Microstructural and morphological changes during ball milling of Copper-Silver-Graphite flake mixtures

The present study reports the microstructural and morphological changes during high energy ball milling of Cu with Ag and Graphite flakes. XRD patterns of ball milled Cu-Ag showed a reduction in the intensity of Ag peaks (1 1 1) and an increase in the lattice parameter of Cu. With an increase in milling time, the formation of metastable Cu-Ag solid solution was observed. Lattice parameter values for 40 h milled Cu (3.6169 Å) and Cu-GF composites (3.6166 Å) indicated that C does not dissolve in Cu. The lattice parameter of Cu in milled Cu-Ag-graphite flake was lower compared to milled Cu-Ag mixture indicating that graphite flakes inhibit solid solution formation. Raman spectra revealed that graphite flakes were converted into multilayer graphene after 10 h of milling. The crystallite size of Cu in the milled powders decreased with increase in milling time and reached a value of ∼25 nm after 35 h of milling. The lattice strain also increased with milling time. The D₁₀, D₅₀ and D₉₀ size reduced appreciably after 5 h of milling.     

Microstructural evolution during combustion reaction between CuO and Al induced by high energy ball milling

The product of the combustion reaction between CuO and Al induced by high energy ball milling has been characterised by using X-ray diffractometry and scanning electron microscopy. It has been observed that the combustion reaction can be ignited very easily by the ball milling. The reaction product consists of polycrystalline Cu in bulk and particle forms and a large number of nanometer sized spherical Al2O3 particles attached to the surface of the Cu. It has been demonstrated that this microstructure is evolved through rapid solidification of Cu and Al2O3 melts and rapid condensation of Cu vapour. Cu and Al2O3 phases are separated in the reaction product. The reason for this is mainly attributed to the large difference in their density and the shaking force of the ball mill.     

Microstructural evolution during mechanical milling of Ti/Al powder mixture and production of intermetallic TiAl cathode target

Titanium aluminides are of great technological interest because of their attractive mechanical properties. Mechanical milling/alloying is a promising powder metallurgical technique, which can achieve ultrafine, uniform and manipulable microstructures. In this study, we employed a recently revisited discus mill to produce a composite Ti–(50–57) at.%Al powder feedstock, which is suitable for hot consolidation to produce bulk cathode targets for physical vapour deposition (PVD) coatings. The effects of milling time, quantity of process control agent (PCA) and discus-to-powder weight ratio (DPR) on the microstructure evolution of the attendant Ti/Al composite powder were investigated in detail. It was found that to produce Ti/Al composite powders with a fine particle size and a uniform microstructure, the practicable processing parameters should be 2 or 3% isopropanol addition as PCA, 12 h of milling time and at least 13:1 DPR weight ratio. Cathode targets were produced by hot isostatic pressing (HIPing) the as-milled powders. The targets were then used to produce a PVD TiAlN coating which had an average microhardness of 2400 HV.     

Amorphization reaction during mechanical alloying: influence of the milling conditions

Amorphous Ni-Zr powders have been prepared by mechanical alloying of elemental crystalline powders. The glass-forming range has been determined in detail at different milling intensities. Depending on the milling conditions, at least partial crystallization of the formerly amorphous material can occur from 66 to 75 at% Ni, due to a temperature rise during milling at high intensity. In comparison with isothermal annealing experiments at various temperatures on completely amorphous powder, a relation between milling temperature and milling time is shown. This confirms the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in alternating crystalline multilayers.     

Microstructural evolution during the supersolidus liquid phase sintering of nickel-based prealloyed powder mixtures

A novel concept for full-density sintering is described. Two prealloyed powders with slight compositional differences are tailored to separate the solidus temperatures into high-melt and low-melt compositions. A mixture of these two powder compositions allows full-density sintering at a temperature between the two solidus temperatures. For these experiments, the two powders were nickel-based alloys, where the low-melt powder contained boron. The mixed powders were sintered at temperatures above the solidus of the low-melt powder to form a transient liquid that promoted rapid densification of the mixture. Microstructure evolution during sintering was assisted using quenching experiments. Variables in this study included the heating rate, peak temperature, hold time, and powder ratio. Interdiffusion between the two powders controls microstructure evolution, with a dominant role associated with boron diffusion and reaction. The transient liquid phase responsible for densification is linked to boron diffusion and subsequent compound precipitation.     

