Precompressed Samples Research Articles

The effect of induced precipitations and pre-twins on the bending neutral layer migration behaviors of AZ80 Mg alloy sheet

To explore the effect of the precipitated phase particles and twins on the microstructure evolution and neutral layer evolution during bending, {10–12} tensile twins were introduced along the transverse direction (TD) on AZ80 Mg alloy sheets at room temperature. Then pre-compressed samples were aged to induce more precipitations. The reverse tensile tests were carried out at room temperature, and the three-point bending deformation was carried out at 100 °C. After aging treatment, precipitations increased. The results showed that twin crystals can promote precipitation formation. And the YS (yield strength) values of the pre-twinned aged samples were significantly larger than that before aging. During the bending process on the pre-compressed samples, the outside of the sample was dominated by the detwinning behaviors, while the inside region was dominated mainly by twinning. For pre-compressed bending samples, the k values were smaller than the initial bending sample. The tension and compression asymmetry in its inner and outer regions was weaker than that of the original bending sample. The existence of precipitations hindered the detwinning behaviors in outside the pre-twinning aged bending samples, and the twinning in inner region was restrained as well so that the inner and outer tension asymmetry decreased. The k values were smaller than that of the unaged samples, which was beneficial to weaken the bending springback of Mg alloy sheets.

Microstructure evolution maps of AZ31 and AZ91–Ca–Y–MM alloys under precompression and subsequent annealing conditions

This study establishes the microstructure evolution maps of a commercial AZ31 alloy and a recently developed Mg–9Al–0.8Zn–0.9Ca–0.6Y–0.5MM (AZXWMM91100) alloy on the basis of their microstructural variations with precompression amount and subsequent annealing temperature. The fully twinned microstructure of precompressed samples of the AZ31 alloy remains unchanged after annealing at 200 °C. At annealing temperatures above 225 °C, the area fraction of twin-free grains gradually increases with increasing precompression amount and annealing temperature. The microstructure evolution map of the AZ31 alloy consists of three regions: those with a fully twinned grain structure, partially twinned grain structure, and completely twin-free grain structure. The twinned microstructure of precompressed samples of the AZXWMM91100 alloy remains unchanged after annealing even at 250 and 300 °C. At annealing temperatures above 350 °C, the microstructure drastically changes from a fully twinned microstructure to a completely twin-free microstructure irrespective of the precompression amount. Consequently, the microstructure evolution map of the AZXWMM91100 alloy does not include the region with the partially twinned grain structure. The large amount of Al solute atoms and numerous second-phase particles in the AZXWMM91100 alloy inhibit grain-boundary movement during annealing, which results in the different microstructural evolution behavior with the AZ31 alloy.

Microstructure evolution and accelerated magnetocaloric phase formation of LaFe13.1Co0.7Si1.4 alloy by forging deformation at room temperature

In order to improve the preparation process and practical process of magnetic refrigerant, a method of accelerating phase formation by cold compression plastic deformation at room temperature is proposed. To determine the significance of this method, the grain structure, phase composition and magneto-thermal effects of LaFe13.1Co0.7Si1.4 alloy were systematically studied. The results show that with the increase of compression ratio, the number of grains per unit volume increases and the grain size tends to be uniform. Compared with the uncompressed sample, the proportion of magnetothermal phase in the pre-compressed samples increases after annealing under the same conditions. When the deformation is 40%, the ratio of 1:13 phase in pre-deformed sample after 1 day annealing is 72 vol%, which is significantly better than that of the undeformed sample. After annealing for 3 days, the Curie temperature (TC) of 40% pre-deformed LaFe13.1Co0.7Si1.4 samples is 330 K, and the magnetic entropy changes to − 4.70 J/kg K and − 5.42 J/kg K under 2 T and 3 T magnetic fields, respectively. The appropriate Curie temperature and value of ∆SM make it an attractive potential candidate for magnetic refrigeration application at room or even high temperature.

Densification of single-walled carbon nanotube films: Mesoscopic distinct element method simulations and experimental validation

Nanometer-thin single-walled carbon nanotube (CNT) films collected from the aerosol chemical deposition reactors have gathered attention for their promising applications. Densification of these pristine films provides an important way to manipulate mechanical, electronic, and optical properties. To elucidate the underlying microstructural level restructuring, which is ultimately responsible for the change in properties, we perform large scale vector-based mesoscopic distinct element method simulations in conjunction with electron microscopy and spectroscopic ellipsometry characterization of pristine and densified films by drop-cast volatile liquid processing. Matching with the microscopy observations, pristine CNT films with a finite thickness are modeled as self-assembled CNT networks comprising entangled dendritic bundles with branches extending down to individual CNTs. Simulations of these films under uniaxial compression uncover a soft deformation regime extending up to an ∼75% strain. When removing the loads, the pre-compressed samples evolve into homogeneously densified films with thickness values depending on both the pre-compression level and the sample microstructure. The significant reduction in thickness is attributed to the underlying structural changes occurring at the 100 nm scale, including the zipping of the thinnest dendritic branches.

