite laths mixed with deformation twins. With increasing of the depth, the numbers of laths decrease. Correspondingly, the micro-hardness continuously decreases with the depth from 450 HV to about 220 HV (the hardness of the matrix). The formed PTS layer has a good thermodynamic stability, so that its microstructure and hardness almost do not change after annealed at 400 ℃ for 1 h.

Utilizing Jominy end quenching test, chemical phase analysis and thermo-dynamical calculation, study of the effect of alloying elements on hardenability and mechanical properties of a B-bearing ultra-heavy plate steel was carried out. The results showed that small amount of Ti addition could form TiN for its much higher bonding ability than B, fixing N element and thus making B free. Normal Al content failed to prevent BN from precipitating due to the weaker competition for N than B. However, when Al content was increased as high as 0.07%, the competition of Al for N was distinctly improved, making solid-solution B increased. For proper chemical combination of B and N-fixing element, hardenability was increased and accordingly both microstructure and mechanical properties were improved so that the quantity and size of martensite/austenite (M/A) islands and granular bainite were decreased markedly, and low-temperature impact toughness and tensile properties were improved by a large degree. The calculation was in a good accord with experimental results.

Development of high strength and high ductility rebar with obvious yield plateau is a tendency for anti-seismic requirement in building steels. In this work, the basic relationship between ferrite grain size and mechanical property, especially the elongation of yield plateau, was investigated by the combination of cold-rolling and heat treatment. After several passes of cold rolling followed by recrystallization annealing for 5~300 min at 600 ℃, ferrite with grain size between 5.2~40.4 μm was obtained in plain steel plates with a carbon content of 0.1%(mass fraction). Mechanical properties were studied by tensile test. With the decrease of ferrite grain size, not only the strength but also the elongation of yield plateau increases. Base on the scanning electron microscopy observation, the deformation becomes more homogeneous with the decrease of grain size, resulting in a higher elongation of yield plateau. A quantitative relationship between grain size d_α and the elongation of yield plateau δ_L was derived from the combination of Hollomon and Hall-Petch equations. The application range of d_α-δ_L equation was also determined, which is well consistent with the experimental results. The narrow-slit spraying equipment was used to treat the HRB400 rebar, and refined ferrite and a considerable non-equilibrium pearlite were obtained due to high cooling rate in treatment. A novel rebar with yield strength of 600 MPa grade was manufactured successfully in commerce, which also verified the above theoretical analysis.

The variation of carbides with tempering temperature in a Fe-Cr-Ni-Mo high-strength steel and their effect on mechanical properties are investigated by means of TEM and three-dimensional atom probe (3DAP). The results show that there mainly appear M₃C and M₇C₃ when tempering temperature is rather low (400 ℃), the former is thick with a length of about 1 μm and the latter is fine and the length is less than 200 nm, and M is composed of Fe, Cr and Mn. Tempering at 500 and 600 ℃, the amount of carbide increases gradually, and there appear M₂C and M₆C types of carbide which are both less than 200 nm in length, simultaneously M₃C becoming fine or disappear. When tempering temperature further increases to 650 ℃, besides M₂C there also appears MC type of carbide. The size of both M₂C and MC are less than 100 nm, meanwhile the amount of carbide decreases. The M of M₂C, M₆C and MC is a combination mainly of Cr, Mo and V. The strength of the Fe-Cr-Ni-Mo high-strength steel gradually reduces with increasing tempering temperature, but the downtrend of strength is rather small when tempering temperature is in the range of 500~600 ℃, owing to secondary hardening induced by the appearance of V-carbide. In short, the high-strength steel could obtain better combination of strength and impact toughness after tempering at about 530~600 ℃.

