Formation Of Subboundaries Research Articles

Partially recrystallized triplex-phase medium Mn steel strengthened by B2 precipitates

We report the annealing time-dependent microstructure–mechanical properties relationship of cold-rolled and annealed Fe–6Mn-0.05C–3Ni-1.5Al (wt.%) steel, containing Ni and Al to form NiAl B2 precipitates. The microstructures of the investigated materials after cold-rolling and intercritical annealing show a mixture of lath and equiaxed zones consisting of a triplex matrix phase (ferrite, austenite, and tempered martensite) with B2 precipitates. Recovery of the deformed martensite in the cold-rolled microstructure during annealing resulted in the formation of sub-boundaries and a continuous transition from partially-recrystallized tempered martensite to recrystallized ferrite. The investigated materials showed a decrease in strength and an increase in ductility and strain-hardening rate with increasing annealing time. A decrease in the fraction of tempered martensite, an increase in the fraction of recrystallized ferrite/austenite grains, and enhanced transformation-induced plasticity (TRIP) kinetics with increasing annealing time led to enhanced ductility and strain hardening of the materials. The mixed presence of less stable equiaxed austenite and more stable lath austenite resulted in sustained TRIP effect during tensile deformation and superior strain hardening capacity of the specimen annealed for 24 h. This study provides a novel microstructure design solution for medium Mn steels, and further optimization of the composition and processing will lead to the development of medium Mn steels with superior mechanical performance.

Microstructure and tempering softening mechanism of modified H13 steel with the addition of Tungsten, Molybdenum, and lowering of Chromium

The rapid development in the advanced manufacturing industry asks for better tempering softening resistance (TSR) of Hot work die steels. In this work, a modified H13 steel (CXN03 steel) with additional tungsten, molybdenum, and lowering chromium was prepared. The TSR of CXN03 is significantly better than H13. After quenching at 1040 °C, the hardness and strength of H13 were larger than those of CXN03. However, the hardness and strength of CXN03 exceeded those of H13 after 2 h tempering at 600 °C. A mathematical model was utilized to correlate microstructural characteristics with yield strength during tempering. The calculated results indicated that the superior tempering softening resistance of CXN03 steel mainly results from the excellent stability of dispersive nano-sized M2C, which could prevent dislocation recovery. Recrystallization softening was observed in H13 but not in CXN03. The recrystallization of H13 is driven by dislocation movement, and the rearrangement of dislocations contributed to the formation of sub-boundaries. These sub-boundaries could divide martensite lath as well as form sub-grains. As the tempering time increased, sub-boundaries transformed into high-angle grain boundaries by absorbing the vicinal dislocations. Therefore, martensite lath collapsed, and massive recrystallized grains occurred. The massive stable M2C in CXN03 hindered the dislocation rearrangement, thus preventing the recrystallization.

Constitutive equation and dynamic recovery mechanism of high strength cast Al-Cu-Mn alloy during hot deformation

The hot deformation behavior and precipitates of cast AlCu4.4Mn0.6 alloy has been investigated by compression tests in a Gleeble-3500 thermodynamic simulator at the temperature range of 200−290 °C and strain-rate range of 0.01−10 s−1. The dynamic recovery (DRV) of AlCu4.4Mn0.6 alloy in hot deformation was sensitive to temperature and strain-rate, and was accelerated by increasing the deformation temperature and strain-rate. The heat-treated AlCu4.4Mn0.6 specimen exhibited high strength as the compressive flow stress was about 310 MPa at 290 °C and the strain-rate of 10−1. A accurate constitutive equation was established with the correlation coefficient (R) of 96.2 % and the average absolute relative error (AARE) of 4.65 %. The predicted flow stress was in good agreement with the experimental results. Nanoscale Ω, θ′ and T phases were uniformly distributed in the matrix of AlCu4.4Mn0.6 alloy and hindered dislocation movement in hot deformation process, leading work hard (WH) of the alloy. Meanwhile the DRV promoted the dislocation annihilation and polygonization, resulting in the formation of sub-boundaries and the softening of the alloy. The flow stress was almost stable or gradually decreased with the increase of strain.

