Severe Plastic Research Articles

Mechanical properties and fracture behavior of ultrahigh temperature ceramics at ultrahigh temperatures

The mechanical properties of an ultrahigh temperature ceramic (UHTC) made up of ZrB2, SiC and graphite flake (ZrB2-SiC-G) are explored by ultrahigh-temperature uniaxial tension, combined with in-situ monitoring of the fracture behavior. The results suggest an inconsistent property degradation of ZrB2-SiC-G at 1200–2000 °C. Significant degradation in tensile modulus is observed at 1400 °C, which differs from the strength degradation that mainly occurs at 1800 °C. Moreover, ZrB2-SiC-G is elastically fractured at 1200 °C, 1400 °C and 1600 °C, which is interrupted at 1800 °C and 2000 °C. At the temperature range, severe plasticity takes place, which reduces the bearing capacity of this material. The brittle-to-ductile transition is well ascertained by in-situ observation of the fracture behavior at 1200–2000 °C. Finally, the constitutive relation of modulus and strength is built by coupling the effect of temperature. It can be integrated in finite element models and is useful for structural design of UHTC based thermal protection systems of hypersonic vehicles.

Shock-induced plasticity and resultant defect analysis in a vacuum kinetic sprayed silver film

High velocity particle impact generates a shock wave that penetrates the material, resulting in severe plasticity and damage. Finite element method simulation demonstrates that Ag particle momentarily undergoes severe deformation with a high strain rate because of stress fluctuation in the material during the shock wave passage. High-resolution transmission electron microscopy analysis reveals that the lattice was severely distorted, flow-like bent with many geometrically necessary dislocations and stacking faults, twinned at the nanoscale, and collapsed into amorphous state.

Effect of material positioning on Si-rich TIG welded joints of AA6082 and AA8011 by friction stir processing

The TIG-welded specimens were friction stir processed (FSPed) to analyze the impact material positioning of the base metals AA8011 and AA6082. The tests included the energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction, microstructure, hardness, tensile test and fracture surface analysis. The mean grain size of 24.83 µm was obtained for the TIG welded joint, 8.68 µm for the FSPed joint with AA6082 alloy on the advancing side (A.S), while 11.14 µm was obtained when the AA8011 was positioned on the A.S. In the TIG + FSPed zone, an intricate vortex flow pattern could be seen. The extremely superplastic flow of plasticized material is seen in the mixed flow zone, which results in a tumultuous intercalation pattern. In situ extrusions of different alloys and severe plastic, deformation was induced by stirring tool action. Lamellae structure was generated by moving the plasticized material layer by layer. In this zone, vigorous mixing and penetration of A6082 and AA8011 were observed. The maximum tensile strength and strain rate of the TIG weldment showed a significant increase after friction stir processing from 83.83 MPa with a strain rate of 23.79% to 90.09 MPa with a tensile strain of 28.04. However, evaluating the FSPed joints, the AA8011-AA6082 specimen had better tensile properties. Moreover, the fracture surface analysis correlated with the tensile tests. The FSPed AA6082-AA8011 joint showed a higher hardness than the AA8011-AA6082 one.

Study on the Effect of Milling Surface Plastic Deformation on Fatigue Performance of 20Cr and TC17 Specimens

In addition to the micro-topography of machined surfaces, plastic deformation is an important factor affecting the machined surface integrity and the fatigue performance of specimens. For 20Cr and TC17 materials, the effect law of milling surface plastic deformation on high- and low-cycle fatigue performance of specimens was studied. Experimental results show that the effect law of milling surface plastic deformation on high- and low-cycle fatigue performance of specimens is different. When the high-cycle fatigue life is about 2 × 105 cycles, severe surface plastic deformation can increase the maximum stress suffered by TC17 specimens from 1060 MPa to 1080 MPa; when the low-cycle fatigue life is about 6 × 104 cycles, severe surface plastic deformation can reduce the maximum stress suffered by 20Cr specimens from 680 MPa to 660 MPa. Therefore, severe surface plastic deformation can improve the high-cycle fatigue performance and, on the contrary, it has a negative effect on the low-cycle fatigue performance. Combined with the analysis of the surface integrity index and the fatigue fracture topography of specimens, it is concluded that the machined surface plastic deformation significantly reduces the plastic deformation capacity of surface layer material and enhances the stress concentration phenomenon caused by the surface micro-topography, so that when a specimen is subjected to a large load, it is very easy to make microcracks appear on the machined surface and form the “over-plastic deformation” phenomenon, resulting in a sharp decrease in the low-cycle fatigue performance of specimens. This suggests that the plastic deformation degree of machined surfaces needs to be optimized depending on the magnitude of the working load and the micro-topography.

