Acicular Grains Research Articles

Effect of the different additives on the fabrication of porous kaolin-based mullite ceramics

Porous mullite ceramics were prepared from kaolin and Al2O3 with different sintering additives TiO2, MgO and MoO3. The sintering aids have a significant influence on the grain morphology and densification process by varying the composition and viscosity of liquid phase during firing. The addition of TiO2 retarded the mullite transformation. The introduction of MgO promoted the secondary mullite transformation and the formation of needle-like mullite grains. Compared with MgO, the samples with MoO3 have lower secondary mullitization temperature. The lower secondary mullitization temperature was beneficial for the secondary mullite to grow anisotropically and then to hinder shrinkage during the sintering process. Thereby, the samples with MoO3 showed a network skeleton structure which consists of acicular mullite grains and exhibited a highly open porosity of 67%.

Effects of Root Opening on Physical and Mechanical Properties of the Double Side Welded of Ship Materials

The effects of root opening process parameters on the phisical and mechanical properties of mild steel specimens of grade LR Gr A having dimensions 200 mm× 100 mm× 12 mm, welded by gas metal arc welding were investigated. The variation of root opening that used were 3 mm, 5 mm and 7 mm. The physical properties examined with regard with microstructure, macrosructure, and corrosion using optical microscope and stereozoom. The measured of mechanical properties with regard to strength, hardness and toughness using, tensile test, Vickers hardness Test, and Charpy impact test. The test results show the base metal had a hardness of approximately 110 VHN and a maximum hardness of approximately 190 VHN that corellates with microstructure of weld metals. Microstructure of base metal and HAZ are ferite and perlite, while microstructure of weld zone are acicular ferrite and grain boundary ferrite. The corrosion rate of weld metals with various root opening categorized as materials having excellent corrosion resistance value. Welding joints with opening roots 3 mm and 5 mm can be used for construction. All welded specimens exhibited fracture at base metals

EFFECTS OF HEAT INPUT ON THE MICROSTRUC-TURE AND IMPACT TOUGHNESS OF WELD METAL PROCESSED BY A NEW FLUXNOVEL FLUX CORED WIRE WELD

In recent years, the components tend to be largescale and largespan. 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 *国家自然科学基金资助项目51174058 收到初稿日期: 2015-11-13,收到修改稿日期: 2016-01-26 作者简介:宋峰雨,男, 1985年生,博士生 DOI: 10.11900/0412.1961.2015.00584 第890-896页 pp.890-896

In-situ observation of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by Zr-Ti combined deoxidation

The effect of Zr-Ti combined deoxidation on the grain refinement in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy, transmission electron microscopy, and electron backscattered diffraction analysis. Owing to the Zr-Ti combined deoxidation, a large amount of fine Zr-Ti oxide particles were formed in the steel and retarded the austenite grain growth during simulated welding thermal cycle. The austenite grains were small and uniform. The Mn can diffuse spontaneously from austenite to Zr-Ti oxide inclusion and MnS precipitated on ZrO2, which can form Mn depleted zone in the vicinity of inclusion. The acicular ferrite grains nucleated on intragranular Zr-Ti oxide inclusions in austenite grains grew in different directions and effectively divided the austenite grain into several finer and separate regions at intermediate temperature. The crystallographic grain size became small in the simulated coarse-grained heat-affected zone of Zr-Ti-killed steel due to the effective pinning effect by Zr-Ti oxide particles and acicular ferrite formation.

Microhardness and microstructure evolution of TiB2 reinforced Inconel 625/TiB2 composite produced by selective laser melting

In this study, micron-size TiB2 particles were utilized to reinforce Inconel 625 produced by selective laser melting. Exceptional microhardness 600–700HV0.3 of the composite was obtained. In further investigation, the microstructure and mechanical properties of Inconel 625/TiB2 composite can be significantly influenced by addition of TiB2 particles during SLM. It was found that the long directional columnar grains observed from SLM-processed Inconel 625 were totally changed to fine dendritic matrix due to the addition of TiB2 particles. Moreover, with laser energy density (LED) of 1200J/m, a Ti, Mo rich interface around TiB2 particles with fine thickness can be observed by FESEM and EDS. The microstructure evolution can be determined by different laser energy density (LED): under 1200J/m, γ phase in dendrite grains; under 600J/m, γ phase in combination of dendritic and acicular grains; under 400J/m, γ phase acicular grains. Under optimized LED 1200J/m, the dynamic nanohardness (8.62GPa) and elastic modulus (167GPa) of SLM-processed Inconel 625/TiB2 composite are higher compared with those of SLM-processed Inconel 625 (3.97GPa and 135GPa, respectively).

