In recent years, the development of material genetic methods, together with multi-scale material design theory and calculation methods has provided new ideas for the alloy design of novel Co-based superalloys. Based on the published results of multi-scale design and the research work of our laboratory, this paper systematically summarizes the present research status of multi-scale design methods in the field of novel Co-based superalloys. A review of multi-scale calculation methods including first-principle calculation, CALPHAD, phase field simulation, and machine learning is presented in this paper. The development trend of multi-scale design in novel Co-based superalloys is prospected.
A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the further optimization of properties, HEAs are multicomponent alloys containing several principle elements (usually ≥5) in equiatomic or near equiatomic ratio. Due to their unique atomic structure, HEAs possess a lot of distinguished properties. Since the discovery of HEAs, a variety of HEA systems have been developed and shown unique physical, chemical and thermodynamic properties, especially the promising mechanical properties such as high strength and hardness, abrasion resistance, corrosion resistance and softening resistance. Here in this short review manuscript, starting from the research challenges for understanding the deformation mechanism of HEAs, this work briefly summarized the mechanical properties and deformation behavior of HEAs, reviewed the proposed strengthening-toughening strategies and their corresponding deformation mechanism in HEAs. A brief perspective on the research directions of mechanical behavior of HEAs was also proposed.
In recent years, the increasing application demand for Mg alloys in automobile, rail transport, aviation and aerospace industries brings about the growing prominence of seeking reliable techniques to join Mg alloys. As a solid state welding method, friction stir welding (FSW) exhibits unique advantages in joining Mg alloys, and thus arouses widespread research interest. This paper emphatically reviewed the research status of conventional friction stir butt-welding of Mg alloys, and highlighted the welding process, microstructure evolution, texture characteristics, mechanical behavior and their interaction mechanisms. It was indicated that the texture plays a vital role in FSW joint performance of wrought Mg alloys, which is quite different from that in the FSW Al alloy joints. The specific strong texture formed in the weld is the main factor that gives rise to the impediment to achieving equal-strength joints to base materials. At the same time, some focuses like the weldability and the factors that influence joint performance in other types of FSW like lap welding, spot welding and double-sided welding; the weldability, interface bonding mechanism, joint performance and its affecting factors and optimization methods in dissimilar FSW between Mg alloys and other materials like Mg alloys of other grades, Al alloys and steels, were summarized and discussed. Finally, the future research and development directions in FSW of Mg alloys were prospected.
Metallic glasses (MGs) have disordered microstructure and no defects like in crystalline materials and possess a suite of outstanding mechanical and functional properties, showing thus promising potential for wide applications. Due to the lack of long range structural order, it is fraught with difficulties to construct the structure-property relationship in amorphous materials. The study of relaxation dynamics provides a very important approach to understand MGs, and is vital to understand their stability and deformation behavior and remains a core issue in the field of condensed matter physics and materials science. In recent years, with the use of more advanced research methods and the deepening of research, it was found that there exists rich dynamics covered by the extremely wide time scale and the different length scales of glassy state. Different dynamic modes not only correlate with each other but also show distinction. This article reviews recent progress in the study of relaxation dynamics in MGs, and its role in understanding and modifying material properties and optimizing material preparation.
