Peak Hardness Research Articles

New quaternary eutectic Al-Cu-Ca-Si system for designing precipitation hardening alloys

The aluminum corner of the previously unexplored quaternary eutectic Al-Cu-Ca-Si system promising for the design of new heat treatable alloys have been studied using thermodynamic modeling and experimental techniques. The experimental data have revealed the presence in equilibrium of a previously undescribed quaternary compound identified as a strict stoichiometric Al2CaSiCu phase with a tetragonal I4/mmm structure (Pearson symbol: tI10) and the lattice parameters a = 4.04 Å and c = 11.00 Å. The phase has a density of 3.36 g/cm3 and a microhardness of 335 Hv. In accordance with the suggested structure of the phase diagram, its region that can be promising for the design of heat treatable alloys contains three (Al)+Al2Cu+Al2CaSiCu, (Al)+Al2Si2Ca+Al2CaSiCu and (Al)+Al2Cu+Al2Si2Ca quasi-ternary sections and two (Al)+Al2Cu+Al2Si2Ca+Al2CaSiCu and (Al)+Al2Cu+Al2CaSiCu+Si four-phase fields. Analysis of the precipitation hardening response for the new quaternary Al-5 wt%Cu-Ca-Si alloys suggests that it is not inferior (the obtained peak hardness is ∼125 Hv) to that for the alloys Al-5 wt% Cu (∼120 Hv) and Al-4 wt%Si-5 wt%Cu (∼125 Hv) based on the conventionally used systems. However, to obtain noticeable hardening at aging, the content of silicon should be at least 1.1–1.2 times higher than that of calcium. A comparative hot tearing susceptibility (HTS) study revealed that the new heat treatable Al-5 wt%Cu-Ca-Si alloys have higher hot tearing resistances (HTS ∼16 mm according to a pencil probe) than that of the Al-5 wt%Cu base alloy (HTS >16 mm).

General investigations on the heat treatment and thermal fatigue behavior of an experimental hot work tool steel tailored for laser powder bed fusion

An experimental hot work tool steel with a leaner chemistry compared to the standard AISI-H13 grade, aimed at enhanced processability, was gas atomized and processed using laser powder bed fusion (L-PBF) to >99.8 % relative density. The lower carbon content (∼0.25 wt% vs. ∼0.4 wt% in H13) resulted in a softer as-built (AB), and quenched martensite (∼485 HV1 vs. 625 HV1 in H13), higher martensite-start temperature (∼360 °C vs. 280–320 °C), and about ∼18 % reduction in dilatation, ascribed to the volume expansive martensitic transformation. Charpy V-notch impact toughness in AB condition (>30 J) was higher than those reported for AB H13. These features account for the improved L-PBF processability. The tempering response in comparison with a wrought H13, was characterized by a larger precipitation of thermally stable Mo- and V-rich secondary hardening alloy carbides at the expense of the easy-to-coarsen Cr-rich ones, attributed to the higher Mo and lower Cr content in experimental alloy. The combination of lower C- and V-contents in experimental alloy, reduced the driving force for the precipitation of coarse vanadium rich carbides during austenitization, as confirmed by electron diffraction spectroscopy (EDS) and synchrotron X ray diffraction. This led to a stronger fine secondary hardening alloy carbide precipitation during tempering, and enhanced temper resistance. As elaborated by isochronal tempering, as well as tempering curves, reduction of the elements Si, and Cr shifted the secondary hardness peak to higher tempering temperatures. Consequently, tempering, and thermomechanical softening resistance was significantly enhanced in the new steel with a maximum hardness of ∼550 HV1 in over-tempered condition (i.e., slightly above secondary hardness peak). Thermal fatigue (TF) tests revealed a denser and finer TF crack network with a larger oxidation in experimental steel. Thereby, the TF crack penetration depth scaling with the local plastic yielding of the matrix was evidently shorter in the new steel compared with wrought H13, thanks to significantly improved hot hardness, thermal conductivity, and enhanced thermomechanical softening resistance. Finally, a proof of concept is demonstrated by processing a relatively large injection molding die (150 mm × 110 mm × 30 mm) with promising tensile and impact toughness properties after direct double tempering to 51 HRC, and 45 HRC hardness levels.