Investigation of Dry Sliding Wear Behavior of Pack Boriding Fe-Based Powder Metallurgy

The powder compact was directly sintered by a pack boriding process to prepare a Fe-based material having a boride layer. The boriding process was carried out at 850 °C, 950 °C and 1050 °C for 3, 5 and 10 h. The morphology of the boride layer was observed by a metallographic microscope, and the thickness of the boride layer was measured. The surface hardness of the boride layer was measured by a Rockwell hardness tester, the phase composition of the boride layer was analyzed by X-ray diffractometer and the friction and wear properties of the sample were tested on the HT-1000 friction and wear tester. The results show that the boride layer was mainly composed of Fe2B single phase, and its thickness was uniform and firmly bonded to the substrate. The thickness and hardness of the layer gradually increased with time and temperature. The binding force between the layer and the substrate would be decreased while the thickness of the layer was too thin or too thick. Boriding at 850 °C and 950 °C resulted with formation of Fe2B phases, while FeB and Fe2B boride phases occurred at 1050 °C. The boride layer has good wear resistance in the friction and wear experiments, the wear mechanism of the boride specimen is mainly the fatigue spalling caused by plastic deformation of microcracks, while the un-bored samples show severe oxidation and plastic deformation.

STRAIN INDUCED PHASE TRANSFORMATION IN Ti-15Mo β ALLOY

β-Ti alloys have been chosen for biomedical applications attributed to a combination of high strength, high fatigue resistance, good corrosion resistance and more importantly low modulus closer than other metallic materials for implants and osseointegrated prosthesis to the cortical elastic modulus (4-30 GPa). However, the phase constituents and phase transformation are still under dispute. A Ti-15Mo alloy after severe cold plastic deformation is studied to reveal the phases and phase transformation by TEM techniques. Athermal ω phase was observed in all samples evidencing the high stability of ω phase compared to β phase. However, the β to ω transformation does not proceed to a completion in heat treatment. Strain induced phase transformation happens in cold-draw wires through a coordinated shuffle of atoms along the {2 1 1}planes of β phase leading to the reduction on the <1 1 1> directions. The atomic level of shear causes the transformation of β to ω. The transformation of β to athermal ω under a train is not in a stable state but having a variable crystal structure between β and ω.

CAMK2-Dependent Signaling in Neurons Is Essential for Survival.

Ca2+/calmodulin-dependent protein kinase II (CAMK2) is a key player in synaptic plasticity and memory formation. Mutations in Camk2a or Camk2b cause intellectual disability in humans, and severe plasticity and learning deficits in mice, indicating unique functions for each isoform. However, considering the high homology between CAMK2A and CAMK2B, it is conceivable that for critical functions, one isoform compensates for the absence of the other, and that the full functional spectrum of neuronal CAMK2 remains to be revealed.Here we show that germline as well as adult deletion of both CAMK2 isoforms in male or female mice is lethal. Moreover, Ca2+-dependent activity as well as autonomous activity of CAMK2 is essential for survival. Loss of both CAMK2 isoforms abolished LTP, whereas synaptic transmission remained intact. The double-mutants showed no gross morphological changes of the brain, and in contrast to the long-considered role for CAMK2 in the structural organization of the postsynaptic density (PSD), deletion of both CAMK2 isoforms did not affect the biochemical composition of the PSD. Together, these results reveal an essential role for CAMK2 signaling in early postnatal development as well as the mature brain, and indicate that the full spectrum of CAMK2 requirements cannot be revealed in the single mutants because of partial overlapping functions of CAMK2A and CAMK2B.SIGNIFICANCE STATEMENT CAMK2A and CAMK2B have been studied for over 30 years for their role in neuronal functioning. However, most studies were performed using single knock-out mice. Because the two isoforms show high homology with respect to structure and function, it is likely that some redundancy exists between the two isoforms, meaning that for critical functions CAMK2B compensates for the absence of CAMK2A and vice versa, leaving these functions to uncover. In this study, we generated Camk2a/Camk2b double-mutant mice, and observed that loss of CAMK2, as well as the loss of Ca2+-dependent and Ca2+-independent activity of CAMK2 is lethal. These results indicate that despite 30 years of research the full spectrum of CAMK2 functioning in neurons remains to be unraveled.

