Showing papers on "Equiaxed crystals published in 2013"

Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys

Abstract: The main types of microstructure are (1) lamellar – formed after slow cooling when deforma‐ tion or heat treatment takes place at a temperature in the single-phase β-field above the socalled beta-transus temperature Tβ (at which the α+β→β transformation takes place), consisting of colonies of hexagonal close packed (hcp) α-phase lamellae within large body centered cubic (bcc) β-phase grains of several hundred microns in diameter, and (2) equiaxed – formed after deformation in the two-phase α+β field (ie, below Tβ), consisting of globular α-phase dispersed in β-phase matrix [7-8]

Microstructure and tensile properties of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near β titanium alloy

Abstract: A near β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe is fabricated by laser melting deposition (LMD). The evolution of its macrostructure and microstructure during LMD process are systematically studied. Then the relationship between the microstructure and tensile properties is further investigated. The macrostructure exhibits unique bamboo-like β grain morphology, which forms during the solidification and remelt treatment during the LMD process. The microstructures exhibit fine basketweave microstructure and continuous grain boundary α (αGB), whose formation is associated with the complex thermal cycling treatment. Interestingly, an unique fork-like α is observed but gradually disappears during the evolution process of microstructure. The underlying mechanism is proposed. Besides, tensile tests show that the fine microstructure results in high strength, and the continuous αGB causes the low ductility and intergranular fracture. Meanwhile, the cracks are prone to propagate along the interface between the columnar and equiaxed regions.

A Successful Synthesis of the CoCrFeNiAl0.3 Single-Crystal, High-Entropy Alloy by Bridgman Solidification

Abstract: For the first time, a face-centered-cubic, single-crystal CoCrFeNiAl0.3 (designated as Al0.3), high-entropy alloy (HEA) was successfully synthesized by the Bridgman solidification (BS) method, at an extremely low withdrawal velocity through a constant temperature gradient, for which it underwent two BS steps. Specially, at the first BS step, the alloy sample underwent several morphological transitions accompanying the crystal growth from the melt. This microstructure evolves from as-cast dendrites, to equiaxed grains, and then to columnar crystals, and last to the single crystal. In particular, at the equiaxed-grain region, some visible annealing twins were observed, which indicates a low stacking fault energy of the Al0.3 alloy. Although a body-centered-cubic CoCrFeNiAl (Al1) HEA was also prepared under the same conditions, only a single columnar-crystal structure with instinctively preferential crystallographic orientations was obtained by the same procedure. A similar morphological transition from dendrites to equiaxed grains occurred at the equiaxed-grain region in Al1 alloy, but the annealing twins were not observed probably because a higher Al addition leads to a higher stacking fault energy for this alloy.

Morphology of Carbon Dioxide−Hydrogen−Cyclopentane Hydrates with or without Sodium Dodecyl Sulfate

Abstract: In this study, effects of cyclopentane (CP) and sodium dodecyl sulfate (SDS) on the hydrate formation morphology were investigated. A gas mixture of 40.0 mol % carbon dioxide and 60.0 mol % hydrogen was used in an unstirred system with subcooling as the driving force. Experimental pressure is at 6.0 MPa and experimental temperatures used are at 275.65 and 277.65 K (ΔT = 15.15 and 13.15 K). Formation of hydrates started at the cyclopentane–liquid water interface. Cloud-like, equiaxed skewed dendritic, equiaxed orthogonal dendritic, long dendritic, and cactus-like crystals could be observed for the experiments in the absence of surfactants. Rapid hydrate formation was observed for the experiments with 0.9 mL CP with or without the presence of surfactants compared to the experiments with 0.45 mL CP system at the same experimental conditions. The addition of SDS had led to a change in the hydrate crystal morphology, forming fiber-like crystals from the hydrate layer. Hydrates had also shown affinity to metal ...

Deformation behavior and microstructural evolution of as-cast 904L austenitic stainless steel during hot compression

Abstract: Hot compression tests of as-cast 904L austenitic stainless steel were carried out at deformation temperatures of 1000–1150 °C and strain rates of 0.01–10 s−1 with different strains. The hot working behavior was investigated by the analyses of flow curves, deformed microstructures and kinetics. The results show that the flow stress depends strongly on the deformation temperature and the strain rate, and it increases with the deformation temperature decreasing and the strain rate increasing. Also, the flow curves combined with microstructural evidence indicate that the dynamic recrystallization process of this material is very sluggish with the increase of strain. High temperature and low strain rate together with large deformation can provide the right changes for obtaining more equiaxed dynamically recrystallized grains. The deformation energy (Q) in the whole range of conditions is calculated to be 459.12 kJ/mol by regression analysis and the constitutive equation embraced the Zener–Hollomon parameter is developed. Furthermore, the processing maps (PMs) are generated to reveal the correlation between microstructural evolution and process parameters based on the flow stress data. It is observed from the PM that two regions of deformation stability and instability are characterized. An optimal processing window available for the hot deformation can be obtained to achieve the desired microstructure with dynamic recrystallization. In addition, the predicted instability regions are verified and the reasons of these instabilities are revealed.