Enhancing compressive mechanical properties of rolled AZ31 Mg alloy plates by pre-compression

{10–12} extension twins were pre-introduced in the hot-rolled AZ31 Mg alloy plates by pre-compression at room temperature (RT) along the transverse direction (TD). Pre-introduced {10–12} extension twins substantially improved the yield strength of AZ31 Mg alloy plates and did not deteriorate the compression ratio. Especially at RT, 100 °C and 200 °C, the pre-compressed samples containing {10–12} extension twins exhibited much larger yield strength (122 MPa vs. 66 MPa, 121 MPa vs. 64 MPa and 118 MPa vs. 63 MPa) than the initial samples. The pre-compressed sample showed more obvious compression flow softening phenomenon than the initial sample at 200 °C, which was attributed to the dynamic recrystallization (DRX) process accelerated by pre-introduced {10–12} extension twins. When compressing at 300 °C, the compression flow curves of the initial and pre-compressed samples tended to coincidence, which was because the discontinuous dynamic recrystallization (DDRX) was the controlling mechanism in both kinds of the samples and the effect of pre-introduced {10–12} extension twins was vanished.

Compressive yield stress improvement using thermomechanical treatment of extruded Mg-Zn-Ca alloy

Thermomechanical treatment consisting of pre-compression and a subsequent isothermal aging was used to improve the compressive properties of extruded Mg-Zn-Ca (ZX10) alloy. Isothermal aging at 150 °C and 200 °C was performed on as-extruded samples for various times to find peak aged conditions. To achieve a different twin volume fraction in the alloy, samples were pre-compressed up to 2%, 3% and 4% along the extrusion direction. Afterwards, the peak-aged conditions were applied to the pre-compressed samples to enhance their mechanical properties. It was found that after the 4 h@150 °C thermomechanical treatment, deformation curves exhibit a distinctive yield plateau independently on the pre-strain level. Using the 2 h@200 °C condition, the yield plateau occurs only after pre-strain of 3%. Higher compressive yield strength values were observed for the thermomechanically treated samples at the lower temperature.

Effects of {10–12} Twins on Dynamic Torsional Properties of Extruded AZ31 Magnesium Alloy

Effects of initial twins on dynamic torsional properties of extruded AZ31 alloy were investigated by introducing {10–12} twins into it through precompression to 3 and 6% strains along the extrusion direction and performing torsional testing at a strain rate of 1.4 × 103 s−1 using a torsional Kolsky bar system. The as-extruded sample without twins showed higher dynamic torsional properties than the precompressed samples with many initial twins; the maximum shear strength and fracture shear strain decreased with increasing amount of initial twins. In the as-extruded sample, twinning occurred vigorously throughout the gage section of the tubular specimen during high-strain-rate torsional tests, resulting in heavily deformed morphology, many macrocracks, and rough fractured surfaces. The increased amount of initial twins suppressed the twinning behavior and localized the applied torsional deformation; this resulted in an almost unchanged sample shape, no secondary cracks, and a flat fracture plane, thereby deteriorating the dynamic torsional properties of the extruded alloy.

Anisotropic in-plane fatigue behavior of rolled magnesium alloy with {10−12} twins

The effects of initial {10−12} twins on the in-plane fatigue characteristics of a rolled Mg alloy were investigated by introducing these twins through precompression along the rolling direction of the material and then performing strain-controlled low-cycle fatigue tests along the rolling direction, 45° to the rolling direction, and transverse direction. The as-rolled sample with a strong basal texture was found to exhibit the same fatigue behavior in all the loading directions, whereas the precompressed samples exhibited anisotropic in-plane fatigue behaviors because the specific crystallographic orientation of the twinned region resulted in a variation in the active deformation modes during cyclic loading in the applied loading directions. An increase in the amount of precompression was also found to enhance the anisotropy of the in-plane fatigue behavior via an increase in the area fraction of the twinned region. The variation in the dominant deformation mechanism with a change in the loading direction was investigated by analyzing the crystallographic angle relationship and activation stresses, and the resultant evolutions of the cyclic flow curve and fatigue life were discussed in detail.

