Twin Zone Research Articles

Deformation nanotwins in a single-crystal Ni-based superalloy at room temperature and low strain rate

The deformation microstructure of single crystal superalloys CMSX-4 was investigated during uniaxial tensile with a strain rate of 5 × 10 −3 s −1 at room temperature. The objection of the current study is to evaluate the deformation twinning and investigate the deformation mechanism. Experimental results followed by transmission electron analysis show that deformation nanotwins and elemental segregation in single-crystal Ni-based superalloys were observed after tensile deformation at room temperature with a strain rate of 5 × 10 −3 s −1 . Furthermore, the relationship among deformation twinning, elemental segregation, and high local stress due to plastic deformation with increasing plastic strain was estimated and discussed. These findings provide valuable insights into understanding twinning and elemental segregation in the plastic deformation of single-crystal Ni-based superalloys. • Deformation nanotwins in CMSX-4 are initiated under tensile tests with a strain rate of 5 × 10 −3 s −1 at room temperature. • Element segregation can be observed at the twin zone at room temperature. • Deformation nanotwins in CMSX-4 is not necessarily accompanied by an atomic reordering process at room temperature.

Through-Thickness Microstructures and Yield Strength Enhancement for AZ31 Mg Sheets Treated by Ultrasonic Nanocrystal Surface Modification

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 µm from the surface) and twin-dominated (200~300 µm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} tensile twins (despite compressive strikes by the hemispherical tip), {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1}-{10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} double twins and {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.

Discontinuous lath martensite transformation and its relationship with annealing twin of parent austenite and cooling rate in low carbon RAFM steel

Lath martensite transformation in low carbon steel relates to the discontinuity of the transformation rate, which exhibits a series of martensite transformation rate maxima (peaks), especially in low cooling rates. In the present work, we put an emphasis on the influence of annealing twins in parent austenite grains (PAGs) and cooling rates on the discontinuity of lath martensite transformation rate in a low carbon reduced activation ferritic/martensitic (RAFM) steel. Lath martensite microstructure is hierarchical and the lath martensite transformation rate peaks are hierarchical during the cooling process. If the cooling rate was not high enough, the lath martensite transformation rate peaks will be divided into two groups by annealing twins in parent austenite. These transformation rate peaks in the first group were mainly caused by lath martensite transformation in the non–twin zones in the PAGs. Then, the second group mainly consisted of a single transformation rate peak, which was primarily caused by the martensite transformation in the twin zones in the PAGs. In the present work, a new “localized stress field” theory is proposed and successfully used for interpreting the classification mechanism of lath martensite transformation rate peaks and the evolution of these peaks with changing cooling rate.

Anomalous detwinning in constrained Cu nanoparticles

In this work, the detwinning process in a 9 nm graphene-constrained Cu nanoparticle was investigated at 1009 °C via in situ high-resolution transmission electron microscopy. Instead of the expected reverse glide of the twinning dislocations, two new twins were formed; the four twin zones rotated synergistically before vanishing. Furthermore, the twin boundary migration energy and the system energy were increased continuously with detwinning. The increased resistance to twin boundary migration in constrained nanoparticles enriches our understanding of the twinning mechanism and may facilitate the design of high-strength and high-ductility nanomaterials.

Twin induced fracture toughness in SiCW/SiC composites processed by spark plasma sintering

The twin boundaries (TBs) was responsible for 30% and 26% enhancement in the fracture toughness of 10% and 20% SiCW/SiC composites spark plasma sintered at 1800 °C, respectively. Crack deflection at TBs, and daughter crack formation at the twin zone were found to be the responsible mechanisms for twin toughening.

Microstructure evolution of depleted uranium impacted by steel projectile at velocity of 50 m/s

The deformed microstructure evolution of depleted uranium impacted by steel projectile at a velocity of 50 m/s was investigated by means of confocal laser scanning microscope, electron backscatter diffraction, transmission electron microscope and indenter technique. The experimental results showed that the spherical cap crater was formed in depleted uranium target impacted by steel projectile, and the diameter and depth of the impacted crater were 5.45 and 1.01 mm, respectively. From crater rim to deep matrix, four deformed zones were classified, including twin fragmentation zone, high density deformation twin zone, low density deformation twin zone and matrix zone. Twinning was considered as the dominant plastic deformation mechanism of depleted uranium subjected to impact loadings. Besides twinning, the dislocation slipping also played an important role to accommodate the plastic deformation. Finally, the deformed microstructure evolution of depleted uranium under high velocity impact was proposed.

