Analysis of the structure formation and properties of the VT6 alloy during upsetting of the symmetrically truncated conical billets

Based on the results of experimental and theoretical investigations, the parameters of deformation and high-speed hardening for the VT6 alloy at deformation temperatures of 780–940°C and rates from 0.5 × 10^?3 to 1 s^?1 are determined. A mathematical model of upsetting for VT6 samples that have a symmetrically truncated conical shape is constructed with the use of Deform3D package. Failures of the stress-strain state at the side surface during upsetting are determined, and a mathematical model of the structure formation is constructed.     

锻造方式对Ti-6Al-4V合金组织及取向分布的影响

在快锻液压机上对Ti-6Al-4V合金进行了锻造变形,采用扫描电镜、背散射电子衍射技术以及X射线衍射技术研究了不同锻造方式下合金组织及晶粒取向的变化规律.在单向镦拔和换向镦拔两种不同锻造方式下,难变形区、小变形区及大变形区中α相及β相的分布差别不大,组织均匀性基本一致,两种变形方式下锻坯不同区域的应变稍有差别.进一步对不同变形区域形变织构的定量分析可知:在应变较小的边缘区域,变形主要以{0001}基面滑移为主,形成基面织构;在应变较大的内部区域,织构明显转向{1120}、{1010}等柱面织构;在应力集中的位置,会产生{1122}、{1011}等锥面织构.两种锻造方式均能提高Ti-6Al-4V合金中形变织构的均匀性,而且换向镦拔优于单向镦拔.      

A modified Johnson-Cook constitutive relationship for a rare-earth containing magnesium alloy

Lightweight magnesium alloy has recently attracted a considerable interest in the automotive and aerospace industries to improve fuel efficiency and reduce CO2 emissions via the weight reduction of vehicles.Rare-earth（RE） element addition can remarkably improve the mechanical properties of magnesium alloys through weakening crystallographic textures associated with the strong mechanical anisotropy and tension-compression yield asymmetry.While the addition of RE elements sheds some light on the alteration in the mechanical anisotropy,available information on the constitutive relationships used to describe the flow behavior of RE-containing magnesium alloys is limited.To establish such a constitutive relationship,uniaxial compressive deformation tests were first conducted on an extruded Mg-10Gd-3Y-0.5Zr（GW103K） magnesium alloy at the strain rates ranging from 1×10–1 to 1×10–4s–1 at room temperature.A modified Johnson-Cook constitutive equation based on a recent strain hardening equation was proposed to predict the flow stresses of GW103K alloy.Comparisons between the predicted and experimental results showed that the modified Johnson-Cook constitutive equation was able to predict the flow stresses of the RE-containing magnesium alloy fairly accurately with a standard deviation of about 1.8%.     

Modeling the mechanical behavior of tantalum

A crystal plasticity model is proposed to simulate the large plastic deformation and texture evolution in tantalum over a wide range of strain rates. In the model, a modification of the viscoplastic power law for slip and a Taylor interaction law for polycrystals are employed, which account for the effects of strain hardening, strain-rate hardening, and thermal softening. A series of uniaxial compression tests in tantalum at strain rates ranging from 10⁻³ to 10⁴ s⁻¹ were conducted and used to verify the model’s simulated stress-strain response. Initial and evolved deformation textures were also measured for comparison with predicted textures from the model. Applications of this crystal plasticity model are made to examine the effect of different initial crystallographic textures in tantalum subjected to uniaxial compression deformation or biaxial tensile deformation.     

Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al–1?wt-%Zr–0.23?wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB₂ were carried out in the temperature range of 675–900°C using the thermochemical package FactSage. It was predicted that ZrB₂ is stable compared to Al–borides (AlB₁₂, AlB₂) hence would form during boron treatment of molten Al–Zr–B alloys. Al–Zr–B alloys were reacted at 750?±?10°C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB₁₂/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB₁₂), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB₁₂/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2–60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (k_m) of Zr in molten alloy is 9.5?×?10^?4?m?s^?1, which is within a typical range (10^?3 to 10^?4?m?s^?1) observed in other metallurgical solid–liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.     

TEXTURES IN BERYLLIUM RODS AND TUBES EXTRUDED FROM CONSOLIDATED POWDER

Abstract

The textures in beryllium rods and tubes extruded from consolidated powder have been studied by X-ray diffraction techniques.

