As an important type of wear-resistant material, the low-alloyed medium carbon wear resistant steel has been widely used in mining, power and metallurgical industries due to its low cost and excellent mechanical properties. However, the coarse as-cast microstructure tends to form in large wear resistant castings because of the long solidification time. As a result, spalling wear resulting from the preferential initiation and propagation of cracks along interdendrite will occur during service process, which severely degrades the wear resistance and service life. In this work, Ti is added to improve the mechanical properties of medium carbon Cr-Mo wear resistant steel. The precipitation behavior of TiN in the solidification process and its effect on the solidification microstructure were investigated by thermodynamic calculation, constant temperature solidification experiment at solid-liquid two phase region and continuous cooling solidification experiment by using OM, SEM, EDS and EPMA. The results show that TiN precipitation temperature gradually increases at solid-liquid two-phase region with the increase of contents of Ti and N. TiN precipitates directly in the liquid region when Ti and N contents (mass fraction) are 0.090% and 0.014%, respectively. Holding at different temperatures of solid-liquid two-phase region, a very small amount of TiN precipitates are present within the dendritic arm, and a large number of TiN precipitates are present at the interdendritic positions and frontiers of dendrites. After quenching, in the remaining liquid most of TiN are present at the boundaries of equiaxed grain and a little amount of TiN stay within the equiaxed grain. During the continuous cooling solidification, TiN precipitation temperature is the main factor affecting the refinement of solidification microstructure. With the increase of Ti content, TiN precipitation temperature increases. At the same time, the actual solidification temperature of liquid steel rises, the solidification temperature range broadens and the local solidification time extends, which results in the increase of secondary dendrite arm spacing. When Ti content exceeds 0.066%, TiN precipitation temperature is near or above the liquidus line. The actual solidification temperature of liquid steel remains unchanged. Therefore, the secondary dendrite arm spacing becomes stable.

Austempered bainitic ductile iron has been widely used in machinery components and parts due to its low fabrication cost, excellent mechanical properties, and abrasive wear resistance. In order to get a fine bainitic matrix, austempering process is usually adopted which consists of austenitizing temperature, austempering temperature and time. For quenched ductile cast iron, tempering plays an important role in subsequent heat treatment process. However, less attention has been paid on the microstructural evolution and mechanical properties of the austempered bainitic ductile iron after tempering treatment. Thus, in this work, 3.55C-1.95Si-0.36Mn-3.58Ni-0.708Cu-0.92Mo-0.65Cr (mass fraction, %) bainitic ductile iron was subjected to austempering and subsequent tempering treatment, and the effect of tempering on microstructures and properties has been investigated by using OM, EP MA, SEM, TEM and XRD. The microstructural evolution during tempering has been investigated, and mechanical properties and wear resistance have also been measured and analyzed. The results show that microstructural evolution of the bainitic ductile iron during tempering contains recovery and recrystallization softening processes of twin martensite and dislocation substructure, decomposition of retained austenite, dissolution of supersaturated carbon and phase transformation in martensite and transformation in eutectic cementite. With increasing tempering temperature, there is a gradual decrease in micro- and macro-hardness of substrate microstructure and compressive strength of austempered low alloyed bainitic ductile iron. When the bainitic ductile iron was tempered at 450 ℃, the eutectic cementite has the lowest micro-hardness value due to the precipitation of α phase in its slice layer and the compressive ratio is thus higher. The mechanical properties of the austempered low alloyed bainitic ductile iron was even worse when tempered at 600 ℃. Under the wear condition of dry sand/rubber wheel, the austempered low alloyed bainitic ductile iron possesses the best wear resistance when tempered at 450 ℃. The worn morphology observation by SEM indicates that the worn surfaces were caused by plastic deformation and micro-cutting. The plastic deformation plays an important role in wear process, while the precipitated and finely distributed Mo₂C contributes a lot to the improvement of wear resistance when tempered at 450 ℃.