Acicular ferrite formation during isothermal holding in HSLA steel

Microstructural observation and high-resolution dilatometry were employed to investigate acicular ferrite formation during isothermal holding in the HSLA steel. A decrease in isothermal temperature suppresses formation of polygonal ferrite and promotes formation of acicular ferrite. Island-like martensite/austenite constituents are dispersed inside of the acicular ferrite grain. The displacive model assuming autocatalytic nucleation was developed to describe the incomplete transformation phenomenon well. Decrease of isothermal temperature lowers the activation energy, and thus enhances the formation of acicular ferrite. Increase of amount of polygonal ferrite and acicular ferrite both results in decrease of M s. Formation of polygonal ferrite causes diffusion of solute alloying atoms into the untransformed austenite, which lowers M s. Acicular ferrite transformation suppresses martensitic transformation by diffusion of solution atoms during isothermal holding, and introduction of internal stress resulting from volume expansion during FCC → BCC transformation.

Magnetic property and stress study of barium hexaferrite thin films with different structures

M-type barium hexaferrite (BaM) thin films with different structures (ds100: single-layer 100 nm thick film; ds200: single-layer 200 nm thick film; db200: bilayer 200 nm thick film, where the thickness of each layer is 100 nm and the total film thickness is 200 nm) were deposited on (001) sapphire (Al2O3) substrate by radio frequency (RF) magnetron sputtering. Effects of different structures on the crystallographic, morphological, magnetic properties and stress were investigated in details. For the structure of ds100, BaM film processes platelet-like grains, which shows excellent magnetocrystalline anisotropy and large squareness ratio S⊥ (0.62). For the structure of ds200, there are a lot of acicular grains. Thus, the c-axis in-plane oriented and/or randomly oriented grains induce poor magnetocrystalline anisotropy. For the structure of db200, the platelet-like grains dominate the microstructure. Both squareness ratio S⊥ (0.67) and coercivity Hc⊥ (98.6 kA/m) of the db200 show the highest values among the three kinds of the film structures. In addition, with the increase of film thickness from 100 nm to 200 nm, the stress in film decreases gradually. Compared with the other structures, the bilayer 200 nm thick film (db200) obtains the smallest stress.

The impact of bonite aggregate on the properties of lightweight cement-bonded bonite–alumina–spinel refractory castables

The lightweight bonite–alumina–spinel (CA6–Al2O3–MA) refractory castables with bonite aggregate and different spinel sources (pre-formed and in situ formation) were prepared in this study. The phase composition, microstructural features, and mechanical and thermo-mechanical properties of CA6–Al2O3–MA castables treated at various temperatures were investigated by techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending method, and thermal shock test. The results indicated that the incorporation of bonite aggregate had a positive influence on the strength, thermal shock resistance and slag corrosion resistance. It especially decreased the thermal conductivity and had a slightly negative influence on the refractory under load and slag penetration resistance of the castables. For the in situ spinel-containing castable, the formation of in situ spinel with finer particle sizes and acicular CA6 grains led to higher overall volume expansion, resulting in higher thermal expansion (∆L/L0), linear change and the apparent porosity of castables. Also, the heat insulation, thermal shock and slag penetration resistance of castables with in situ spinel improved, while the strength, displacement, refractory under load and slag corrosion resistance decreased sharply.