Short bursts of cyclic mechanical compression modulate tissue formation in a 3D hybrid scaffold

Among the cues affecting cells behaviour, mechanical stimuli are known to have a key role in tissue formation and mineralization of bone cells. While soft scaffolds are better at mimicking the extracellular environment, they cannot withstand the high loads required to be efficient substitutes for bone in vivo. We propose a 3D hybrid scaffold combining the load-bearing capabilities of polycaprolactone (PCL) and the ECM-like chemistry of collagen gel to support the dynamic mechanical differentiation of human embryonic mesodermal progenitor cells (hES-MPs). In this study, hES-MPs were cultured in vitro and a BOSE Bioreactor was employed to induce cells differentiation by mechanical stimulation. From day 6, samples were compressed by applying a 5% strain ramp followed by peak-to-peak 1% strain sinewaves at 1Hz for 15min. Three different conditions were tested: unloaded (U), loaded from day 6 to day 10 (L1) and loaded as L1 and from day 16 to day 20 (L2). Cell viability, DNA content and osteocalcin expression were tested. Samples were further stained with 1% osmium tetroxide in order to investigate tissue growth and mineral deposition by micro-computed tomography (µCT). Tissue growth involved volumes either inside or outside samples at day 21 for L1, suggesting cyclic stimulation is a trigger for delayed proliferative response of cells. Cyclic load also had a role in the mineralization process preventing mineral deposition when applied at the early stage of culture. Conversely, cyclic load during the late stage of culture on pre-compressed samples induced mineral formation. This study shows that short bursts of compression applied at different stages of culture have contrasting effects on the ability of hES-MPs to induce tissue formation and mineral deposition. The results pave the way for a new approach using mechanical stimulation in the development of engineered in vitro tissue as replacement for large bone fractures.

Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium

Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow reaching unprecedented high densities and moderately high ∼103–104 K temperatures. We describe here a complete analysis framework for the velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments. Since the precompression increases the initial density of both the sample of interest and the quartz reference for pressure-density, reflectivity, and temperature measurements, we describe analytical corrections based on available experimental data on warm dense silica and density-functional-theory based molecular dynamics computer simulations. Using our improved analysis framework, we report a re-analysis of previously published data on warm dense hydrogen and helium, compare the newly inferred pressure, density, and temperature data with most advanced equation of state models and provide updated reflectivity values.

De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction (ND) of pre-compressed samples along the extrusion direction (ED). {10 1 ¯ 2} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain, and the complete de-twinning occurred at a strain of ∼7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {10 1 ¯ 2} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {10 1 ¯ 2} twinning corresponded to the formation of new texture components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and a decrease in the initial texture components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and an increase in the components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>.

The Effects of Pre-strain and Subsequent Annealing on the Yielding Behavior in a Rolled Magnesium Alloy AZ31

In present work, pre-compression along the transverse direction and the normal direction (ND) was conducted on the rolled AZ31Mg alloy. Subsequently, the pre-compressed samples were annealed at 250 °C for different times. The effects of pre-compression and post-annealing on the yielding behavior during subsequent compression along ND were investigated. The results revealed that a lot of non-recrystallized extension twin remained in 5% PT sample after annealing for 4 h, while no twin was observed in the 5% PN sample, under the same annealing time. Along with, no prominent changes occurred in the texture of post-annealed as compared to pre-compressed samples. Due to the grain refinement caused by static recrystallization, yield strength increase of 5% PNA sample is found for an annealing time of 30 min. For the 5% PTA sample, the strength increase is attributed to annealing hardening (i.e., annealing retards the detwinning activity and consequently increase the yield strength).

Influence of pre-deformation and subsequent annealing on strain hardening and anisotropy of AM30 magnesium alloy

This study was aimed at evaluating the recrystallization kinetics and re-compression properties of a pre-compressed and annealed AM30 magnesium alloy with special attention to the strain hardening behavior, microstructure and texture change. Pre-compression along the extrusion direction (ED) facilitated the occurrence of {101¯2} extension twinning due to the presence of two sets of basal textures with the c-axes aligned nearly parallel to the normal direction of the extruded plate. Most grains twinned and the c-axes of the twinned regions were rotated towards the anti-compression direction (i.e., ED) after a 4.3% pre-compression. A distinctive strain hardening stage B, characterized by an increasing strain hardening rate with a positive slope, was present. The subsequent annealing of the pre-compressed samples led to recovery, recrystallization and grain growth, and the recrystallization kinetics could be described via the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. With increasing annealing time the twins in the pre-compressed samples were observed to gradually vanish, as demonstrated by a decreased volume fraction of twins and weakened texture which became more randomly distributed. As the annealing time increased at a given temperature of 450°C, the ultimate compressive strength (UCS) and the slope of strain hardening stage B decreased, while the compressive yield stress (CYS) remained nearly unchanged. With increasing annealing temperature, CYS, UCS, the extent of stage B and the slope of stage B all decreased, and the fracture strain increased.