Effects of welding heat input on the microstructure and dynamic fracture toughness (J_Id) of the CO₂ shielded arc welded joints of Q460 high strength low alloy steel were investigated. The mechanism of effects on the dynamic facture behavior of the welded joint was also discussed. The results showed that there existed the allotriomorphic ferrite at the columnar interface in the fusion zone of welded joint under the condition of low heat input. The morphological characteristics of columnar crystal in the fusion zone gradually decreased and the allotriomorphic ferrite disappeared as the heat input increased. The fusion zone was mainly composed of acicular ferrite, and its average size increased with increasing heat input. The welded joint exhibited the optimal dynamic fracture toughness under the condition of medium heat input while it showed the lowest value under low heat input within the temperature range of -70 ℃ to room temperature. When the temperature decreased from room temperature to -70 ℃, the dynamic fracture mechanism of Q460 welded joint changed from ductile fracture to brittle cleavage fracture. Under the condition of low heat input, the allotriomorphic ferrite characterized by the planar growth at the columnar interface in the fusion zone of welded joint can lead to the rapid intergranular crack propagation at low temperature. The fine acicular ferrite in the fusion zone of the welding joint obtained at medium heat input which can hinder the crack propagation during the dynamic fracture at low temperature to the greatest extent is the reason why the welded joint exhibits high dynamic fracture toughness.

High-entropy alloys (HEAs), defined as solid solution alloys which have at least 5 principal elements but no more than 13 elements, with concentrations of each principal element ranging from 5% to 35% in atomic fraction, are emerging as one of the hot research frontiers in the metallic materials field. The significance of HEAs originates from their various combinations of high strength, good thermal stability and excellent resistance to corrosion, wear and oxidation. HEAs exhibit simple solid solutions with bcc and/or fcc structure(s) due to the effect of high mixing entropy in the solid solution state of HEAs, which may make the HEAs with improved mechanical and physical properties. However, a small quantity of intermetallic compounds can also form in certain HEAs, indicating that the formation of simple solid solutions cannot solely depend on the high mixing entropy. Then, the theory of HEAs based on the concept of entropy-enthalpy competition to judge whether or not simple phases will form was proposed. However, even if an alloy meets these criterions, it can still contain intermetallic phases. Why and how these intermetallics form in HEAs needs much more clarification. In this work, Co-Al-Cu-Ni-Mo_x (x=0, 0.5, 1) powder system with close-to-equiatomic ratios was mixed and laser surface alloyed onto 2Cr13 stainless steel substrates, and then the FeCoCrAlCuNiMo_x HEA coatings were obtained by reaction synthesis of Fe, Cr with Co-Al-Cu-Ni-Mo_x powder. The phase transition mechanism, microstructure and microhardness of FeCoCrAlCuNiMo_x coatings were investigated by XRD, SEM, EDS and microhardness tester. Experimental results showed that the principal elements of Fe, Cr in 2Cr13 stainless steel substrate participated in surface alloying process during the laser irradiation, forming FeCoCrAlCuNiMo_x laser high-entropy alloying coatings. With the increase of Mo content, the crystal structures of FeCoCrAlCuNiMo_x laser high-entropy alloying coatings evolved from fcc+bcc two-phase solid solution to fcc+bcc solid solution with hcp phase precipitations. The hcp phases were mainly Ni₃Mo and Co₇Mo₆, and the content of Ni₃Mo phase was higher than that of Co₇Mo₆. The phase formation analysis indicated that besides Ω and δ parameters, solidification temperature of the molten pool must be considered during the phase selection, instead of melting point as suggested previously. The microstructure of the coatings exhibited a typical dendrite structure. With the increase of Mo content, the block-shaped Ni₃Mo and Co₇Mo₆ precipitated in the innerdendritic regions. The microhardness of the FeCoCrAlCuNiMo_x laser high-entropy alloying coatings was 390~490 HV, which significantly increased with the increase of Mo content.