The temperature of extrusion has little effect on the texture, but the extrusion reduction determines which of two main types of texture is developed in tubes. In low-reduction textures (? 6 : 1), the basal planes tend to concentrate parallel to the surface of the tube; in high-reduction textures (? 12 : 1), the basal planes still lie parallel to the extrusion direction, but tend to concentrate at angles ranging from 45–90° to the surface. A prism-plane fibre texture is present in all tubes.

The deformation mechanisms possibly responsible for these textures are discussed.     

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ～100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 10¹⁰ cm^?3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 10¹¹ cm^?3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al₃Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al₃Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.     

Influence of graphite and copper in mechanical properties of aluminum silicon alloy

The present work deals with the mechanical behaviour of the Al-Si-Cu/graphite composite at different strain rates. Stir casting technique was used to fabricate the Al-Si alloy and composite. In order to improve the wettability of graphite with the alloy, graphite particulates were ball milled with Copper and preheated to 400°C. The uniaxial tensile tests were conducted at various strain rates in the range of 10^?4s^?1 to 10^?2s^?1. The ultimate tensile strength of the composite was higher than that of the alloy for all strain rates. The hardness of the composite was also better than that of alloy. The elongation of the alloy was more than that of composite for all strain rates. The uniform distribution of graphite and silicon particles were observed from the microstructure. The fracture mode of alloy and composite were analyzed.     

Strain rate effect on the evolution of deformation texture for α-Fe

The effect of strain rate on the deformation texture of alpha-iron (α-Fe) is studied at different strain levels during the deformation. Two shear compression specimens (SCS) were deformed in three consecutive stages at room temperature, one at 10⁻³/s and the other at 10⁺³/s, to the same strain level. The crystallographic textures were determined using electron backscattered diffraction. The textures at each deformation stage were found to be unaffected by the wide variations in strain rate. By comparing the stress-strain curves and crystallographic texture at the two strain rate levels, it is realized that for α-Fe there is no marked increase in strain hardening at high strain rate with respect to quasi-static tests, so that the deformation texture remains unchanged. The temperature increase that develops at high strain rates is deemed to sharpen the texture.     

Effect of initial texture on texture evolution in 1050 Al alloys under simple shear

The effect of the initial textures prior to dissimilar channel angular pressing (DCAP) on the texture evolution of the 1050 Al alloy sheets, processed by the continuous confined strip shearing (C2S2) process, were studied. The four different specimens, i.e., cold rolled, heat treated, warm rolled, and as-cast 1050 Al alloy sheets, having various initial textures were prepared using different thermomechanical routes. Although the major texture types were significantly affected by the initial textures prior to DCAP, DCAP always promoted both the 〈111〉//normal direction (ND) textures and the {001}〈110〉 rotated cube texture regardless of the initial texture status. Effects of the texture evolutions due to equal channel angular pressing (ECAP) on deep drawbility and planar anisotropy were analyzed based on the -r value and the Δr value determined from the measured pole figures. A feasibility for producing the 1050 Al alloy sheets having high deep drawbility and low planar anisotropy was demonstrated.     

Characteristics of rapidly solidified Cu–10%Sn alloy powders produced by water jet cooled rotating disc atomisation

Abstract

In the present work, an experimental water jet cooled rotating disc centrifugal atomiser was designed and constructed and used to produce rapidly solidified Cu–10%Sn alloy powders. The characteristics of rapidly solidified Cu–10%Sn alloy powders have been investigated with respect to powder size and disc surface condition. Uncoated and ZrO₂ coated copper discs were used to investigate the effect of disc surface conditions on the microstructure and cooling rate of the powders. The produced powders appeared in the shape of sphere, rounded, ligament, irregular and flaky, depending on the particle size. The powders exhibited fine grained microstructure, cell size increased with increasing powder size and higher cooling rates were obtained using uncoated disc. The results indicated that cooling rates of 20 μm powder produced with uncoated and ZrO₂ material coated discs were estimated as 5·82×10⁵ and 1·44×10⁵ K s^?1 respectively.     

