Uniaxial Deformation Research Articles

Grain Size Evolution of Nickel-Based Superalloys after Hot Deformation

In this work, the uniaxial compression deformation test of nickel-based powder superalloys was carried out in a vacuum isothermal adiabatic environment using the Thermocmaster-Z thermal processing simulation test machine. The evolution of the microstructure in the two stages of dynamic softening and strain hardening in the true stress-true strain curve in the process, including the interaction of grain size, local misorientation and their relative mobility. During most of the plastic deformation time, the flow stress is gradually increased, that is, the strain hardening dominates, while in the remaining plastic deformation process parameters, dynamic softening dominates. Under different plastic deformation process parameters, The average grain size fluctuates to a certain extent under the condition of partial strain values, but the final evolution law is gradually decreasing, while the final trend of the average local misorientation is increasing.

Calculation of Deformation-Related Quantities in a Hot-Rolling Process

The hot deformation of metal as a nonlinear system is mathematically described by a local linear model associated with the working conditions using a transfer function (TF) in the Laplace domain. Experimental data (true stress vs. true strain curves) are obtained using the established compressive uniaxial deformation test, where experimental conditions (strain rate and temperature) define the working conditions of the local linear TF model, which is intrinsically a function of strain. Based on the TF model, three important physical quantities of the tested metal are determined exactly: the work done per unit deformation, the average flow stress, and the flow-stress derivative with respect to the strain based on a particular TF. The exactly determined quantities, determined as a function of strain, can replace the previously used approximations in some rolling force and torque calculations.

Evolution of the Lamellar Pearlite Structure of Rail Steel under Tension

In this paper, the properties of rail steel subjected to fracture under uniaxial tension deformation are studied. Mechanical properties of rail steel, defective substructure of lamellar pearlite, and fracture surface of rail steel are investigated using methods of modern physical materials science.
 Tensile strength is found to vary from 1247 to 1335 MPa, and the fracture strain of the samples changes from 0.22 to 0.26. The deformation of the rail steel is accompanied by the process of breaking the ferrite plates into fragments by low angle boundaries along with the significant increase in the scalar dislocation density to 7.9-1010 cm-2 (the scalar dislocation density of the original rail steel is 3.2 -1010 cm-2). Also, there is a formation of internal stress fields appearing in the form of bend extinction contours. The sources of stress fields are identified. It is revealed that cementite plates are fractured by cutting and dissolution with subsequent transfer of carbon to ferrite plates by moving dislocations and formation of round-shaped nanodimensional (8.3 nm) round shaped particles of tertiary cementite. It is shown that the dissolution of cementite plates is accompanied by fragmentation (into coherent scattering regions with an average size of 9.3 nm).

Deformation Mechanism of Amorphous Plasticized Poly(vinyl butyral)

Understanding the deformation mechanism of amorphous polymeric materials is indispensable for their applications but is quite challenging. Here, with amorphous plasticized poly(vinyl butyral) (PVB) as a model system, we studied its structural change in situ during uniaxial deformation with ultrasmall-angle X-ray scattering (USAXS). We observed a stretch-induced phase separation behavior characterized by a butterfly scattering pattern in plasticized PVB for the first time. This phase separation, featured by a concentration fluctuation along the stretch direction, is a result of stress–concentration coupling under stretch. The phase size at its formation is around 100 nm, which increases almost linearly with strain. The strain rate has a weak effect on the phase size. This suggests that the elastic deformation of the network, cross-linked by entanglements and hydrogen-bonding clusters, governs the phase size. We proposed a tentative model to explain the deformation behavior of plasticized PVB, where the breaking and reforming of hydrogen bonds, the formation and growth of phase separation against osmotic pressure, and the plastic deformation and rupture of phase structure dissipate a large amount of energy and endow the plasticized PVB with excellent mechanical performance. We believe our findings are general and should be applicable to other amorphous polymers with solvents or plasticizers.

