Showing papers on "Equiaxed crystals published in 2008"

Efficient Melt Stirring Using Pulse Sequences of a Rotating Magnetic Field: Part II. Application to Solidification of Al-Si Alloys

Abstract: The present study considers the solidification of an Al-7 wt pct Si alloy under the influence of electromagnetic melt stirring using a rotating magnetic field (RMF). The effect of a continuously applied RMF is compared with an RMF pulse sequence of alternating direction (RMF-PSAD). The resulting flow structure in a cylindrical liquid metal column has been measured by isothermal experiments using the ternary alloy GaInSn. The solidification experiments performed with the Al-7 wt pct Si alloy confirm our numerical predictions concerning the temperature field during solidification and the distribution of primary crystals and eutectic phase in the solidified samples. The application of the RMF-PSAD regime at suitable frequencies of the reversals of the magnetic field direction fP delivers an equiaxed microstructure without macrosegregation.

Influence of processing temperature on the microstructures and tensile properties of 304L stainless steel by ECAP

Abstract: L austenitic stainless steel was successfully processed by equal channel angular pressing (ECAP) in the temperature range of 500-900 ◦ C, and the influences of processing temperature on the microstructures and tensile properties were investigated. At temperature below 700 ◦ C, the microstructures were characterized by lamellar structures and many bundles of deformation twins, which led to a high tensile strength but low elongation-to-failure. With increasing the processing temperature up to 800 ◦ C, dynamic and also static recovery took place and more equiaxed subgrains with low dislocation density were obtained. Deformation twins were found to form only in some grains in the form of individual bands. A low strength and a high elongation-to-failure were achieved compared with those processed at low temperature. The best combination of both high strength and large elongation took place at the processing temperature of 800 ◦ C. © 2007 Elsevier B.V. All rights reserved.

Direct Rolling of As-Cast Ti–6Al–4V Modified With Trace Additions of Boron

Abstract: Trace boron additions to titanium alloys produce an order of magnitude reduction in as-cast grain size, leading to the possibility of significant simplification of ingot breakdown and thermomechanical processing procedures. In this study, the boron modified titanium alloy Ti–6Al–4V + 0.1B (wt.%) was hot rolled from the cast + HIP condition to thickness reductions of 50% and 75% in multi-step rolling sequences. Baseline alloys (without boron) in the cast and wrought (mill product) states were also processed under identical conditions for comparison. After 50% reduction in thickness at 750–950 °C (1382–1742 °F), the deformation behavior of cast Ti–6Al–4V + 0.1B is not noticeably different from that of standard Ti–6Al–4V mill product, whereas cast Ti–6Al–4V without boron exhibited extensive cracking. The boron-containing alloy could be deformed further to 75% reduction in thickness at 950 °C (1742 °F) without producing any macroscopic defects. The alpha phase shows a tendency to globularize during heat treatment after a 50% reduction in thickness to produce an equiaxed microstructure. The potential, therefore, exists for the production of slab and sheet stock with an equiaxed microstructure by directly rolling the as-cast titanium alloys modified with trace boron additions.

Influence of High-Pressure Torsion Straining Conditions on Microstructure Evolution in Commercial Purity Aluminum

Abstract: The influence of heat generation during severe plastic deformation on microstructure evolution was investigated in commercial purity aluminum (Al 1050, CP-Al) by using high-pressure torsion (HPT) process. The microstructure was characterized by the observations of the torsion and the longitudinal planes. CP-Al disks were deformed by HPT-straining up to 20 turns (equivalent strain, "eq ,o f� 600) at 0.2 or 5 rpm at room temperature. To prevent the increase in specimen temperature, HPT-straining was also carried out in liquid nitrogen. In the all conditions, the value of Vicker’s microhardness, Hv, was saturated around 0.65 GPa and the microstructure consisted of the equiaxed grains of about 500 nm with quite low dislocation density. The microstructure in the early stage of HPT-straining showed the deformed (sub)structure, and then the equiaxed grain structure with high-angle boundaries formed by grain subdivision, recovery, continuous recrystallization and grain growth with increase in strain amounts and specimen temperature. [doi:10.2320/matertrans.ME200713]

Effect of precipitates on damping capacity and mechanical properties of Ti–6Al–4V alloy