Duplex stainless steels consist of a two phase microstructure involving α-ferrite and γ-austenite. These alloys have a remarkable combination of mechanical properties together with good corrosion resistance under critical working conditions and are suitable for marine and petro-chemical applications. However, the poor hot workability of these materials makes the industrial processing of flat products particularly critical. Many investigations focus on the mechanisms and behaviors of hot deformation on these materials. Several factors are frequently reported give rise to hot cracking: phase proportions, size and morphology of both phases, softening mechanisms in constituting phases, microstructural evolution during hot work, and strain partitioning between α and γ. On the contrary, few studies have been carried on cold rolling performance. Hot cracking should be avoid during forming process of duplex stainless steel, the more effective way of manufacturing in such materials is also needs research. In this work, the formability of 0Cr32Ni7Mo4N duplex stainless steel was studied in the hot rolling and directly cold rolling processes. The deformation mechanism of α and γ phase at room temperature, the microstructure evolution after hot rolling, cold rolling and solution treatment were investigated. Mechanical properties and corrosion resistance of two kinds of cold-rolled sheets were tested. The metallography and corrosion morphology were observed by OM and SEM. The results show that cracks emerged along the edge of hot-rolled plate even it was reheated three times, and it has good cold rolling formability after cutting edge of the plate. On the other hand the as-cast billet solution-treated at 1100 ℃ has good cold rolling performance. Deformation mechanism of α phase at room temperature is that multi-slip system form dislocation cell structure, while single slip model and mechanical twins appear in γ phase. As the temperature of heat-treatment raised, microstructure became more homogeneous and the amount of precipitate particles decreased. The experimental results show that the tensile strength of cold-rolled sheet after heat-treatment reaches 1082.9 MPa and the elongation is 29.3%. Critical pitting potential of the specimen in 3.5%NaCl liquor is 1060 mV; weight loss after intergranular corrosion in 65%HNO3 solution is 0.05 g/(m2·h).
Bicrystal slabs with different grain boundary angles were cast to study the effect of varied grain boundary angle on stress rupture properties of a Ni-based bicrystal superalloy. It was found that the stress rupture lives of single crystal specimens were superior to those with grain boundaries. With the increase of grain boundary angle, the stress rupture life was decreased and the fracture type was transferred from trans-granular to inter-granular fracture. The reduced rupture properties was attributed to the inhabitation of grain boundary on slip deformation. With the rise of temperatures, the effect of grain boundaries on rupture properties was enhanced and the critical value of grain boundary angle from trans-granular to inter-granular fracture was decreased. Inter-granular fracture occurred from 12° grain boundary in the rupture test of 871 ℃ and 552 MPa, and it occurred from 4.5° grain boundary in the rupture test of 1100 ℃ and 120 MPa. Since the grain boundary became weaker at higher temperature, the angle of low-angle boundary in single crystal superalloys should be controlled strictly.
Transformations of grain boundaries often strongly influence both the structure and the properties of polycrystalline and nanocrystalline materials. Thus, plastic deformation processes in fine-grained polycrystals and nanocrystalline solids are associated with transformations of grain boundaries, which crucially affect the structure and mechanical characteristics of such solids. Motion of grain boundary dislocations in plastically deformed materials is commonly considered to be the absorption of lattice dislocations by grain boundaries. In order to reveal the mechanism of motion of a low-angle symmetric tilt grain boundary (STGB) associated with the emission and absorption of lattice dislocation, the emission and evolution of a STGB under strain were simulated by phase-field crystal (PFC) model. The decay of STGB and dislocation reactions of separation, annihilation and mergence and their mechanisms were analyzed from the energy point of view, furthermore, the active energy of the dislocation separation was calculated. The research results show that the low-angle STGB is composed of pair dislocations in a line arrangement in two dimensions of triangular atomic lattice, in which there are two sets of basic Burgers vectors. The evolution process of STGB decay can be divided into six typical stages which includes the detail features as: dislocation climbs firstly along the STGB under strain, then the dislocation occurs to break up into two new dislocations after it gets enough energy to overcome the active potential barrier of dislocation, at this time the STGB emits pair dislocations to move in gliding in grain instead of climbing along STGB; gliding for while, the dislocation crosses the grain until it is annihilated by another dislocation at the STGB right in the front, i.e. the Grain boundary absorbs or merges the gliding dislocation. The remain of dislocation in the STGB can still climb along the grain boundary in which splits off again into two dislocations when it gets enough energy, at the same time it looks as if STGB emits the dislocations and changes the dislocation movement from climbing to gliding again. The dislocation continues gliding until it meets another gliding dislocation in grain to be annihilated, finally the total dislocations are annihilated and the STGB disappears. The two grain systems with STGB become one grain system. The two sets of basic Burgers vectors of lattice dislocation in triangular lattice can validly be used to express the dislocation reaction of emission, separation, mergence, absorption, annihilation, and also can reveal the creation of new Burgers vector and the annihilation of old Burgers vectors and mechanism of the directional change of Burgers vectors during the dislocation reaction.