Nitrogen Incorporated Ultrananocrystalline Diamond Microstructures From Bias‐Enhanced Microwave N2/CH4‐Plasma Chemical Vapor Deposition

Microstructural evolution as a function of film thickness of nitrogen incorporated ultrananocrystalline diamond (NUNCD) films, grown using bias‐enhanced microwave plasma chemical vapor deposition with gas mixtures of N2/CH4, is systematically investigated. It is observed that by controlling the growth time, the morphology, the microstructure, and the electrical properties of NUNCD films can be manipulated. The growth of NUNCD films starts with the formation of amorphous carbon on Si surface prior to the nucleation of diamond. In the growth time of 10 min, the films retain rod‐shaped diamond grains, whereas in the films grown for 30 min, needle‐like diamond grains are formed, which comprises a diamond core encased in a sheath of sp2‐bonded graphite phase. On increasing the growth time to 60 min, the growth of acicular grains ceases and large proportion of graphite clusters or defective diamond clusters (n‐diamond) is formed. The salient features of such materials with unique granular structure are that their electrical properties can be tuned in wide range such that they are especially useful in practical applications.

Effect of annealing treatment on microstructure and mechanical properties of hot isostatic pressing compacts fabricated using Ti–6Al–4V powder

The effects of annealing temperature and annealing time on microstructure evolution and mechanical properties of Ti–6Al–4V titanium alloy prepared using powder hot isostatic pressing were investigated. The results showed that when annealing temperature was < 1173 K, the globularisation of primary α was observed, and the morphology of secondary α changed from acicular shapes to acicular and continuous lathy grains. Scanning electron microscopy fracture surface showed that the fracture mechanism changed from transcrystalline dimple type to a mixture of transcrystalline and intercrystalline dimple types with increasing annealing temperature. The tensile strength and elongation varied with the temperature in an opposite trend. However, they varied similarly with increasing annealing time. Microhardness was sensitive to the annealing time, resulting into a maximum microhardness of 396.95 HV at 1173 K for 3 h.

Highly Conductive Diamond-Graphite Nanohybrid Films with Enhanced Electron Field Emission and Microplasma Illumination Properties.

Bias-enhanced nucleation and growth of diamond-graphite nanohybrid (DGH) films on silicon substrates by microwave plasma enhanced chemical vapor deposition using CH4/N2 gas mixture is reported herein. It is observed that by controlling the growth time, the microstructure of the DGH films and, thus, the electrical conductivity and the electron field emission (EFE) properties of the films can be manipulated. The films grown for 30 min (DGHB30) possess needle-like geometry, which comprised of a diamond core encased in a sheath of sp(2)-bonded graphitic phase. These films achieved high conductivity of σ = 900 S/cm and superior EFE properties, namely, low turn-on field of 2.9 V/μm and high EFE current density of 3.8 mA/cm(2) at an applied field of 6.0 V/μm. On increasing the growth time to 60 min (the DGHB60), the acicular grain growth ceased and formed nanographite clusters or defective diamond clusters (n-diamond). Even though DGHB60 films possess higher electrical conductivity (σ = 1549 S/cm) than the DGHB30 films, the EFE properties degraded. The implication of this result is that higher conductivity by itself does not guarantee better EFE properties. The nanosized diamond grains with needle-like geometry are the most promising ones for the electron emission, exclusively when they are encased in graphene-like layers. The salient feature of such materials with unique granular structure is that their conductivity and EFE properties can be tuned in a wide range, which makes them especially useful in practical applications.

Pressure effect on magnetic hysteresis parameters of single-domain magnetite contained in natural plagioclase crystal

This study investigates pressure effects on the magnetic properties of non-interacting single-domain (SD) magnetite. Using a high-pressure cell specially designed for a Magnetic Property Measurement System, magnetic hysteresis measurements were conducted under high pressures of up to 1 GPa on natural plagioclase crystals containing much acicular SD magnetite. Coercivity and saturation magnetization were nearly constant with pressure, while saturation remanent magnetization and coercivity of remanence decreased with pressure at moderate rates of −8 per cent GPa–1 and −18 per cent GPa–1, respectively. These results suggest that temperature effects govern the magnetic behaviour of acicular SD magnetite grains in the middle and lower crusts.