The Microstructure and Mechanical Properties of Ferritic-Martensitic Steel EP-823 after High-Temperature Thermomechanical Treatment

The effect of high-temperature thermomechanical treatment (HTMT) with plastic deformation by rolling in austenitic region on the microstructure and mechanical properties of 12% chromium ferritic-martensitic steel EP-823 is investigated. The features of the grain and defect microstructure of steel are studied by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). It is shown that HTMT leads to the formation of pancake structure with grains extended in the rolling direction and flattened in the rolling plane. The average sizes of martensitic packets and ferrite grains are approximately 1.5–2 times smaller compared to the corresponding values after traditional heat treatment (THT, which consists of normalization and tempering). The maximum grain size in the section parallel to the rolling plane increases up to more than 80 µm. HTMT leads to the formation of new sub-boundaries and a higher dislocation density. The fraction of low-angle misorientation boundaries reaches up to ≈68%, which exceeds the corresponding value after HTMT (55%). HTMT does not practically affect the carbide subsystem of steel. The mechanical properties are investigated by tensile tests in the temperature range 20–700 °C. It is shown that the values of the yield strength in this temperature range after HTMT increase relative to the corresponding values after THT. As a result of HTMT, the elongation decreases. A significant decrease is observed in the area of dynamic strain aging (DSA). The mechanisms of plastic deformation and strengthening of ferritic-martensitic steel under the high-temperature thermomechanical treatments are also discussed.

Deformation structure in Alloy-type creep of Cantor alloy

Compression creep tests were conducted in order to investigate the creep behavior of high entropy alloy Cantor at high temperatures. It was found that the creep behavior of Alloy-type was observed in low stress region and that of Metal-type was observed in high stress region. In addition, the activation energy of creep increased with increasing stress. This suggests that a kind of rate controlling diffusion atoms is changing depending on the stress. Microstructures after creep were observed using the Electron Back Scatter Diffraction method. The generation of dynamic recrystallized grains and formation of sub-boundaries were observed in high stress region. This is known as a typical deformation structure of Metal-type creep. In addition, a lot of low-angle grain boundaries were observed. These characteristic structures are similar to that observed in FCC metals in which stacking fault energy is relatively low.

Enhanced creep properties of Y2O3-bearing Ti-48Al-2Cr-2Nb alloys

Abstract This study examined the influence of Y2O3 addition on the creep behavior of Ti-48Al-2Cr-2Nb alloy. Creep tests of Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2Nb-0.05Y2O3 alloys were performed under 275–325 MPa at 800 °C. The results show Y2O3 can significantly enhance the creep properties of the titanium aluminide (TiAl) alloys. Under applied stress, the creep rupture life can be increased three-fold with the addition of Y2O3 and the minimum creep rate drops by around one-third. Stress exponents of the TiAl alloy matrix and Y2O3-bearing alloy were found to be 6.77 and 4.49, respectively, suggesting creep is controlled by the dislocation climb mechanism. The excellent creep properties of the doped alloy can be attributed to the thin lamellar structures and dispersed second phases. The fine Y2O3 particles can impede dislocation climb and prevent sub-boundary formation. Microstructure instabilities occur in both alloys during creep exposure. Y2O3 particles accelerate recrystallization of γ phases and precipitation of B2 phases. The combination of fine grains, increasing massive γ phases and more precipitates in the Y2O3-bearing alloy may promote the development of cracks and cavities, leading to the low fracture strain.

Change in Structural-Phase States and Properties of Lengthy Rails during Extremely Long-Term Operation