Deformation-induced martensite transformation and variant selection in AISI 316L austenitic stainless steel during uniaxial tensile deformation

Present work has investigated the orientation dependence of deformation-induced martensite (DIM) transformation and contemporaneous variant selection in AISI 316L austenitic stainless steel during uniaxial tensile loading. The combined effect of slip and deformation-induced martensite transformation have been analysed comprehensively using experimental macrotexture analysis, visco-plastic self-consistent (VPSC) simulation and reconstruction of prior-austenite orientations from electron backscatter diffraction (EBSD) data. It has been shown that when the polycrystalline austenitic material is deformed, grains with orientations like Brass, Rot-Goss, Rot-Cu undergo plastic deformation by slip and rotate towards {110}<111> orientation. Grains of this orientation eventually undergo DIM transformation at sufficiently higher engineering strain (≥69%) levels. Variant selection during DIM transformation from {110}<111> component of austenite has been critically examined using a model developed based on the phenomenological theory of martensite crystallography (PTMC). It has been observed that in general, variants with higher interaction energies are preferred over the others. The results in turn indicate that Patel-Cohen theory, which is believed to be valid for stress-induced martensite (i.e. during elastic deformation), holds equally good for strain-induced martensite (i.e. during plastic deformation) as well.

Microscopic mechanism of distinct mechanical properties of divinylbenzene reinforced cross-linked polystyrene revealed by molecular dynamics simulations

Molecular dynamics method was employed to establish the model of three-dimensional cross-linked polystyrene (PS) formed by divinylbenzene (DVB) and PS chains. Density, radial distribution function, free volume fraction, mean square displacement of systems with different cross-linking degrees (DVB contents of 0%, 3.8%, 7.1% and 11.1%) were studied, as well as macroscopic properties such as glass transition temperature, elastic mechanical properties, uniaxial tensile deformation. The results showed that cross-linking algorithm proposed was feasible for constructing cross-linked PS. With the increase of cross-linking degree, cross-linked PS became denser, and glass transition temperature increased, indicating an increase of heat resistance. Compared with uncross-linked PS, elastic modulus of three cross-linked systems increased by 19.26%, 29.56% and 40.19%; bulk modulus increased by 2.9%, 20.98% and 44.03%; and shear modulus increased by 21.05%, 29.82% and 42.98%. Tensile stress-strain curves showed that network structure formed by adding DVB improved yield stress and tensile resistance of PS.

Simulating Stress-Strain Behavior by Using Individual Chains: Uniaxial Deformation of Amorphous Cis- and Trans-1,4-Polybutadiene.

This work develops a probability-based numerical method for quantifying mechanical properties of non-Gaussian chains subject to uniaxial deformation, with the intention of being able to incorporate polymer-polymer and polymer-filler interactions. The numerical method arises from a probabilistic approach for evaluating the elastic free energy change of chain end-to-end vectors under deformation. The elastic free energy change, force, and stress computed by applying the numerical method to uniaxial deformation of an ensemble of Gaussian chains were in excellent agreement with analytical solutions that were obtained with a Gaussian chain model. Next, the method was applied to configurations of cis- and trans-1,4-polybutadiene chains of various molecular weights that were generated under unperturbed conditions over a range of temperatures with a Rotational Isomeric State (RIS) approach in previous work (Polymer2015, 62, 129-138). Forces and stresses increased with deformation, and further dependences on chain molecular weight and temperature were confirmed. Compression forces normal to the imposed deformation were much larger than tension forces on chains. Smaller molecular weight chains represent the equivalent of a much more tightly cross-linked network, resulting in greater moduli than larger chains. Young's moduli computed from the coarse-grained numerical model were in good agreement with experimental results.

Pressure and temperature dependence of the electric modulus and loss factor of Poly(vinyl alcohol)/Poly(vinylidene fluoride) blends reinforced with Nano - Graphene platelets