The keyhole plasma arc welding (K-PAW) is widely applied in engineering project now as a high energy beam welding with its advantages of low-cost and easy operation. However, the arc instability may arise and welding defects will be produced in K-PAW due to the high current and strong plasma penetrating force when medium thickness plates are welded, finally weakening the efficiency of K-PAW. Furthermore, it is found that the flow field of liquid metal in the molten pool and the stability of keyhole have a critical influence on welding quality. Therefore, modeling and simulating molten pool, keyhole and flow field in the K-PAW quasi steady process except for arc starting and ending phases are helpful to understand the welding process theory completely and promote its application further. But to date, there is little study on the coupled analysis of molten pool and keyhole in the quasi steady welding process due to the difficulty to make keyhole stable. In this work, based on the principles of fluid dynamics with considering arc pressure, surface tension, electromagnetic force, buoyancy and gravity, a three dimensional transient model is established to reveal the secondary changing of heat and force effect regularly as the keyhole depth increases. To describe the welding heat process, a combined type volumetric heat source model of 'double ellipsoid+conical body' is employed. A keyhole inside solid agitated (KISA) calculated method is proposed to maintain the keyhole stability in the quasi steady welding process. To improve the computational efficiency, the calculated region is limited within the action region of a cone-symmetrical weld heat source. With volume of fluid (VOF) method to track the keyhole boundary, the dynamic behavior process of molten pool, keyhole and flow field are calculated using FLUENT software. The stability of K-PAW is analyzed and the factors affecting keyhole production are discussed. The calculated results show that under the welding current 140 A and plasma gas flow 3.5 L/min, it needs 3.0 s to reach the quasi-steady state in which the average thickness of molten pool in keyhole front wall is 0.6 mm. The offset range of the keyhole center between top side and bottom side is 0.46~0.97 mm. There is the anticlockwise heat vortex appearing in molten pool of back side. The calculated width of keyholes on the bottom side is in good agreement with experimental results.

Due to the increasing demand of low density and high strength Mg alloys for the automobile, railway, and aerospace industries, the exploration of cost-effective RE-free Mg alloys becomes more and more attractive. Instead of Mg-Sn based system, the Mg-Bi alloy system seems to satisfy this requirement as a potential candi date, since it shows typical precipitation-type phase equilibrium and contains thermal stable Mg₃Bi₂ phases, which exhibit a high melting temperature of 821 ℃ comparable to those in RE-bearing Mg alloy. In addition, the fine Mg₃Bi₂ plates on the prismatic plane were reported to be more effective than the more commonly observed basal plates for precipitation-hardening. In this work, pure Mg with/without 6% Bi (mass fraction) additions were extruded, and the corresponding microstructure and mechanical properties were investigated. The results show that dynamic recrystallization (DRX) occurs in both alloys after extrusion and these two kinds of specimens exhibit similar extrusion texture. However, the as-extruded Mg-6Bi alloy represents finer and homogenous microstructure, and the average grain size (AGS) decreases from 30 μm to 4 μm when 6% Bi added. In addition, the Mg-6Bi alloy contains strip-like fragmented Mg₃Bi₂ particles along the extrusion direction and fine Mg₃Bi₂ precipitates, and demonstrates superior mechanical properties with tensile yield strength of 189 MPa, ultimate tensile strength of 228 MPa, and an elongation of 19.9%. There is a large number of nano-scale Mg₃Bi₂ particles in the tensile fracture surface of Mg-6Bi alloy. And there is a large number of twins in the microstructure of compression fractured pure Mg sample; while for the Mg-6Bi alloy specimen, with a large number of second phase particles on the α-Mg matrix, little twins are observed. Moreover, the Mg-6Bi alloy also gives a low tension-compression yield asymmetry with yield asymmetric ratio of 1.01. These significantly improvement of mechanical properties are mainly attributed to the combined effects of grain refinement and large quantity of co-exist micro/nano-size Mg₃Bi₂ particles.

