Effect Of Thermal Cycling Research Articles

Effects of thermal aging on the performance of ordinary and novel superhydrophobic and oleophobic ultra-fine dry powder extinguishing agent

Powder-based fire extinguishing agents have become a kind of promising substitutes for halon extinguishing agents in civil aircrafts. However, their storage lifespan, significantly influenced by the thermal aging, emerges as a crucial yet overlooked aspect for aviation use. This study investigates the effects of thermal aging cycles on various parameters of ordinary dry powder extinguishing agent (ODPEA) and novel superhydrophobic and oleophobic ultra-fine dry powder extinguishing agent (SHOU DPEA), including surface microscopic morphology, D90 (the diameter at which 90% of the cumulative volume of particles are equal to or smaller than this value), chemical structure, hydrophobic and oleophobic angles, flowability, extinguishing time and effectiveness. The results indicate that SHOU DPEA exhibits smaller particle size, more regular particle shape, significantly superior heat stability and flowability compared to ODPEA. Furthermore, the D90 value evolution of ODPEA with aging time exhibits two stages: (a) a slow and linear growth stage (0–160 days), and (b) a rapid and substantial growth stage (160–200 days). However, SHOU DPEA shows a gradual increase in D90 value throughout the entire accelerated aging process. After 160 days of aging or more, the performance of ODPEA has significantly deteriorated, while SHOU DPEA has shown less degradation. Specially, the extinguishing concentration for the 160-day-aged ODPEA exceeds that of non-aged ODPEA by 10%, whereas the concentration of the 200-day-aged SHOU DPEA is less than 10% of the non-aged counterpart. Additionally, the predicted lifespans for ODPEA and SHOU DPEA at room temperature (25 °C) are 2715 days and over 4525 days, respectively. These findings can provide valuable guidance for assessments and the selection of aviation fire extinguishing agents.

Effect of friction stir processing on wire arc additively manufactured ER70S-6/SS308L bimetallic FGM

In this research, wire arc additive manufacturing (WAAM) is employed to fabricate a bimetallic steels component using ER70S-6 and SS308L. However, this process involves high power densities and significant heat inputs, which elevate the risk of unfavorable grain structures and defects. To mitigate this issue, friction stir processing (FSP) is applied, utilizing a unique tool featuring a flat shoulder without a pin. The resulting samples undergo testing for mechanical and metallurgical properties. The microstructure of ER70S-6 reveals ferrite with small amounts of pearlite at the grain boundaries. The upper portion of the deposited sample exhibits more pronounced grain refinement compared to the lower section, largely due to the rapid cooling rate and thermal cycling effects during layer deposition. This grain refinement is attributed to dynamic recrystallization, which enhances the hardness of the FSP-treated sample relative to its initial as-deposited state. Tensile tests indicate a significant increase in tensile strength, accompanied by a reduction in percentage elongation after FSP. The fatigue crack growth rate is higher along the longitudinal direction of bi-metallic joint as compare to transverse to the joint.

Development of an Alternative Bitumen Rejuvenator Consisting of Waste Oils and Reacted and Activated Rubber (RAR)

After a long-term in service on pavement, bituminous material exposure to environmental conditions and traffic loads leads to inevitable aging, causing an increase in binder viscosity and making it stiffer. On the other hand, the absence of proper elimination and illegal dumping of waste oil and tire rubber can lead to serious environmental risks. This research aims to investigate the possibility of using waste engine oil (WEO) and waste cooking oil (WCO) separately combined with reactive and activated rubber (RAR) as new alternative rejuvenating agents. In this study, 5% of waste oil (WCO, WEO) and 15% of RAR were added to the aged bitumen with a penetration grade of 20/30. The virgin, aged, and rejuvenated bitumen were investigated for their physical and chemical properties using penetration, softening point, and Fourier transform infrared spectroscopy (FTIR) tests, as well as the effect of thermal cycles (heating and cooling) on the cracking resistance of bituminous mixtures containing regeneration bitumen using the Fénix test. The study outcome indicated that these rejuvenators could effectively soften and restore the basic properties of the aged bitumen to a normal level of 40/50. Meanwhile, the bituminous mixture with the aged bitumen rejuvenated with both oils and tire rubber (AB-WRCO and AB-WREO) showed the best cracking behavior. Therefore, these alternative solutions involve not only reusing the aged bitumen but also reusing the waste oil and tire rubber in order to attain environmental protection and economic benefits.

