Scaling Analysis of Solidification of Liquid Aluminum Alloy Flowing on Cooling Slope

Among all methods of metal alloy slurry preparation, the cooling slope method is the simplest in terms of design and process control. The method involves pouring of the melt from top, down an oblique and channel shaped plate cooled from bottom by counter flowing water. The melt, while flowing down, partially solidifies and forms columnar dendrites on plate wall. These dendrites are broken into equiaxed grains and are washed away with melt. The melt, together with the equiaxed grains, forms semisolid slurry collected at the slope exit and cast into billets having non-dendritic microstructure. The final microstructure depends on several process parameters such as slope angle, slope length, pouring superheat, and cooling rate. The present work involves scaling analysis of conservation equations of momentum, energy and species for the melt flow down a cooling slope. The main purpose of the scaling analysis is to obtain a physical insight into the role and relative importance of each parameter in influencing the final microstructure. For assessing the scaling analysis, the trends predicted by scaling are compared against corresponding numerical results using an enthalpy based solidification model with incorporation of solid phase movement.

Effects of different modes of hot cross-rolling in 7010 aluminum alloy: part I. evolution of microstructure and texture

The current study describes the evolution of microstructure and texture in an Al-Zn-Mg-Cu-Zr-based 7010 aluminum alloy during different modes of hot cross-rolling. Processing of materials involves three different types of cross-rolling. The development of texture in the one-step cross-rolled specimen can be described by a typical beta-fiber having the maximum intensity near Copper (Cu) component. However, for the multi-step cross-rolled specimens, the as-rolled texture is mainly characterized by a strong rotated-Brass (Bs) component and a very weak rotated-cube component. Subsequent heat treatment leads to sharpening of the major texture component (i.e., rotated-Bs). Furthermore, the main texture components in all the specimens appear to be significantly rotated in a complex manner away from their ideal positions because of non-symmetric deformations in the two rolling directions. Detailed microstructural study indicates that dynamic recovery is the dominant restoration mechanism operating during the hot rolling. During subsequent heat treatment, static recovery dominates, while a combination of particle-stimulated nucleation (PSN) and strain-induced grain boundary migration (SIBM) causes partial recrystallization of the grain structure. The aforementioned restoration mechanisms play an important role in the development of texture components. The textural development in the current study could be attributed to the combined effects of (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component after each successive pass, (b) recrystallization resistance of Bs-oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids.

Evolution of microstructure and texture in friction stir processed Al-Mg-Mn alloy

Friction stir processing was carried out on the Al-Mg-Mn alloy to achieve ultrafine grained microstructure. The evolution of microstructure and micro-texture was studied in different regions of the deformed sample, namely nugget zone, thermo-mechanically affected zone (TMAZ) and base metal. The average grain sizes of the nugget zone, TMAZ and base metal are 1.5 mu m +/- 0.5 mu m, 15 mu m +/- 8 mu m, and 80 mu m +/- 10 mu m, respectively. The TMAZ exhibits excessive deformation banding structure and sub-grain formation. The orientation gradient within the sub-grain is dependent on grain size, orientation, and distance from nugget zone. The microstructure was partitioned based on the grain orientation spread and grain size values to separate the recrystallized fraction from the deformed region in order to understand the micromechanism of grain refinement. The texture of both deformed and recrystallized regions are similar in nature. Microstructure and texture analysis suggest that the restoration processes are different in different regions of the processed sample. The transition region between nugget zone and TMAZ exhibits large elongated grains surrounded by fine equiaxed grains of different orientation which indicate the process of discontinuous dynamic recrystallization. Within the nugget zone, similar texture between deformed and recrystallized grain fraction suggests that the restoration mechanism is a continuous process.

Evolution of microstructure and texture in Ni49.4Ti38.6Hf12 shape memory alloy during hot rolling

This paper deals with the evolution of microstructure and texture during hot rolling of hafnium containing NiTi based shape memory alloy Ni49.4Ti38.6Hf12. The formation of the R-phase has been associated with the precipitation of (Ti,Hf)(2)Ni phase. The crystallographic texture of the parent phase B2 as well as the product phases R and B19' have been determined. It has been found that the variant selection during the B2 -> R phase transformation is quite strong compared to the case of the B2 -> B19' transformation. During deformation, the texture of the austenite phase evolves with strong Goss and Bs components. After transformation to martensitic structure, it gives rise to a 011]parallel to RD fiber. Microstructure and texture studies reveal the occurrence of partial dynamic recrystallization during hot rolling. Large strain heterogeneities that occur surrounding (Ti,Hf)(2)Ni precipitates are relieved through extended dynamic recovery instead of particle stimulated nucleation.

