To find out the reason of poor secondary recrystallization behavior in thin-gauged grain-oriented steel, EBSD technique is applied to reveal the grain growth behavior of thin-gauged steel processed by HiB steel method under high cold-rolling reduction. Nitriding treatment with different times is conducted to ensure the occurrence of secondary recrystallization in thin-gauged grain-oriented steel and to determine the effect of nitrogen content. Attention is put on the influence of 114 418 texture on the abnormal growth of Brass and Goss grains. Results show that, at initial stage of secondary recrystallization, 114 418 grains in the surface region of sheets possess obvious growth advantage than the other oriented grains. If these grains in the surface region grew to the central layer of sheet and swallowed the nucleus of secondary recrystallization, abnormal growth could not occur. In contrast, reinforcing the inhibitors at surface region of sheet by nitriding treatment will avoid the excessive growth of surface grains and therefore improve magnetic properties of steel significantly. The {114}<418> grains are adverse to the abnormal growth of Brass-oriented and scattered Goss grains in way of island grains, but their effect on the abnormal growth of Goss grains is weaker.

Based on our earlier preliminary work, a model was developed for prediction of the critical grain size where the plasticity would be decreased as the grain refined. In the model the effect of grain size on the fracture strength was combined. The prediction of the model exhibited that in the range of grain size of 10 mm to 0.2 mm as an example, the total elongation of the steels would be firstly increased. But when the grain size was refined to 2.5 mm and below, the total elongation of the steels was not increased but decreased sharply, which was good agreement with the experimental results published recently. Present work illustrated that the dominant mechanism of the elongation decreased in the ultra-fine grain size materials is due to increase in resistance force of grain boundaries on the dislocation sources resulting in the difficulty of activation of dislocation movements. Its expression would be the decrease of the plastic strain in macro-level.

High strength low-alloy (HSLA) steel has been widely used in buildings, bridges, ships and automobiles because of the remarkable high strength and forming property. Conventional HSLA steels are strengthened by a combination of grain refinement, solid-solution strengthening and precipitation hardening, and the contribution of precipitation hardening is considered to be minor, since many of the alloying elements are added to HSLA steels in the past basically for the strengthening of grain refinement. However, in recent research, yield strengths up to 780 MPa have been achieved in Ti and Mo bearing HSLA sheet steels by producing microstructures that consist of a ferritic matrix with nanometer-sized carbides, and the precipitation strengthening has been estimated to be approximately 300 MPa. Nowadays, thermo mechanical controll process (TMCP) is widely used to process HSLA steels, the final temperature of ultra-fast cooling (UFC) plays a decisive role for microstructure evolution and precipitation behavior, and finally determines the mechanical properties of the steels. In this work, the effects of final temperature after UFC on microstructural evolution, precipitation behavior and micro-hardness of Nb-V-Ti bearing low alloy steel were studied by using the thermal mechanical simulator, OM, HRTEM and micro-hardness instrument. The results showed that the microstructure and nucleation sites of micro-alloy carbides changed with final temperature after UFC. The microstructure changed from bainite to pearlite and ferrite and the nucleation sites changed from bainite to ferrite with final cooling temperature increasing. The number density of the precipitates in ferrite matrix was greater than that in bainite. Furthermore, the number density of the nanometer sized carbides got the maximum values at 620 ℃. The aspect ratios of the precipitates were close to 1, which meat that the precipitation morphology close to spherical. The sizes of the carbides were all less than 10 nm and became smaller with the decrease of final cooling temperature. Through the calculation by Orowan mechanism, the contributions of the precipitation strengthening to yield strength could reach 25.6% at the final cooling temperature of 620 ℃.

In order to increase the magnetic properties and realize the essential applications in magnetic recording and spintronics devices, it is significant to control the growth mode and grain size of Fe films. In this work, the effects of a high magnetic field (HMF) on the growth and magnetic properties of Fe thin films with different grain sizes by using physics vapor deposition were explored. The decreased grain sizes are obtained by increasing the evaporation source temperatures. It is found that when the evaporation source temperature is 1440 ℃, the grains of film are fine. The growth mode is changed from layered to columnar by HMF. And HMF effectively reduces the defects of Fe film. When the evaporation source temperature is 1400 and 1350 ℃, the grains of films are large. HMF does not change the columnar growth mode of films. However, the width of columns is improved by a HMF. Additionally, HMF increases the average particle (composed of the grains) and grain size of Fe films with different grain sizes. And the surface roughness of all the films is remarkably reduced by a HMF. With the decrease of grain sizes, the ability of HMF on increasing the coercivity, saturation magnetization and squareness ratio of the Fe films is enhanced.