The crystal structures and phase compositions of Ti_0.7Zr_0.3(Cr_1-xV_x)₂ (x=0.1, 0.2, 0.3, 0.4) alloys are analyzed by the XRD and SEM. The hydrogen storage properties, activation performance, thermodynamics and high-temperature desorption process of the alloys are investigated by pressure-composition-temperature (P-C-T) and DTA-TG measurements. The results show that the Ti_0.7Zr_0.3(Cr_1-xV_x)₂ alloys contain multi-phases, i.e. C36 (P6₃/mmc) and C15 (Fd3m) Laves phases and V-based bcc solid solution phases with different lattice constants. When the content of V in the alloy is low, the alloy basically consists of C36 type of Laves phase and small amount of bcc solid solution phase. As the content of V increases, the C36 type transfers into C15 type of Laves phase, where the probability of forming third type of stacking layers (C layers) increases, and the content of the bcc solid solution also increases. The alloys in bulk can be easily activated at 2 MPa and room temperature. The x=0.1, 0.2 alloys present excellent activation performance even after exposure in air for 20 d. As V content increases, the hydrogen absorption capacity of the alloy increases whereas the plateau pressure decreases. The relative partial molar enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption for the alloys are found to be in the ranges of -7~-28 kJ/mol and -35~ -95 J/(mol·K). The DTA-TG analysis indicates that the hydrogen release from the hydrides of the alloys occur in two dissolving temperatures within the range of 500~600 K, and some residual hydrides have completely decomposed at heating temperature up to 800 K.

The dynamic-load mechanical of laser solid forming (LSF) GH4l69 Ni-based superalloy is influenced by the coarse columnar grains formed in LSF processing and then its application in jet engines like turbine disks is limited. The previous study showed that the grain structure of the LSF GH4l69 superalloy can be refined by recrystallization processing during post heat treatment. In this work, the microstructure, interface and crystal orientation of the as-deposited and heat treated LSF GH4169 superalloy samples were investigated by EBSD analysis. It was found that the crystal orientations of grains become more randomly organized as the grain structure of the LSF GH4169 superalloy was refined by heat treatment and the holding time of heat treatment was increased. Subsequently, microstructural anisotropy existed in the as-deposited samples was disappeared in the heat treated samples and the fraction of high angle grain boundary was increased greatly. < 111> 60° twin crystal boundary was presented during the later period of recrystallization and the volume fraction of the twin crystal boundary reached up to 44%. This indicated that the formation of twin crystal boundary during static recrystallization of LSF GH4169 samples played an important role in the refinement of grain structure. Sub-grain and grain-boundary migration nucleation were the main nucleation mechanisms at the beginning of the recrystallization and nucleation related to twin was one of the most important mechanisms in the later period of the recrystallization.

It is a significant foundation of superplastic free bulging analytic theory to accurately measure the surface profile and the deformation of each point on surface. Based on an important feature of the profile of the free bulging part which can be depicted as axisymmetric rotating surface, in this study, the measurement of the surface was translated into that of the feature points on the part surface, and the three-dimensional coordinates of these points were measured by the binocular stereo vision system for the first time. Then, through curve fitting, contour surface and geometric parameters were determined, and the strain were characterized through analyzing coordinates change of the feature points and distance change between two adjacent points. The setup and the measuring method of the vision system were introduced, as well as several key steps, such as system model, calibration method, elimination of influence of high temperature, and image processing. The methods were proposed for measuring geometric parameters and deformation of bulging part , and the related experiments were performed.

Influences of induction heating continuous annealing (IHCA) on the microstructure of both copper sheath and aluminum core, and intermetallic compound at the Cu/Al interface of cold-drawn copper-clad aluminum wire were investigated, compared with the traditional isothermal annealing in furnace (TIA). The results showed that recovery of both the copper sheath and aluminum core happened when the temperature of IHCA was 250 ℃. Recrystallization began to occur in the copper sheath at 300 ℃ and in the aluminum core at 330 ℃, respectively. Complete recrystallization of both the copper sheath and aluminum core took place at 430 ℃, whose average grain size were 6.0 and 7.3 μm, respectively. An intermetallic compound CuAl₂ discontinuously formed at the interface at 360 ℃, and continuous CuAl₂ layer formed at 390 ℃. Both CuAl₂ layer and Cu₉Al₄ layer formed at the interface at 430 ℃, with average thickness of 0.52 and 0.48 μm, respectively. With further raising the temperature, the grains of both copper sheath and aluminum core grew, and the thickness of the intermetallic compound layer increased slightly. The appropriate IHCA temperature of the cold-drawn copper-clad aluminum wire was 430 ℃. Compared with TIA, IHCA was able to not only refine recrystallized grain of both copper sheath and aluminum core remarkably, but also reduce the thickness of the interfacial intermetallic compound layer in the copper-clad aluminum wire.