Investigation of the Effects of Thermal Cycling and Surface Treatments on Interim Resin Materials Produced with Additive Manufacturing

This study aimed to compare the impact of thermal cycling on the flexural strength (σfs) of 3-dimensional (3D) printer resins and polymethyl methacrylate (PMMA). It also aimed to evaluate different surface treatment protocols and surface sealant applications on the surface roughness (Ra) and surface hardness (VHN) of the resins used. Rectangular (25�2�2 mm) and disc (Ø10�2-mm) specimens were fabricated from auto-polymerized PMMA, and 3D-printed resins were tested for mechanical properties (ISO 10477) using thermal cycling (n=10). Disc specimens were separated into three groups (n=10) based on the surface treatments: conventional sanding (C), disk polishing (P), and surface sealant coating (O). Ra and VHN values were statistically analyzed with the Kolmogorov Smirnov, Shapiro Wilk, Independent Samples t-test, Mann-Whitney test, one-way ANOVA test and Kruskal Wallis test (α =.05). A significant main effect was found on the flexural strength analysis for each of the two factors: thermal cycling procedure (p[0.001) and materials (p 0.001). A significant main effect was found on the surface roughness and hardness analysis for each of the two factors: surface treatment protocols (p[0.001) and materials (p[0.001). Based on the results; interim materials produced with 3D-printed resins have better mechanical properties than conventionally polymerized materials. Coated materials had lower surface roughness values than polished ones, and adding a surface sealant agent increased their hardness significantly.

Effect of thermal cycling on microstructure and mechanical properties of nano-silver microsolder joint

Effect of thermal cycling on microstructure and mechanical properties of nano-silver microsolder joint

Effects of Thermal and Stress Cycles on the Mechanical Properties and Acoustic Emission Characteristics of Rocks

Abstract The surrounding rock of underground rock chamber is frequently affected by disturbance load and circulating temperature; it is meaningful to study the mechanical properties of surrounding rock under these conditions for the construction of a safe and effective underground chamber. This study investigates the mechanical properties, failure modes, and acoustic emission (AE) characteristics of basalt and sandstone under various pretreatments, including water saturation pretreatment (rock samples [SR]), rock samples subjected to cyclic temperature pretreatment (SR-CTT), rock samples subjected to cyclic loading pretreatment (SR-CLT), and rock samples subjected to combined cyclic loading and temperature pretreatment (SR-CTT-CLT). A series of uniaxial compression tests (UCTs) were conducted to analyze how these pretreatments affect the mechanical properties of basalt and sandstone. These two kinds of rock exhibit distinct failure modes, SR-CLT and SR-CTT-CLT make the failure of basalt change from inclined shear to X-shaped shear, while SR-CLT makes it turn into splitting. AE data reveal that basalt generally exhibits lower AE counts than sandstone, with the highest counts observed under SR-CTT-CLT. Energy analysis indicates that basalt accumulates more total energy (Et) and elastic energy (Ee) compared to sandstone, with different pretreatments affecting energy dissipation (Ed) and damage severity differently in each rock type. These findings contribute to understanding the complex interactions between pretreatment methods and rock behavior in engineering applications.

Thermocycling test of a titanium-carbon fiber adhesive joint produced using laser texturing technology

In spacecraft load-bearing structures, adhesive bonding of titanium alloy and composite material parts is often used. To increase the strength of the adhesive bond of the titanium-carbon fiber reinforced plastic pair, preliminary treatment of the bonded surfaces is necessary. In this paper, it is proposed to use laser texturing to process the metal surface. The main objective of the study is to experimentally determine the strength characteristics of the adhesive bond of carbon fiber reinforced plastic and titanium alloy with different modes of laser processing of the metal surface and to determine the effect of thermal cycling on the samples of the adhesive bond. The surface of the OT-4 titanium alloy was laser processed in different modes, after which the samples were glued with VK-9 and LOCTITE® EA 9394 AERO glue. The glued samples were subjected to thermal cycling in a vacuum chamber in the temperature range from –150 to +150 °C. Shear testing of adhesive bond samples showed that laser texturing increases bond strength by an average of 60% for LOCTITE® EA 9394 AERO adhesive and by 142% for VK-9 adhesive. Samples with laser texturing have a cohesive nature of failure on carbon fiber. During thermal cycling, most samples show a slight decrease in adhesive bond strength by an average of 6...8%. The results show that the use of laser processing to prepare titanium alloy before bonding with composite material is a promising method for increasing the strength of adhesive bonds for spacecraft components.