Significant efforts on development of advanced ultra-supercritical (A-USC) fossil fired power plants with steam conditions of 700 ℃ and 30 MPa or higher have been made in recent years. The most important consideration is the development of materials for superheater and reheater tubes with working temperature as high as 760 ℃. During the design and application of these materials, phase stability, creep rupture strength and corrosion performance at 700~760 ℃ should be evaluated. A new type Ni-Cr-W-Fe alloy has been designed for A-USC power plants and the microstructure and mechanical properties of Ni-Cr-W-Fe alloy after long-term aging at 760 ℃ was investigated using OM, SEM, TEM and tensile testing in this work. The fractographs of tensile samples were observed. The results show that the average gain size of specimen after solution-annealing at 1100 ℃ is about 80 μm with twin planes present in the matrix. The major precipitates after aging at 760 ℃ for 16 h are M₂₃C₆ and g'. The average particle size and the volume fraction of g' phase are approximately 29 nm and 19%, respectively. The coarsening behavior of g' during long-term aging at 760 ℃ follows Ostwald ripening theory. The solution-annealed Ni-Cr-W-Fe alloy performs excellent ductility at room temperature and the fracture mode of is ductile. The room temperature tensile strengths increase obviously with the decreasing of elongation and reduction of area after aging treatment. The yield strengths at both room and elevated temperatures decrease gradually with the extending aging time at 760 ℃. The tensile ductility at room temperature of Ni-Cr-W-Fe alloy decreases after aging from 1000 to 3000 h, while the elevated temperature ductility varies mildly and keeps at approximately 15%.

This paper reviews the research works of effects of elements on the microstructure and mechanical properties of Nb-Si alloys conducted by the authors in recent years, including effect of Hf on Ni-16Si, Hf and Sn on Ni-20Ti-5Cr-3Al-18Si, Zr on Ni-22Ti-16Si, Ta on Nb-22Ti-16Si-7Cr-3Al, and rare earth elements Dy and Ho on Ni-23Ti-10Ta-2Cr-18Si and Ni-22Ti-16Si-7Cr-3Al-3Ta-2Hf, respectively. The addition of elements Hf, Zr, Sn+Hf, Ta, Dy and Ho in Nb-Si binary system and multi-component system enhances room and high temperature strength, plasticity and fracture toughness obviously. The enhancement of strength is related with the solution strengthening of elements, and the improvement of plasticity and ductility is related with the fining of microstructure and the increase of particles which exceed the critical size of (Nb, Ti)ss.

NiAl base eutectic alloy is an attractive material and promising to use in high temperature environment. However, the inadequate high temperature strength limits its application. In order to improve its strength, Zr was added in the Ti and Hf doped NiAl/Cr(Mo) base eutectic alloys and the effect of Zr addition on microstructure and mechanical properties of the eutectic alloy was investigated in this work. The results show that small addition of Zr can refine the NiAl/Cr(Mo) lamella inside the eutectic cell and optimize NiAl and Cr(Mo) phase morphology in the intercellular zone. Moreover, the Zr addition promotes the precipitation of bulk Heusler phase along eutectic cell boundary. With the increase of Zr addition, the eutectic cell of the alloys becomes fine, but the NiAl and Cr(Mo) phases in the intercellular zone become coarse and the Heusler phases exhibits semi-continuously distribution along the eutectic cell boundary. When the Zr content increases to 1% (atomic fraction), the NiAl and Cr(Mo) phases in eutectic cell and intercellular zone are all coarsened obviously. Additionally, coarse Cr-rich phases precipitate in the intercellular zone and Heusler phase forms the continuous network along eutectic cell boundary. The addition of Zr promotes the precipitation of coarse b-NiAl and a-Cr phase in Cr(Mo) phase and NiAl phases, respectively. Moreover, the segregation of Heusler phase forming elements along the precipitate interface leads to the formation of a large number of interfacial dislocations. In addition, the addition of Zr results in the precipitation of fine Heusler particles in NiAl phase. It is shown that appropriate addition of Zr can improve the compression strength of Ni-33Al-28Cr-5.5Mo-1.0Ti-0.3Hf eutectic alloys significantly at room temperature and high temperature without reducing its compression plasticity, but more addition of Zr reduces the compressive plastic of the alloy inevitably.

