Showing papers on "Equiaxed crystals published in 2011"

Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy

Abstract: The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size.

Effect of microstructure on retained austenite stability and work hardening of TRIP steels

Abstract: Retained austenite is a metastable phase in transformation induced plasticity (TRIP) steels that transforms into martensite under local stress and strain. This transformation improves sheet formability, allowing this class of higher strength steels to be used for applications such as automotive structural components. The current work studies two distinct TRIP steel microstructures, i.e. equiaxed versus lamellar, and how microstructure affects the austenite transformation during uniaxial tensile loading. Different heat treatments were employed to obtain the two microstructures, and the bainite hold times of the treatments were varied to change the volume fraction of retained austenite. Based on uniaxial tensile response and magnetic saturation measurements, the bainite hold time of 100 s was determined to produce the best results in terms of largest strain at the ultimate tensile strength and highest volume fraction of retained austenite. The work hardening of the samples with a 100 s bainite hold was evaluated by calculating the instantaneous n value as a function of strain. It was found that the lamellar microstructure has a lower maximum instantaneous n value than the equiaxed microstructure, but has higher work hardening values for strain levels greater than 0.05 and up to the ultimate tensile strength. This difference in work hardening behaviour corresponds directly to the transformation rate of retained austenite in the two microstructures. The slower rate of transformation in the lamellar microstructure allows for work hardening to persist at high strains where the transformation effect has already been exhausted in the equiaxed microstructure. The different rates of transformation can be attributed to the location, carbon content, and size of the retained austenite grains in the respective TRIP microstructures.

Room-temperature ductility of Ti–6Al–4V alloy with α′ martensite microstructure

Abstract: The microstructures, cold-rolling abilities, and mechanical properties of a Ti–6Al–4V alloy with a α′ microstructure were examined, and compared with these properties in alloys with the (α + β) microstructure. The microstructure of a Ti–6Al–4V alloy solution treated at 1373 K followed by quenching (referred to as 1373 K STQ in the following) exhibits the acicular α′ martensite microstructure. The alloy exhibits the (equilibrium α + α′ martensite) bimodal microstructure when 1223 K solution treatment is carried out followed by quenching (referred to as 1223 K STQ). In contrast, Ti–6Al–4V alloys heat treated at 1373 K or 1223 K, followed by furnace cooling (referred to as 1373 K ST-FC and 1223 K ST-FC), exhibit a lamellar (α + β) microstructure and an equiaxed (α + β) microstructure, respectively. The strength of the STQ specimens was found to be higher than that of the ST-FC specimens. The 1223 K STQ specimen had excellent cold-rolling ability (more than 40% reduction) and higher tensile ductility than those of the ST-FC specimens. The excellent ductility of 1223 K STQ is thought to be due to the constituent phase of the single HCP phase (α′ and α) and the fine equiaxed bimodal morphology. The compositional distribution of the V content enrichment and the decrease in the Al content in the α′ martensitic region during solution treatment at 1223 K should contribute to the excellent ductility. This work therefore suggests a useful method for improving room-temperature ductility by using industrial Ti–6Al–4V alloys with the α′ martensite microstructure.

In situ TiC particles reinforced Ti6Al4V matrix composite with a network reinforcement architecture

Abstract: TiC particles reinforced Ti6Al4V (TiCp/Ti6Al4V) composite with a network TiCp distribution has been successfully fabricated by reaction hot pressing of coarse Ti6Al4V particles and fine carbon powders. TiC particles are in situ synthesized around the boundaries of the Ti6Al4V particles, and subsequently formed into a TiCp network structure. Contrary to the typical Widmanstatten microstructure for the monolithic Ti6Al4V alloy, an equiaxed (α + β) microstructure for the Ti6Al4V matrix of the composite is formed. This is due to the isotropic tensile stress generated by the network TiCp structure and the mismatch of coefficients of thermal expansion (CTE) during the phase transformation. The prepared composite exhibits superior compressive strength before and after heat-treatment due to the reinforcement network architecture and the relatively large matrix region with an equiaxed microstructure.

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Abstract: Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.

Mechanical properties of sintered ZrB2–SiC ceramics

Abstract: ZrB2 ceramics containing 10–30 vol% SiC were pressurelessly sintered to near full density (relative density >97%). The effects of carbon content, SiC volume fraction and SiC starting particle size on the mechanical properties were evaluated. Microstructure analysis indicated that higher levels of carbon additions (10 wt% based on SiC content) resulted in excess carbon at the grain boundaries, which decreased flexure strength. Elastic modulus, hardness, flexure strength and fracture toughness values all increased with increasing SiC content for compositions with 5 wt% carbon. Reducing the size of the starting SiC particles decreased the ZrB2 grain size and changed the morphology of the final SiC grains from equiaxed to whisker-like, also affecting the flexure strength. The ceramics prepared from middle starting powder with an equiaxed SiC grain morphology had the highest flexure strength (600 MPa) compared with ceramics prepared from finer or coarser SiC powders.