The regularities and formation mechanisms of structural-phase states and properties at different depths in the rail heads along the central axis and fillet after differential quenching of 100-meter rails and extremely long operation (with passed tonnage of 1411 million tons gross weight) have been revealed by the methods of the state-of-the-art physical materials science. As revealed, the differential quenching is accompanied by the formation of morphologically multi-aspect structure presented by grains of lamellar perlite, ferrite–carbide mixture, and structure-free ferrite. The steel structure is characterized by the α-Fe lattice parameter, the level of microstresses, the size of coherent-scattering region, the value of interlamellar distance, the scalar and excess dislocation densities. As shown, the extremely long operation of rails is accompanied by the numerous transformations of metal structure of rail head: firstly, a fracture of lamellar pearlite structure and a formation of subgrain structure of submicron (100–150 nm) sizes in the bulk of pearlite colonies; secondly, a precipitation of carbide phase particles of nanometer range along the boundaries and in the bulk of subgrains; thirdly, a microdistortion growth of steel crystal lattice; fourthly, a strain hardening of metal resulting in the increase (by 1.5-fold) in scalar and excess dislocation densities relative to the initial state. A long-term operation of rails is accompanied by the formation of structural constituent gradient consisting in a regular change in the relative content of lamellar pearlite, fractured pearlite, and structure of ferrite–carbide mixture along cross-section of railhead. As the distance to the rail fillet surface decreases, a relative content of metal volume with lamellar pearlite decreases, and that with the structure of fractured pearlite and ferrite–carbide mixture increases. As determined, the characteristic feature of ferrite–carbide mixture structure is a nanosize range of grains, subgrains and carbide-phase particles forming it. The size of grains and subgrains forming the type of structure varies in the limits of 40–70 nm; the size of carbide-phase particles located along the boundaries of grains and subgrains varies in the limits of 8–20 nm. A multiaspect character of steel strengthening is detected that is caused by several factors: firstly, the substructural strengthening due to the formation of fragment subboundaries, whose boundaries are stabilized by the carbide-phase particles; secondly, the strengthening by carbide-phase particles located in the bulk of fragments and on elements of dislocation substructure (dispersion hardening); thirdly, the strengthening caused by the precipitation of carbon atoms on dislocations (formation of Cottrell atmospheres); fourthly, the strengthening being introduced by internal stress fields due to incompatibility of crystal-lattices’ deformation of α-phase structural constituents and carbide-phase particles.

Dynamic recrystallization behavior and interfacial bonding mechanism of 14Cr ferrite steel during hot deformation bonding

In this study, hot compression bonding was first applied to join 14Cr ferrite steel at temperatures of 950–1200 °C and strains of 0.11–0.51 under strain rates of 0.01–30 s−1. Subsequently, tensile tests were performed on the joints to evaluate the reliability of the joints formed. Detailed microstructural analyses suggest that two different competing dynamic recrystallization (DRX) mechanisms occur during the bonding process depending on the strain rate, and the joints obtained at different strain rate exhibits distinct healing effect. At a low strain rate (0.01 s−1), continuous DRX occurs, as expected in high-stacking-fault-energy materials, and is characterized by the progressive conversion of the sub-boundaries into larger-angle boundaries, which involves very limited grain boundaries migration. In addition, strain-induced precipitation (SIP) is sufficient under this condition, further impeding the healing of bonding interface. Hence, the joints obtained at low strain rate fractured at the bonding interface easily. Whereas discontinuous DRX is activated at high strain rates (10 and 30 s−1). Under this condition, the formation of sub-boundaries is severely suppressed, resulting in the piling-up of dislocations and hence the storage of a greater amount of stored energy for nucleation and subsequent nuclei growth via the long-distance grain boundaries migration. Meanwhile, the SIP process is sluggish, making the conditions much more favorable for grain boundaries migration which plays a key role in the healing of the original bonding interface. Thus, the joints can be successfully bonded when a high strain rate is applied, with the joints exhibiting tensile properties similar to that of the base material.

SOFTENING OF HARDENED MEDIUM-CARBON STEEL DURING HEATING

Purpose. The work is aimed to clarify the softening mechanism during the heating of martensite hardened carbon steel, which is of practical importance, especially in the development of the production technology of rolled products with different levels of hardening. Methodology. The samples after martensite hardening were tempered at the temperatures of 300-500˚С. The microstructure was investigated under the electron microscope. Thin foils were made using the Bolman and tweezer methods in chlorous-acetic solution and Morris reagent. Phase distortions of crystalline lattice were determined by the methods of X-ray structural analysis, using the diffractometer. The cold-worked layer of metal after grinding was removed by electrolytic dissolution. Tensile strength brake of the metal was determined using the tensile diagrams of samples using the Instron type machine. Microhardness was measured using the PMT-3 device with indentation load 0.49 N. Findings. When heating the hardened steel to a temperature of 300˚C, the softening effect is mainly related to the rate of reduction of the accumulated as a result of martensitic transformation, density of the crystalline structure defects. The total result is caused by the development of dislocations recombination and strengthening because of the emergence of additional number of cementite particles during the martensitic crystals decomposition. Starting from the heating temperatures of 400˚C and above, the development of polygonization processes in the ferrite is accompanied by the emergence of additional sub-boundaries, which enhance the effect of metal strengthening. With increase in the heating temperature of the hardened steel, the level of strength properties is determined by the progressive softening from the decrease in carbon atoms saturation degree of the solid solution, dislocations density and increase in the size of cementite particles over the effect of strengthening from hindering of mobile dislocations by carbon atoms and the emergence of additional sub-boundaries. Originality. For the tempering temperature of 300-400˚C, the absence of the phase distortion change indicates the emergence of additional factor in strengthening the metal from the formation of sub-boundaries and the dispersion strengthening from the carbide particles. Practical value. The given explanation of the mechanism of structural transformations in the process of tempering in the average temperature range of the hardened carbon steel can be used to optimize the technology of thermal strengthening of rolled metal.