A low carbon steel containing Cu addition was treated by Q&P process using a CAS-200 continuous annealing simulator. The microstructure of the steel was characterized by means of SEM, EBSD, XRD and TEM and its mechanical properties were investigated by tensile testing at room temperature. Cu-rich precipitates formed during the Q&P process were observed as spherical particles in martensitic laths and are 9 nm to 20 nm in diameter. According to the Orowan mechanism, those fine particles may have a contribution to the yield strength of the steel about 134 MPa. Also observed are three different morphologies of the retained austenite phase in the test steel, i.e. thin film--like, fine granular and blocky, formed at different locations. The test steel has a good comprehensive mechanical properties, of which the product of tensile strength and elongation, the tensile strength and the total elongation are as high as 21.2 GPa·%, 1326 MPa and 16 %, respectively. The excellent combined properties can be attributed to the effect of transformation induced plasticity (TRIP) caused by the retained austenite.
The matter of fracture in tension is also the issue of fracture elongation. The ability of superplasticity of materials is mainly characterized by excellent fracture elongations. Since first superplastic phenomenon was recorded, the investigations of superplasticity have not halted. Most of the existing literatures focused on physical or microstructural mechanisms while less attention was paid to mechanical theories on the superplastic deformation. However, superplastic phenomena on large elongation in superplastic tension are closely related to the mechanical stability and are finally dependent on the special fracture mechanism. Correspondingly, in this article, the studies of fracture mechanism of the superplastic deformation are reviewed, which involved nucleation, growth and coalescence of cavities. Then, the literatures related to the mechanical stability in superplastic tension are classified and reviewed, which involved the mechanical analysis and numerical simulation of the fracture elongation or the limit strain induced from neck’s initiation and development. The conclusions indicate that there has yet been no united and confirmed opinion on the superplastic fracture mechanism which has numerous versions from the microstructural or physical view, and the superplastic fracture mechanism would have maken no significant progress unless many long-term investigations will be carried out in the future. In order to interpret the essence of large fracture elongation, the current task should be thoroughly investigate the mechanical stability in superplastic tension based on the advanced technology of numeric analysis. In numeric analysis, the precise and quantitative constitutive equation should be adopted and the deformation conditons involving strain paths should be taken into account.
The microstructure of a low alloy martensitic steel has been investigated using TEM. It was indicated that the as-quenched plate and lath martensites consist of ferrite matrix and high density of nanometer-scaled ultrafine particles embedded in the matrix. These particles were designated to beω phase with a primitive hexagonal crystal structure. Theω particles exhibit an orientation relationship with the ferrite (α-Fe) matrix as follows: α//ω,(110)α //(1101)ω and (211)α//(0110)ω, with lattice parameters of aω=21/2aα , cω=31/2/2aα. The results of the present study suggested that the carbon atoms in the steel are not homogenously distributed in the martensites. The ferrite matrix possesses very low content of carbon, and most of the carbon atoms are concentrated in the ω phase.