Effect of Process Variables on Transformation-Texture Development in Ti-6Al-4V Sheet Following Beta Heat Treatment

The effect of preheat time, prestrain, cooling rate, and concurrent deformation during cooling on the preferential selection of hcp alpha variants during the decomposition of the high-temperature, bcc beta phase in two-phase titanium alloys was established using Ti-6Al-4V sheet material. For this purpose, sheet tension samples were pre-soaked in the beta phase field for 0 or 10 minutes (to vary the beta grain size), subjected to a prestrain of 0 or 0.1, and cooled at a rate of 11 or 155 K/min (11 or 155 °C/min) under conditions comprising free ends, fixed ends, or concurrent deformation at a strain rate between ~10^?5 and 3 × 10^?4 s^?1. Electron-backscatter diffraction was used to determine the orientations of the alpha variants so formed, from which the underlying high-temperature, beta-grain microstructure and orientations were reconstructed. These measurements revealed that the parent beta texture changed due to grain growth during preheating. A comparison of the alpha- and beta-phase textures indicated that preferential variant selection was most noticeable under conditions involving a slow cooling rate especially when prestrain or concurrent straining was imposed.     

Negative Strain-Rate Sensitivity of Mg Alloys Containing 18R and 14H Long-Period Stacking-Ordered Phases at Intermediate Temperatures

The strain-rate sensitivity of a Mg-10Dy-1Zn (wt pct) alloy containing different long-period stacking-ordered (LPSO) phases has been investigated in the strain rate range of 10^?3 to 1 s^?1 from room temperature to 673 K (400 °C). Both alloys containing 18R-LPSO and 14H-LPSO phases show negative strain-rate sensitivity (–0.02  to –0.01) at intermediate temperatures [423 K to 523 K (150 °C to 250 °C)]. The serration behavior of the Mg-10Dy-1Zn alloy containing 18R-LPSO phase is related to dynamic strain aging. However, the appearance of serrated flow in the Mg-10Dy-1Zn alloy containing 14H-LPSO phase is mostly rooted in the formation of microcracks in \( \left\{ {10\overline{1} 2} \right\} \) planes.     

Analysis of heat flow and microstructure evolution during spray deposition of Fe-3C-1.5Mn alloy

A theoretical model for the concomitant solidification of droplets and preform during spray deposition has been proposed, based on heat-flow analysis. It has been unambiguously demonstrated that cooling rates approaching those in the rapid solidification (RS) regime can only be achieved when the droplets are still in free flight during the deposition process. The cooling rates in the droplets range from 10⁴–10⁶ Ks^?1 depending upon their size for the experimental conditions employed in the present studies. In contrast, the model predicted cooling rates for the deposits in the region of 10³–10⁴ Ks^?1. A hypoeutectic Fe-3C-1.5Mn-0.3Si has been chosen as an experimental alloy for studies relating to microstructural characterization. The microstructure of powder developed fully during solidification of droplets in free flight revealed dendritic morphology of the metastable austenitic phase, whereas the spray-deposited alloy exhibited characteristic homogeneous and refined substructure. The evolution of microstructure during spray deposition as also during atomization has been compared and discussed by invoking the proposed model.     

Relationship between contractile strain ratioR and texture in zirconium alloy tubing

The crystallographic textures of zirconium alloy tubing used as cladding in nuclear reactor fuel are commonly characterized by the quantitative texture numbersF (Källström) and f_r (Kearns) which are derived from the direct and inverse pole figures. The texture numbers of zircaloy 2 and 4 tubes have been correlated experimentally with the value of the contractile strain ratioR which is a measure of the plastic anisotropy of the tube. The correlations were based on the results of 20 different tubing lots. Thef _r-R correlation shows much less data scatter than theF-R correlation. By assuming a simple plastic deformation model for zirconium alloys the following relations between texture and anisotropy are obtained:F=R- 1/R+1 and f_r = R/R+1 The theoretically derived relations are in good agreement with the experimental data. The procedure of correlating texture with plastic anisotropy is not limited to zirconium alloy tubing, but should be equally applicable to textured sheet and plate materials and other alloys with a limited number of slip systems.     