Evolution of twins and sub-boundaries at the early stage of dynamic recrystallization in a Ni-30%Fe austenitic model alloy

Evolution of twins and sub-boundaries at the early stage of dynamic recrystallization (DRX) is investigated in a Ni-30%Fe austenitic alloy. It is found that the primary nucleation of DRX is accomplished by formation of annealing twins along the bulged pre-existing grain boundaries and at the triple junctions at the early stage of deformation. With the strain increasing, the DRX nucleation due to the formation and evolution of sub-boundaries also plays a significant role. The results also indicate that the ∑9 twin boundaries are generated by mutual interactions between the encountered ∑3 twin boundaries at the initial stage. The pre-existing twin boundaries can gradually convert to general boundaries or twin steps owing to the interaction of twin boundaries with dislocations. Continuous DRX (CDRX) mechanism associated with the sub-boundaries evolution is found to be an assistant mechanism at the early stage of DRX. Besides, formation of sub-boundaries inside the growing DRX grains can stimulate a new round DRX nucleation by impeding migration of grain boundaries and making them serrated.

Annealing Behavior and Kinetics of Primary Recrystallization of Copper

The microstructure evolution during the annealing treatment of a recycled copper after cold rolling to total strain of 2.6 was investigated. The cold deformation resulted in the elongation of initial grains along rolling direction and the strain-induced formation of subboundaries. Annealing recovery occurred in the temperature range 100-250 °C. The recrystallized microstructures were observed after annealing at 300-400 °C. The hardness of partially recrystallized copper samples was interpreted in terms of dislocation strengthening. The recrystallization kinetics was estimated according to a Johnson–Mehl–Avrami–Kolmogorov equation using different methods for recrystallized fraction determination, i.e., the fractional softening, the grain orientation spread, and the Kernel average misorientation.

Enhanced Surface Mechanical Properties and Microstructure Evolution of Commercial Pure Titanium Under Electropulsing-Assisted Ultrasonic Surface Rolling Process

The effects of electropulsing-assisted ultrasonic surface rolling process on surface mechanical properties and microstructure evolution of commercial pure titanium were investigated. It was found that the surface mechanical properties were significantly enhanced compared to traditional ultrasonic surface rolling process (USRP), leading to smaller surface roughness and smoother morphology with fewer cracks and defects. Moreover, surface strengthened layer was remarkably enhanced with deeper severe plastic deformation layer and higher surface hardness. Remarkable enhancements of surface mechanical properties may be related to the gradient refined microstructure, the enhanced severe plastic deformation layer and the accelerated formation of sub-boundaries and twins induced by coupling effects of USRP and electropulsing. The primary intrinsic reasons for these improvements may be attributed to the thermal and athermal effects caused by electropulsing treatment, which would accelerate dislocation mobility and atom diffusion.

Influence of Mo Addition on High Temperature Deformation Behavior of L12 Type Ni3Al Intermetallics

The high temperature deformation behavior of Ni3Al and Ni3(Al,Mo) single crystals that were oriented near was investigated at low strain rates in the temperature range above the flow stress peak temperature. Three types of behavior were found under the present experimental conditions. In the relatively high strain rate region, the strain rate dependence of the flow stress is small, and the deformation may be controlled by the dislocation glide mainly on the {001} slip plane in both crystals. At low strain rates, the octahedral glide is still active in Ni3Al above the peak temperature, but the active slip system in Ni3(Al,Mo) changes from octahedral glide to cube glide at the peak temperature. These results suggest that the deformation rate controlling mechanism of Ni3Al is viscous glide of dislocations by the {111} slip, whereas that of Ni3(Al,Mo) is a recovery process of dislocation climb in the substructures formed by the {001} slip. The results of TEM observation show that the characteristics of dislocation structures are uniform distribution in Ni3Al and subboundary formation in Ni3(Al,Mo). Activation energies for deformation in Ni3Al and Ni3(Al,Mo) were obtained in the low strain rate region. The values of the activation energy are 360 kJ/mol for Ni3Al and 300 kJ/mol for Ni3(Al,Mo).