Discontinuously reinforced aluminum matrix composites (AMCs) have been widely applied in structures of aerospace industry. Wide industrial applications of AMCs depend on effective joining methods, which are dependent on the use of a specific material and process. As a new solid-state welding technique, friction stir welding (FSW) has been attempted for joining the AMCs in last few years. However, few attentions have been paid to the effect of initial heat treatment tempers of the AMCs on the FSW joints. In this work, 6 mm thick SiC_p/2009Al composite plates in both soft (solution temper) and hard (natural aging temper) conditions were successfully friction stir welded at a rotation rate of 800 r/min and a welding speed of 100 mm/min (named as Sol-FSW and T4-FSW samples). In the nugget zone (NZ) of both samples, the grain size and the distribution of the coarse Al₂Cu phases were similar. In the heat affected zone, two low hardness zones (LHZs) were observed. LHZ I adjacent to the NZ had the lowest hardness. Both samples had the similar hardness in this zone. For the Sol-FSW sample, LHZ II far away from the NZ had a higher hardness and was closer to the NZ compared to that of the T4-FSW sample. The ultimate tensile strength of both the samples was similar and reached 83% of T4-tempered base metal. Both samples failed in LHZ I adjacent to the NZ due to the lowest hardness in this zone. This indicates that for the SiC_p/2009Al composite under solution temper it is possible to produce similar joints to that under natural aging temper using FSW technique. FSW of the composites under soft condition is beneficial to enlarging the welding process window and reducing the tool wear.

Near 50 years ago, transformation induced plasticity (TRIP) effect was proposed and TRIP steels as an advanced high strength one are widely investigated. However, the mechanism of TRIP effect can be only qualitatively explained, and has not been experimentally and theoretically verified so far. In this work, a strain equivalent model for strain-induced martensitic transformation was built in a microstructure-based finite element model of novel quenching-partitioning-tempering (Q-P-T) steel. With the model, the TRIP effect under the condition of uniaxial tension was simulated, from which the micro-mechanism of TRIP effect is revealed. Stress relaxation from TRIP relieves the stresses within untransformed retained austenite and its adjacent martensite and blocks the formation of cracks, meanwhile, a considerable retained austenite still exists at higher strain level, which is the origin of TRIP effect. Compared with original (thermal-induced) martensite, fresh (strain-induced) martensite bears higher stress. Therefore, it could be predicted that cracks form at first in fresh martensite or its boundaries. Moreover, stress relaxation makes strain-induced martensite formed in intermittent and slow way, and this is consistent with experimental results. However, in stress-free relaxation state fresh martensite appears in successive and quick way, not consistent with experiments, and thus this verifies in opposite way that TRIP effect inevitably produces stress relaxation.

With the thermodynamic analysis on directional solidification of metal-hydrogen eutectic, a theoretical model was developed to predict the effect of the Gasar processing parameters on the pore diameter and inter-pore spacing. The model was applied for the Gasar porous Cu fabricated by continuous casting process. The average pore diameter and inter-pore spacing decrease as increasing withdrawal rate. The theoretical relationship between the inter-pore spacing l and the withdrawal rate v can be described by a simple equation vl²=B, where B is a constant depending on the melt temperature and hydrogen gas pressure. The model can predict the overall tendency of the experimental results. The deviation between the calculated and experimental values in the case of lower withdrawal rate is considered to be associated with the difference between the real pore structure and ideal pore structure, and the melt convection in the vicinity of solid/liquid interface.