Formation of low-angle boundaries accompanying the deformation process by twinning in titanium

The structure of the transition zone of the wedge twin formed inside the grains of polycrystalline titanium was experimentally investigated. The accumulation of low-angle grain boundaries formed during the deformation by twin growing was discovered. Mathematical modeling of the stress state close to the twin zone was described in context of the macroscopic dislocation model. The model allows describing physical processes responsible for the formation of low-angle grain boundaries in a form of dislocation walls close to the transition zone of the wedge twin.

Full scale experimentation on a new translucent passive solar wall combining silica aerogels and phase change materials

This paper presents the first passive solar wall providing simultaneously super insulation, heat storage and daylighting to the inner space. The wall’s external layer is composed of a silica aerogels bed for high insulation and solar radiation transmission. The second layer, internal, is composed of glass bricks filled with a eutectic phase change material (PCM) for heat storage and restitution. The whole wall is translucent. The experimentations carried out to characterize thermal and optical properties of the materials used are described. Then results are given for a full scale comparative experimentation on a twin zones building located in the south of France for the solar wall and a standard opaque high thermal capacity wall. Results show the heat losses through the wall are very low while the heat and light gains are high: The U value of the solar wall is 0.59Wm−1K−1 and 0.72Wm−1K−1 respectively when the PCM is in liquid and solid state. With the test building in free floating condition, the temperature difference between the outdoor and indoor air provided by the wall is about 9°C in winter. The wall can provide up to 500lux to the inner environment, which is sufficient for conference rooms. The tested wall has proven more effective in winter and shoulder season, particularly for cold sunny climates, but may cause overheating in summer.

Research on Boron Extraction Rate of Boron-Containing Slag and its Industrial Experimental Conditions

With the boron-containing slag separated from ludwigite in the 13m3 blast furnace as crude material, cooling experiments in practice are fulfilled, where reasonable boron-containing slag cooling technic and industrial cooling installation conditions in industrial practice are obtained. Relationship between cooling rate and extraction rate of boron is observed. That is, as long as cooling rate keeps below 2°C/min in the temperature range of 1200°C~900°C , average boron extraction rate increases with the rise of cooling rate in the temperature range of 1500°C~1200°C and could reach up to 80%. This result is coincided with that of laboratory research.The experimental result demonstrates that twin zone cooling technology could be fulfilled in practice. Meanwhile, the way for the future cooling equipments design is pointed out. This study has scientific values for both practice and theory on multipurpose utilization of mineral resources.

The Effect of Cooling Rate to the Extraction Ratio of Boron-Containing Slag

Twin zone cooling process with high extraction ratio is established by studying the effect of cooling rate on the boron extraction ratio of boron-containing slag. The viscosity increases rapidly by continuous cooling and the prior crystallization of forsterite, which hinders the nucleation and growth of suanite, result in the low extraction ratio of boron. With the twin zone cooling process, the rapid cooling in 1500°C-1200°C, restrains the formation of forsterite and reduces the inhibition effect of forsterite on the nucleation and growth of suanite. Heat is preserved at 1200°C to ensure the sufficient interaction time among the particles, which is favorable toward the nucleation and growth of suanite. As a result, the extraction ratio of boron-rich slag is improved.

Deformed microstructure evolution in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s −1

Deformed microstructure in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s −1 were investigated through optical microscope, scanning electron microscope and transmission electron microscope. The results show that four deformed zones around the crater can be classified based on the different deformed microstructure, including ultrafine grain zone, ultrafine grain and deformation twin zone, high and low density deformation twin zones. The dislocation slipping, deformation twins and ultrafine grains are the dominant components in the four deformed zones, and the evolution of deformed microstructure is speculated based on the deformed microstructure observed in four zones. Slipping and twinning play a critical role for the formation of the dynamic recrystallized grains, and twinning-induced rotational dynamic recrystallization mechanism is thought to be the main mechanism for the formation of ultrafine grains. The microhardness and dynamic compressive strength in different deformed zones were measured, and the high microhardness and yield strength in ultrafine grain zone should be attributed to the strain hardening and grain refining.

Deformed microstructure and mechanical properties of AM60B magnesium alloy under hypervelocity impact at a velocity of 4 km s −1

Deformed microstructure and mechanical properties of AM60B magnesium alloy under hypervelocity impact at a speed of 4 km s −1 were studied through optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and indenter technique. The results show that the deformed microstructure below the crater can be classified as three zones, i.e. dynamic recrystallization zone, high density deformation twin zone and low density deformation twin zone. The refined and equiaxed recrystallized grains adjacent to the crater were formed, which should be attributed to the twining-induced dynamic recrystallization mechanism. The { 1 0 1 ¯ 2 } , { 1 0 1 ¯ 1 } deformation twins are confirmed through selected area electron diffraction (SAED) technique. The microhardness and yield strength in the deformed microstructure zone near the crater are much higher than that of the matrix, which should be attributed to strain hardening and grain refining.