Influence of Machining on the Microstructure, Mechanical Properties and Corrosion Behaviour of a Low Carbon Martensitic Stainless Steel

The influence of cutting conditions of a low carbon martensitic stainless steel (15.5wt.% Cr, 4.8% Ni, 1% Mo, 0.9% Mn, 0.24% Si, 0.1% Cu, < 0.06% C and 77.46% Fe) on the microstructure is first studied using electron backscatter diffraction and transmission electron microscopy. The mechanical and corrosion behaviours of this stainless steel are then investigated. It was found that the microstructure is significantly affected near the machined surface. The formation of grain sub-boundaries or a refinement of the microstructure is observed depending on cutting conditions. Both lead to an increase of the hardness. In addition, the microstructure refinement yields to a huge increase of the average pit surface area. By contrast the average pit density is not affected by machining conditions.

Structural changes during severe hot forging of the aluminum alloy 1570C

Microstructure evolution in the cast Al-Mg-Sc-Zr alloy 1570C during multidirectional forging (MDF) to the strain 10.5 at 450оС (~0.77Тm) and 10-2 s-1, was investigated. The alloy belongs to advanced structural materials exhibiting high strength at ambient temperature. It can be easily hot worked, exhibits superior superplasticity in a fine-grained state, while its cold deformation causes difficulties due to high yield-stress and relatively low ductility. Consequently, an evaluation of grain refinement potentiality in this alloy at high temperatures may be important for industrial application. Before MDF, the alloy had the equiaxed grains ~25 µm in size with near uniformly distributed nanoscale coherent dispersoids Al3(Sc,Zr). In the early MDF stages, new fine (sub)grains with low- and high-angle boundaries were formed near original grain boundaries. With increasing strain, the fraction of these crystallites increased, finally resulting in almost fully recrystallized structure with grain size ~3 µm and fraction of high-angle boundaries (HABs) ~0.9. The strain dependency of microstructural parameters showed that grain refinement occurred mainly in accordance with continuous dynamic recrystallization. There was a strong interaction between lattice dislocations, (sub)grain boundaries and coherent dispersoids Al3(Sc,Zr). This implies that the latter effectively hindered (sub)grain boundary migration and provided dislocation accumulation, resulting in formation of high-density subboundaries and their transformation to HABs, even at high temperature. Moreover, hot MDF did not result to any degradation of the alloy high strength. The data suggested that there was a great potential for extensive grain refinement in this alloy at elevated temperatures alleviating difficulties of its thermo-mechanical processing.

Hot deformation behavior and dynamic recrystallization of a β-solidifying TiAl alloy

Hot deformation behavior and dynamic recrystallization (DRX) of a β-solidifying TiAl alloy Ti–43Al–2Cr–2Mn–0.2Y (at%) were investigated by means of isothermal compression tests in a temperature range of 1100–1225°C and strain rate range of 0.01–0.05s−1. A processing map at a true strain 0.8 was developed based on the dynamic materials model. The current alloy possesses a wide processing window, and the optimum deformation condition is 1175–1225°C/0.05s−1. The main softening mechanism is DRX of γ grains. The DRX proceeded preferentially at triangle boundaries, and subsequently in the lamellae. Both deformation temperature and strain rate have an obvious impact on the microstructural evolution. The partial DRX occurred when the alloy deformed at low temperature and high strain rate. Some remnant lamellar structures existed due to the plastic anisotropy of lamellar colony. With the increase of temperature and /or the decrease of strain rate, the lamellae were effectively broken down into fine grains, and a relatively complete DRX occurred. Dynamic recovery is the main deformation mechanism for β0 phase, which promotes the compatible deformation between γ and α2 phases and prevents premature cracks. Additionally, the DRX mechanism of γ phase was investigated through TEM observations. The DRX of γ grains begins with the formation of sub-boundaries, followed by a process of rearrangement of sub-boundaries to form new grains.