Abstract: The present study was concerned with the effects of over-aging on damping property and fracture toughness in Ti–6Al–4V alloy. Damping property and toughness become important factors for titanium implants, which have big modulus difference between bone and implant, and need high damping capacity for bone-implant compatability. Widmanstatten, equiaxed, and bimodal microstructures containing fine α 2 (Ti 3 Al) particles were obtained by over-aging a Ti–6Al–4V alloy. Over-aging heat treatment was conducted for 200 h at 545 °C. Fracture toughness, Charpy impact, and bending vibration tests were conducted on the unaged and the over-aged six microstructures, respectively. Charpy absorption energy and apparent fracture toughness decreased as over-aging was done, even if the materials were strengthened by precipitation of very fine and strong α 2 -Ti 3 Al particles. On the other hand, damping properties were enhanced by over-aging in Widmanstatten and equiaxed microstructures, but was weakened in bimodal microstructure due to the softening of tempered martensite and the decreasing of elastic difference between tempered martensite and α phase contained α 2 particles, etc . These data can provide effective information to future work about internal damping and fracture properties of Ti–6Al–4V alloy.

Characterization of nanostructured TiN coatings fabricated by reactive plasma spraying

Abstract: The nanostructured TiN coatings are fabricated by means of reactive plasma spraying micrometers titanium powders in the atmosphere, and the microstructure and performance of the coatings are analyzed by XRD, SEM and TEM. The experimental results show that the coatings are mainly composed of TiN and Ti3O phases, and the coatings have the typical sprayed lamellae structures. In parallel to substrate surface direction, the nanoscale grains with particle diameters ranging from 60 to 120nm are observed in the coatings, and both fine equiaxed and columnar grains are found in some zones of the nanostructured TiN coatings. But in vertical to substrate surface direction, the contrary is the case. Thus it can be concluded that the TiN coatings are composed of the columnar grains, and the columnar grains are nanostructural equiaxed grains in their cross-section. In addition, a large number of deformation twins caused by the stresses concentration are found in TiN coatings. Meanwhile, the nanostructured TiN coatings have a higher bonding strength and better fracture toughness than other observed as-sprayed coatings.

Modeling of equiaxed growth in multicomponent alloys accounting for convection and for the globular/dendritic morphological transition

Abstract: A lagrangian model has been developed to predict the evolution of a single equiaxed crystal moving into an infinite medium, and intended to be coupled with a macroscopic code devoted to the solidification of large cast products. A particular attention is paid to the transition from globular to dendritic crystals because both morphologies can be observed in cast products. First, the model has been compared with experiments of settling NH 4 Cl crystals in NH 4 Cl-H 2 O melts. Good agreements have been achieved provided that a suitable value is chosen for the stability constant of the dendrite tip growth model. The model is then applied to a multicomponent steel so as to outline the link between the morphology and the sign of the chemical segregation. Finally, the trends drawn from the model concerning the change in morphology are discussed so as to explain the concomitancy of the structural and chemical patterns observed in the equiaxed zone of heavy steel ingots.

Mechanical Blocking Mechanism for the Columnar to Equiaxed Transition

Abstract: Mechanical blocking of the columnar front during the columnar to equiaxed transition (CET) is studied by quantitatively comparing the CET positions obtained with one stochastic model and two deterministic models for the unidirectional solidification of an Al-7 (wt pct) Si alloy. One of the deterministic models is based on the solutal blocking of the columnar front, whereas the other model is based on the mechanical blocking. The solutal-blocking model and the mechanical-blocking model with the traditional blocking fraction of 0.49 give columnar zones larger than those predicted with the stochastic model. When a blocking fraction of 0.2 is adopted, however, the agreement is very good for a range of nucleation undercoolings and number density of equiaxed grains. Therefore, changing the mechanical-blocking fraction in deterministic models from 0.49 to 0.2 seems to model more accurately the mechanical-blocking process that can lead to the CET.

Change in Mechanical Properties and Microstructure of ARB Processed Ti during Annealing

Abstract: Ultrafine grained commercial purity titanium (CP-Ti) was fabricated by accumulative roll-bonding (ARB) process up to 6 cycles at ambient temperature. The microstructure was composed of the equiaxed grain structure having a mean grain size of 90 nm and the lamellar boundary structure having a mean lamellar spacing of 70 nm. The specimen ARB processed by 6 cycles were subsequently annealed at various temperatures for 1.8 ks. After annealing at 400 � C, the ARB specimen showed the partially recrystallized microstructure composed of recrystallized grains with grain size of approximately 0.5 mm and the recovered ultrafine structure. After annealing at 500 � C, the microstructure was filled with the equiaxed recrystallized grains having a mean grain size of approximately 2 mm. The mechanical properties of the ARB processed and subsequently annealed specimens were investigated by tensile test. The tensile strength decreased and the total elongation increased continuously with increasing the annealing temperature. It was found that the tensile strength decreased linearly with increasing the total elongation in a strength-ductility balance plot, which was significantly different from the cases of Al and the interstitial free (IF) steel where the strength-elongation balance showed a trade-off relationship. The result indicates that ultrafine grained Ti has an excellent strength-ductility balance compared with Al and IF steel. [doi:10.2320/matertrans.ME200710]