Al-Li alloys are considered as the ideal structural materials for aerospace industry because of their low density, high specific strength and specific elastic modulus as well as low fatigue crack growth rate and good low temperature performance. 2055 Al-Li alloy among new Al-Li alloys developed recently is a super-high strength Al-Li alloy. An important method to improve the performance of Al-Li alloys is to add micro-alloying elements. Er-microalloying in Al alloy has been investigated much, but the study on Al-Li alloy is still seldom reported. In this work, the effect of 0.2%Er and 0.4%Er addition on the microstructure and mechanical properties of 2055 Al-Li alloy sheet with T8 aging (6% cold rolling pre-deformation and aging at 160 ℃) were investigated. The results show that the addition of 0.2% Er significantly decreases the strength by about 50 MPa, but enhances the elongation; the strength is further decreased by about 100 MPa with the addition of 0.4%Er. The precipitate types in Er micro-alloyed Al-Li alloy are not changed with the addition of Er, and the aging precipitates are still T1 (Al₂CuLi) and θ' (Al₂Cu) phases. In the Er-microalloyed Al-Li alloy, the incubation time of T1 precipitate is longer, and its precipitation rate is decreased, accordingly the aging response is slowed. Meanwhile, under peak-aging condition, the fraction of T1 precipitates, especially θ' precipitates in the Er-microalloyed Al-Li alloy is decreased, which results in a decrease of strength. As Er is added to the Al-Li alloy, Er-contained particles Al₈Cu₄Er are formed during solidification process, and their amount is increased with the addition increasing. These particles cannot be completely dissolved into the alloy matrix during homogenization process. After solution treatment following cold rolling, they are not yet dissolved into the solid solution. These particles contain Cu and Er simultaneously, and the concentration of dissolved Cu in solid solution is therefore decreased. With increasing Er addition, the Cu concentration in solid solution is further decreased. The precipitation rate of T1 is consequently decreased, slowing the aging response of the Er-microalloyed Al-Li alloy. And this factor also decreases the fraction of T1 and θ' precipitates and lowers the alloy strength.

Most titanium alloys have been designed for aeronautical applications, where their excellent specific properties are fully employed and weldability is a classic problem with Ti and its alloys. Microstructure and mechanical properties of the electron beam weldments of TC17 alloy were investigated in this work. The results showed that there exhibited three zones across the TC17 electron beam weldment: the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). It was also observed that the as-welded FZ consisted of metastable β columnar grains, while the HAZ consisted of acicular α/α′ phase, equiaxed α phase and metastable β phase. Furthermore, it was indicated that the transformation from metastable β phase to α+β phase happened when the FZ and HAZ were post-weld heat treated at 630~800 ℃, the coarsening of α laths and the grain boundary α were also observed when the heat treatment temperature increased. The increasing of 450 ℃ ultimate tensile strength of FZ was ascribed to the precipitation of secondary acicular α platelets during tensile testing in the as-welded and 800 ℃ heat treated conditions, which led to the low yield ratio of FZ. The tensile failure location of the weldments was found to occur in preference in the low tensile yield strength area, or in the low hardness area when the difference between yield strength across the weldments is very small. It was concluded that the optimal post-weld heat treatment for the TC17 alloy weldment was 630 ℃, 2 h, A.C., at which the weldments showed good combination of tensile strength and elongation.

Near-net-shape forming through powder metallurgy (PM) route is a cost-efficient approach to produce the hard-to-machining materials such as titanium alloys. Hot isostatic pressing (HIPing) is a common technique to fabricate PM titanium alloys and components. Prealloyed powder metallurgy through HIPing is considered as upgrade of precision casting for titanium alloys. Ti-5Al-2.5Sn ELI (extra-low interstitial) is a typical α-Ti alloy, which is widely used at cryogenic temperature. In this work, the Ti-5Al-2.5Sn ELI prealloyed powder was produced by electrode induction melting gas atomization. Characterization of the prealloyed powder was carried out to understand the following HIPing process. The influence of HIPing parameters on microstructure and mechanical properties of Ti-5Al-2.5Sn ELI powder compact was studied. The results show that the relative density of powder compact increases with the increasing of HIPing temperature and pressure. The mechanical properties of powder compact can achieve those of wrought materials, when the relative density is more than 99.5%. To balance the relative density, microstructure and mechanical properties of the powder compacts, the optimized HIPing parameters for Ti-5Al-2.5Sn ELI powder are temperature in the range of 890~940 ^oC, pressure above 120 MPa and holding for 3 h. The shielding effect of capsule will hinder the powder densification during HIPing process, which will likely cause non-uniform densification and degrade the relative density of powder compact. However, the shield effect can be weakened through proper tooling design and optimization of the HIPing procedure.