Method for determining parameters of two-way shape memory effect in amorphous-crystalline TiNiCu alloy ribbons

This article addresses an urgent challenge in the development of effi cient micromechanical tools for the manipulation of microobjects related to microelectromechanical systems. Recently, layered amorphous-crystalline ribbons of rapidly quenched TiNiCu alloy exhibiting a two-way shape memory effect have been successfully used to create microtweezers. The study outlines a method and presents an original experimental setup designed to determine the parameters associated with the two-way shape memory effect in such ribbons. The thermomechanical properties of an amorphous-crystalline composite, characterized by an amorphous layer thickness of 25 μm and a crystalline layer thickness of 7 μm, produced by melt spinning technique, were investigated. The effect of multiple thermal cycles in the shape memory effect temperature range was studied. The maximum reversible bending strain observed in the ribbon exceeds 0.2 %, which facilitates the development of micromechanical devices such as microtweezers. An increase in the number of thermal cycles from 1 to 100 resulted in a signifi cant enhancement of the reversible strain (by approximately 1.2 times) and a nearly twofold reduction in the temperature hysteresis associated with the two-way shape memory effect. The characteristic temperatures exhibited minimal variation, with the exception of the onset temperature of shape change during cooling, which showed a marked increase. The results obtained make it possible to improve the functional characteristics of micromechanical devices, based on amorphous-crystalline TiNiCu alloy ribbons by increasing their reliability and operational speed, as well as reducing their dimensions.

Influence of the combined effect of long hygrothermal aging and thermal cycling on mode I and II interlaminar fracture toughness of carbon/epoxy composites

Carbon fiber/epoxy composites (CFRP) are nowadays extensively used in aircraft structures. During the lifecycle of an aircraft, composites are subjected to temperature and humidity variation over time and that can lead to performance loss. Efforts to understand the environmental conditioning impact on CFRP properties have been widely investigated, leading to the main objective of this study, which was investigating whether damages caused by different environmental conditioning individually are intensified when they are combined. During an aircraft lifetime, several different environmental conditions may occur, and to simulate these effects, this research tested coupons that were exposed to long-term hygrothermal conditioning, drying and thermal cycling and their combined effects on fracture toughness in mode-I and mode-II. Dynamic mechanical and fractographic analysis were also performed on the specimens. Thermal cycling on unaged samples (reference) resulted in average reductions of 5% and 15%, for mode-I and mode-II fracture toughness, respectively, compared to the reference samples. Furthermore, combined effect of thermal cycling and hygrothermal conditionings resulted in reductions around 44% (mode-I) and 15% (mode-II) compared to the reference. The combination of different environmental conditionings was more detrimental to fracture toughness in Mode-I, as confirmed by fractographic analysis. In contrast, Mode-II showed no change in results when exposed to a combination of conditionings, suggesting that isolated analysis of the results can be misinterpreted. Combined effects of thermal cycling and hygrothermal conditioning affected fracture toughness more than either effect alone. Mode-I testing proved to be more sensitive to identifying environmental exposure effects on properties than mode-II.

Influence of arc energy on the microstructure and corrosion behavior of ER2209 duplex stainless steel deposited on Q345B low-alloy steel by GMAW

In order to eliminate the complex effects of thermal cycling and the thermal effects of the next pass on multi-layer multi-pass welding of low-alloy steel and duplex stainless steel, ER2209 welding wire was single-layer single-pass deposited on the surface of Q345B hot-rolled plate by gas metal arc welding (GMAW). The microstructure and corrosion resistance of welded joints were studied by different arc energies. The results showed that the arc energy had a significant impact on the “tongue-shaped” morphology and the width of heat affect zone in the welded joint, which in turn affected the corrosion performance of the welded joints. Due to the least distribution of “tongue-shaped” morphology and minimum width of heat affected zone in the welded joint with arc energy of 0.645kJ/mm, it owned the best corrosion resistance. The weld metals of the welded joints were composed of ferrite and austenite phases. The ferrite phases were distributed along grain boundaries and intergranular. As the heat input increases, the distribution of ferrite phase along grain boundaries gradually decreased. The corrosion products and corrosion morphology of the welded joint were also investigated and the corrosion mechanism was discussed in detail. The research results of this article provide a theoretical basis for improving the corrosion performance of multi-layer multi-pass welded joints of low-alloy and duplex stainless dissimilar steels.