Investigation on Tribological Properties of the Pre-oxidized Ti2AlN/TiAl Composite

Different oxidation layers on the Ti2AlN/TiAl substrate which was fabricated by in situ synthesis were prepared through thermal oxidation process. The microstructure, phase identification and elements distribution of the oxidation layers were analyzed. The tribological performance of pre-oxidized composites against Si3N4 ball at 25 and 600 °C, as well as the effect of pre-oxidation layers on tribological performance was systematically investigated. The results show that, compared to Ti2AlN/TiAl, the pre-oxidized composites present more excellent tribological properties, especially the wear resistance at 600 °C. It is a significant finding that, different from severe abrasive wear and plastic deformation of Ti2AlN/TiAl, the tribo-films formed by the pre-oxidation layers on the worn surface of pre-oxidized composites weaken abrasive wear and suppress the development of plastic deformation to protect the underlying composite substrate from wear. Moreover, the stable cooperation on the interface between tribo-films and Si3N4 ball results in the relatively steady friction coefficient.

Multiscale structural changes of atomic order in severely deformed industrial aluminum

The regularities of multiscale structural changes in the atomic order of the aluminum alloy AD-1 after a severe cold plastic deformation by conventional rolling in smooth rolls or in rolls with relief recesses favorable for shear deformation have been investigated. It has been found that there are four types of structural fractions that differ in scale and perfection of atomic order: crystallographic planes with a long-range order; nanoscale fragments of the planes (D = 100–300 A) with an incipient long-range order; smaller groups of atoms (D = 20–30 A) of amorphized structure; and the least ordered structural fraction of intercluster medium, keeping only a short-range atomic order (2–3 interatomic distances, 10 A). The presence of diffuse halo bands in the region of intense Debye lines indicates phase transitions of the order → disorder type with the formation of one to three groups of amorphous clusters with the dominance, in the nanometer scale, of the atomic order characteristic of the family of planes (111), (220), and (311) of crystalline aluminum. We have found a dynamic phase transition with the changing crystallographic order of aluminum, with the matrix structure of a face-centered cubic (FCC) lattice, in the form of nanosized local groups of atoms, that is, the deformation clusters of aluminum with a simple cubic K6 lattice. In the case of conventional rolling, the development of large clusters 50–500 A in size is observed; however, in the use of rolls with relief recesses, the difference in the sizes of the clusters is one half as much: 50–250 A. Based on the analysis of the integrated intensity of incoherent X-ray scattering by the samples, we have elucidated the nature of the lowest measured density for the sample subjected to conventional rolling, which consists in the volume concentration of disorderly arranged atoms, the highest of the compared structures, which indicates the formation therein of the greatest amount of fluctuation “voids.”

Deep Hardening of Inner Cylindrical Surface by Periodic Deep Rolling - Burnishing Process

Severe surface plastic deformation is one of the methods to improve the mechanical behavior of metallic components, because such a treatment generates a hardened near-surface layer with compressive residual stresses. As is well known, deep rolling ( DR ) stands out against a background of the others mechanical techniques owing to maximum values of hardness, residual stress and hardening depth attained. In this work, an accomplished version of DR is proposed, which may be regarded as a superposition of DR and burnishing ( B ) processes in order to elevate those characteristics up to larger values. In contrast to consecutive application of DR and B techniques, the new approach allows to alter them multiply within one and only technological process with no increasing the working duration and without the need of any additional equipment. The working principles of the device are described. The dependences “depth-hardness” are presented. Numerical assessment of the residual stress is performed. The micro-photographies demonstrate an increase in the structural homogeneity of the near-surface layer material. The results indicate that the newly developed technique attains its main goals.

