Acicular Grains Research Articles

MoO3 and AlF3 effect on the properties of porous mullite ceramics with needle‐like microstructure

AbstractPorous mullite ceramics were synthesized via ceramic processing using a local industrial kaolin, alumina, and with AlF3 and MoO3 precursor additives to catalyze the formation of pores and acicular mullite grains. The textural properties, crystalline phases, microstructures, and mechanical properties of the obtained samples were comprehensively analyzed and compared with a reference mullite formulated without additives. The obtained ceramics have a well‐defined porous microstructure, with a porosity of about 50%, and a pore size distribution in the 0.1–2 µm range. Ceramics that involve the addition of MoO3 present well‐defined acicular mullite grains, with slightly higher porosity, larger pores, and better mechanical resistance compared to those processed using AlF3.To correlate the ceramic porosity with its mechanical properties, a simple model of spherical pores was proposed to assess the flexural strength of the dense ceramic. It was found that the inclusion of additives promoting needle‐like microstructures increases the mechanical resistance up to four times the value determined for the ceramic without additives (flexural strength up to 390 MPa for zero‐porosity extrapolation). These results, together with the refractoriness of mullite, allow inferring the potential applications of the developed materials as structural ceramics with relatively low density and for filtering applications.

Correlation between heterogeneous micromechanical properties and fracture toughness in X80 girth weld

The safety accidents caused by the service of pipelines will cause huge economic losses due to the deterioration of the fracture toughness of the girth weld. To clarify the fracture behaviors of girth weld, X80 girth welds with heterogeneous microstructure consisted acicular ferrite (AF) and grain boundary ferrite (GBF) with different micromechanical properties were investigated. The results show that the typical microstructure of X80 girth weld is heterogeneous, which is consist of high strength AF and low strength GBF. With the decrease of the difference in micromechanical properties between AF and GBF, the crack tip opening displacement of three girth welds is 0.13 ± 0.04 mm, 0.18 ± 0.04 mm and 0.29 ± 0.05 mm. Meanwhile, the type of microcracks gradually changed from secondary cracks with GBF aggregation to micro-voids without obvious aggregation, which means the fracture mechanism transforms from cleavage fracture to micro-voids aggregation fracture. The finite element analysis indicates that the change of fracture mechanism is mainly due to the reduction of the gap between the micromechanical properties of AF and GBF, the stress and strain distribution are homogenized, resulting the fracture toughness is significantly improved.

Multi-pass butt welding of thick TA5 titanium-alloy plates by MIG: Microstructure and properties

A 35 mm-thick TA5 titanium alloy plate was successfully joined by metal inert-gas (MIG) welding. The welded joints were analyzed in a more integrated and comprehensive manner. The joint microstructure was analyzed by OM, SEM, EBSD, and TEM. Mechanical properties were analyzed by tensile, impact, and microhardness tests. The corrosion resistance of the joints was determined by FeCl3 immersion corrosion. Experimental results showed that the mechanical properties and corrosion resistance of the fusion zone (FZ) were poorer than those of the base material (BM). The mechanical properties of the welded joints were related to the acicular grains in the FZ. The 260 A joints showed relatively good mechanical properties. With increased welding current, the proportion and size of the acicular grains increased, and the strength of the joint increased, but the plastic toughness decreased. When the welding current was too small, anisotropy occurred in the FZmiddle, which affected the mechanical properties of the joints. The corrosion resistance of the FZ was lower than that of the BM, and the corrosion resistance of the three groups of joints decreased in the order 300 A > 230 A > 260 A.The welding current affected the corrosion resistance of the FZ by influencing the grain size of the FZ, the frequency of the HAGBs, and the GND density.