Improvement of low temperature toughness of ferritic Mn steels by alloy modification

A study has been made on the effects of Mn content and addition of Al on the microstructure and mechanical properties of (4-6)Mn steels, with the aim of developing ferritic Mn steels having good combination of tensile properties and impact toughness. It has been shown that the reduction of Mn content to 6 wt% and lower from 8-12 wt% of conventional steels can effectively prevent the occurrence of intergranular fracture, which has been a major problem of (8-12)Mn steels with poor toughness at cryogenic temperature. While increasing Mn content in the binary (4-6)Mn steels results in an increase in strength and a decrease in impact toughness, the addition of 1Al to (4-6)Mn steels results in improvements in both strength and impact toughness as compared to those of the binary (4-6)Mn steels and the best impact toughness has been obtained in the as-rolled 1Al added 4Mn steel, whose microstructure consists of fine acicular ferrite grains.

In situ observation of acicular ferrite formation and grain refinement in simulated heat affected zone of high strength low alloy steel

The phase transformation from austenite to acicular ferrite in the simulated coarse grained heat affected zone of a high strength low alloy steel was investigated by means of analytical characterisation techniques such as in situ microscopy, transmission electron microscopy and electron backscattered diffraction analysis. The acicular ferrite grains nucleated on inclusions (Zr–Ti oxides) in coarse austenite grain grew in different directions and effectively partitioned coarse austenite grain into several finer and separate regions. The crystallographic grain size became small for coarse austenite grains due to the effective partitioning by acicular ferrite laths or plates.

Electron emission from the zirconium coated suspension plasma sprayed bioglass

The suspension plasma sprayed bioglass coatings were covered with electrically conducting zirconium film obtained by sputtering. The microstructure of the coatings, observed by scanning electron microscope (SEM) shows the spherical droplets and some acicular grains formed at the bioglass recrystallization. The coatings were submitted to the electron emission investigations. The measurements enabled to find out the critical electric field and electron emission coefficient. The values of the critical field as low as Eth=12V/μm and of the field enhancement coefficient being as high as β=280 are comparable to the values reached for carbon nanotubes. The possible application of the electron field emitting coatings is the large-area emission source used as e.g. information display.

PENGEMBANGAN METODE STRESSED SHEETING WELD PADA PENGELASAN PLAT BERPENGUAT DENGAN VARIASI TEMPERATUR PREHEAT

This research investigates stressed sheeting welded (SSW) effects of stiffened thin plate structure with preheat temperature variations. Stiffened thin plate structure is a structure that consists of frame and sheet that used on vehicle structure because of requirement low volume materials. The principle of SSW is installation of sheeting on the frame is done by pulling the sheeting in hot conditions so the sheeting after joined with the frame has the flat surface. Lap joints of SS 400 with thickness of 1 mm dan 3 mm were made using Gas Metal Arc Welding. The preheat temperature that used were 50°C, 60°C and 70°C. The mechanical properties were measured with respect to strength and hardness while physical properties were examined based on the distortion, micro and macrostructure. Results show that the distortion of weld metal with preheat temperatures 50°C, 60°C and 70°C are 1.02; 2.06; and 2.22 mm respectively. This values is better than weld metal without SSW that has distortion 3.02 mm. The tensile strength of weld metals with SSW method is 20% higher than the weld metal without SSW. Ferrite and pearlite structure appeared in based metal and HAZ region while weld metal had acicular ferrite and grain boundary ferrite structure.

Fracture toughness in the circumferential–longitudinal and circumferential–radial directions of longitudinal weld API 5L X52 pipeline using standard C(T) and nonstandard curved SE(B) specimens