Poly(vinyl alcohol)/Poly(vinylidene fluoride) blends reinforced with Nano - Graphene Platelets render piezoelectric by combining uniaxial plastic deformation end polarization. Comparative Broadband Dielectric Spectroscopy on fresh and piezoelectric specimens were carried oud at various temperature and hydrostatic pressure states. Dc conductivity, which results from inter-NGP fluctuation induced tunneling of electrons through the effective potential barrier set by the polymer, involves two charge transfer mechanisms. The shift of the glass transition temperature and the temperature triggering a kinetic process due to the enhancement of the mobility of water molecules bound to PVA, upon pressure are detected indirectly through associated changes to the conductivity modes. NGP filling induces a couple of interfacial charge trapping. Relaxations exhibit different thermodynamics and different dependencies for fresh and piezoelectric specimens on pressure. Hydrostatic compression of heterogeneous matter results in an inhomogeneous internal stress field, whereas stress is locally amplified. In PNC, small scale relaxation is disturbed by the spatial fluctuation of the internal stress field. Rendering the PVdF grains piezoelectric, local stress amplification induces fluctuations of the local electric field. Fluctuations (of the internal stress or electric field) affect rather a short-range charge relaxation, rather than macroscopic dc conductivity.

Theoretical modeling for piezoresistive behavior of aligned carbon nanotube/polymer nanocomposites accounting for evolution of agglomerates morphology

In this study, the electromechanical response of carbon nanotube (CNT)-reinforced polymer nanocomposites subjected to uniaxial deformation was investigated theoretically. For this purpose, a comprehensive analytical model that incorporated critical factors affecting the piezoresistive properties of CNT/polymer nanocomposites was proposed. To determine the effects of field-induced CNT alignment and the corresponding prolonged agglomeration morphology, an effective resistor network was established to predict piezoresistivity defined as the normalized resistance change of CNT/polymer nanocomposites. The proposed model accurately described the piezoresistive properties of randomly aligned CNT-based composites compared with a previously developed model, and its further validation was performed using experimental data obtained from the literature for CNTs aligned in a polymer matrix. A similar approach was used for the determination of a percolation threshold, which strongly correlated with piezoresistivity. In addition, the effect of each individual critical factor was examined to establish a desirable set of parameters that would either decrease the percolation threshold or increase the piezoresistivity of composites. • The theoretical model is proposed for the piezoresistive behavior of carbon nanotube (CNT)/polymer composites. • The effects of electric field-induced alignment of CNTs and their aggregation morphology have been taken into account. • An optimal set of critical parameters is determined to increase the piezoresistivity of CNT/polymer nanocomposites.

Diffusion Creep Characteristics of Anorthite Revealed by Uniaxial and Pure Shear Deformation Experiments

AbstractPlagioclase, a major rock‐forming mineral in the Earth's crust, often shows microstructural evidence of ductile deformation in crustal rocks, suggesting that crustal flow is largely controlled by the high‐temperature deformation behavior of this mineral. Here, we present uniaxial and pure shear deformation experiments that reveal various diffusion creep characteristics of anorthite (CaAl2Si2O8), a calcium‐rich plagioclase endmember. We synthesized fine‐grained (∼1 μm) anorthite aggregates with different Al/Si ratios (Al/Si = 1 and 0.97) that were either undoped or doped with 1 wt% MgO. We determined that the synthesized samples deformed by interface (reaction)‐controlled and grain‐boundary diffusion creep mechanisms. At similar conditions of stress, temperature, and grain size, strain rates varied by ∼4 orders of magnitude among the samples. A reduction in Al/Si ratio weakens the aggregate by ∼2 orders of magnitude, while doping with MgO, which probably becomes concentrated at grain boundaries, further weakens samples with different Al/Si ratios to the same low strength level. The resulting low viscosity due to MgO is comparable to that of grain‐boundary diffusion creep in anorthite in previous studies. Grain boundary sliding (GBS)‐induced rigid‐body‐like grain rotation was identified by analysis of a marker‐etched sample surface after deformation. Crystallographic preferred orientation and shape preferred orientation developed after the samples were deformed to strains of ≥0.7, which is well explained by preferential GBS along grain boundaries parallel to (010) and also in the [100] direction within the grain‐boundary plane. These important, but previously unknown, characteristics of anorthite diffusion creep originate from the nature of the grain boundaries.