Correlations Between XRD Peak Broadening and Elastic and Plastic Deformation for Mild Steel Sheets Under Tensile Loading

The polycrystalline nature of steel sheets plays a fundamental role in their plastic behavior during forming, especially under complex stresses, such as the deep drawing process. Heterogeneous microstructures cause variable internal stress levels, which strongly impact the material’s macroscopic mechanical properties. X-ray diffraction (XRD) technique can be applied to determine microscopic internal stresses by peaks broadening analysis, which can be related to the degree of cold work at the microscopic level. As deep drawing is a complex deformation mode, an important motivating factor to perform such a study is to find the correlation between anisotropy and XRD peak broadening through tensile deformation tests. Therefore, the initial mechanical state and different deformed states were analysed by in-situ XRD peak broadening under tensile tests in various directions in the rolling plane of mild steel sheet, for both the elastic and plastic deformation domains. The results showed that the XRD peak broadening gives accurate estimations of deformation degrees. In the elastic domain, under realistic assumptions, a relation between the relative variation of peak broadening and the applied stress σ, like Hooke’s law, was obtained by exploiting the linearity between the peak broadening and the elongation measured by the strain gauge. This formula may be used to deduce The Young’s modulus. Interesting empirical relations between peak broadenings, macro deformations and hardness have been also established for the studied sheet in the plastic deformation region. Moreover, XRD peak broadenings were able to characterize the anisotropy of deformation in the rolling plane of the sheet. They are like the anisotropy coefficients in indicating the expected behavior under complex processes such as deep drawing by using ordinary tensile tests.

Natural and artificial aging behaviour of Al-Cu-Mg-Ag-Ti-B (A205) alloy processed by laser powder bed fusion: Strengthening mechanisms and failure analysis

The Al-Cu-Mg-Ag-Ti-B (A205) alloy has gained success in the additive manufacturing domain due to its good processibility. It is a precipitation hardening alloy, and because of this, its mechanical properties are greatly influenced by post-processing treatments. In the present work, A205 samples processed by laser powder bed fusion were first solution treated and then aged by: (i) natural aging (NA) and (ii) artificial aging (AA). The effects of different aging treatments on the microstructure and mechanical behaviour were evaluated. Both NA and AA heat treatments improved the mechanical behaviour of as-built A205 alloy. Peak hardness was achieved in 20 days (≈ 137 HV) for the NA specimen, whereas it took 4 h at 190 °C (≈ 144 HV) for the AA specimen. The average yield strength, ultimate tensile strength and elongation to fracture for NA and AA were, 276 MPa, 434 MPa, 20% and 397 MPa, 463 MPa, 11%, respectively. The structure-property corelation was analysed by evaluating various strengthening factors. Considerable grain boundary strengthening was observed in both NA and AA specimen. The major strengthening contributions in the NA specimen were due to the disordered Ag-Mg co-clusters and solid solution strengthening by Cu. In AA instead, strengthening due to the precipitation of plate-type Ω/θ’- Al2Cu phase was dominant. In the end, failure mechanisms in the NA and AA conditions were also briefly discussed. The laser powder fused A205 alloy demonstrated a significant response to the NA and possessed a high-ductility and good-strength combination.

Effects of pre-stretch on microstructure, mechanical properties and corrosion resistance of 2A14 aluminum alloy

The microstructure, mechanical properties and corrosion behaviors of the peak-aged 2A14 aluminum alloys subjected to pre-stretch prior to artificial aging were investigated. The results indicate that the hardness and strength of the peak-aged alloy can be greatly enhanced via introducing pre-stretch due to the formation of dense and finer θ′ precipitates with dispersed distribution in alloy matrix. Pre-stretch prior to peak aging evidently enhances the peak aging hardness and shortens the peak aging time of the alloy. A pre-stretching degree of 7.5% leads to a distinct inhomogeneous distribution of θ′ phase, which decreases the ultimate tensile strength. As pre-stretching degree increases, the plasticity, impact toughness and exfoliation corrosion (EXCO) resistance of the peak-aged alloy all gradually decrease, but its resistance to intergranular corrosion (IGC) increases and then decreases.

Influence of Zn Addition on the Aging Precipitate Behavior and Mechanical Properties of Al-Cu-Li Alloy.