In this work the fracture toughness of a longitudinal submerged arc-welding (SAW) seam weld API 5L X52 pipeline of 36 in. diameter and 1 in. thickness, was evaluated and compared in the circumferential–longitudinal (CL) and circumferential-radial (CR) directions. These tests were evaluated by means of compact tension C(T) specimens and nonstandard curved SE(B) specimens, respectively. The K function of the nonstandard curved SE(B) specimens was experimentally calibrated using the compliance experimental technique. The crack initiating notch was aligned in the longitudinal weld direction so the crack propagated through the deposited metal zone for both specimens. The laboratory tests were performed at room temperature. The K\(_{IC}\) average value in the CL direction was \(75.4 \pm 3.22\,\hbox {MPa}\sqrt{\hbox {m}}\), and for the CR direction was \(56.3 \pm 2.9\,\hbox {MPa}\sqrt{\hbox {m}}\). The results showed a large difference of K\(_{IC}\) values for each direction analyzed, which was attributed to the anisotropic properties of the weld metal, particularly in the CR direction, where the predominance of acicular ferrite grains (fragile zones) and the porosity provided a favorable crack propagation direction for separation of the fracture plane, resulting in low fracture toughness values in this direction. Finally the fractographic analysis showed that the deposited metal zone consisted of a mixture of acicular ferrite and ferrite grains, presenting a ductile behavior with dimple coalescence in the CL direction and a fragile behavior exhibiting cleavage regions that corresponded to acicular ferrite grains in the CR direction.

The effects of lateral variations in rock composition and texture on anhydrite caprock integrity of CO2 storage systems

We investigated the effect of rock texture and composition on the mechanical strength and volumetric behaviour of anhydrite-rich caprock. Conventional triaxial experiments were performed at 80°C, confining pressures of 1.5–35MPa and strain rates of ~10−5s−1, both dry and in the presence of fluids. We determined the failure envelope, and the effect of fluids upon it, for anhydrite displaying an equigranular texture consisting of euhedral grains. We observed a general pattern of increasing peak compressive strength with increasing confining pressure, consistent with a previous study on anhydrite from the same formation, which showed an irregular texture consisting of interlocking, acicular grains embedded in a fine-grained matrix. The peak strength of euhedral anhydrite was found to be 15–65% lower than that of acicular anhydrite. No chemical effects were observed for the fluids tested. Hydraulic fracturing experiments showed the tensile strength to be approximately 4MPa for euhedral and 6MPa for acicular anhydrite. We combined the failure envelopes with simple, analytical calculations on the effect of cooling on in-situ stress, showing that tensile caprock failure due to cooling would be unlikely for both types of anhydrite, assuming a hypothetical reservoir at a depth of 3800m. However, it should be noted that tensile failure may pose an issue for shallower reservoirs or weaker caprock formations.

Critical Stress for Cleavage Fracture in Continuously Cooled Medium Carbon V-microalloyed Steel

Cleavage fracture of the medium carbon V-microalloyed steel with structure consisting of acicular ferrite, pearlite and grain boundary ferrite has been investigated by means of four-point bending of the notched samples at -196°C. It was found that cleavage fracture initiation has not been related to the coarse second phase particles cracking. Calculated values of the effective surface energy of 49J/m2 and critical cleavage fracture stress normalized by yield stress of 1.84 are in agreement with the results for the steels with ferrite-pearlite and bainite structures.

Properties of Ba-Hexaferrite Thin Films with Different Layer Structures

M-type Ba-hexaferrite (BaM) thin films with two different structures (single layered and double layered) were deposited on (001) Al2O3 substrates by RF magnetron sputtering. The changes in microstructural and magnetic properties of the films corresponding to different layer structures and substrate temperatures (Ts) were investigated. Experimental results indicated that for the single layered films deposited directly on substrates at Ts = 300 °C and Ts = 500 °C, most of the grains are acicular type grains with their c-axis in-plane and/or randomly oriented. However, in the double layered film with first interfacial layer deposited at Ts = 300 °C and second layer deposited at Ts = 500 °C, good crystallographic characteristics and excellent perpendicular c-axis orientation were obtained. The c-axis dispersion angle (Δθc) decreased to 0.49°, while the squareness ratio and coercivity of the out-of-plane respectively increased to 0.85 and 4.67 kOe in the double layered film. The mechanism for improving perpendicular c-axis orientation with the interfacial BaM layer was attributed to an increase in the perpendicularly oriented nucleation sites and the release of the stress that comes from the film-substrate interface.