Quantitative Evaluation of Bulk and Interface Microstructures in Al-3003 Alloy Builds Made by Very High Power Ultrasonic Additive Manufacturing

Abstract: Ultrasonically consolidated 3003 aluminum alloy builds were prepared with constituent tapes by using a very high power ultrasonic additive manufacturing (UAM) process. Microstructures of interface and bulk regions were quantitatively characterized using the electron backscattered diffraction technique. The interface microstructure consists of equiaxed grains. The 〈111〉 crystallographic directions of these grains were aligned with the normal direction of the specimen, confirming a shear deformation mode at these regions. In addition, due to recrystallization, the density of low-angle grain boundaries also significantly decreased. In contrast, original elongated grains and partially recrystallized grains were observed in the bulk region of the tape. These elongated grains correspond to rolling texture components of face-centered-cubic materials. The preceding microstructure gradients are rationalized based on the accumulated thermomechanical cycles during processing.

Microstructural characterization and evolution mechanism of adiabatic shear band in a near beta-Ti alloy

Abstract: The adiabatic shear band (ASB) was obtained by split Hopkinson pressure bar (SHPB) technique in the hat-shaped specimen of a near beta-Ti alloy. The microstructure and the phase transformation within the ASB were investigated by means of TEM. The results show that the elongated subgrains with the width of 0.2–0.4 μm have been observed in the shear band boundary, while the microstructure inside the ASB consists of fine equiaxed subgrains that are three orders of magnitude smaller than the grains in the matrix. The β → ω(althermal) phase transformation has been observed in the ASB, and further analysis indicates that the shear band offers thermodynamic and kinetic conditions for the ω(althermal) phase formation and the high alloying of this alloy is another essential factor for this transformation to take place. The thermo-mechanical history during the shear localization is calculated. The rotational dynamic recrystallization (RDR) mechanism is used to explain the microstructure evolution mechanism in the shear band. Kinetic calculations indicate that the recrystallized fine subgrains are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Abstract: Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.

Development of nanocrystalline structure in Cu during friction stir processing (FSP)

Abstract: The characteristics of microstructures at various locations behind the pin tool extraction site were studied in copper after FSP that had been conducted with continuous quenching to enhance cooling rates. The substructures initially formed around the pin tool consist of very small crystallites having sizes of a few tens of nanometers. It is proposed that the processing conditions result in formation of microband structures around pin tool in the presence of severe strain heterogeneity. The microbands appear as nano-scale elongated crystallites surrounded by high-angle boundaries. The elongated crystallites transform to nearly random oriented and equiaxed grain structures by shape adjustment during the initial stages of cooling from the peak temperature. Nanocrystalline structures ∼174 nm in size were produced in OFHC copper by FSP.

Effect of inclusions on the solidification structures of ferritic stainless steel: Computational and experimental study of inclusion evolution

Abstract: The effect of oxide and nitride inclusions in a steel melt on the formation of the equiaxed grain structure during solidification of ferritic stainless steel has been investigated. The solidified grain size decreased with an increasing content of titanium. In steel samples with large solidified grains, the inclusions were generally a two-phase system in which the titanium oxide was precipitated in the liquid TiOx–Cr2O3–SiO2 matrix during cooling. Alternatively, in steel samples with fine equiaxed grains, single TiN and MgAl2O4–TiN complex particles were observed. MgO–Al2O3–TiOx ternary compounds formed in molten steel, and the spinel crystals grew at the expense of the liquid phase as the temperature decreased. Concurrently, the TiN nucleated on the surface of the MgAl2O4 particles because the lattice disregistry between MgAl2O4 and TiN was low. The formation behaviors of non-metallic compounds were successively predicted via thermochemical computation. Single mode log-normal distributions with mode particle diameters ( d m o d e ) were observed in many samples, whereas a bimodal distribution was obtained in solidified samples with a fine-grained equiaxed structure. The grain sizes of the solidified samples decreased when the mean diameter of the inclusions increased. Consequently, the solidification structure can be interpreted based on the effectiveness of TiN and MgAl2O4–TiN complex inclusions as inoculants for the nucleation of δ -Fe.

The Influence of a Small Boron Addition on the Microstructure and Mechanical Properties of Ti-6Al-4V Fabricated by Metal Injection Moulding†

Abstract: The effect of boron additions on the sintering behavior, microstructural development, and mechanical properties of a Ti-6Al-4V alloy fabricated by metal injection moulding (MIM) was studied. The addition of boron promotes a significant refinement of the microstructure by changing the microstructure from the typical lamellar to a more equiaxed morphology. The presence of both features: α colonies and α grains were confirmed by electron backscatter diffraction (EBSD) experiments. Furthermore, the pinning effect of TiB particles on grain boundary motion enhances the densification process due to the fact that the separation of pores and grain boundaries is suppressed. As a result of the refinement of the microstructure achieved by adding 0.5 wt% boron to the Ti-6Al-4V alloy, excellent tensile (σ 0.2 = 787 MPa, UTS = 902 MPa and e = 12 %) and fatigue (endurance limit = 640 MPa) properties were obtained.