Microstructural evolution during high energy ball milling of Fe2O3-SiO2 powders

The reaction of a 25 mol% Fe₂O₃-SiO₂ (hematite-amorphous silica) powder mixture during high energy ball milling in both closed and open containers has been studied by x-ray diffraction and Mössbauer spectroscopy. After around 21 h of milling, the α-Fe₂O₃ powders with an average particle size of 15 nm have formed and no reaction between α-Fe₂O₃ and SiO₂ is found in the two types of milling containers. This demonstrates that the high energy mechanical milling technique is able to prepare a dispersion of ultrafine α-Fe₂O₃ particles. After extended milling in the open container all iron atoms are found in a hematite phase. In the closed container the hematite phase transforms into an iron-rich spinel phase and some of the iron atoms react with the amorphous SiO₂, forming a new Fe(II)-containing silicate compound.     

Microstructural evolution and mechanical properties of CoCrFeNiMnTix high-entropy alloys

This study was conducted to investigate the effect of titanium addition on the microstructure and properties of an equitaomic CoCrFeNiMn high-entropy alloy. Homogenized microstructures of CoCrFeNiMnTi_x (x = 0.1 and 0.3) alloys consist of face-centered cubic phase; however, addition of more titanium led to formation of a (chromium, titanium)-rich σ phase in CoCrFeNiMnTi_0.4 alloy. The average electron hole number calculations indicate the higher possibility of σ phase formation by adding more titanium. Furthermore, addition of an atom like titanium with a larger atomic radius in comparison with other elements can affect stability of face-centered cubic structure. Chromium and manganese has a destabilizing influence on the single face-centered cubic phase and manganese may reject chromium to facilitate the formation of a (chromium, titanium)-rich phase in alloys containing more than 5.5 at.% titanium (x>0.3). The mechanical properties revealed an improvement in strength without losing the ductility drastically by adding titanium up to 5.5 at.% (x = 0.3). Nevertheless, the strength remarkably increased and ductility significantly decreased in CoCrFeNiMnTi_0.4 alloy due to formation of brittle σ phase in the microstructure.     

Microstructural evolution and mechanical properties of a nickel-based honeycomb sandwich

A nickel-based superalloy honeycomb sandwich was manufactured by high-temperature brazing. The microstructural evolution and the out-of-plate mechanical properties were determined for honeycomb sandwiches aged at 800 °C. The maximum tensile strength was 28.5 MPa and the compressive yield strength was 29.6 MPa for the original specimens. These parameters decreased to 24.7 MPa and 23.5 MPa for specimens aged for 10 h, to 24.9 MPa and 21.5 MPa for specimens aged for 20 h and to 26 MPa and 24.8 MPa for specimens aged for 30 h, respectively. With increased aging time the tensile elongation decreased, the intermetallic compounds and the eutectic structure in the brazing region disappeared, and the solid solution approaching the matrix gradually increased.     

高能球磨固态扩散反应研究

从扩散理论出发,结合结合Ａｌ－Ｃｕ合金高能球磨实验结果,分析了高能球磨过程中的扩散特点,提出了固态合成反应模型并进行了分析计算,结果表明,高能球磨过程中固态反应能否发生取决于体系在球磨过程中能量升高程度,而反应完成与否则受体系中的扩散过程控制,即受制于晶粒细化攻粉末碰撞温度。     

Microstructural evolution and mechanical properties of 55NiCrMoV7 hot-work die steel during quenching and tempering treatments

55NiCrMoV7 hot-work die steel is mainly used to manufacture heavy forgings in the fields of aerospace and automobile.This study aims to clarify the effects of h...     