Manufacturing evaluation of seven medical device companies during the production of a 3D-printed titanium pelvic implant

Powder bed fusion (PBF) technology has become a popular manufacturing method to fabricate custom metallic implants. This trend has generated some regulatory concerns because traditional developing guidelines are not suitable for three-dimensional (3D)-printed implants. This is due to the layered microstructure of additive manufactured parts that produces mechanical properties different than those of traditionally manufactured parts. The inappropriate choice of the process parameters and postprocessing methods can lead to fabrication errors that could negatively affect mechanical properties and dimensional accuracy. The objective of the study was to perform a preliminary evaluation of the quality of manufacturing provided by the medical device industry and identify the best 3D printing practices. We designed a pelvic bone reconstructing implant and asked seven companies to manufacture it in Ti6Al4V with PBF technology. We inspected some important aspects of the manufacturing quality of the prototypes received by evaluating geometrical precision and microstructural integrity in the surface and in the matrix, including a qualitative assessment of voids and grain morphology. Results demonstrated a great difference among the implant prototypes. Two companies proved to be superior and provided defect-free implants. The other five produced evidence of some defects including: geometrical deviations (maximum values of up to 5 mm); heterogeneous acicular grain morphologies; broken sections of lattice structures; and internal and superficial voids and cracks that could potentially compromise functional and clinical performance. To our knowledge, this is the first study analyzing the production of several custom implant additive manufacturers based on a geometrical and microstructural evaluation of the same pelvic implant fabrication. The imperfections found in some prototypes produced by companies certified to commercialize personalized implants highlight the urgent need for technical standards that regulate the safe development of 3D-printed implants. Further analyses are required to determine the actual clinical and mechanical consequences of such imperfections. The results also show that when additive manufacturing is adequately managed, it can be a valid manufacturing method to fabricate defect-free implants.

Crack-Tip Opening Displacement of Girth Welds in a Lean X70 Pipeline Steel.

Crack-tip opening displacement (CTOD) tests were conducted on girth welds of two API 5L X70 pipeline steels (pipe A and pipe B) to investigate the influence of base metal composition on the fracture toughness of the joint. CTOD measurements across the weld showed that the weld fusion zone had the lowest CTOD values for both pipes, with pipe B having a higher CTOD value than pipe A. Detailed microstructure characterization of the multi-pass weld showed that the fusion zone in both pipes consisted of three distinct zones: the columnar zone, the coarse equiaxed zone, and the fine equiaxed zone. Both the columnar zone and coarse-grained equiaxed zone had acicular ferrite and grain boundary ferrite microstructures, whereas the fine-grained equiaxed zone had a finer ferrite microstructure compared to the other two zones. The main difference between the two pipes was the variation in ferrite grain sizes and the volume fractions of grain boundary ferrite and acicular ferrite. In comparison to pipe B, pipe A, with a higher concentration of Mo, Ni, and Cu in both the base metal and the weld fusion zones, consisted of a higher volume fraction of grain boundary ferrite and a lower volume fraction of acicular ferrite in the columnar and coarse-grained equiaxed zones. The lower concentration of Mo, Ni, and Cu in pipe B likely resulted in the formation of a predominantly acicular ferrite microstructure in the fusion zone, thereby improving the toughness of the weld joint in comparison to pipe A.

Colloidal processing, sintering and properties of aluminum borate Al18B4O33 porous ceramics

Aluminum borate (Al18B4O33) is one of the mixed oxides with extensive applications as due to its good thermomechanical properties. Its application requires of accessible processing techniques to provide controlled microstructures and shapes. This work presents an aqueous colloidal route to manufacture porous aluminum borate ceramics from micrometric Al18B4O33 powder (∼2.0 μm) obtained by high temperature reaction of alumina and boric acid. Zeta potential measurements determine that isoelectric point of the Al18B4O33 powders suspended in water happens at pH = 8.5 achieving a maximum positive value of 60 mV for pH < 4. Rheological characterization of concentrated slurries showed a pseudoplastic flowing behavior with a thixotropic cycle, being the yield point and the thixotropy value strongly dependents on the solid content. Shear thinning curves were fitted to a Windhab model that confirmed a face-edge interactions of the particles in the suspension.Porous ceramic with a green density of 50 % and a narrow micronic (1–2 μm) porosity are obtained by slip-casting of the optimal concentrated suspension (33 vol%). Sintering above the formation temperature showed a decrease in the density of the compacts as a consequence of the acicular grain growth. Four sintering conditions were tested to follow the structural and compositional evolution. It has been observed that thermal treatments up to 1300 °C do not affect the composition but the microstructure. At 1400 °C presence of alumina is detected indicating the initial decomposition of Al18B4O33 and evaporation of boron. At 1500 °C decomposition and evaporation phenomena are massive. The porous compacts sintered below the decomposition temperature are characterized by scanning electron microscopy, mercury intrusion porosimetry and mechanical strength, indicating that at 1300 °C Al18B4O33 achieves the more uniform microstructure and the better mechanical response.It should be noted that for the first time the Zeta potential curve for aluminum borate powder is reported. The processing methodology employed in this study, has potential application to the manufacture of porous materials with insulating applications to replace traditional ceramic materials working in harsh conditions, especially in nuclear energy systems with complex geometry by casting techniques and even by 3D printing.