Martensitic transformation and effects of thermal cycling in Ni50Mn33Al17 magnetic shape memory alloy

The phase transformations of a Ni50Mn33Al17 alloy in the range from room temperature to 1000 K are investigated. Differential scanning calorimetry, electrical resistometry and thermomagnetic measurements are presented. The critical temperatures and entropy change for the direct and inverse martensitic transformation are determined. The effects of thermal cycling are studied by performing repetitive measurements on different temperature ranges after quenching. For cycling below 500 K martensitic transformation/retransformation do not show significant changes. However, if the cycling reaches higher temperatures, modifications appear in the values ​​of electrical resistivity and magnetization at room temperature, and the volume fraction that transforms martensitically is reduced. These effects could be attributed to an excess of vacancies retained by quenching, which are only eliminated above 500 K, and to the growth of equilibrium phases with high-temperature magnetic properties.

Determining the Permeability Coefficient of Concrete Using New Fractal Model and Examining the Effect of Different Thermal Cycles on Concrete Permeability

Determining the Permeability Coefficient of Concrete Using New Fractal Model and Examining the Effect of Different Thermal Cycles on Concrete Permeability

Effect of heat treatment on microstructure and mechanical properties of WE43 alloy fabricated by wire arc additive manufacturing

Heat treatment is an effective method to improve the mechanical properties of Wire Arc Additive Manufacturing (WAAM) WE43 alloy. This study investigates the effects of heat treatment on the microstructure and mechanical properties of the WE43 alloy processed by WAAM. The as-deposited samples exhibit a uniform equiaxed crystal structure, with grain size gradually increasing as the number of deposited layers grows. The as-deposited microstructure comprises eutectics and a small amount of cubic phases, and the phase content escalates as the amount of deposited layers increases, influenced by the cumulative effect of multiple thermal cycles. At lower solid solution temperatures (480 °C × 8 h), most of the eutectic phase remains undissolved. During tensile testing, undissolved eutectics tend to cause stress concentrations, leading to cracks that result in sample fractures, negatively affecting the mechanical properties. At higher solid solution temperatures (520 °C × 8 h), most of the eutectic dissolves, but abnormal grain coarsening (AGC) in the interlayer fine-grain region occurs, causing a significant reduction in the sample's mechanical properties. After solid solution treatment at 500 °C for 8 h, only a small amount of eutectics remains undissolved, and the AGC phenomenon is not observed, resulting in an optimal solid solution effect. Subsequently, after aging treatment at 225 °C for 18 h, dense nanoscale β′′ phases precipitated within the alloy grains, significantly improving the mechanical properties. The samples achieved optimal performance, with UTS, YS, and EL of 323 ± 15.2 MPa, 225 ± 11.3 MPa, and 8.4 ± 1.1 %, respectively.

Bonding effectiveness of multi-step adhesive resin cements to CAD/CAM blocks: impact of thermal cycling and surface treatment methods

BackgroundTo investigate the effects of thermal cycling and surface treatment methods on the bonding effectiveness of multi-step resin cements to CAD/CAM blocks.MethodsA total of 198 slices, 66 each from CAD/CAM blocks (feldspathic ceramic: Vitablocs TriLuxe Forte, V; resin matrix ceramics (RMCs): Cerasmart, C; and Shofu Block HC, S), were obtained and randomly divided into two subgroups for etching with hydrofluoric acid (HFA) and sandblasting with Al2O3 (SB). After the surface treatments, one etched and one sandblasted sample of each CAD/CAM block was observed via SEM analysis at 500× magnification. The remaining 32 etched and 32 sandblasted samples of each CAD/CAM block were divided into two subgroups to be cemented with total-etch (TE) and self-etch (SE) resin cements. Then, half of the 16 samples in all the subgroups were subjected to aging (TC) for 5000 cycles (n = 8). The shear bond strength (SBS) of each sample was measured. Four-way analysis of variance (ANOVA) and Tukey tests were used to analyze the data (p < 0.05).ResultsWith or without TC, the highest SBS values for V were obtained with the HFA-TE and HFA-SE interactions, respectively. C presented the highest SBS values with HFA-SE and SB-TE interactions, whereas S presented the highest SBS values with SB-TE and HFA-TE interactions. Except the SB-SE interaction, C presented lower SBS values after TC than other materials. HFA created less porosity on the C and S surfaces than V. SB visibly roughened the surfaces of all the materials but caused fractures, cracks, and damage to the surfaces.ConclusionSimilar SBS values can be achieved between feldspathics, RMCs, and multi-step adhesive resins with both HFA and SB treatments. However, the SBS values obtained from the SB-SE interaction may be below the recommended threshold values for all materials after TC. SB can cause distinctive cavities, fissures, and damage, especially on the surfaces of RMCs.