Mechanism of lamellae deformation and phase rearrangement in ultrafine β-Ti/FeTi eutectic composites

We report the microscopic mechanism of lamellae deformation in a series of (Ti0.705Fe0.295)100−xSnx (0⩽x⩽4) eutectic comprising of ultrafine FeTi and β-Ti phases with varying lamellae thickness. Furthermore, the microstructural refinement and phase rearrangement upon severe forging have been explored using X-ray diffraction and transmission electron microscopy to reveal the dislocation activity inside the lamellae interior and at the β-Ti/FeTi interface. Nanoindentation tests have been performed at control loading rate and control strain rate modes to evaluate the strain rate sensitivity, activation volume, and to compare them with that of the constituent single phases. The strain rate sensitivity slightly decreases upon Sn addition and with the decrease of the lamellar spacing. However, severe forging increases the hardness, reduces the activation volume without any change of the strain rate sensitivity. The role of dislocation accumulation in the ultrafine lamellae, inter-lamellar sliding, formation of sub-boundaries, evolution of shear bands, cutting, and phase rearrangement towards the deformation-induced nanostructuring of the ultrafine lamellar composites, have been discussed using a micromechanical model.

Dynamic Strain - Induced Boundary Migration during Dynamic Recovery at a High Temperature Deformation with a Lower Strain Rate

Conventional hot compression deformation and water quenching experiments were applied to investigate the evolution of austenite grain structures before the initiation of dynamic recrystallization. The experimental results reveal an interesting phenomenon that dynamic strain induced boundary migration can lower dislocation density and coarsen austenite grains. The results show that dynamic recovery is not the only way to decrease dislocation density, the mechanism of which for dynamic recovery is related to dislocations climb and annihilation, resulting in the formation of sub-grains and regular sub-boundaries. However, the mechanism of decreasing dislocation density for dynamic strain induced boundary migration is different from dynamic recovery. Therefore, dynamic strain induced boundary migration should be another softening mechanism before the initiation of dynamic recrystallization.

Creep-Induced Microstructural Changes in Large Welded Joints of High Cr Heat Resistant Steel

The creep damage process of high-Cr steel welded joints is characterized by the formation and growth of creep voids prior to the initiation of cracking, and this formation and growth accounts for a large proportion of creep life. Therefore, there has been much investigation into the detection and quantitative evaluation of creep voids and microcracks in welded joints for use in remaining life assessment. However, the microstructure around creep voids and microcracks is still not well known. In this study, a creep test on a large welded joint in Mod.9Cr-1Mo steel was conducted under 60 MPa at 650 °C. The observation of the microstructure in the heat-affected zone (HAZ) was made for the specimen interrupted the creep tests at 25% of a rupture life. The microstructure around the creep void was characterized using an electron backscatter diffraction pattern (EBSD) method. It was founded that creep voids formed and developed along random high angle grain boundaries that were not subject to K-S orientation relationship in the martensitic transformation. In addition, the initially formed void promoted preferential dynamic recovery and dynamic recrystallization in its surrounding microstructure, followed by sub-boundary formation.

Fracture Process of Aluminum/Aluminum Nitride Interfaces during Thermal Cycling

Al circuit substrates, which are composed of a sintered AlN plate and pure Al plate joined to both sides of the AlN plate, are used for semiconductor power devices. It is important to prevent fracture of the Al/AlN interface to ensure normal and stable device operation. In this study, the fracture process of Al/AlN interface during thermal cycling was investigated using advanced scanning electron microscopy (SEM). Al circuits joined to an AlN plate were plastically deformed with thermal cycling. Al grains were divided with the formation of sub-boundaries due to the plastic deformation. After 2000 thermal cycles, a crack was generated at edges of the Al/AlN interface and propagated gradually to the center of the substrate. Cross-sectional observation, using an angle selective backscattered electron detector (AsB), revealed that the Al grain size near the Al/AlN interface decreased to 3 m or less, and the crack proceeded along the Al grain boundaries. To clarify the temperature dependence of the fracture process, a repeated bending test was performed at various temperatures. Shear strains were induced at the Al/AlN interface by the repeated bending. The rate of crack propagation tends to be higher at higher temperatures for bending test. In substrates bent at 373 K or higher, the crack proceeded after the Al grains had been refined. These results indicate that fine-grained Al resulting from thermal cycling is formed by creep deformation and recrystallization at higher temperatures. Thus, improving the creep strength of the Al plate is thought to be effective for prevent cracking during thermal cycling. The effect of additive elements in the Al plate was also discussed in this study.