In the present work, the effect of Zn on the aging precipitates and mechanical properties of Al-Cu-Li alloys was investigated by Vickers hardness, tensile tests, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results indicated that the addition of Zn reduced the activation energy of the T1 phase and makes it easier to precipitate. The activation energy of the T1 phase, which was 107.02 ± 1.8 KJ/mol, 94.33 ± 1.7 KJ/mol, 90.33 ± 1.7 KJ/mol and 90.28 ± 1.6 KJ/mol for 0Zn, 0.4Zn, 0.8Zn and 1.2Zn alloy, respectively. The area number density of the T1 precipitate ranged from 97.0 ± 4.4 pcs/μm2 to 118.2 ± 2.8 pcs/μm2 as the Zn content increased from 0 to 1.2 wt.%. Consequently, the addition of Zn promoted the precipitation of the T1 phase. Therefore, the peak hardness and tensile strength of the alloy also increased with the increase in the Zn content, and the hardness of the alloy with Zn content of 1.2 wt.% increased by 16.5 ± 1.4 HV; meanwhile, the ultimate tensile strength increased by 46.5 ± 2.5 MPa. Therefore, the area number density of precipitates increased and improved the strength of the Zn-containing alloy.

Effects of Mo content on the microstructure and mechanical properties of laser cladded FeCoCrNiMox (x = 0.2, 0.5) high-entropy alloy coatings

The laser cladding additive manufacturing technology and high-entropy alloys can serve as ideal techniques and materials for the surface repair of high-speed train axles. Coatings of FeCoCrNiMox (x = 0.2, 0.5) were prepared on EA4T axle steel using laser cladding technology, and their phase, structure, and mechanical properties were analyzed using X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) characterization techniques. Additionally, the mechanical properties of the coatings were tested, and first-principles calculations were used to verify and calculate the material's mechanical performance. The research findings indicate that an increase in Mo content leads to a greater degree of lattice distortion, causing a leftward shift in diffraction peak positions. Significant differences in the microstructure from the substrate to the coating surface were observed, with columnar grain structure at the bottom of the cladding layer and equiaxed dendrites dominating the coating surface. The increase in Mo content promotes the formation of σ phase while also refining the grain size. ECCI results show that both types of coatings consist of a high-density dislocation cell structure and Mo-rich particles. With an increase in Mo content, the peak hardness of the coating increased from 456.5 HV to 469.4 HV, while the impact energy decreased from 56 J to 16 J, attributed to the increase in dislocation density and the greater quantity of σ phase. First principles calculations verified that the comprehensive mechanical properties of FeCoCrNiMo0.2 are superior, providing theoretical guidance for the optimization and design of the coatings.

Effects of synergistic modification using alkalis and guar gum on the pasting, rheological, and microstructural properties of germinated highland barley starch gels

Germination treatment of highland barley enhances its nutritional value while weakening the starch gel properties. This study aims to enhance the characteristics of germinated highland barley starch (GBS) by exploring the synergistic effects of two alkalis (Na2CO3 and NaHCO3) and guar gum (GG) on GBS gel properties. The combined action of alkalis and GG significantly improved the peak viscosity, setback viscosity, and hardness compared with GG alone. The highest G' and G" reached 998 and 204 Pa at 0.4% Na2CO3 addition, which were increased by nearly 44% and 50%, respectively. Fourier-transform infrared spectral analysis revealed that the alkalis strengthened interaction forces, particularly with intensified absorption peaks at 3200–3700 cm−1 and 1550–1750 cm−1. The Na2CO3 and NaHCO3 reduced the spin–spin relaxation time (T2), resulting in a dense starch gel network. This study contributes to enhancing the market application of GBS and offers innovative insights for modifying other starches.

Atomic structure of clusters and GP-zones in an Al-Mg-Si alloy

The structures of atomic clusters and GP-zones preceding precipitation in an Al-0.70Mg-0.85Si-0.15Fe-0.25Mn (wt. %) alloy have been investigated by annular dark field scanning transmission electron microscopy imaging and density functional theory calculations. One analysed condition consisted of one-month natural aging, and another was in a pre-baked state after 24 h aging at 90 °C. To quantify the effect of clusters and GP-zones on microstructure development during a subsequent isothermal artificial aging at 185 °C, hardness evolution was measured, and precipitate statistics was performed at the peak hardness corresponding to 5 h aging. Several types of clusters and GP-zones have been observed and based on structure and visual appearance named 'Disordered Frank-Kasper', 'Square', 'Binocular', and 1β", 2β", 3β", 4β". The first four are most common in the natural aged condition and are associated with a well-known influence on the initial hardness evolution. The last three are fractions of the hardening β" phase and refer to the number of structural units they contain. These GP-zones have an increased density in the pre-baked condition, leading to a rapid increase in hardness during artificial aging and to a finer microstructure of high-density small needle precipitates at peak hardness.