Relationship between grain structure evolution and tensile anisotropy in Al-Zn-Mg-Cu cylindrical part formed by additive friction stir deposition

The thick-walled Al-Zn-Mg-Cu cylindrical part was obtained by additive friction stir deposition, and the relationship between grain structure evolution and tensile anisotropy was investigated in detail. The intra-layer grains are significantly refined due to continuous dynamic recrystallization (CDRX), and the grains at the interface are further refined by 30%–45 % due to geometric dynamic recrystallization (GDRX). The early cracking and ductility deterioration under Z direction loading are caused by strain localization due to coarse and fine grain distribution and pre-existing strain at the interface. The grain growth rates for specific orientations (Cube, P, Q, and Rotated Goss) are higher than average, and the mismatch in elastic modulus between these grown grains and the matrix results in ductility differences in the X and Y directions. Texture components and residual stresses affect yield strength in the X and Y directions. Meanwhile, dissolution of η/η′ and the coarsening of the particles due to the thermal cycling effect leads to a decrease in the yield strength in the building direction. The average ultimate tensile strength (UTS) and elongation (El) of the part could reach 370 MPa and 20 %, respectively. The tensile properties of the as-deposited Al-Zn-Mg-Cu cylindrical part are generally higher than those of melt-based additive manufacturing, but lower than those of the extruded and forged states due to the dissolution and coarsening of the strengthening precipitates during thermal cycling.

Full-scale in-situ tests on a displacement cast in situ energy pile: Effects of cyclic thermal loads under different mechanical load levels on pile stress and strain

Numerous full-scale in situ tests have been conducted to assess the effect of thermal cycles on the pile response. However, those studies investigated the response of only precast and cast in-situ energy piles, with limited focus on the impact of the applied mechanical load on the pile response. This study presents the results of a field test conducted on a new type of energy pile, i.e. a displacement cast in-situ energy pile in multilayered soft soils, subjected to different fixed mechanical loads while undergoing simultaneous thermal cycles. Four tests were carried out, each corresponding to various axial loads ranging from 0 % to 60 % of the pile’s estimated bearing capacity. After applying the axial load on the pile head (0 %, 30 %, 40 %, or 60 % of the bearing capacity), the pile was subjected to up to ten thermal cycles. The highest magnitudes of thermal axial strains were observed near the pile top due to the lowest restraint provided by the made ground layer in all tests. Under zero (0 %) mechanical load, the thermal axial strains near the pile head were elastic and recoverable, while residual strain was observed near the toe. Under reasonable working mechanical loads (30 %, 40 %, or 60 %) residual strains were observed near both the pile head and the toe, with higher residual strains observed under higher mechanical loads. The results indicate that the cyclic thermal loadings could induce an increase in the compressive stress in the energy pile, attributed to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects counteracts thermally induced tensile stress and thus leads to an insignificant effect on the energy pile during cooling. A limited impact of the shaft capacity was observed and was mainly attributed to the drag-down of the surrounding soil and thermal creep along the pile-soil interface.