Achieving recrystallization of prior β in Ti-6Al-4V alloy by electroshock

Fine grain strengthening is an important approach to improve mechanical properties in titanium alloys. The martensite (α′) of Ti-6Al-4V alloy generally shows fine acicular grain and has high strength. However, the grain size of martensite is influenced by the prior β grain, thereby affecting the overall strength-plasticity combination of the alloy. Generally, the prior β grains grow rapidly above β-transus temperature (∼1000 ℃). It is difficult to refine the prior β grain through traditional heat treatment alone if it coarsened. In this work, we found that electroshock treatment could directly recrystallize the coarse prior β grain in α+β titanium alloy without plastic deformation process. EBSD and TEM analyses found that the prior β grain size reduced from 44 µm to 29 µm and the remained textures had been eliminated after electroshock, therefore the tensile strength and elongation were simultaneously improved from 1196 MPa and 4.7 % to 1259 MPa and 10.4 %. The residual subgrain boundaries within prior β grain, electric current-induced extra driving force and extremely high heating rate (106 ℃/s) are the main reasons for the occurrence of recrystallization as well as no grain growth of prior β. These findings may be used for repairing the mechanical properties of coarse overheated microstructure or eliminating the anisotropy of columnar grain that produced in additive manufactured α+β titanium alloy without deformation processing.

Influence of initial microstructure on reaustenitization behavior in low alloy steel by in-situ high-temperature EBSD characterization

In-situ high-temperature EBSD characterization was carried out to directly observe reaustenitization in a low alloy steel with different initial microstructures. For lath bainite, globular austenite mainly nucleated at prior austenite grain boundaries, and grew gradually to form uniform small grains from 780 to 860˚C. For granular bainite, martensite-austenite constituents accelerated acicular austenite formation via martensitic reversion transformation. Then, acicular grains coalesced. Finally, recrystallization occurred due to high dislocation density in martensitic revered austenite.

Material properties of selective laser melting additive-manufactured Ti6Al4V alloys with different porosities

In this study, Ti6Al4V specimens with a variety of porosities were prepared using selective laser melting (SLM) by changing its laser hatch spacing. The pores on the surfaces perpendicular (SPER) to the stacking direction (SD) and surfaces parallel (SPAR) to SD of the cube samples were totally different from each other. The topography of SPER was dominated by the laser hatch spacing, whereas SPAR was almost entirely covered with non-molten powder particles having different looseness values. Thus, an optimal modeling direction exists for the real-time application of the alloys. The SLM-manufactured samples were scanned by computed tomography (CT) and then analyzed using the binarization method to investigate their internal porosity, which ranged from 20% to 45%. Tensile tests were conducted to describe their tensile behavior and evaluate their elastic moduli. These analyses revealed an inverse relationship between the porosity and elastic modulus as laser hatch spacing increased. An elastic modulus less than 2000 MPa was obtained using the SLM process, which is sufficient for applications requiring a low elastic modulus. Furthermore, their grain details were observed using electron backscattered diffraction (EBSD). The samples had typical fine acicular grains but with smaller average diameters as the laser hatch spacing increased. Furthermore, the average fraction of misorientation decreased because of the increasing small-angle grains and decreasing large-angle grains, indicating a significant influence of the laser hatch spacing on the grain orientation. Additionally, surface qualities were quantitatively assessed to reveal how laser hatch spacing affected the morphology. By comprehensively analyzing the surface morphology and quality, porosity and tensile properties, and grains details, a vivid material forming theory of the SLM process was concluded.

Synergistic Effects of 2-Butyne-1,4-Diol and Chloride Ions on the Microstructure and Residual Stress of Electrodeposited Nickel.