Microstructural evolution and mechanical behavior of warm multi-axially forged HSLA steel

Detailed studies are conducted on the microstructural evolution and mechanical behavior of a high strength low alloy (HSLA) steel processed by warm multi-axial forging (MAF). After nine MAF strain steps, the initial ferrite grains of average 13-μm size reduced to submicron-sized grains with over 0.7 fraction of high angle grain boundaries. Pearlitic cementite is fragmented and refined to about 50–100 nm size particles. The microstructure evolution with respect to fraction of HAGB with increase in number of strain steps is more sluggish in HSLA steel as compared to plain carbon steel of comparable carbon content. This is ascribed to the Zener pinning effect of (Ti, Nb) carbide particles. Tensile strength and hardness values of MAFed steel increased by more than 45 and 58 %, respectively, after nine warm MAF strain steps, whereas the fracture strain was reduced from 21 to 12 %.     

Microstructural evolution in nickel during rolling and torsion

Abstract

Microstructural evolution in high purity nickel was investigated by comparing the dislocation substructures formed following torsion deformation with those following rolling. Results from transmission electron microscopy observations and measurements of microdiffraction show a common evolutionary path in rolling and torsion from (i) dislocation tangles to (ii) equiaxed cells bounded by dense dislocation walls (DDWs) which form cell blocks, then to (iii) microbands which, together with DDWs, bound smaller and smaller cell blocks, and, finally, to (iv) subgrains. During this evolution, grains are continually subdivided by the formation of new cell blocks. New cell blocks arise through the formation of new DDWs between cell blocks or of first generation microbands by the subdivision of a single DDW Since the dislocation content of a microband is principally derived from that of the parent DDW, microbands can take various forms, such as a string of small pancake shaped cells in highly recovered DDWs, short double walls in discontinuous DDWs, or long paired dislocation sheets from continuous DDWs which can be very straight if formed close to {111} planes. At moderate strains and subsequent to microband formation, localised shear and shear offsets were observed in some first generation microbands. A few second generation microbands, in which localised shear is part of the formation process, were also observed. Localised shear was much more prevalent in rolling than in torsion. These results provide evidence for the theory that grains are divided into regions in which slightly different slip systems operate. Collectively, these regions provide evidence for strain accommodation, although in individual regions the number of slip systems is less than that required by the Taylor criterion.

MST/1333     

Microstructural evolution of bulk nanocrystalline Ni during creep

Experiments were conducted on electrodeposited (ED) nanocrystalline (nc) Ni with an average initial grain size of about 20 nm at 393 K to study the shape of the creep curves. In addition, microstructure was examined by means of transmission electron microscopy (TEM). The results show that the creep curves are characterized by the presence of a well-defined steady-state stage. An examination of the microstructure indicates that while grain growth occurs during deformation, the grain size attains a constant value once steady state creep is approached. A comparison between grain size measurements obtained by the TEM technique and those obtained via the X-ray diffraction method shows that the use of the latter method may lead to an underestimation of the value of the average grain size.     

Microstructural evolution during thermal stabilization of PAN fibers

Thermally stabilized polyacrylonitrile (PAN) fibers were produced by multi-stage thermal stabilization between 190 and 270 °C. X-ray diffraction (XRD) and Fourier Transform infrared (FT-IR) spectroscopy were used to investigate the structure of PAN fibers and the stabilized fibers. A new diffraction peak appears at 2θ = 23.2° in XRD pattern of the stabilized fibers besides the peaks in that of PAN fibers. The distinct structural changes were also observed from FT-IR spectra, the intensities in the 2,936–2,918, 2,244–2,236, and 1,451–1,446 cm⁻¹ regions decrease remarkably, and new peaks occur at 1,594 and 1,700 cm⁻¹ (shoulder-like peak) which correspond to the stretching vibration of C=N and C=O groups, respectively. The ultramicrotomy to prepare samples of PAN fibers and the stabilized PAN fibers was explored to characterize their microstructures by high-resolution transmission electron microscopy (HRTEM). The HRTEM images further verified the microstructural changes of PAN fibers during thermal stabilization, and the detailed information was given that thermal stabilization is accompanied by a new structure forming inside the crystallites in PAN fibers, and the (110) plane of PAN fibers transforms preferentially.     