Deformation Driven Deswelling of Brush Gels

We studied equilibrium swelling in brush gels undergoing large uniaxial deformations in contact with solvent by using a combination of the Flory–Rehner and scaling models of gels and coarse-grained molecular dynamics simulations of swollen brush networks. Our analysis showed that the swelling ratio of the brush gels Qeq(λ), describing the volume change upon swelling, is a nonmonotonic function of the gel uniaxial deformation ratio λ characterizing the gel shape. It first increases with increasing strand elongation from its value for the free-standing gel then begins to decrease. This behavior is a result of the optimization of polymer/solvent interactions and conformational entropy of brush strands in the nonlinear deformation regime. The location of the maximum in the Qeq(λ) function is directly related to the value of the strain-stiffening parameter β. This parameter depends on the degree of polymerization of the brush backbone and the backbone Kuhn length defined by both the grafting density and degree of polymerization of the side chains. Analysis of the nonlinear gel modulus confirmed that the strain-stiffening parameter β is identical in both dry networks and gels. However, the structural modulus of the gels, Gs, is smaller than that of the dry networks, Gdr, by a factor equal to the ratio of the Kuhn length of brush strands in the dry state, bK, and in the gel, bK,s, such that Gs = GdrbK/bK,s.

The strength and microstructure of cemented sand-gravel (CSG) mixture containing fine-grained particles

The hardfill embankments are composed of cemented sand-gravel (CSG) mixtures. In some cases, however, sand and gravel used in the mixture contain fine-grained fraction. This paper aimed to study the mechanical behavior and microstructure of CSG mixtures in which the base soil contains fine-grained particles with different plasticity specifications. Various contents of kaolinite and bentonite were added to a sand-cement mixture with different cement contents and uniaxial strength and deformation modulus in the mixtures were measured at two curing ages. According to the results, in low cement mixtures, the mixture containing 10% of the additive exhibited the maximum strength and deformation modulus, regardless of the additive type. In a family of bentonite-containing mixtures with higher cement, the strength of mixtures decreased with increasing the bentonite content. However, the mixture with higher cement and 10% of kaolinite presented the best performance from in terms of strength and stiffness. Analyzing the mineralogy and SEM images of mixtures showed that the structure of base soil attained the best packing and a strong matrix was devolved inside the mixture with 10% of fine additive. Furthermore, the kaolinite particles in the mixture containing 10% kaolinite contributed to the hydration process of cement.

In-situ EBSD study of deformation behavior of nickel-based superalloys during uniaxial tensile tests

In this paper, we study the deformation behavior of nickel-based superalloys by in-situ EBSD and uniaxial tensile tests. This study mainly focuses on the microstructure evolution in the same area of the sample during deformation, including the evolution of various types of grain boundaries, geometrically necessary dislocations, grain orientations and texture, and their effect on the mechanical properties. The results show that the rotation of grains is closely related to their initial orientation during deformation. The lattice of grain is rotated to a certain extent to accommodate the more favorable orientation under low strain, leading to higher Schmid factors for the sample. Under large stresses, the grains occur deformation, multiple slip systems start simultaneously or alternately and the Schmid factors value gradually tends to stabilize at a lower level. Meanwhile, the texture orientation density of the initial Rotating cubic {001} < 110 > texture, α-fiber (RD//<110 >) texture, and γ-fiber (ND // <111 >) gradually decrease or even disappears, while the texture orientation density of Copper {112} < 111 > and Goss {011} < 100 > increases during uniaxial tensile deformation for Cr20Ni80 alloy. Grain rotation may be a result of large strain due to stress triaxiality and resulting plane stress condition owing to specimen dimension, which is beneficial to improve the plastic deformability of the alloy. Interestingly, with the increase of strain, the average misorientation angle first decreases and then increases, which is associated with the dislocation density. In addition, serious torsion occurs in the grains during this process, forming a large number of substructures. These substructures result in grain refinement and the Hall-Petch effect. In fact, during the deformation process, multiple strengthening mechanisms act in coordination, leading to an increase in the strength of the alloy.