Synergetic effect of diamond particle size on thermal expansion of Cu-B/diamond composite

Diamond particles reinforced Cu matrix (Cu/diamond) composites have promising applications for heat dissipation of high-power electronic devices because of their high thermal conductivity and suitable coefficient of thermal expansion. The effect of diamond particle size on thermal conductivity has been addressed; however, the effect of diamond particle size on thermal expansion still needs to be clarified. In this study, Cu-B/diamond composites with various diamond particle sizes ranging from 66 μm to 701 μm were fabricated to assess the impact of diamond particle size on the thermal expansion behavior. The composites exhibit low and adjustable coefficient of thermal expansion (CTE) values of 4.58–6.63 × 10−6 K−1, which align with 4–8 × 10−6 K−1 of widely employed semiconductors. The CTE of the Cu-B/diamond composites first decreases and then increases with increasing diamond particle size, which arises from a synergetic effect of interfacial bonding strength and matrix strengthening effect. As the diamond particle size is smaller than 272 μm, the interfacial bonding strength rises with increasing particle size, enabling the diamond particles to restrain the expansion of the Cu matrix more effectively and to reduce the CTE. As the diamond particle size exceeds 272 μm, the dislocation density in the Cu matrix continually decreases with increasing particle size, reducing the strength increment in the Cu matrix and increasing the CTE. The effect of thermal cycling on the thermal expansion of the Cu-B/diamond composites was also investigated, and all the composites show an increase in the CTE after 100 thermal cycles. Notably, the composite with 272 μm diamond particle size exhibits the lowest CTE increment. It shows a CTE value of 5.29 × 10−6 K−1 after thermal cycling, still compatible with the semiconductors for electronic packaging applications.

Effects of thermal cycles and time-dependent behavior of the deck on steel network arch bridges

Network arches are a type of arch bridges with inclined hangers, in which some hangers cross each other at least twice. This paper aims to evaluate the effects of thermal fluctuations and volumetric changes of the concrete deck on steel network arches. Fifty-nine variants of a reference network arch bridge were developed, assuming identical arch geometries but varying hanger arrangements. Each bridge was modeled using the finite element method and subjected to heat flux due to solar radiation and changes in ambient temperature in addition to heat transfer via conduction, convection, and radiation. The time-dependent deformations of the concrete deck were implemented using a rate-type formulation. Results obtained during a one-year period after the assumed end of construction showed that stress changes in the arch bridge due to ambient and time-dependent effects were not very sensitive to the hanger layout. Neglecting concrete creep and shrinkage was found to cause a 15-20% underestimation of maximum stresses in the concrete deck, whereas a 40-60% error was observed in both the deck and rib when thermal effects were ignored. The radial arrangement of hangers, already known to perform favorably under live loads, was also found to provide the smallest bending moments and the fewest number of relaxed hangers due to thermal and time-dependent effects.

Performance Evaluation of Cold Metal Transfer-Based Wire Arc Additive Manufacturing of SS316L: An Effect of Weld Thermal Cycles and Heat Inputs

Performance Evaluation of Cold Metal Transfer-Based Wire Arc Additive Manufacturing of SS316L: An Effect of Weld Thermal Cycles and Heat Inputs

Study on Dynamic Recrystallization under Thermal Cycles in the Process of Direct Energy Deposition for 316 L Austenitic Stainless Steel

In the process of directed energy deposition (DED), the grain structure of the deposited samples is determined by two aspects. The first is the initial solidification grain structure; the second is the effect of the upper thermal cycle on the solidified grain structure of the lower layer. Dynamic recrystallization and grain growth can be activated under suitable strain and the temperature resulting from thermal cycles. The evolution of grain size and the geometric dislocation density (GND) of austenitic stainless steel 316 L under different strains and temperatures caused by thermal cycles was investigated. It is found that dynamic recrystallization requires an appropriate level of accumulated strain, temperature, and initial grain size. Under &lt;2% accumulated strain and 400–1200 °C conditions caused by 30 layers of thermal cycles, fully dynamic recrystallization occurs with coarse initial grains (CIG), leading to the complete coarsening of grains. However, relatively fine initial grains (FIG) under the same conditions only display partial dynamic recrystallization. The next 2–4% strain and 400–700 °C by 60 layers of thermal cycles make up the driving force of fully dynamic recrystallization, and the grains coarsen completely. Larger accumulated strain (4–6%) and lower temperature (400–600 °C) by 90 layers of thermal cycles and FIG provide more nucleation sites for dynamic recrystallization, which leads to little coarsening of grains even after fully dynamic recrystallization. Temperature, accumulated strain, and the amount of δ-ferrite promote the formation of sub-grains during dynamic recrystallization caused by thermal cycles, which leads to the increase in GND.