GH4169 superalloy is one kind of important metallic materials used for manufacturing turbine discs in aero-engine. In order to meet the demand of higher strength, high ratio alloying elements have to be added, resulting in the complex microstructure evolution during the long-term service at elevated temperature. Furthermore, the turbine disc usually bears overloading which will lead to the low cycle fatigue (LCF) damage in real working and result in fatal security problem. Besides, it is meaningful to decide the relationship between the microstructure evolution and performance degradation. In the present work, microstructure evolution and LCF behavior of GH4169 alloy during long-term aging were investigated. The microstructure evolutions of GH4169 alloy during long-term aging at 750 ℃ for 500, 1000, 1500 and 2000 h and the influences of long-term aging on the LCF behavior were investigated. The results show that the size of g″ phases increases and the volume fraction decreases with the increase of aging time, compared with the increase of both size and volume fraction of d phases. Both the fatigue strength and fatigue life of the alloy decrease with the increase of aging time. For the specimen aged for the same time, the cyclic stress firstly contributes to cyclic hardening, then cyclic stability, and finally cyclic softening with the increase of cyclic numbers. It is found that the decrease of cyclic stress contribution is slightly effected by the size of g″ phases increase and volume fraction decrease after long-term aging. Therefore, the LCF life of the alloy decreases since the crack easily propagates along with the long needle-like d phases and the g″ phases precipitate free zones.

The researches on the grain refinement by applied pulsed magnetic field (PMF) during solidification have received much attention in recent years and lots of positive experimental results indicate that it is a potential method for controlling solidification process. Various grain refinement mechanisms under PMF are proposed and most of them are considered to be relevant to the convection of melt driven by the electromagnetic force. An obvious fact is that the forced convection caused by PMF is strongly limited by the shape of the melt. However, most of previous studies were focused on the cylindrical samples rather than rectangular ones, and actually the later one was widely used in industry. The aim of this work is to investigate the influence of PMF on the grain refinement of K4169 superalloy rectangular samples with various aspect ratios. Grain refinement of K4169 superalloy under PMF was experimentally investigated in the rectangular samples with the aspect ratios of 1.0, 2.0, 4.5 and 5.5 on the transverse section. In order to study the influence of aspect ratio on the forced convection, the distributions of the electromagnetic field, electromagnetic force and melt flow caused by PMF were numerically simulated by finite element software ANSYS. The experimental results show that the grains of the K4169 rectangular samples are coarse equaxied grains without PMF and the grain size slightly decreases with the increase of aspect ratio . Under the PMF with same excitation voltage and frequency, the grains are refined remarkably in the sample with the aspect ratio of 1.0. As the aspect ratio is increased, the grain refinement effect can still be observed but not such obvious. The numerical simulation results indicate that the periodic pushing-pulling electromagnetic force is induced by the PMF, which drives the melt to vibrate and flow circularly. Under the same PMF, the electromagnetic force and fluid rate decreases with the increase of aspect ratio. When the aspect ratio increases from 1.0 to 5.5, the average electromagnetic force and fluid rate in the melt is reduced to 40% and 60%, respectively. The strongest fluid flow and vibration occur in the sample with section aspect ratio 1.0 in the present experiment, which is beneficial for grain refinement due to detachment of the solidified nuclei from mould wall and the break of dendrite arms from dendrite trunks.

Superalloy components are always produced by the way of investment casing. During investment casting, interfacial reactions may take place and bring about metal contamination and defect formation on the surface of the components. The influence of C content on the interfacial reaction and wettability between a Ni-based superalloy and ceramic mould was investigated by using a sessile drop method. The interfacial morphology and elements distribution were studied by SEM and EPMA. Activities of C, Cr and Al were calculated by using Thermo-Calc software. The relationship between interfacial reaction and wettability was discussed. It was found that when C content was higher than 0.1%, activity of C increased greatly and interfacial reaction took place. The wettability varied from non-reactive wetting to reactive wetting. In the reactive wetting systems, sand adhesions appeared and Al and Cr diffused to the ceramic surface.

High Nb-TiAl alloys, which being regarded as a new generation TiAl alloy, had attracted more and more attention for their higher operating temperature and better oxidation resistance than conventional TiAl alloys. It was found that silicide particles in high Nb-TiAl alloys were Nb₅Si₃ rather than Ti₅Si₃ precipitated in TiAl alloys. In this work, the effect of Nb₅Si₃ phase on the microstructure and room-temperature tensile properties of high Nb-TiAl alloy was studied. The experimental results showed that the precipitation temperature of silicide was between 1000~1200 ℃. Precipitates located in the colony boundary, b(B2) segregation and between g/a₂ lamella. The tensile properties of as-cast alloy with Si addition increased. Because the formation of Nb₅Si₃ precipitates resulted in the reduction of Nb content, which was one of b(B2) phase stable elements. Therefore, the volume fraction of b(B2) phase obviously decreased due to Si addition. However, after heat treatments, the tensile properties of Si containing high Nb-TiAl alloy gradually reduced with the increasing of heat treatment temperature. Silicide particles which precipitated along lamella leaded to generation and propagation of cracks. Moreover, silicide particles further precipitated due to tensile stress which increased the rate of crack propagation. Si addition leaded to g phase area expanded. g single-phase region formed between 1280~1300 ℃. Silicide precipitated in colony boundary resulted in bulk g+b(B2) phases, which weaken the grain boundaries. While silicide precipitated in lamella leaded to formation of secondary g lath which split the initial lamella microstructure.