Effect of unequal channel angular pressing on microstructure and mechanical properties of as cast Mg–5·8Zn–1·2Y–0·7Zr alloy

The microstructure and mechanical properties of Mg–5·8Zn–1·2Y–0·7Zr alloys before and after unequal channel angular pressing (UCAP) have been investigated in detail using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, differential scanning calorimetry and mechanical property testing. The SEM and optical microscopy showed that the grain size was refined more at lower UCAP temperatures. When squeezed below 300°C, the average grain size was refined from the initial 110 μm to below 1·5 μm; the tensile strength increased to 390 MPa, and the elongation to break was over 13%. The high mechanical properties could be attributed to the reduced grain size and the strengthening effects of broken secondary phase particles and fine precipitates formed as a result of the severe shear and plastic deformation during pressing at high temperature.

Microstructure and Mechanical Properties of AZ81 Magnesium Alloy after Unequal Channel Angular Pressing

The microstructure and mechanical properties of AZ81magnesium alloys after unequal channel angular pressing (UCAP) was investigated using self-made 90° die under different temperature. The results showed that under the same pressing speed, there was a better grain refinement and mechanical property improvement with the decreasing of pressing temperature, however, the tiny cracks on the surface of the processed samples increased. When squeezed at 250 , the average grain was refined from the initial 150um to10um. The tensile strength was changed to 350Mpa, and the elongation was of 10. Tensile fractures presented the increment of dimples structure, which was the result of refined α-Mg matrix and intermetallic compound β-Mg17Al12 phase by severe shear and plastic deformation at high temperature press.

Microstructure and Mechanical Properties of Medical Magnesium Alloy Fabricated by Unequal Channel Angular Pressing

A new unequal channel angular pressing (UCAP) procedure is proposed for ultrafine-grained metals and alloys. The microstructures and mechanical properties of Mg-5.8Zn-1.2Y-0.7Zr alloys subjected to unequal channel angular pressing (UCAP) are investigated. It is found that the optimum condition in UCAPed alloy is obtained at 523 K with a largest elongation to failure of 13.1% and ultimate tensile strength of ~400 MPa. Microstructural observations show that the grain size is refined to ~1.0 μm during UCAP. The probable mechanisms for these high mechanical properties are attributed to grain size and destroyed secondary phase strengthening effects and fine precipitates formed during pressing at high temperature by severe shear and plastic deformation.

Preliminary Study on Ductile Fracture of Imperfect Lattice Materials

The ductile fracture behavior of two-dimensional imperfect lattice material under dynamic stretching is studied by finite element analyses (FEA). Three isotopic lattice materials, including the regular hexagonal honeycomb, the Kagome lattice and the regular triangular lattice, are taken into account, which are made of an elastic/visco-plastic metal material. Two typical imperfections (vacancy defect and rigid inclusion) are introduced separately. The numerical results reveal novel deformation modes and crack growth patterns in the ductile fracture of lattice material. Various crack growth patterns as defined according to their profiles, such as “X”-type, “Butterfly”-type, “Petal”-type. Crack propagation could induce severe material softening and plastic dissipation of the lattices. Subsequently, the effects of the strain rate, relative density, microstructure topology, and defect type on the crack growth pattern, the associated macroscopic material softening and the knock-down of total plastic dissipation are investigated.

Comparing specific features of the structural formation of aluminum alloys during severe and intense plastic deformation

We compare the deformation behavior and specific features of the structural formation of two aluminum alloys, AMTs and V95, upon megaplastic (MPD) and severe plastic (SPD) deformation. It is established that upon SPD by dynamic channel angular pressing, a submicron crystalline structure is formed with grain sizes of 200 to 600 nm; and that upon MPD by high quasi-hydrostatic pressure shearing, a nanostructure is formed with grain sizes of 55 to 100 nm. The sequence of the phase and structure transition is established for an increase in the rate of deformation and velocity of the materials. Mechanisms of elastic energy relaxation are determined as a function of the extent of dislocation mobility.