An equivalent undercooling model to account for flow effect on binary alloy dendrite growth rate

A theoretical analysis is carried out to observe the influence of important flow parameters such as Nusselt number and Sherwood number on the tip speed of an equiaxed dendrite growing in a convecting alloy melt. The effect of thermal and solutal transfer at the interface due to convection is equated to an undercooling of the melt, and an expression is derived for this equivalent undercooling in terms of the flow Nusselt number and Sherwood number. Results for the equivalent undercooling are compared with corresponding numerical values obtained by performing simulations based on the enthalpy method. This method represents a relatively simple procedure to analyze the effects of melt convection on the growth rate of dendrites. (C) 2013 Elsevier Ltd. All rights reserved.

Crystallographic texture and microstructure evolution during hot compression of Ti-6Al-4V-0.1B alloy in the (alpha plus beta)-regime

Microstructure and texture are known to undergo drastic modifications due to trace hypoeutectic boron addition (similar to 0.1wt.%) for various titanium alloys e.g. Ti-6Al-4V. The deformation behaviour of such an alloy Ti-6Al-4V-0.1B is investigated in the (+) phase field and compared against that of the base alloy Ti-6Al-4V studied under selfsame conditions. The deformation microstructures for the two alloys display bending and kinking of lamellae in near and softening via globularization of lamella in near phase regimes, respectively. The transition temperature at which pure slip based deformation changes to softening is lower for the boron added alloy. The presence of TiB particles is largely held attributable for the early softening of Ti-6Al-4V-0.1B alloy. The compression texture of both the alloys carry signature of pure phase defamation at lower temperature and phase transformation near the transus temperature. Texture is influenced by a complex interplay of the deformation and transformation processes in the intermediate temperature range. The contribution from phase transformation is prominent for Ti-6Al-4V-0.1B alloy at comparatively lower temperature.

Optimization of degree of sphericity of primary phase during cooling slope casting of A356 Al alloy: Taguchi method and regression analysis

The present work presents the results of experimental investigation of semi-solid rheocasting of A356 Al alloy using a cooling slope. The experiments have been carried out following Taguchi method of parameter design (orthogonal array of L-9 experiments). Four key process variables (slope angle, pouring temperature, wall temperature, and length of travel of the melt) at three different levels have been considered for the present experimentation. Regression analysis and analysis of variance (ANOVA) has also been performed to develop a mathematical model for degree of sphericity evolution of primary alpha-Al phase and to find the significance and percentage contribution of each process variable towards the final outcome of degree of sphericity, respectively. The best processing condition has been identified for optimum degree of sphericity (0.83) as A(3), B-3, C-2, D-1 i.e., slope angle of 60 degrees, pouring temperature of 650 degrees C, wall temperature 60 degrees C, and 500 mm length of travel of the melt, based on mean response and signal to noise ratio (SNR). ANOVA results shows that the length of travel has maximum impact on degree of sphericity evolution. The predicted sphericity obtained from the developed regression model and the values obtained experimentally are found to be in good agreement with each other. The sphericity values obtained from confirmation experiment, performed at 95% confidence level, ensures that the optimum result is correct and also the confirmation experiment values are within permissible limits. (c) 2014 Elsevier Ltd. All rights reserved.

Microstructural Features of Hot Deformed Nb-1Zr-0.1C Alloy

Niobium-based alloys are well-established refractory materials; as a result of their high melting temperature and good creep properties, these alloys find their applications in nuclear reactors. The present study deals with a microstructural response of these materials during hot working. The evolution of microstructure and texture during high-temperature deformation has been investigated in the temperature range 1500-1700A degrees C and strain rate range of 0.001-0.1 s(-1). For each deformed sample, the microstructure has been examined in detail. The microstructural features clearly revealed the formation of a substructure and the occurrence of dynamic recrystallization in a proper temperature-strain rate window. At low strain rates, the necklace structure formation was more prominent.

Effect of shrinkage induced flow on binary alloy dendrite growth: An equivalent undercooling model

This paper presents a theoretical model for studying the effects of shrinkage induced flow on the growth rate of binary alloy dendrites. An equivalent undercooling of the melt is defined in terms of ratio of the phase densities to represent the change in dendrite growth rate due to variation in solutal and thermal transport resulting from shrinkage induced flow. Subsequently, results for dendrite growth rate predicted by the equivalent undercooling model is compared with the corresponding predictions obtained using an enthalpy based numerical method for dendrite growth with shrinkage. The agreement is found to be good. Published by Elsevier Ltd.

Synthesis of a new tungsten-free gamma-gamma ` cobalt-based superalloy by tuning alloying additions