Microstructural evolution of pure iron during hot rolling

Abstract

The microstructural evolution of commercially pure iron has been investigated via hot rolling. Results from optical microscopy, transmission electron microscopy, and microdiffraction show that, at the initial stage of deformation, equiaxed subgrains form at the original grain boundaries. Long layered substructures composed of parallel dense dislocation walls form within grains, and in some cases microbands are also observed. With the increase of strain, the dense dislocation walls persist and their misorientations increase. At the same time, new subboundaries with smaller misorientations form between the dense dislocation walls, and eventually fully equiaxed subgrains develop throughout the specimen. The development of the microstructures and the formation mechanism of micro bands are discussed.

MST/1740     

Microstructural evolution and mechanical properties of NiCrAlYSi+NiAl/cBN abrasive coating coated superalloy during cyclic oxidation

Cyclic oxidation behavior of NiCrAlYSi + NiAl/cBN abrasive coating at 900 ℃ and the mechanical properties of the coating-substrate system were investigated.Results indicated that elemental interdiffusion occurred between the coating and substrate,which caused the formation of interdiffusion zone (IDZ) and second ary reaction zone (SRZ) during aluminization,while their compositions and structures changed with oxidation.AlN interfacial layer formed at cBN/metallic matrix interface during aluminization,while it transformed into multilayer oxides during oxidation.Due to the microstructural evolution of these interfaces,the fracture behavior and bending toughness of the system changed greatly during three-point bending tests.Besides,the damage mechanisms were discussed.     

Solid-state reaction between crystalline nickel and amorphous Fe78Si12B10 powders during mechanical milling

The solid-state reaction between crystalline nickel and pulverized melt-spun amorphous Fe₇₈Si₁₂B₁₀ powders during mechanical milling has been investigated. For the powder mixtures with low nickel content ( 30 at%), the reaction results in the production of a new amorphous phase. For the powder mixture with a high nickel content (up to 50 at%), crystalline (Fe, Ni) and new amorphous phases are obtained. During the milling process, nickel atoms can dissolve in the amorphous matrix by diffusion due to severe deformation. The existence of free volume in the melt-spun amorphous phase may favour the diffusion of nickel atoms. At the same time, the elemental atoms (such as iron) in the amorphous phase may also dissolve into the nickel matrix. The amorphization between the amorphous and crystalline phases is attributed to the asymmetry of interdiffusion of the atoms in different matrices.     

SmCo5和FeCo两种快淬薄带混合物机械研磨的研究

在钼辊表面线速度±=35m/s条件下制备SmCo5和FeCo快淬薄带,对70(SmC05)/30(FeCo)%(质量分数)的薄带混合物进行15～240min的短时闻机械研磨实验,研究了该过程中研磨磁粉的微观组织演化和磁性能变化.XRD分析表明,机械研磨后的磁粉由SmCo5与α-(Fe,Co)两相构成;经过15min研磨后,SmCo5达到约6.5nm,而α-(Fe,Co)达到约23.9nm,而经过120min研磨的磁粉中两相的尺寸都<10nm.VSM分析表明,经过60min机械研磨的磁粉的磁滞回线已呈现单一硬磁性相磁化的特征;在15～240min的机械研磨时间范围内,磁粉的饱和磁化强度随研磨时间延长而不断增大,从研磨15min的0.92T到研磨240min后达到1.3T,而磁粉的剩磁和矫顽力都在研磨90min时达到极大值,分别为0.32T和9.6×104A/m.