Blending or Bonding? Mechanochromism of an Aggregachromic Mechanophore in a Thermoplastic Elastomer

A straightforward way for the preparation of mechanochromic polymers consists of incorporating a suitable content of a mechanophore in the polymeric matrix either by physical dispersion or via covalent functionalization. Although covalent incorporation may require demanding chemical efforts, this approach can offer significant advantages over physical dispersion. In this work, a common thermoplastic elastomer, styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer grafted with maleic anhydride (SEBS-MAH), was covalently functionalized with 1-aminomethylpyrene (AMP). MAH functional groups are covalently linked to the ethylene-co-butylene blocks, thus allowing a precise and selective confinement of the chromogenic AMP units in the soft block. Flat, fully conjugated pyrene units undergo the reversible formation of π–π aggregates, readily distinguishable by their red-shifted emission. These aggregates were heavily affected by the application of mechanical stimuli. Despite the low degree of mechanophore functionalization (about 1 wt %), uniaxial deformation of the polymer was reliably monitored via fluorescence and a clear drop in the excimer to monomer emission ratio (IE/IM) was observed starting from 50% of strain. The marked mechanochromism was confirmed by emission lifetime measurements and also by near-field investigations. In addition, the mechanoresponse showed good reversibility after repeated stress–relaxation cycles. Control experiments performed on formulations comprising a physical dispersion of pyrene in unfunctionalized SEBS showed faint excimer emission and a negligible mechanochromic response up to 5 wt % of doping, in substantial agreement with the scanning near-field optical microscopy analysis. An evident drop of the IE/IM ratio occurred for 10 wt % of pyrene, albeit the excimer emission remained predominant even at the highest deformation, being a smaller fraction of pyrene moieties involved. Overall, the covalent approach appeared as an elegant procedure to confine the chromogenic unit in the soft phase of block copolymers and thus to provide an elastomeric film showing a detectable and reversible mechanochromic response with a modest (i.e., ∼1 wt %) amount of pyrene molecules, i.e., 10 times smaller compared to the dispersed system.

Synthetic salt rock prepared by molten salt crystallization and its physical and mechanical properties

Physical simulation is an important research method for salt cavern energy storage construction technology. However, its progress is constrained by lack of effective model salt. Exploratory experiments were conducted to prepare salt rock by molten salt crystallization. A well-made synthetic salt rock was obtained from repeated adjustment of staged cooling conditions and controlled crystal growth procedures. Samples were taken for microscopic, physical, and mechanical tests, and compared with natural salt rock. SEM, CT and p-wave velocity results show that synthetic salt rock achieved crystallization. Its crystal structure, grain size and intercrystalline relationship are consistent with those of natural salt rock. Its density distribution is uniform and compaction is good. X-ray results prove a high crystallinity and stable mineral composition. Porosity of synthetic specimens ranges from 2.3% to 5.9%, permeability is in the range of E−13 to E−16 m2. Uniaxial compression stress-strain curves, strengths, deformation modulus and poisson's ratios are close to those of natural salt rock. It has obvious ductility and damage occurs after dilation deformation. We initially illustrate energy storage and heat storage-transfer principles of molten salt. The mechanism of layered growth in molten salt crystallization is revealed. The findings provide new insight on salt rock preparation, crystallography, molten salt and salt cavern study.

Investigation of the Hot Deformation Behavior in VDM® Alloy 780 by In Situ High-Energy X-Ray Diffraction

Ni-based superalloys are indispensable for applications in demanding environments, such as the heavily stressed rotating discs in the hot sections of modern gas turbines or jet engines. In this paper, the microstructure evolution during hot deformation to mimic the forging process was investigated in the polycrystalline VDM® Alloy 780 viain situ X-ray diffraction at temperatures of 950, 1000, and 1050 °C. For the tested temperatures, the hot forming led to subgrain formation, the built-up of a texture by rotation of the matrix grains into preferred orientations, and dynamic recrystallization. The influence of the deformation was analyzed depending on the direction of the lattice plane normals to the load direction, for the first five γ-reflections in the diffraction pattern. During uniaxial compressive deformation intensity, maxima develop in the loading direction solely for the γ-(220) reflections, while intensity minima develop for the other reflections which correspond to the formation of a <110> fiber texture. In the transverse direction, all γ-reflections except the (220) have an increased intensity at the maximum specimen strain of 20 pct. Directly after the hot forming, three different cooling rates of 10, 100, and 1000 °C/min and their influence on the microstructure were investigated. The fast and medium cooling rates lead to low recrystallized fractions and a largely preserved deformation texture, whereas the low cooling rate leads to a high recrystallized fraction and a slight remaining texture. Additionally, the diffraction data are complemented by electron microscopy measurements.