To assess the individual and synergistic effects of 2-butyne-1,4-diol (BD) and chloride ions on the microstructure and residual stress of electrodeposited nickel, various nickel layers were prepared from sulfamate baths comprising varying concentrations of BD and chloride ions by applying direct-current electrodeposition. And their surface morphologies, microstructure, and residual stress were tested using SEM, XRD, EBSD, TEM, and AFM. While the nickel layers composed of pyramid morphology were prepared from additive-free baths, the surface flattened gradually as the BD concentration of the baths was increased, and the acicular grains in the deposits were replaced with <100> oriented columnar grains or <111> oriented nanograins; additionally, the residual tensile stress of the deposits increased. The addition of chloride ions to the baths containing BD significantly increased the residual stress in the nickel layers, although it only slightly promoted surface flattening and columnar grain coarsening. The effects of BD and chloride ions on the growth mode and residual stress of nickel deposits were explained via analysis of surface morphologies and microstructure. And the results indicate that the reduction of chloride ion concentration is a feasible way to reduce the residual stress of the nickel deposits when BD is included in the baths.

Effect of gadolinium doping on structural, ferroelectric and electrical properties of strontium bismuth niobate ceramics

The substitution of gadolinium (Gd3+) at the Bi-site of SBN (SB1.6G0.4N, SB1.4G0.6N, and SB1.2G0.8N) to realize the technical importance of the materials has been presented. The density of the polycrystalline compositions was obtained establishing sintering conditions from solid-state treatment. The monophasic orthorhombic crystal structure was revealed from the XRD study. The SEM morphology indicated well-defined acicular grains with an induced porosity in all the studied samples. Ferroelectric studies were conducted on SBN and Gd3+ modified SBN ceramic samples and observed that the parameters of 2Ps, 2Pr, and 2Ec, have been found to decrease in gadolinium-substituted SBN compared to SBN. The Cole-Cole plots from the impedance measurement are utilized to evaluate Rg, Rgb, Cg, and Cgb. The conduction process in these materials is through Rg but not Rgb as received from the calculations of Rg and Rgb. The Z' versus frequency plots resulted in an incremental change in temperatures. The constant character of Z' at higher temperatures indicates DC conductivity. The Z“ values have been found to drop and move towards the higher frequency side with rising temperature, which may be ascribed to the inhomogeneity of the polycrystalline ceramic samples. An asymmetric change in M' and M” to zero was observed and the broadening peaks indicate non-Debye relaxation in these materials.

Porous acicular mullite ceramics produced from well and poorly crystallized kaolinite

This work studies how the crystallinity of a raw kaolinite clay affects the properties of mullite ceramics produced from it. To this purpose, two well and poorly crystallized kaolinite samples from the repository of the Clay Mineral Society were assessed. Also, other two samples were produced in which the growth of mullite acicular grains were promoted by addition of a MoO3 precursor. The role of the kaolinite crystallinity and the MoO3 precursor on the properties of the final ceramics was studied. For each sample, the microstructure and the textural, crystalline, and mechanical properties were analyzed.The poorly crystallized kaolinite is more reactive than the well crystallized one and promotes better mullitization. In addition, when the MoO3 precursor is used, the formation of grains with acicular shape inhibits the ceramic shrinkage and promotes higher porosities. On the other hand, the flexural strength is slightly lower for the mullite produced from poorly crystallized kaolinite. Nevertheless, when this property is extrapolated to the case of zero porosity it is found that in such cases the ceramic solid fraction presents higher flexural strength, indicating that the porous mullite with the acicular grain morphology is a reinforced lightweight ceramic. The obtained results show that the crystallinity of the starting clay should be considered in the ceramic fabrication process due to its effects on the properties of the final product.

Effect of microstructure tailoring on the deformation coordination of welded duplex stainless steel

With the improvement of the strength of stainless steel, the proportion of stainless steel used for structural components has increased significantly, and the welding problem has become increasingly important. In this study, the microstructure features of gas tungsten arc welding (GTAW) welded duplex stainless steel (DSS) before and after deformation were characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and nanoindentation. The results show that base metal (BM) consists of bimodal ferrite grains and fine austenite grains while weld zone (WZ) features fine acicular austenite grains scatter between irregularly shaped ferrite. Coarse equiaxed ferrite grains were observed in heat affected zone (HAZ) which have a relatively poor capacity for dislocation multiplication and storage. The appropriate distribution of fine δ-ferrite and γ-austenite and almost no precipitate together guarantee the high yield strength (768.6 MPa), ultimate tensile strength (935 MPa), and enough plasticity (total elongation 32.6%) for the welded DSS. Fractography presents that welded DSS is in ductile rupture mode composed of various dimple patterns in the different regions. The drop in the elongation of welded DSS is attributed to the weakened deformation coordination involving dislocation slipping and multiplication. Investigating the microstructure origin related to the mechanical properties of welded joint may provide ideas for improving the welding process from the microstructure design level.