Effect of aging process on precipitated phase and properties of mechanical extruded aluminum alloy

As the industrial sector develops, the performance requirements for aluminum alloys are also constantly improving. The study explores how aging and rolling treatment affect aluminum alloys' precipitates and mechanical properties by controlling the parameters of aging process and rolling deformation variables. 7N01 aluminum alloy was selected as the experimental object, and the samples were treated with non-aging, natural aging, artificial aging, and rolling deformation. How aging processes and rolling deforming affected alloys’ mechanical properties was evaluated through performance testing (mechanical and tensile testing) and material fiber characterization methods (advanced electronic instruments). These results confirmed that the combination of three aging pre-treatments + R20 % + 120 °C re-aging could significantly improve the hardness of aluminum alloys and maintain high ductility. As the deformation decreased, the time for the sample to reach the hardness peak was shorter and the hardness was higher. The 20 % deformation sample’s strength was better than the 80 % deformation sample’s. This confirmed that appropriate aging process and deformation combinations could improve the strength and hardness of aluminum alloys. These results have certain guiding significance for optimizing the aging process of mechanical extruded aluminum alloys and provide reference for the study of related alloy properties.

Enhancing joint performance in friction stir welding through tailored double-butt-lap geometry

The study investigates the effectiveness of a novel joint configuration, the double-butt-lap (DBL), in friction stir welding (FSW) to join 6-mm-thick AA 6061-T6 plates. Four welds, including a simple square butt (SSB) configuration and three DBL variations (with UB to LB ratios of 1:2, 1:1, and 2:1), were executed. Using consistent parameters, the investigation evaluates the force distribution, macrostructure, and mechanical properties of the joints. Results show varied force profiles; spindle torque and Z-force decrease notably in DBL configurations compared to SSB, while X-force decreases overall for all DBL variations. Macrostructural analysis indicates increased weld area and finer microstructures in DBL joints. A correlation between peak temperature, grain size, and hardness highlights larger grain sizes at higher temperatures, influencing joint hardness. The DBL configuration with a 2:1 ratio exhibits the highest joint efficiency (83.76%). Fractographic analysis indicates ductile failure in all samples. Bend testing reveals increased flexural strength and bend angles in two DBL-configured joints compared to SSB. The results indicate that using a 2:1 ratio for joining 6-mm AA6061-T6 plates through FSW is viable, providing valuable insights into enhancing mechanical properties by optimizing joint geometry.

Laser powder bed fusion of high-strength low-alloy steels with post-heat treatment design for enhanced mechanical performance

A commonly used structural steel, namely high-strength low-alloy (HSLA) steel, HSLA-100, was fabricated using the laser powder bed fusion process in this work. Critical processing parameters such as laser power and scanning speed for achieving the least porosity were identified. The designed post-heat treatment differs from the traditionally manufactured HSLA steels. The optimum homogenization time was 80 min at 950 °C with the lowest prior austenite grain size. The peak hardness was achieved after tempering at 550 °C for 5 h, and atom probe tomography showed that the fraction of Cu and M2C (M: Mo, Cr) was the highest along with the co-precipitation of these strengthening phases at this condition. The yield strength (YS) and elongation (%El) of the builds printed with optimized parameters (porosity∼0.5%, YS = 875 MPa, and %El = 23%) were superior to those printed with factory-default parameters (porosity∼3%, YS = 772 MPa, and %El = 15%) after applying the designed post-heat treatment. Despite the anisotropy observed in the low-temperature toughness, the ductile-to-brittle transition temperature is below −40 °C for samples with a notch in the XZ plane and between −20 and −40 °C for samples with a notch in the XY plane. This work demonstrates the successful adaptation of HSLA-100 steel in additive manufacturing, and thus is critical for scaling up engineering applications at a higher technology readiness level.

Carbide precipitation during tempering of hybrid steel 60

The effects of carbide precipitation on mechanical performance of Hybrid Steel 60, known as a novel bearing steel, have not been investigated. In this study, the austenite transformation temperatures of Hybrid Steel 60 during heating were revealed by the thermal expansion curve. The temperature and effective activation energy of the second phase precipitation were determined by the differential scanning calorimetry (DSC) curve. Different solid solution structures after austenitization were detected using various cooling rates. The solubility temperature was determined based on hardness and residual austenite content. The carbides precipitated at the peak temperature were qualitatively identified using XRD. It was discovered that the temperature points Ac1 and Ac3 of the steel were 786 °C and 864 °C, respectively. In addition, the effect of solid solution temperature on quenching hardness is minimal, while the cooling rate has a greater impact on hardness, reaching a peak at 5 °C s−1. The primary carbide phase in Hybrid Steel 60 is the M7C3 and VC. When the temperature ranges from 500 °C to 550 °C, M23C6 begins to precipitate. As a result, after tempering at 525 °C, the hardness peak value reached 566 HV.