Microstructural evolution and mechanical properties of a Cu–Zr alloy processed by high-pressure torsion

Abstract: Experiments were conducted on a Cu–0.1% Zr alloy in order to examine the evolution of hardness and microstructure after processing by high-pressure torsion. Disks were pressed through different numbers of revolutions up to a maximum of 10 turns using an applied pressure of 6.0 GPa. It is shown that there is a gradual evolution in both the hardness and the microstructure with increasing numbers of turns. After 5 and 10 turns there is a high degree of hardness homogeneity and the microstructure consists of well-defined equiaxed grains. The measured grain size after 5 turns was ∼180 nm in the peripheral region of the disk. Tensile testing at 673 and 723 K after processing through 5 and 10 turns showed a maximum elongation to failure of ∼280% at 723 K using a strain rate of 1.0 × 10−4 s−1.

Microstructure and texture studies on twin-roll cast AZ31 (Mg–3 wt.%Al–1 wt.%Zn) alloy and the effect of thermomechanical processing

Abstract: The microstructure and texture of the twin-roll cast (TRC) AZ31 (Mg–3 wt.%Al–1 wt.%Zn) sheet, with a thickness of 6 mm, have been investigated. The TRC AZ31 exhibits a dendritic microstructure with columnar and equiaxed grains. These contain Al–Mn and Mg–Al–Zn second-phase particles that are approximately 1 μm in size. This heterogeneous structure is attributed to the effect of the cooling rate, which varies from 325 °C/s on the surface to ∼150 °C/s in the mid-thickness of the sheet. No surface segregation, but a certain degree of macrosegregation is observed in the mid-thickness which persists after annealing and rolling. Recrystallization at 420 °C leads to a bimodal grain-size distribution, while a fine-grain structure is obtained after rolling and annealing. The TRC AZ31 sheet exhibits basal textures in the (i) as-received, (ii) rolled and (iii) rolled-annealed conditions. However, post-annealing of the TRC AZ31 at 420 °C produces a relatively random texture that has not been previously observed in the conventional AZ31 sheet. The texture randomization is attributed to the particle-stimulated nucleation of new grains in the TRC structure. The preliminary evaluation of mechanical properties indicates that such annealing treatment slightly increases the ultimate tensile strength (UTS), but significantly improves elongation.

Microstructural features, texture and strengthening mechanisms of nanostructured AA6063 alloy processed by powder metallurgy

Abstract: Nanostructured AA6063 (NS-Al) powder with an average grain size of ∼100 nm was synthesized by high-energy attrition milling of gas-atomized AA6063 powder followed by hot extrusion. The microstructural features of the consolidated specimen were studied by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques and compared with those of coarse-grained AA6063 (CG-Al) produced by hot powder extrusion of gas-atomized powder (without using mechanical milling). The consolidated NS-Al alloy consisted of elongated ultrafine grains (aspect ratio of ∼2.9) and equiaxed nanostructured grains. A high fraction (∼78%) of high-angle grain boundaries with average misorientation angle of 33° was noticed. Microtexture evaluation by plotting pole-figures and orientation distribution function (ODF) analysis showed Copper and P texture components for both the consolidated Al alloys. Tensile test at room temperature and microhardness measurement revealed that a significant improvement in the strength of AA6063 alloy is obtained through refinement of the grain structure. The strengthening mechanisms are discussed based on the dislocation-based models. The role of high-angle and low-angle grain boundaries on the strengthening mechanisms is discussed.

Deformation behavior and microstructure evolution in multistage hot working of TA15 titanium alloy: on the role of recrystallization

Abstract: Interrupted compression tests of TA15 titanium alloy with initially equiaxed microstructure were carried out at deformation temperatures between 1173 to 1273 K and strain rates between 0.001 to 0.1 s−1 to investigate the deformation behavior and microstructure evolution under multistage deformation. The TA15 alloy exhibits significant flow softening in both β and (α + β) working. It is found that the flow softening relates to dynamic recrystallization of β phases under current experimental conditions. In multistage β working, metadynamic recrystallization is the main softening mechanism during inter-pass holding. The grain refinement by metadynamic recrystallization leads to the decrease in peak stress upon reloading. In multistage (α + β) working, static recrystallization is the main softening mechanism during inter-pass holding. The static recrystallization kinetics increases with temperature and strain rate. The inter-pass holding has little influence on the morphology of the primary α phases. The β grain size is determined by spacing of primary α phases, which is more affected by working temperature but less dependent on strain rate and inter-pass holding time.

Thermo-mechanical and microstructural issues in dissimilar friction stir welding of AA5086–AA6061

Abstract: In this research, thermo-mechanical behavior and microstructural events in dissimilar friction stir welding of AA6061-T6 and AA5086-O have been evaluated. The thermo-mechanical responses of materials during the process have been predicted employing a three-dimensional model together with a finite element software, ABAQUS. Then, mechanical properties and microstructures of the weld zone were studied with the aid of experimental observations and model predictions. It is found that the mixing of material in the weld nugget is performed more efficiently when AA5086 is in the advancing side and also the temperature field is distributed asymmetrically resulting in larger thermally affected region in the AA6061 side. Besides, the microstructural studies shows that the microstructures of stirred zone consist of fine equiaxed grains where finer grains are produced in AA6061 side compared to AA5086 side.