Evolution of Microstructure and Texture during Hot Torsion of Ferritic Stainless Steels Stabilized by Nb and Ti

The study was carried out to understand the mechanisms occurring during dynamic recrystallization of hot deformed 11% chromium stabilized ferritic stainless steels and to compare the behaviour induced by various types of stabilization. The experimental temperatures ranged from 800 to 1150°C and strain rates from 10^?2 to 15 s^?1. The development of the textures and microstructures was analysed using EBSD maps. It was observed that continuous dynamic recrystallization occurs in all materials starting at the onset of straining. Niobium has a more pronounced influence on hardening than titanium during hot deformation. The D2 component was found as the major texture component at the steady state for the torsion tests conducted along the negative shear direction. It was likely to be formed by the combination of straining and growth of the grains exhibiting both low stored energy and low rotation rate of the crystallographic axes. The texture evolution was the main reason for the flow stress behavior during the hot torsion tests. A complementary study was carried out to understand the texture formation occurring during the direct (negative shear direction) and reversed (positive shear direction) hot torsion. After reversion of the shear direction, i.e. during positive shear, the above major texture component is gradually changed into the D1 component. Using the method of Continuum Mechanics of Textured Polycrystals, the stress evolution is explained by the volume fraction changes of each component at various strains, associated with their respective Taylor factors. This simplified approach leads to a good agreement with experimental results.     

Effect of Nd Additions on Extrusion Texture Development and on Slip Activity in a Mg-Mn Alloy

Two Mg-1 wt pct Mn alloys containing 0.5 wt pct and 1 wt pct Nd have been processed by indirect extrusion at temperatures ranging from 548 K (275 °C) to 633 K (360 °C) and speeds between 2.8 and 11 mm/s. The microstructure and the texture of the extruded bars were analyzed in order to understand the effect of the processing parameters and of the rare-earth (RE) alloying additions on the texture development. Increasing the Nd content results in weak textures in which the predominant orientations are a function of the extrusion conditions. This may be explained by the occurrence of particle pinning of grain boundaries and by the nucleation of grains with a wider range of orientations. Mechanical tests were carried out in tension and in compression in all the processed samples at 10^?3 s^?1 and room temperature. It was found that larger RE amounts give rise to the disappearance of the yield asymmetry and to an anomalously high activity of tensile twinning, especially at the lowest extrusion temperatures. This has been attributed to an increase of the critical resolved shear stress of basal slip due to the presence of Mg₃Nd coherent and semi-coherent intermetallic prismatic plates.     

Quantitative analysis of texture evolution of cold-rolled direct-chill-cast and continuous-cast AA5052 and AA5182 aluminum alloys during isothermal annealing

The as-received direct-chill-cast (DC) and continuous-cast (CC) AA5052 and AA5182 hot bands were preheated at 454 °C for 4 hours, followed by cold rolling to an 80 pct reduction in thickness. The texture evolution of these cold-rolled samples during isothermal annealing was investigated by X-ray diffraction. The variation in texture volume fractions with annealing time was quantitatively analyzed by using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The differences in recrystallization textures between the AA5052 and AA5182 alloys and between the DC and CC alloys were compared. It was found that the AA5052 alloy possessed a stronger cube recrystallization texture than the AA5182 alloy for the DC and CC materials. The recrystallization textures of the AA5182 alloy were affected strongly by the annealing temperature. As the annealing temperature increased, the cube recrystallization texture strengthened, whereas the R texture weakened. The annealing temperature had little influence on the recrystallization textures of the AA5052 alloy. The DC AA5052 and 5182 alloys also exhibited stronger cube recrystallization textures than the corresponding CC alloys. For the DC and CC AA5052 alloys, the n value in the JMAK-type equation increased with an increase in the annealing temperature, while the n values varied only slightly with the annealing temperature for the DC and CC AA5182 alloys.     

Texture-Based Optimization of Crystal Plasticity Parameters: Application to Zinc and Its Alloy

Evolutionary algorithms have become an extensively used tool for identification of crystal plasticity parameters of hexagonal close packed metals and alloys. However, the fitness functions were usually built using the experimentally measured stress–strain curves. Here, the fitness function is built by means of numerical comparison of the simulated and experimental textures. Namely, the normalized texture difference index is minimized. The evolutionary algorithm with the newly developed fitness function is tested by performing crystal plasticity parameter optimization for both pure zinc and zinc-magnesium alloy. These materials are promising candidates for bioabsorbable implants due to good biocompatibility and optimal corrosion rate. Although their mechanical properties in the as-cast state do not fulfill the requirements, they can be increased by means of hydrostatic extrusion. The developed modeling approach enabled acquisition of the crystal plasticity parameters and analysis of the active deformation mechanisms in zinc and zinc-magnesium alloy subjected to hydrostatic extrusion. It was shown that although slip systems are the main deformation carrier, compressive twinning plays an important role in texture evolution. However, the texture is also partially affected by dynamic recrystallization which is not considered within the developed framework.