Evaluation of Structural Reliability of Steel Frames: Interstory Drift versus Plastic Hysteretic Energy

The structural reliability in terms of maximum interstory drift—and, alternatively, in terms of plastic hysteretic energy—is evaluated for six regular moment-resisting steel frames designed according to the Mexico City Building Code, and located in the Lake Zone of that city. While the maximum interstory drift was used because of its relevance within the format of current seismic design codes, the plastic hysteretic energy was considered due to its importance for the performance of structures when subjected to severe cumulative plastic deformation demands. The demand hazard curves of the frames in terms of drift and energy are compared to provide a general idea of the reliability levels associated to the models, and to provide insights into which response parameter dominates their dynamic behavior and structural performance. In some cases, large differences are observed in the reliabilities computed by measure of the two different response parameters under consideration.

Scale levels of deformation and fracture and mechanical properties of 25CrlMolV steel before and after nonisothermal loading

In the work, the peculiarities of tensile deformation and fracture of 25CrlMolV steel subjected and not subjected to thermal cycling were studied on the basis of measured mechanical characteristics and data of optical and electron microscopy. It is shown that the deformation of the steel specimens in the initial state features very low strain hardening and considerable elongation at the stage of necking. After thermal cycling, the yield stress decreases, the plasticity grows markedly, and the strain hardening becomes more pronounced. The results of study are discussed with resort to metallographic data. Fractographic analysis suggests that the fracture pattern is governed by regularities of the nucleation of micropores, their coalescence, and formation of a central opening mode macrocrack. According to the experimental results, the specimen is fractured by a mixed “local shear - cleavage” mechanism and the character of fracture is strongly affected by secondary microcracking and severe intragranular plastic deformation. On the basis of the obtained results, a model of structural levels of deformation and fracture of the specimens under static tension is proposed.

Nanostructured phase boundaries in aluminum under severe cyclic plastic deformation

The mechanisms of plastic strain observed at high degrees of alternating bending of thin aluminum foils glued to elastically strained substrates have been investigated. As extrusion and intrusion develop on the surface of the aluminum foils, multiscale fragmentation of the structure is found to take place in the bulk of the materials to form nanostructured phase boundaries between subgrains. The width of the phase boundaries varies between 200 and 300 nm, with the size of the structure elements within the subgrain boundaries being 30–50 nm. Formation of the nanostructured phase boundaries between nonequilibrium subgrains is regarded to be the fragmentation mechanism operative at the submicrometer scale level in the foils subjected to bending-torsion at very high degrees of plastic strain.

Nanostructure formation and phase transformations in nitrided stainless steel Kh18N8 during severe cold deformation

The phase transformations and nanostructure formation in metastable stainless steel Kh18N8 are studied during nitriding and subsequent severe cold plastic deformation by high-pressure torsion in Bridgman anvils. Ion-plasma nitriding of the steel leads to the nitrogen saturation of austenite, CrN nitride formation, and the destabilization of the structure with respect to the γ-α transformation. Subsequent deformation is accompanied by the reverse α-γ phase transformation, the dissolution of nitrides, and the formation of nitrogen-supersaturated solid solutions and secondary nanonitrides. These fine particles restrict the grain growth in the matrix and stabilize the nanostructure formed.

Phase transformations in the hematite-metal system during mechanical alloying

Mossbauer spectroscopy and X-ray diffraction are used to show that the phase transformations in hematite α-Fe2O3-metal (M = Fe, Ni, Ti, Zr) powder mixtures induced by severe cold plastic deformation in ball mills occur via the formation of M-Fe-O solid solutions, redox reactions with the reduction of metallic iron, and the formation of secondary M x O y oxides and M x Fe y intermetallics. Mechanical activation in a ball mill is compared to that under high-pressure shear in Bridgman anvils. The transformations that take place in a ball mill are found to have several stages and to be accelerated.