Strain localization and damage development during elevated temperature deformation of AA7075 Aluminum sheet

Elevated temperature forming of AA7075 aluminum alloy is of considerable interest in the automotive industry due to its potential for light weighting and several other environmental benefits. This study presents elevated temperature uniaxial tensile deformation and damage behavior of AA7075-W sheet through a set of experiments at the microstructural scale. A multi-scale experimental methodology consisting of in-situ SEM tensile testing, use of micro digital image correlation (µ-DIC) method for obtaining strain field at the microstructural scale and post-test X-ray computed tomography was adopted with temperature held constant at 300°C and 400°C. The deformation temperature affected the precipitate formation and their distribution and subsequently the precipitate-induced void damage development in the material. At 300°C, precipitates as well as precipitate free zones were formed at the grain boundaries during heating and following deformation. Intergranular void initiation and catastrophic void coalescence with relatively small void growth occurred along the grain boundaries during plastic straining resulting in intergranular fracture and poor ductility at this temperature. In contrast, at 400°C, fewer intergranular precipitates were formed during heating and deformation resulting in delayed void nucleation during plastic straining. The diffuse necking occurred earlier with significant void growth occurring during the long regime of diffuse necking with very little evidence of void coalescence leading to considerably higher ductility at fracture. The final fracture occurred due to transgranular microcrack formation and propagation at this temperature.

Characterization of gradient plastic deformation in gradient nanotwinned Cu

Gradient nanostructured (GNS) metals exhibit high overall extra strengths relative to their non-gradient counterparts. However, the spatial distribution of local extra strengths stemming from plastic strain gradients remains elusive. This work is focused on characterizing the gradient distribution of plastic strains in a representative GNS metal of gradient nanotwinned (GNT) Cu. Full-field strain mapping reveals the gradient distributions of lateral strains in the transverse cross section of GNT Cu samples undergoing uniaxial tensile deformation. We find that the lateral strain gradient increases but the maximum lateral strain difference decreases in GNT samples with increasing structural gradient. The latter arises because the softest layer with the lowest initial yield strength gains the largest local extra strength during tensile deformation, and vice versa. Such a gradient distribution of local extra strengths results from the combined strengthening effects of plastic strain gradient and grain size. These experimental results are used to inform a strain gradient plasticity model for revealing the gradient distributions of local extra back stresses and local extra strengths with increasing load. The coupled experimental and modeling characterization of gradient plastic deformation provides an in-depth mechanistic understanding of the spatial-temporal evolution of gradient strengthening effects in gradient nanostructures.

Quasi-elastic neutron scattering study on dynamics of polymer gels with controlled inhomogeneity under uniaxial deformation.

We study the segmental and monomer dynamics of polymer gels with controlled inhomogeneity under uniaxial deformation by means of quasi-elastic neutron scattering (QENS). In order to clarify the effect of the homogeneity of a network structure on the polymer dynamics in gels, we compare two types of polymer gels with controlled homogeneity: a homogeneous tetra-PEG gel (homo-gel) prepared from uniform tetra-arm pre-polymers and a heterogeneous tetra-PEG gel (hetero-gel) with a small amount of shorter tetra-PEG pre-polymer. The different inhomogeneity in the homo-gel and the hetero-gel has little effect on the average relaxation time of the chain dynamics in the undeformed state. The difference in the local dynamics in the gels is emphasized under uniaxial deformation: while the homo-gel shows a single relaxation mode, the hetero-gel exhibits a bimodal distribution of relaxation times with a slow dynamic mode ascribed to highly stretched short strands, which causes a more brittle macroscopic fracture compared with that in the case of the homo-gel.

Determination of Yield Strength of Single-Crystals with a Hexagonal Lattice at a Given Strain Tensor and Hydrostatic Pressure

Within the framework of the Mises yield criterion generalized to hexagonal crystals proposed by the authors, the problem of determining the stress tensor depending on the strain rate tensor and the applied external pressure is solved. The cases of plane and uniaxial deformation are considered in detail for an arbitrary orientation of the crystal lattice. For the cases of plane and uniaxial deformation, diagrams of stress level lines for the onset of plastic flow are plotted. Diagrams are given for the components of the stress tensor deviator