Surveying the Effects of Aging a High C-Containing Co-Based Superalloy From the As-Cast and Solution Heat-Treated Conditions

The microstructure of the high carbon-containing cobalt-based superalloy, Co-101, has been studied in the as-cast state and following a variety of heat treatments. In the as-cast state both M7C3 and Mo-rich M23C6 carbides were observed in the interdendritic regions. After thermal exposure at temperatures between 1000 °C and 1250 °C for 1, 10, and 100 hours, the M7C3 interdendritic carbide network was observed to transform into a Mo-lean M23C6 carbide. These changes were rationalized with thermodynamic calculations. The carbide transformation liberated carbide-forming elements that resulted in the precipitation of intragranular carbides in the dendritic regions at temperatures below 1150 °C. These carbides in the cast-aged material preferentially formed at the dendrite peripheries early during exposure, leading to wide particle size distributions. Peak hardness in the cast-aged material was attained within the first 10 hours of exposure and softening was observed thereafter. After solution heat treating at 1250 °C for 10 hours, the microstructure of Co-101 comprised an M23C6 interdendritic carbide network and solid solution dendrites supersaturated with carbide-forming elements. Subsequent aging of this microstructure for 100 hours at 900 °C led to a high number density and narrow particle size distribution of intragranular carbides. The characteristics of these carbides in the solution-aged material resulted in greater hardness, which was retained for longer durations of exposure, than the cast-aged specimens.

Development of an experimental-analytical method for obtaining optimal two-layer welding window of a Ni–Cr–Mo–V alloy steel

This study investigates the feasibility of using two-layer tungsten inert gas (TIG) welding to repair damaged parts made of 3.5NiCrMoV low-alloy steel, which is widely used in various industries for complex and thick components. Experimental and analytical methods are used to determine the optimal heat input for the second layer of welding, which can eliminate the need for post weld heat treatment (PWHT). The first layer is welded with a heat input of 0.42 kJ/mm and the second layer is welded with heat inputs ranging from 0.59 to 0.92 kJ/mm. The effects of the second layer heat input on the microstructure, peak temperature profile, cooling rate, hardness, and impact energy of the welded alloy are evaluated. The optimal heat input for the second layer is found to be 0.76 kJ/mm, which is 1.8 times the heat input of the first layer. This heat input tempers the coarse-grained region in the heat affected zone (HAZ) caused by the first layer, maintains the hardness of the weld metal, and improves the impact energy of the welded alloy. This method is novel and useful because it can save time, money, and energy for repairing the damaged parts made of high strength low alloy steel. This method can also provide useful information for controlling the microstructure and mechanical properties of high strength low alloy steels during welding. Rosenthal's analytical models are used to simulate the temperature and cooling rate profiles of the welded metal and the HAZ, and the results are compared with the experimental data.

Microstructure and mechanical properties of ultrasonic spot welding of AA7075-T6 and A380 casting aluminum alloy

Ultrasonic spot welding (USW) was employed to join AA7075-T6 and casting Al alloy A380. Under a constant welding power, the influence of the welding time duration (from 1 s to 3 s) on peak temperature, microstructure, hardness, and mechanical strength of the joints were studied. It was found that the peak temperature increased with welding time. A sound solid-state metallurgical bonding was obtained at the joint interface. The cracks occurred at the edge of the joints were eventually eliminated as the welding time increased. A layer of refined grains formed in A380 adjacent to the bonding line. Hardness reduction occurred in AA7075 at the joint interface and the degree of hardness reduction and the volume increased with the peak temperature. By increasing welding time, lap shear tensile tests show that the peak load increased from 5.3 kN to 6.55 kN. The corresponding interfacial shear strength ranged from 96.29 MPa to 99.72 MPa, lower than the shear strength of both base materials (331 MPa for AA7075 and 185 MPa for A380). All joint samples exhibited interfacial fracture after lap shear tensile testing, with a thin layer of A380 remnant attached to AA7075 at the center of the fractured surface.