Tomographic quantitative phase measurement by hard X-ray micro-interferometer with 250 nm spatial resolution

Tomographic quantitative phase measurement has been carried out by using a hard X-ray micro-interferometer in Hyogo-ID at SPring-8. The micro-interferometer combines an imaging microscope and a wavefront-division-type interferometer by using a novel optical element called a “twin zone plate.” A three-dimensional quantitative phase distribution of a glass capillary with an outer diameter of 6 μm was successfully obtained with the tomographic technique. The measured refractive index corresponded to the ideal value. The spatial resolution of the tomographic image was estimated to be 250 nm and the density resolution was estimated to be 0.6 g/cm 3.

Image analysis of plagioclase crystals in rock thin sections using grey level homogeneity recognition of discrete areas

Complex optical properties of plagioclase, such as twinning, present a particularly difficult challenge to image processing techniques. Conventional image processing methods attempt to recognize mineral grain boundaries by grey level gradients and are likely to classify optical twin zones as different grains. To overcome this problem, automated grey level processing of small areas under different orientations of cross polarization is used to identify both twinned and un-twinned plagioclase areas as seeds. In an interactive procedure, seeds are projected onto polarized images and are built-up to individual grains. An independent grey level utility supplements automatic processing, allowing grey level profiling down to groups of a few pixels, providing a means for preliminary examinations of thin sections, and to establish typical differences between grain types.

Ti Coating of Nanocrystalline Diamond by Atomic Layer Deposition

Nanocrystalline diamond compact possesses not only the advantageous performance of polycrystalline diamond but also the high strength and the high toughness of nano-ceramics. However, single-phase nanocrystalline diamond compact is very difficult to sinter because of a huge amount of oxygen-containing and nitrogen-containing functional groups absorbed on the surface of nanocrystalline diamond. In this paper, atomic layer deposition (ALD) method has been used to coat nanocrystalline diamond with titanium, which will promote the bonding of nanocrystalline diamond as the bond in polycrystalline diamond. In vacuum, the H2 and TiCl4 reactants were employed alternately in an ABAB… binary reaction sequence to achieve Ti layer, which reacted with diamond matrix and formed TiC in the coating, realizing strong chemical bonding between the coating and the diamond. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were utilized to study the structure and the morphology of the coating. The results confirmed the formation of titanium carbide at the depositing temperature 500°C. The darker spots and strips observed on nanocrystalline diamond particles by TEM were proved to be TiC and the nucleation and subsequent growth of TiC preferentially occurred in the defects as twin zones and dislocation areas on diamond surfaces.

Differential interference contrast X-ray microscopy with twin zone plates at ESRF beamline ID21

We describe a novel approach for converting the specimens phase information into strong image contrast in the x-ray region, the differential interference contrast x-ray microscopy (X-DIC). In the used setup, X-DIC operation was accomplished by a twin zone plate (TZP), i. e. two zone plates on both sides of the same substrate, laterally shifted by about one outermost zonewidth. Once a TZP has been successfully produced by means of especially developed nanofabrication processes, it is as easy to use as a single zone plate, without any alignment difficulty or further requirements to the coherence of the illumination. The tremendeous contrast enhancement was demonstrated at the microscopy beamline ID21 at ESRF in Grenoble for test objects and biological samples. TZPs allow for full field X-DIC imaging as well as for DIC scanning transmission x-ray microscopy. The first experiments were carried out at 0.31 nm wavelength (4 keV photon energy), and features down to approx. 160 nm could be resolved in X-DIC.

Differential interference contrast x-ray microscopy with twin zone plates.

X-ray imaging in differential interference contrast (DIC) with submicrometer optical resolution was performed by using a twin zone plate (TZP) setup generating focal spots closely spaced within the TZP spatial resolution of 160 nm. Optical path differences introduced by the sample are recorded by a CCD camera in a standard full-field imaging and by an aperture photodiode in a standard scanning transmission x-ray microscope. Applying this x-ray DIC technique, we demonstrate for both the full-field imaging and scanning x-ray microscope methods a drastic increase in image contrast (approximately 20x) for a low-absorbing specimen, similar to the Nomarski DIC method for visible-light microscopy.