The mechanical properties of materials are related to the integrity of interfaces (phase and grain boundaries). For substitutional alloys, the Kirkendall voids tend to form more easily at the phase boundary or grain boundary when the atomic mobilities of different species are unequal, which will degrade the bounding quality of interfaces. So far, there have many experimental studies on the evolution of Kirkendall voids and the formation mechanism. However, allowing for the fast process of the Kirkendall voids from formation to evolution, it is hard to capture such process in real experimental conditionals. So the formation and evolution mechanism of the Kirkendall void need to be studied. A binary phase field crystal model was used to simulate the process of void formation and expansion at phase boundaries induced by the Kirkendall effect. Simulated results show that for the low misorientation phase boundary (PB), the void moves toward the side with large atomic mobility (a phase) and the void shape evolves from the initial parallelogram to hexagon. The atomic annihilation rate around a void is faster than that of growth rate, which results in void expansion. The PB migration, phase growth and shrinkage can also be observed in void expansion. For the large misorientation PB, voids can also expand along the PB direction, resulting in the connection of voids, therefore, the PB is separated and presents zigzag shape. In the interdiffusion system, the free energy decreases. The descending speed of the free energy is almost equal for the low misorientation PB while is increasing for the large misorientation PB when the atomic mobility difference becomes larger. The descending speed of the free energy is proportional to PB misorientations. The PB void predicted from our computer simulation is consistent with the experiment observation.

With the development of electronic products towards further miniaturization, multifunction and high-reliability, the packaging density has been increasing and the dimension of solder joints has been scaling down. In electronic packaging, during the soldering process of Sn/Cu system, an intermetallic compound (IMC) layer is formed at the interface between the molten solder and pad (substrate), the interfacial microstructure plays an important role in the reliability of solder interconnects. Generally, during the reflow soldering and subsequent aging process, a large number of Kirkendall voids may form at the Cu/Cu₃Sn interface and in the Cu₃Sn layer. The existence of Kirkendall voids may increase the potential for brittle interfacial fracture of solder interconnects and reduce the thermal conductivity. Thus, characterization of formation and growth of Kirkendall voids is very important for the evaluation of performance and reliability of solder interconnects. In this work, the formation and growth of Kirkendall voids at the Cu/Cu₃Sn interface and in the Cu₃Sn layer of Sn/Cu solder system have been investigated by means of phase field crystal modeling. The growth mechanism of Kirkendall voids was analyzed. The effects of thickness of Cu₃Sn layer and impurity particles in the Cu₃Sn layer on the growth of Kirkendall voids were discussed. Phase field simulation results show that the growth of Kirkendall voids exhibits four stages during the thermal aging, including the formation of atomic mismatch areas at the Cu/Cu₃Sn interface, the rapid growth of the atomic mismatch areas leading to the formation of Kirkendall voids, the growth of Kirkendall voids and the subsequent coalescence of Kirkendall voids. Kirkendall voids nucleate preferentially at the Cu/Cu₃Sn interface and their sizes increase with the aging time, and the coalescence of the voids can be observed obviously in the later stage of thermal aging. It has also been shown that the increase of the Cu₃Sn layer thickness and the amount of impurity particles lead to an increase in both number and size of Kirkendall voids, as well as an increased growth exponent; and the number of Kirkendall voids increases initially and then decreases with the aging time.

In this work, the hot rolling process of SiCp/2009Al composites is simulated using the fully coupled thermal-stress analysis in Abaqus/Explicit. By the investigation of formation process for rolling along with different fields of temperature, strain rate, strain and stress and their evolutionary history, the hot rolling mechanisms under complicated stress states is achieved. The results show that the maximum principal stress changes from compressive stress to tensile stress at the stage of rolling entrance and a reverse trend replaces it at the exit, and that the compressive stress is dominant in the deformation zone at the steady rolling stage. The temperature drop effect due to heat transfer is far greater than the temperature rise effect due to friction on the plate surface while the temperature rise is embodied in the center due to plastic deformation. Besides, the effect of strain rate on flow stress plays a leading role at the entrance and exit stage, and the flow stress on the plate surface in the deformation region is mainly determined by strain and temperature except the stick zone which is controlled by strain rate, however, the center flow stress in deformation is mainly affected by temperature.