The microstructure evolution and influence factors of acicular ferrite in low alloy steels

The excellent combination of strength and toughness renders acicular ferrite to be a desirable microstructural contexture in C-Mn and high-strength low-alloy steels. The formation of acicular ferrite in steels is generally affected by factors such as composition, cooling rate, prior austenite grain size (PAGS), and impurity inclusions. Due to the complexity of multi-factor coupling, it is difficult to explore the influence of a single factor through actual experiments. In the current work, the influences of cooling rates, PAGS, and impurity inclusion density on the microstructure evolution of austenite-acicular ferrite transformation in low alloy steels are quantitatively evaluated by multi-phase-field simulation. The numerical results demonstrate that reducing PAGS during austenite-ferrite transformation, increasing cooling rate and increasing inclusion nucleation density can effectively refine acicular ferrite grain size. Moreover, the C element diffuses from acicular ferrite to austenite and accumulates at the phase interface during the phase transformation from austenite to ferrite. The findings in the current work contribute to better regulating and designing acicular ferrite in steels.

Laser Beam Welding of CubeSat 1U Structure Parts Obtained by Powder Bed Fusion

This study contributes to a possible methodology for manufacturing CubeSats using additive manufacturing and laser beam welding. Titanium connectors were constructed by selective laser melting and electron beam melting and characterized from a topological point of view. The connectors can be joined to titanium tubes for the construction of CubeSats via laser spot welding. The fiber laser welds exhibited full penetration using pulses with 400 J of energy. The welds showed titanium acicular martensite grains with recesses and pores. The average hardness of the cast zone was 350 HV, which is close to the hardness of the connectors (400 HV) and more rigid than that of the tubes (100 HV). Spot welding has proven to be useful in resisting forces above 2000 N, which is sufficient for CubeSat frame space applications.

Apatite in brachinites: Insights into thermal history and halogen evolution

Abstract Apatite is an important petrogenetic indicator in extraterrestrial materials. Here, we report the mineralogical features of apatite and associated phases in three brachinites Northwest Africa (NWA) 4969, NWA 10637, and NWA 11756. Two types of apatite are observed: intergranular apatite and apatite inclusion within chromite and silicate minerals. The intergranular chlorapatite is enclosed by or penetrated by irregular porous merrillite, indicating chlorapatite replacement by merrillite. The intergranular chlorapatite is closely associated with a fine-grained pyroxene-troilite intergrowth along olivine grain boundaries, which is a sulfidization product of olivine. High-Ca pyroxene is observed as a constituent phase in the intergrowth for the first time. The apatite inclusions are either monomineralic or closely associated with subhedral-euhedral pore-free merrillite. In NWA 4969, the apatite inclusions show a large compositional variation from chlorapatite to fluorapatite and are systematically more F-rich than intergranular apatite; while the apatite inclusions in NWA 10637 and NWA 11756 are chlorapatite. Most of the two apatite types in brachinites contain oriented tiny or acicular chromite grains, suggesting the exsolution of chromite from apatite. We propose that apatite replacement by merrillite, formation of pyroxene-troilite intergrowth, and exsolution of chromite in apatite were caused by a shock-induced, transient heating event (~930–1000 °C) on the brachinite parent body. This heating event resulted in halogen devolatilization during replacement of the intergranular apatite by merrillite, which probably disturbed the Mn-Cr isotopic system in brachinites as well. We also propose that the apatite inclusions could be a residual precursor material of the brachinites.