Modified twinning behaviour induced by texture evolution in hot rolled Mg–3Al–1Zn alloy

Compressive deformation along the rolling direction (RD) of a hot rolled Mg–3Al–1Zn alloy is applied to investigate the texture evolution and the recompressive yield strength (RYST) along the transverse direction (TD). Preferential orientation of the basal and prismatic planes is generated by the plastic deformation. Precompression along RD results in one plane of {10–10} aligned nearly perpendicular to the normal direction to the rolling plane. As the compressive strain along RD increases, the RYST shows an earlier raised and later decreased trend. The modified twinning mechanism is investigated using X-ray diffraction and electron backscattered diffraction observations. The results reveal that {10–12} twinning in the matrix dominates the recompression along TD, while the formation of {10–12}–{10–12} twins becomes comparatively easier to occur in the previous {10–12} twins for large precompressed samples.

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {10−12} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.

Reducing the anisotropy of a pre-twinned hot-rolled Mg–3Al–1Zn alloy by plane-strain compression

In the current paper, pre-compression is performed along the rolling direction (RD). Subsequently, the samples are deformed in plane-strain compression along the transverse direction (TD) and not allowed to expand in the normal direction (ND). The effect of plane-strain compression on the texture and anisotropy modification of a pre-twinned hot-rolled Mg–3Al–1Zn alloy is investigated by EBSD observation. The results reveal that the generation of {101¯2}−{101¯2} double twins in the previous {101¯2} twins dominates the plane-strain pre-compression along TD while the {101¯2} twinning is difficult to occur in the matrix due to the constraint along ND. Additionally, the {101¯2}−{101¯2} double twins generated in the previously formed {101¯2} twins effectively modify the compression behavior and enhance the compressive yield strength of the pre-compressed samples along ND. The anisotropy along RD, TD and ND is decreased.

Effect of pre-compression on the kinetics of thermal decomposition of pure and doped sodium oxalate under isothermal conditions

Pure and doped samples of sodium oxalate (Na2C2O4) were subjected to pre-compression and their thermal decomposition kinetics was studied at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry. The pre-compressed samples decomposed in two stages governed by different rate laws; the Prout–Tompkins model best describes the acceleratory stage of the decomposition while the decay region is best fitted with the contracting cylinder model as in the case of uncompressed sodium oxalate samples. The rate constants k1 and k2 of the acceleratory and deceleratory stages of the thermal decomposition were dramatically decreased on pre-compression. However, the activation energies, evaluated by model fitting kinetic method, E1 and E2 for the respective stages of decomposition remained unaltered by pre-compression. The results favor ionic diffusion mechanism proposed earlier on the basis of doping studies.

A research study on the spallation strength of LY12 aluminum under the pre-compression condition

To pre-compress the disk-shaped LY12 samples along the radial direction can be done with the aid of overstress assembly by heating or by mechanical clamping, which can also generate the deviatoric stress fields under different states. The spallation signals of these pre-compressed samples are measured by VISAR in the light-gas gun shock experiments. The experimental results show that even under the same impact velocity, the pullback amplitudes of the velocity at the free surface of the samples vary significantly. According to the experimental data, we propose a distinct concept that the material spallation strength is closely related to the deviatoric stress fields in the material. Based on the numerical simulation, we develop a damage constitutive model, which reveals that the deviatoric stress reduces the tensile threshold of the void growth. The numerical investigations also demonstrate that the spallation strength decreases as pre-compression increases. The experimental idea proposed in this paper can also be used to study the spallation process in other structures.

Significant static pressure increase in a precompression cell target for laser-driven advanced dynamic compression experiments

Laser shock compression experiments on precompressed samples offer the possibility to explore extreme material states unreachable by static or single-shock compression techniques alone. We have found significant increases in static compression pressure in a wide-opening and thin diamond precompression cell. This suggests that the precompression target is adaptable to advanced coupling techniques with laser-driven dynamic compression methods. The novel coupling techniques proposed give the potential to access outstanding material states required in planetary and condensed-matter physics.

Achieving high-density states through shock-wave loading of precompressed samples

Materials can be experimentally characterized to terapascal pressures by sending a laser-induced shock wave through a sample that is precompressed inside a diamond-anvil cell. This combination of static and dynamic compression methods has been experimentally demonstrated and ultimately provides access to the 10- to 100-TPa (0.1-1 Gbar) pressure range that is relevant to planetary science, testing first-principles theories of condensed matter, and experimentally studying a new regime of chemical bonding.