Influence of melon shell ash reinforcement on the mechanical and microstructural characteristics of recycled aluminium matrix composites

This study produced a composite using melon shell ash as reinforcement on recycled Aluminium waste cans matrix. The melon shell particulate additions to the matrix were done in wt.% of 0, 10, 5, 15 and 20. The mechanical behaviour of the composite demonstrates that the tensile values increased as the particulate addition increased to 15% wt. to reach a maximum of 113MPa but dropped at the 20% addition. The impact energy on the other hand increases marginally at 5% particulate addition and increases slightly higher with the addition of reinforcement of 10% wt to attain a maximum value of 77J. However, a further increase in the melon shell ash to 15% wt and 20% wt reduced the impact energy. The hardness values of the composites at all the % wt. additions were generally higher than that of the unreinforced Aluminium with the peak hardness value of 57.2BHN obtained at 10% wt. addition. The density of the composites decreases as the % wt. of the melon shell ash particles increases. The improved mechanical properties and lower density of the composites achieved in this study are significant factors in developing strong and lightweight engineering materials for industrial applications.

Effect of destability time on microstructure and properties of hypoeutectic high chromium cast iron.

The present work investigated the effect of destabilization time on the mechanical properties and microstructure evolution of high chromium cast iron, and scanning electron microscopy and electron probe microanalysis techniques were employed. The results show that the hardness of hypoeutectic high chromium cast iron is related to the size and volume fraction of secondary carbides precipitated from the matrix. The hardness of the alloy continues to rise due to the continuous increase of the volume fraction of the secondary carbide at the initial stage of destabilization. The alloy reaches its peak hardness value at 950 °C and 1000 °C for 1 hour holding time. The solid solubility of carbon and alloying elements in the matrix increases as the holding time extends, resulting in a large number of carbides redissolved into the matrix, making the hardness of the alloy decrease; the hardness of the alloy at 14 h is less than that at 10 min. Under 1050 °C, the size and density of the secondary carbide increase significantly; extending the holding time will lead to the continuous reduction of the carbide rod that provides strength, thus, the hardness curve shows a downward trend.

低压静电场辅助冷藏对草莓贮藏品质的影响

In order to maintain the storage quality of strawberry after harvest and study the effect of low voltage electrostatic field (LVEF)-assisting refrigerated storage on strawberries quality, the study applied the low voltage electrostatic field (3kV 4±1℃ in LVEF group) to store strawberries in refrigeration environment, and setting refrigerated storage at 4±1°C as the control (CK group). The results showed that the freshness days of strawberries refrigerated by low voltage electrostatic field were twice as long as those of refrigerated group. The brightness and red-green degree of strawberries in LVEF group were 1.5 times and 2 times of those in CK group, respectively. The peak hardness of strawberry in LVEF group was 1.43 times of that in CK group, and the hardness decline range was 18.97% lower than that in CK group. The decreasing ranges of Total soluble solids (TSS), Titratable acids (TA) and Vitamin C(VC) in LVEF group were 43.91%, 60.01% and 46.17% lower than those in CK group, respectively. The increment of Malondialdehyde (MDA) of strawberries in LVEF group was 19.77% lower than that in CK group, and the activity peak of Superoxide dismutase (SOD) of strawberries in LVEF group was 1.22 times of that in CK group, and the decreasing range was 22.86% lower than that in CK group. After 10 days of storage, the weight loss rate and decay rate of strawberries in LVEF group and CK group were 16.69%, 15% and 70.65%, 59.45%, respectively. In conclusion, applying low voltage electrostatic field to refrigerated storage can effectively inhibit the quality deterioration of strawberries during refrigerated storage period.

Spectral Hardness and Evolution of Swift Gamma-Ray Bursts and X-Ray Afterglows

Spectral hardness of gamma-ray bursts can be characterized by either hardness ratio or peak energy, and the inconsistency of these parameters can reflect spectral diversity and complexity. We have chosen 310 Swift bursts with well-measured X-ray afterglows to analyze the distributions of their peak energies and hardness ratios before and after absorption of neutral hydrogen. We categorize them into four types according to X-ray temporal patterns and calculate the hardness ratios of both γ-rays and X-rays. It is found that the hardness ratios of X-rays are systematically smaller than those of γ-rays. We also find that both γ-ray hardness ratios and peak energies of bursts of types I, II, and III are identically distributed and are significantly lower than the corresponding measurements of type IV bursts in statistics, which hints that the evolving spectra of type IV bursts are most probably affected by the geometry of outflows. This proves that the influence of neutral hydrogen absorption on the estimation of hardness ratio is negligible even though it can reduce the brightness of γ/X-rays somewhat. Furthermore, we verify that almost all the observed spectra of gamma-ray bursts can be described by a broken power-law function and confirm a magnetar origin for the bursts with internal X-ray plateaus.