Friction Stir Processing of Investment-Cast Ti-6Al-4V: Microstructure and Properties

Abstract: Investment-cast titanium components are becoming increasingly common in the aerospace industry due to the ability to produce large, complex, one-piece components that were previously fabricated by mechanically fastening multiple pieces together. The fabricated components are labor-intensive and the fastener holes are stress concentrators and prime sites for fatigue crack initiation. The castings are typically hot-isostatically-pressed (HIP) to close internal porosity, but have a coarse, fully lamellar structure that has low resistance to fatigue crack initiation. The as-cast + HIP material exhibited 1- to 1.5-mm prior β grains containing a fully lamellar α + β microstructure consistent with slow cooling from above the β transus. Friction stir processing (FSP) was used to locally modify the microstructure on the surface of an investment-cast Ti-6Al-4V plate. Friction stir processing converted the as-cast microstructure to fine (1- to 2-μm) equiaxed α grains. Using micropillars created with a dual-beam focused ion beam device, it was found that the fine-grained equiaxed structure has about a 12 pct higher compressive yield stress. In wrought products, higher strength conditions are more resistant to fatigue crack initiation, while the coarse lamellar microstructure in the base material has better fatigue crack growth resistance. In combination, these two microstructures can increase the fatigue life of titanium alloy castings by increasing the number of cycles prior to crack initiation while retaining the same low-crack growth rates of the colony microstructure in the remainder of the component. In the current study, high-cycle fatigue testing of investment-cast Ti-6Al-4V was performed on four-point bend specimens. Early results show that FSP can increase fatigue strength dramatically.

The influence of the as-hot rolled microstructure on the elevated temperature mechanical properties of magnesium AZ31 sheet

Abstract: An AZ31 alloy was rolled at temperatures between 350 and 450 °C, after soaking at rolling temperature for either 1 or 10 h, using various rolling reductions per pass and rolling speeds. Two extremes of as-hot rolled microstructure were generated: (i) a banded structure comprised of very fine equiaxed grains, but with a few ‘pancaked grains’, i.e. ‘partially recrystallized’, and (ii) much coarser equiaxed grains with no banding or pancaked grains, i.e. ‘recrystallized’. These hot-rolled sheets were tensile tested at temperatures between 300 and 450 °C, with strain rates from 0.1 to 0.001 s−1. Grain boundary sliding appeared to dominate at higher test temperatures and lower strain rates, and dynamic recrystallization dominated at lower temperatures and higher strain rates. In terms of good high temperature formability (i.e. low flow stress and high ductility) the partially recrystallized structure was more beneficial than the fully recrystallized structure, under test conditions where grain boundary sliding dominated.

Microstructure evolution in adiabatic shear band in fine-grain-sized Ti-3Al-5Mo-4.5V alloy

Abstract: Dynamic testing of Ti–3Al–5Mo–4.5V (TC16) alloy was carried out in a hat-shaped specimen by a split Hopkinson pressure bar (SHPB) at ambient temperature. The microstructure and phase transformation in adiabatic shear band (ASB) produced in TC16 alloy were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). ASB in TC16 alloy is a “white” band of width about 13 μm. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation indicates that the maximum temperature within ASB is about 1069 K. The phase transformation and the microstructure evolution within ASB in TC16 alloy are explained.

Effect of heating rate on the microstructure and texture of continuous cast AA 3105 aluminum alloy

Abstract: The hot band of a continuous cast (CC) AA 3105 aluminum alloy was annealed at different temperatures for 6 h in an air furnace with a heating rate of 11 °C/min and annealed at different temperatures for 30 min in a salt bath (∼50 °C/s), respectively. The effect of heating rate on the microstructure and texture of the CC AA 3105 aluminum alloy was investigated. In the case of slow anneal, the coarse elongated recrystallized grains were observed at the temperatures of 482–599 °C. The recrystallization texture was characterized by relatively strong P orientation and 22.5° ND rotated cube orientation. In the case of rapid anneal, the size and shape of recrystallized grains depended on the annealing temperature. As the annealing temperature increased, the size of recrystallized grains decreased and the recrystallized grains became gradually equiaxed. At 599 °C, a fine equiaxed grain structure was obtained. The recrystallization texture consisted of a weak cube texture. Moreover, the fine-grain alloy produced by the rapid anneal exhibited higher tensile strength and elongation than the slow anneal sample that had a larger grain size.