Underwater wet flux-cored arc welding (FCAW) has great potential prospects of wide application in ocean engineering due to its easiness of automation and high weld quality. However, the thermal process of underwater wet welding is more complicated: the arc energy distribution is more concentrated in high-pressure environment of underwater, the convection heat transfer coefficient of the weldment under water is much larger than that in air. This study focuses on establishing the numerical model for analyzing the thermal process and the temperature field in underwater wet FCAW by using the FEM software SYSWELD. Both the generalities and peculiarities of the conventional GMAW (gas metal arc welding) in air and underwater wet FCAW processes are taken into consideration, especially the two remarkable characteristics of underwater wet welding, i.e., the water compressing action to the arc, and the enhanced heat losses caused by the surrounding water. Based on the calculated temperature profiles, the weld bead shape and sizes are predicted in underwater FCAW, which lays the foundation for the process optimization. It is found that under 4 groups of typical welding conditions the calculated weld bead dimensions are in agreement with the experimental ones, which validated the energy distribution pattern of the heat source and the numeric model for underwater wet welding. Experiments showed that the weld bead was thinner and deeper in underwater wet welding than that in conventional GMAW under the same welding parameters, while the variation regularity of weld bead profile is similar.
The near isothermal canned hot extrusion at a temperature close to α transus temperature was used to fabricate Ti－45.5Al－2Cr－2Nb－0.15B alloy rod. Microstructures and tensile properties of samples taken from different locations of the extrudate were compared with each other, and the formation mechanism of extrusion microstructure was investigated in combination with the finite element simulation. It was found that lamellar grains were significantly refined by hot extrusion. Microstructure and tensile elongation were homogeneous along the axial direction of extruded rods, but heterogeneous along the radial direction. The center of rods with coarse fully－lamellar microstructure had low tensile elongation, and the edge of rods with fine near lamellar microstructure had high tensile elongation. Such heterogeneities could not be eliminated in subsequent α solid solution treatment. Lamellar grain size decreased with increasing effective strain. There existed the refined homogeneous microstructure in the regions with effective strain larger than 2.25. The difference of microstructure type was mainly due to different temperatures of different parts of rods during extrusion process. In the edge of rod tails, the γ phase lamellar structure precipitated from α phase was formed due to the chilling effect caused by contacting with the cold die, then the lamellar structure with tortuous boundary was formed in subsequent deformation. Tensile elongation was found to decrease with increasing lamellar grain size, but the poor tensile elongation in the center was mainly attributed to the existence of lamellar grains which lamellar boundaries were nearly perpendicular to the extrusion direction.
Surface modification experiment of the commercial purity aluminum (α-Al) and Al-Cu-Mg alloyed aviation aluminum alloy 2A02 by laser shock processing (LSP) was implemented. The surface strengthening effect of both the target materials was investigated from dislocation mechanisms of microstructural response by means of TEM method. The results show that the strengthening effect of the two kinds of materials by laser shock processed is significantly different. The strengthening mechanism of α-Al by laser shock can be attributed to the multiplication of a large number of dislocations. With the increase of the impact number of laser shock and the degree of deformation, the new-generated dislocations will pile up and interact with the forest dislocations, and the dislocation lines will gradually evolve into waved-like, or wind into dislocation tangles and dislocation networks. But the hardness curve of the laser shocked (α-Al) will fast and linearly decline due to Bauschinger effect (BE) and stress wave damping. The laser shock strengthening mechanisms of the aging-hardened aluminum alloy 2A02 can be summarized to the enhancement of the matching between the elastic energy of dislocations with the ultra-high energy of laser shock processing due to the higher matrix strength and the dislocation-pinning effect of large number of dispersed precipitates, as well as the complex dislocation networks in between the precipitates constructed by the dislocations induced by laser shock. The matrix strengthened by laser shock processing and the precipitates keep the extra-semi-coherent relationship to coordinate the total deformation, with the number of laser shock increase, dislocation multiplication and the vacancy motion constitutes geometrically necessary boundaries (GNBs), which consists of the sub-grain boundaries to refine the matrix into the nanometer-grains. The strengthening mechanism of surface modification of aluminum alloy by laser shock processing is formed of the internal stress state caused by the combination of the complex dislocation configurations and the Hall-Petch effect of the nanocrystalline grains.