The effect of microstructure on the mechanical properties of TC4-DT titanium alloys

Abstract: This paper presents the results on the microstructure and mechanical properties of damage tolerance TC4-DT titanium alloy after different heat treatments. The influence of volume fraction of primary α-phase and the morphology of lamellar microstructure on the tensile properties and fracture toughness of the alloy was studied. Static tensile, fracture toughness tests and microstructure investigations were performed. The result shows that heat-treatment can adjust the microstructure feature, and cooling rate and aging conditions have remarkable effect on the microstructure parameters, such as the content of equiaxed α, dimension of β grains and thickness of lamellar α, which has a significant effect on the properties of the alloys tested.

Microstructural Characterization of Laser-Deposited Al 4047 Alloy

Abstract: Direct metal deposition (DMD) technology is a laser-aided rapid prototyping method that can be used to fabricate near net shape components from their CAD files. In the present study, a series of Al-Si samples have been deposited by DMD in order to optimize the laser deposition parameters to produce high quality deposit with minimum porosity and maximum deposition rate. This paper presents the microstructural evolution of the as-deposited Al 4047 sample produced with optimized process parameters. Optical, scanning, and transmission electron microscopes have been employed to characterize the microstructure of the deposit. The electron backscattered diffraction method was used to investigate the grain size distribution, grain boundary misorientation, and texture of the deposits. Metallographic investigation revealed that the microstructural morphology strongly varies with the location of the deposit. The layer boundaries consist of equiaxed Si particles distributed in the Al matrix. However, a systematic transition from columnar Al dendrites to equiaxed dendrites has been observed in each layer. The observed variation of the microstructure was correlated with the thermal history and local cooling rate of the melt pool.

Influence of Material Microstructures in Micromilling of Ti6Al4V Alloy

Abstract: In the most recent decades the introduction of unconventional machining processes allowed the development of micromachining techniques. In this work, the influence of material microstructures on the micromilling process was investigated. Ti6Al4V alloy was selected as workpiece material since it is a very common material for micro applications and because its duplex microstructure can be easily changed by proper thermal treatments. Four different microstructures (namely bimodal, fully equiaxed, fully lamellar and mill annealed) were obtained through recrystallization annealing treatments carried out at different times and temperatures. The mechanical properties of the samples were assessed by microhardness measurements. Nano-indentations were also performed on single grains to understand how the different hardness of phases and structures present in the Ti6Al4V alloy can affect the micromilling process. Microchannels using two flute flat end mills with a diameter equal to 200 µm were realized on the treated samples. Two different feed-per-tooth values were used during the tests. Cutting force, channel shape and burr dimension were investigated. Morphological and energy dispersive spectroscopy (EDS) analyses were performed on tools by means of a scanning electron microscope (SEM): in this way the phenomena mainly influencing the tool status were also identified. Lower cutting forces and reduced tool wear were observed when working fully lamellar microstructures compared to the other ones.

Effect of minor Er and Zr on microstructure and mechanical properties of Al–Mg–Mn alloy (5083) welded joints

Abstract: Samples of Al–Mg–Mn and Al–Mg–Mn–Er–Zr alloys were welded using the method of laser welding. The influence of Er and Zr on microstructure, microhardness and mechanical properties of the Al–Mg–Mn alloy welded joints were investigated. It has been found that addition of Er and Zr refines the grain size in the fusion zone, due to the formation of primary Al3Zr and Al3Er. Fine equiaxed grains are dominated near the fusion boundary of the Al–Mg–Mn–Er–Zr alloy joint, which is contrary with the columnar crystal in the Al–Mg–Mn alloy joint. Microhardness of the center of the fusion zone rises from 74HV0.1 to 84HV0.1 owing to the grain refinement by Er and Zr. The tensile test result shows that the ultimate tensile strength and yield strength are improved by adding Er and Zr. The main reason for this is related to grain refining strengthening.

Effect of Cooling Rate on the Microstructure of Laser-Remelted INCONEL 718 Coating

Abstract: The rapid cooling rate was achieved during laser remelting with high scanning speed. The microstructure and precipitations in the INCONEL 718 remelted layer were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), and solid phase microextraction (SPME). The phase transition temperatures were carried out by differential thermal analysis (DTA). The results showed that columnar-dendritic and equiaxial structures appeared in different regions of the remelted layer. The dendritic spacing of the columnar dendrite and equiaxed grain size decreased with increasing scanning speed. The precipitations in the remelted layer consisted of Laves, granular phase, and a small quantity of quadrangular nitride (Ti, Nb)N. The granular phase Nb(Al, Ti) was precipitated at about 1272 K (999 °C) with the spontaneous decomposition of the supersaturation Laves during the cooling stage, and the small-size granule became coarsened to 0.2 to 0.9 μm during the cooling stage. The noncoherent relationship existed between the granular phase and austenite, and the coarsening of granule was related to the cube root of the diffusion coefficient, interfacial energy, and diffusion time. The microhardness of the remelted layer was increased by increasing the cooling rate due to the Nb atomic solid solution strengthening caused by the distorted elastic stress field and the short-range internal stress.