Implementation of Exceptional Microstructures and Mechanical Properties of Structural Carbon Steel Tubes by Friction Welding

In this study, friction welding was applied to carbon steel tubes at a high speed condition to achieve an eco-friendly welding process and a superior mechanical performance. Friction welding is a very efficient process for joining materials without defects, especially with joint rods or tubes. For this study, cold drawn carbon steel tubes (AISI 1020) were introduced as the base material and successfully joined by friction welding. Friction welding was performed with parameters of a burn-off length (1, 2 and 5 mm). In order to analyze the welds, the electron backscattering diffraction method was introduced and observed the grain boundary characteristic distributions such as grain size, shape and orientation. To evaluate the relationship between the mechanical properties and microstructure, Vickers microhardness and tensile tests were introduced. As a result, the yield strength of the welds significantly increased relative to the base material at all conditions, and which derived from the microstructure development like a refined acicular ferrite grains through the dynamic recrystallization during the welding. Consequently, we suggest the optimum conditions of the friction welding with the interdependence of the microstructures and mechanical properties in this study. This demonstration substantially offers the possibility of an eco-friendly environment and superior tube welding quality without any defect and harmful gases at the high speed welding process as well as at the lowest cost. This technology can be extended to mass production processes with exceptional benefits for various industries.

Redox Condition and Mineralogical Evidence of the Magma Mixing Origin of the Mafic Microgranular Enclaves (MMEs) from Sircilla Granite Pluton (SGP), Eastern Dharwar Craton (EDC), India

Abstract Mafic microgranular enclaves (MMEs) and Syn-plutonic dykes (SPD) are commonly observed in granitoids and are believed to have a similar period of origin as that of the host. Here the petrographical and mineralogical characteristics of the MMEs and Syn-plutonic dykes from Sircilla granite pluton (SGP), eastern Dharwar craton (EDC), Southern India is presented. The contact relationships of MMEs with the host granite are diffusive or gradational, implying under-cooling and disaggregation of MMEs. Petrographic features such as quartz ocelli, bladed biotite, and acicular apatite grains in mafic enclaves suggest magma mixing/ mingling-related processes. Consequently, crystallization temperature and pressure of hornblende and biotite from SPD indicate rapid undercooling and hence suggest their emplacement towards the final stage of pluton evolution. Mineral chemistry studies reveal that the plagioclase in host SGP compositionally varies from An 23 -An 27, while the Fe# in biotite vary from 0.47-0.52. The composition of plagioclase and mafic phases in MMEs and the SPDs exhibit variation in compositional range, plagioclase (An 26 -An 30), hornblende Mg# (0.42-0.55), and biotite Fe# (0.46-0.54). A decreasing trend in fO2 value in MMEs and SPD during progressive crystallization suggest an orogenic tectonic setting of magma formation. Intermediate composition of biotite and plagioclase from MMEs suggest control of magma mixing processes for their formation.

Fatigue behavior of X65 pipeline steel welded joints prepared by CMT/GMAW backing process

The fatigue behavior of X65 welded joints with cold metal transfer (CMT) backing process and gas metal arc welding (GMAW) backing process has been studied in comparison. The fatigue life of CMT-joints is improved by about 67% due to the combined effect of the low stress concentration in weld root and CMT-produced microstructure. Furthermore, the homogeneously distributed acicular ferrite and grain boundary ferrite arrested crack propagation and increased the fatigue propagation life of CMT-joints, but the necklace-type martensite/austenite constituents and lath bainite would reduce fatigue crack initiation and propagation life of GMAW-joints.

Magma Mixing and Mingling during Pluton Formation: A Case Study through Field, Petrography and Crystal Size Distribution (CSD) Studies on Sirsilla Granite Pluton, India

Abstract Field, petrography, and crystal size distributions (CSD) of different lithological variants from Sirsilla granitic pluton (SGP), southern India, is described here to understand operative magmatic processes. The SGP contains many mafic microgranular enclaves (MMEs) and syn-plutonic dykes. The contact relationship between MMEs and the host granite is often diffusive or gradational and rarely sharp, implying disaggregation and under-cooling of MMEs. Petrographic features like resorption textures, quartz ocelli, and the poikilitic nature of the large K-feldspar grains enclosed within plagioclase indicate interaction and magma mixing/mingling processes in an open magma chamber. Bladed biotite and acicular apatite grains in MMEs are due to rapid crystallization during the magma mingling process. The CSD curves generated for plagioclase provide an inverse relationship between population density and crystal size. Multiple crystal populations, i.e., a gently sloping line for the core samples and a steeply sloping line for margin samples, are interpreted to be caused by the mafic - felsic magma mixing and mingling processes.