Up to now, considerable effort has been expended in attempts to investigate the influences of Re on Ni-based single crystal superalloys. However, few study had elucidated the influences of Re on carbide, boride and grain boundary. Therefore, the influence of a 2%Re (mass fraction) addition on the as-cast and heat-treated microstructures of a Ni-based directionally solidified superalloy was investigated by comparison with Re-free alloy using SEM, EPMA and TEM. The results show that Re accelerates the precipitation of μ phase in the periphery of eutectic and at grain boundary for as-cast microstructure. After heat treatment, Re also accelerates the precipitation of phase in the vicinity of primary MC carbide and at grain boundary. For 0Re alloy, there are small number of M₆C carbide in the vicinity of primary MC carbide and M₂₃(C, B)₆ boro-carbide at grain boundary. For 2Re alloy, a large amount of blocky μ phase enveloped by thick γ′-film is found in the vicinity of primary MC carbide and at grain boundary. Enrichment of B along the grain boundary is observed in 0Re alloy. On the contrary, relatively uniform distribution of B is found in 2Re alloy. The precipitation mechanism of μ phase during the process of heat treatment is also analyzed.

The intermetallic compound has been widely introduced in alloys as a reinforced phase due to its high strength, high hardness and enhanced heat stability. The size, morphology, distribution and volume fraction of these intermetallic compounds affect the mechanical properties of materials significantly. In this work, the microstructures evolution and growth behoviors of primary intermetallic Al₃Y phase have been investigated in directionally solidified Al-15%Y (mass fraction) hypereutectic alloy at a wide range of pulling rates (1~100 μm/s). The as-cast Al-15%Y alloy is composed of primary intermetallic Al₃Y phase and Al₃Y/Al eutectic structure. At relatively low pulling rates (1~5 μm/s), primary Al₃Y phase exhibits irregular and having a branching structure on the top of the specimens. Primary Al₃Y phase also precipitates in a faceted growth with sharp edges and corners. As the pulling rate increases, the morphologies of Al₃Y phase transit to elongated prism. Al₃Y phase distributes dispersively in the eutectic structure at a higher pulling rate, presenting a crossing shape with two prisms crossed vertically. Further increasing the growth rate to 100 μm/s, the cross morphology such as two six prismatic vertical cross structure of primary Al₃Y appear, similar to the growth in the form of dendrites. During the increase of pulling rates, the leading-phase at solid-liquid interface appear gradually, and the growth distance of primary phase increases with the pulling rates increase.

Peritectic reaction is frequently encountered in many technologically important materials (e.g., steels, brass, bronze, intermetallic compounds, magnetic materials and YBa₂Cu₃O_x superconductors). Many interesting microstructures have been found during directional solidification of peritectic alloys, which have drawn much attention since last decade. In this work, in order to investigate the growth behavior of Al₃Y phase as a peritectic phase, directioanal solidification experiments at different pulling rates have been performed on Al-53%Y (mass fraction) peritectic alloy. The results show that the primary phase and the peritectic phase both grow continuously, the microstructure, which is parallel to the solid-liquid interface has been found and explained at a low pulling rate (V=1 μm/s). With the growth distance increase, the precipitating solid phase from the liquid at the quenching solid-liquid interface transforms from primary Al₂Y phase to peritectic Al₃Y phase. The interface consists of coarse Al₃Y phase without Al₂Y phase. At relatively high pulling rates, the morphologies of primary Al₂Y phase transit from continuous growth to cellular phase, and further to dendrites with the pulling rate increase. The results also show that the primary phase is enclosed with the serrate peritectic phase, and Al₃Y phase precipitates from the liquid in needle shape at the same time. With the growth distance further increase, Al₃Y phase become thicker and more numerous. In addition, the Al₃Y phase precipitated from liquid transit from needle shape to short rod and lump shape, which distributes around the peritectic structure.