The paper presents the synthesis of a new class of gamma-gamma' cobalt-based superalloy that is free of tungsten as an alloying addition. It has much lower density and higher specific strength than the existing cobalt-based superalloys. The current superalloys have a base composition of Co-10Al and are further tuned by the addition of a binary combination of molybdenum and niobium, with the optimum composition of Co-10Al-5Mo-2Nb. The solvus temperature of the alloy (866 degrees C) can be further enhanced above 950 C by the addition of Ni to give the form Co-xNi-10Al-5Mo-2Nb, where x can be from 0 to 30 at.%. After heat treatment, these alloys exhibit a duplex microstructure with coherent cuboidal L1(2)-ordered precipitates (gamma') throughout the face-centred cubic matrix (gamma), yielding a microstructure that is very similar to nickel-based superalloys as well as recently developed Co-Al-W-based alloys. We show that the stability of the gamma' phase improves significantly with the nickel addition, which can be attributed to the increase in solvus temperature. A very high specific 0.2% proof stress of 94.3 MPa g(-1) cm(-3) at room temperature and 63.8 MPa g(-1) cm(-3) at 870 degrees C were obtained for alloy Co-30Ni-10Al-5Mo-2Nb. The remarkably high specific strength of these alloys makes this class of alloy a promising material for use at high temperature, including gas turbine applications. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A new tungsten-free gamma-gamma ` Co-Al-Mo-Nb-based superalloy

We present the first report of a tungsten-free cobalt-based superalloy having a composition Co-10Al-5Mo-2Nb. The alloy is strengthened by cuboidal precipitates of metastable Co-3(Al,Mo,Nb) distributed throughout the microstructure. The precipitates are coherent with the face-centred cubic gamma-Co matrix and possess ordered Ll(2) structure. The microstructure is identical to the popular gamma-gamma' type nickel-based superalloys and that of recently reported Co-Al-W-based alloys. Being tungsten free, the reported alloy has higher specific proof stress compared to existing cobalt-based superalloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Microstructure dependent elastic modulus variation in NiTi shape memory alloy

Modulus variation of NiTi shape memory alloy has been investigated at microstructural level through nano dynamical mechanical analysis and compared with bulk experimental measurements. The differences between the modulus values at the macro and micro level as well as within the micro level are discussed and the corresponding variations have been explained based on the crystal structure, orientation and misorientation. The experimental results confirm a higher modulus value for the martensite phase that is in agreement with the theoretical predictions. (C) 2015 Elsevier B. V. All rights reserved.

Deformation behaviour of Rheocast A356 Al alloy at microlevel considering approximated RVEs

A micromechanical approach is considered here to predict the deformation behaviour of Rheocast A356 (Al-Si-Mg) alloy. Two representative volume elements (RVEs) are modelled in the finite element (FE) framework. Two dimensional approximated microstructures are generated assuming elliptic grains, based on the grain size, shape factor and area fraction of the primary Al phase of the said alloy at different processing condition. Plastic instability is shown using stress and strain distribution between the Al rich primary and Si rich eutectic phases under different boundary conditions. Boundary conditions are applied on the approximated RVEs in such a manner, so that they represent the real life situation depending on their position on a cylindrical tensile test sample. FE analysis is carried out using commercial finite element code ABAQUS without specifying any damage or failure criteria. Micro-level in-homogeneity leads to incompatible deformation between the constituent phases of the rheocast alloy and steers plastic strain localisation. Plastic stain localised regions within the RVEs are predicted as the favourable sites for void nucleation. Subsequent growth of nucleated voids leads to final failure of the materials under investigation.

Polyvinylidene fluoride based lightweight and corrosion resistant electromagnetic shielding materials

Various NixCo1-x alloys (with x varying from 0-60 wt%, Ni: nickel, Co: cobalt) were prepared by vacuum arc melting and mixed with polyvinylidene fluoride (PVDF) to design lightweight, flexible and corrosion resistant materials that can attenuate electromagnetic radiation. The saturation magnetization scaled with the fraction of Co in the alloy. Two key properties such as high-magnetic permeability and high-electrical conductivity were targeted. While the former was achieved using a Ni-Co alloy, multiwalled carbon nanotubes (CNTs) in the composites accomplished the latter. A unique approach was adopted to prepare the composites wherein PVDF powder along with CNTs and Ni-Co flakes were made into a paste, using a solvent, followed by hot pressing. Interestingly, CNTs facilitated in uniform dispersion of the Ni-Co alloy in PVDF, as manifested from synergistic improvement in the electrical conductivity. A significant improvement in the shielding effectiveness (41 dB, >99.99% attenuation) was achieved with the addition of 50 wt% of Ni40Co60 alloy and 3 wt% CNTs. Intriguingly, due to the unique processing technique adopted here, the flexibility of the composites was retained and more interestingly, the composites were resistant to corrosion as compared to only Ni-Co alloy.

Effects of Interface Treatment on the Fatigue Behavior of Shape Memory Alloy Reinforced Polymer Composites

Interfacial properties of Shape Memory Alloy (SMA) reinforced polymer matrix composites can be enhanced by improving the interfacial bonding. This paper focuses on studying the interfacial stresses developed in the SMA-epoxy interface due to various laser shot penning conditions. Fiber-pull test-setup is designed to understand the role of mechanical bias stress cycling and thermal actuation cycling. Phase transformation is tracked over mechanical and thermal fatigue cycles. A micromechanics based model developed earlier based on shear lag in SMA and energy based consistent homogenization is extended here to incorporate the stress-temperature phase diagram parameters for modeling fatigue.