Role of Grain Refinement in the Hot Tearing of Cast Al-Cu Alloy

Abstract: The effects of grain refinement on hot tear formation and contraction behavior in a modified Al-Cu alloy 206 (M206) have been studied. The experiments were conducted using a newly developed mold which could simultaneously measure the contraction force/temperature during solidification for a restrained casting, and thereby could be used to investigate hot tear formation. Quantitative information on crack initiation and propagation was obtained by analyzing load measurement data. Al-Ti and Al-Ti-B master alloys were added to the melt to refine the grains to obtain grains ranging from columnar dendritic to equiaxed dendritic and globular structures. Effects of grain structure and grain size on hot tearing susceptibility were investigated. The correlations between microstructure evolution in grain-refined castings at various levels and hot tear formation were determined and discussed. Grain refinement was found to have a complex effect on load onset. Hot tearing tendency was significantly affected by both grain size and grain morphology as reflected by the measured data.

Effect of Microstructural Variation on Erosion Wear Behavior of Ti-6Al-4V Alloy

Abstract: The present article describes the effect of microstructural variations—that is, lamellar, bimodal, and equiaxed—on solid particle erosion wear behavior of Ti-6AL-4V alloy at room temperature. Erosion tests were carried out at various test conditions using an air jet–type test rig and Taguchi's orthogonal array experimental design. The results indicated that impact velocity is the most significant controlling factor influencing the solid particle erosion wear of Ti-6Al-4V alloy followed by impact angle, microstructural variation, and size of erodent. The lamellar microstructure of Ti-6Al-4V alloy has excellent erosion resistance, followed by bimodal and equiaxed microstructures. Ploughing or pile-up leading to platelet formation was found to be the primary mechanism of material loss in erosion of Ti-6Al-4V alloy. This mechanism of material loss is independent of its microstructural variation. These results were determined after observation of the eroded surface under a scanning electron microscope. Optical...

Metallographic and fracture characteristics of resistance spot welded TWIP steels

Abstract: In this study, the microstructural characterisation, mechanical testing and fractography investigation were performed on twinning induced plasticity (TWIP) steels, fabricated with resistance spot welding. Failure mode during the cross-tensile test was found to follow the sequences of strain localisation of both sheets, crack initiation at notch tip, crack following along the fusion boundary and, finally, ductile shear fracture along the sheet thickness direction. On the other hand, failure in the tensile shear test was always directed along the sheet/sheet (s/s) interface; the interfacial failure and shear deformation were observed at the weld centreline. Solidification occurred as a primary austenitic solidification mode, and no martensitic transformations were detected through electron backscatter diffraction analysis. The fusion zone was mainly composed of austenite with directional solidification towards the centreline; the columnar dendritic and equiaxed structures were identified. Interdendr...

Effect of heat treatment on microstructure and microhardness evolution in a Ti–6Al–4V alloy processed by high-pressure torsion

Abstract: A Ti–6Al–4V alloy was heat-treated to give two types of microstructures with different volume fraction of equiaxed α phase and lamellar (α + β) microstructure. Disks were cut from the heat-treated rods and processed by quasi-constrained high-pressure torsion (HPT) at room temperature with an applied pressure of 6.0 GPa and torsional straining from 1/4 to 20 turns. The results show that there is a gradual evolution of homogeneity in microhardness and grain size with increasing numbers of revolutions in HPT such that the microhardness values attain a maximum constant value across the disk after processing by HPT for 10 turns and the measured equilibrium grain sizes after 20 turns are ~130 nm in Ti64-1 and ~70 in Ti64-2. The results show also that a larger fraction of lamellar (α + β) in the microstructure of Ti–6Al–4V leads to a higher hardenability after processing by HPT.

Micromechanical behavior study of α phase with different morphologies of Ti–6Al–4V alloy by microindentation

Abstract: In order to investigate four typical morphologies of α phase, including Platelet α, α case, Equiaxed α, and Martensite α, microindentation tests of Ti–6Al–4V under different heat treatment conditions were performed with various indentation loads from 50 mN to 4900 mN and two loading speeds of 9.6841 mN/s and 103.7053 mN/s by a sharp Berkovich indenter. The microhardness H, young's modulus E, initial yield stress σy and strain hardening exponent n were obtained by means of Oliver–Pharr method and reverse analysis algorithms. The results show that H of four typical morphologies decreases with the increase of indentation load. When loads are above around 2000 mN, H of Equiaxed α is the largest. However, when loads fall down below 450 mN, H of α case is the largest. This phenomenon can be explained by indentation size effects (ISE) and degree of ISE can be indicated by variable h H ⁎ . E of four typical morphologies also decreases with the increase of indentation load, which can be explained by damage during indentation. Among four morphologies, damage could be easily generated and spread within Equiaxed α. But it is difficult for α case. Moreover, σy and n remain constant with different loads in four morphologies. The working hardening effect becomes weaker as n becomes larger.