Ni-based single-crystal superalloy DD5 has excellent high temperature properties, which is the preferred raw material for aero-engine turbine blade in recent year. In this research, DD5 superalloy was brazed by different contents of Ni-Co-Cr-W-B+DD99 mixed powder filler alloy. The microstructure evolution and interfacial formation mechanism of DD5 superalloy brazing joint were analyzed by SEM and EPMA. The mechanical properties of joint after solid solution treatment and aging treatment were tested. The results show that γ-Ni primary phase formed firstly in the Ni-Co-Cr-W-B/DD99 interface during the brazing process, and then B element segregated and precipitated to fine granular M₃B₂ type boride. The residual liquid phase solidified and formed lastly to the M₃B₂ phase, γ+γ′ eutectic phase and γ-Ni+Ni₃B+CrB eutectic phase during cooling. With increasing the ratio of DD99 in mixed powder filler alloy, the low melting point eutectic phase and borides in the joint decrease and the uniformity of composition and microstructure of joint improve. When the ratio of DD99 increased to 70% (mass fraction) in the mixed powder filler alloy, it can be observed that element of B diffused to DD99 additive powder which result ed in the decrease of low melting point eutectic phases and brittle compounds. The high temperature tensile properties of joint is 1010 MPa at 870 ℃.

Based on the understanding of oxide spent fuel reprocessing, the AlCl₃ was selected as chlorinating agent to directly chloridize spent fuel. At the same time, Nd could be extracted by co-deposition with the intro duction of Al³⁺. This process could be realized by the purpose of electrochemical extraction. The electrochemical behavior of Nd(III) ions was investigated in LiCl-KCl-AlCl₃-Nd₂O₃ melt on W and Al electrodes at 753 K. Simultaneously, the chlorination effects of Nd₂O₃ by AlCl₃ was also studied. Using Nd₂O₃ as raw material, Al-Nd alloy was obtained via electrolytic extraction Nd on W and Al electrodes. On the W electrode, no reduction signal of Nd(III) was detected in the cyclic voltammogram of LiCl-KCl-Nd₂O₃ melt. After the addition of AlCl₃, three electrochemical signals for three Al-Nd intermetallic compounds from the underpotential deposition of Nd on pre-deposited Al substrate were observed. On the Al electrode, two formation signals of Al-Nd intermetallic compounds were observed in the LiCl-KCl-AlCl₃-Nd₂O₃ melt. The results show that AlCl₃ can effectively chloridize Nd₂O₃. Nd was extracted by galvanostatia electrolysis on the W electrodes under -2 A, and Al-Nd alloy was obtained. The XRD results suggest that Al₂Nd phase is formed. However, when the electrolytic extraction of Nd was carried out on active Al electrode, Al₃Nd phase was formed in Al-Nd alloy.

In recent years, the components tend to be large-scale and large-span. In order to improve the welding construction efficiency and reduce production costs, the high input welding methods, such as automatic gas electric vertical welding, submerged arc welding, electro slag welding, etc., have been widely used in manufacturing fields, like shipbuilding, buildings, bridges, petrochemical and marine structures, etc.. The domestic iron and steel enterprises and research institutes have cooperated successively to develop a number of heat input welding steel with heat input greater than 400 kJ/cm. However, at present, the welding materials which can be matched with these special steels are still dependent on import. In order to change this passive situation, a new type of flux cored wire has been independently developed in this research. The effects of heat input on the microstructure and impact toughness of the weld metal have been investigated through laboratory tests. The results demonstrate that under the condition of large heat input welding, a large number of fine inclusions are formed and distributed randomly in the weld metal. Substantial amount of interlocked acicular ferritic grains are found around the inclusions, contributing to the high impact toughness for the weld metal. With the increase of heat input value, the number of fine inclusions (smaller than 1 μm) decreases and the tendency of inclusion assembly and growth is found to accelerate. Simultaneously, the nucleation points of acicular ferrite decreased and the grain size of acicular ferrite increased slightly in the weld metal. The impact toughness was deteriorated mildly as well.