Papers by Keyword: Crack Healing

Abstract: The effect of heat treatment on microstructure and hardness of internal crack healing in a low carbon steel was studied. The internal cracks were produced into the samples by a drilling and compression method. The microstructure of crack healing zone was examined using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness of crack healing zone was measured using a Vickers micro-hardness testing machine (FM-800). The results show that healing temperature plays a more significant role in internal crack healing than holding time. Compared as-quenched samples with as-normalized samples under the same healing parameters, it is found that cooling speed is also an important factor for internal crack healing. The migration and enrichment of iron atoms provide material source for recrystallization and grain growth of crack healing zone. The existence of micro-voids leads to the hardness of the ferrite in the crack healing zone lower than that in the matrix.

Abstract: The molecular dynamics method is used to simulate microcrack healing in copper nano-plate during heating. During microcrack healing, the tip of microcrack is blunted and deforms to round shape, the microcrack becomes smaller and smaller until it is healed through slip bands emitting from the pre-crack tip and expanding to the top and bottom of the copper nano-plate. The healing time is different in different temperature. The healing processes in different temperature present different slip bands for crack healing. When temperature is below 650K, the healing time decreases dramatically with temperature increase. When temperature is above 650K, the healing time decreases smoothly with temperature increase. The critical temperature of microcrack healing in copper nano-plate without pre-existing dislocations is about 400K.

Abstract: Internal cracks in 30Cr2Ni4MoV steel were produced by compressing cylindrical sample with drilled hole. In order to consider the effects of deformation degree, temperature, strain rate and holding time on the crack healing, the hot compression tests were performed on Gleeble-1500 thermo-simulation machine under different process conditions. By in situ observation under optical microscope and scanning electron microscope, effects of process condition on crack healing and the behaviours of crack healing were revealed successfully: (1) the result of crack healing improves as the melt temperature, reduction and holding time increase, (2) the effect of crack healing decreases as the strain rate increases. By the energy dispersive spectroscopy, it is also found that the chemical composition of the healed crack area is different from that of the matrix.

Abstract: Crack-healing effectiveness was investigated on 5 vol% nano-Ni dispersed Al₂O₃ hybrid materials. Influence of the Y or Si doping or SiC co-dispersion was also studied on the crack healing behavior. Cracks were introduced by a Vickers indentation to be a crack length of approximately 60 μm. Cracks of nano-Ni/Al₂O₃ were completely disappeared, for example, by oxidation at 1200°C for 6 h in air, Y/Si doped one and SiC co-dispersed one have similar performance of crack disappearance. Bending strength of crack-disappeared samples showed about 550 MPa and was comparable or improved with that of as-sintered one. Mechanism of crack healing was considered as filling up of cracks by NiAl₂O₄ oxidation product which is developed by outward diffusion of cations at grain boundary of Al₂O₃ matrix. Nano-Ni/Al₂O₃ with Y or Si doping or SiC co-dispersion are realized to have crack-healing effectiveness with improved high-temperature oxidation resistance.

Abstract: Using Al, MgO and Al₂O₃ as raw materials, Al₂O₃-MgAlON composite was synthesized in N₂ atmosphere by hot-press sintering at 900°C. Cracks prepared on the sample surface were healed at 1200°C×6hrs. Phase composition and element chemical state of sample were studied before and after healing. The results showed that: after healing process Al and MgO phase disappeare, and AlN and MgAlON phase appear, all kinds of elements in Al₂O₃-MgAlON material change their chemical state after the healing treatment, the changed electronic binding energy is Al 0.05ev, Mg 0.08ev, O 2.58ev or N 1.02ev respectively, and the chemical composition and crystal structure of MgAlON phase also change, all these change are helpful for crack healing process of Al₂O₃-MgAlON material.

Abstract: The elongation and the strength of rolled steel are severely affected by cracks in production. Reduction of cracks is important for the improvement of slab quality. In this paper, a series of experiments on internal crack healing in slab during rolling was carried out on the experimental rolling mill. The crack shape and the fracture morphology of the crack healing zones were observed by using the scanning electron microscope, and the shear strength of crack healing zone was tested through the tensile testing machine. With increase of the reduction ratio during rolling, the crack healing degree increases.

Abstract: A drilling and compression method was designed to produce samples with artifical inner crack. According to the morphology, two types of crack were defined. After heating and holding at different temperatures, the crack healing results were observed under SEM. The starting temperature of crack healing is between 900°C and 950°C. Pseudo-equilibrium phase diagram of 12%Cr was calculated by the Thermo-Calc software to investigate the influence of transformation on the crack healing. It is found that the diffusive transformation has a positive influence on the crack healing behavior. Through experimental results and analysis, we conclude that crack healing is a physical process in association with diffusion, nucleation and grain growth. Any factor which can accelerate diffusion, nucleation and grain growth rate is favorable for the crack healing.

Abstract: With the development of the embedded microcapsule concept for self-healing material, the preparation of microcapsule has been paid more attentions. A new series of microcapsules were prepared by in situ polymerization technology in an oil-in-water emulsion with polyoxymethylene urea (PMU) as shell material and a mixture of epoxy resins as core material. The PMU microcapsules were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electronic microscopy (SEM), particle size analyzer and thermo gravimetric analyzer (TGA) to investigate their chemical structure, surface morphology, size distribution and thermal stability, respectively. The results indicate that PMU microcapsules containing epoxy resins can be synthesized successfully. The optimized reaction parameters were obtained as follow: agitation rate 600 rpm, 60°C water bath, pH=3.5, core material 20ml and hot water dilution by in-situ polymerization. The size is around 116 μm. The rough outer surface of microcapsule is composed of agglomerated PMU nanoparticles. The microcapsules basically exhibit good storage stability at room temperature, and they are chemically stable before the heating temperature is up to approximately 200°C.

Abstract: Gas turbine components are subjected to high temperature and high stress during engine operation, so they often become physically damaged due to the formation of cracks, voids and worn surfaces. In order to develop a new process for repairing gas turbine components with high strength filler materials, an attempt has been made to prepare IN738/NiCrSi/IN738 sandwich-like coating on thermal sensitive superalloy IN738 substrate and heat-treat it in a vacuum furnace. The microstructure morphologies, hardness and element distribution of as-deposited and as- heat-treated coatings have been investigated. The results showed that there were lot of cracks and voids in the deposited ‘sandwich-like’ coating but they could be eliminated by heat treatment. The diffusion of activated element Si in the sublayer NiCrSi of ‘sandwich-like’ coating was beneficial to healing cracks.

Abstract: After preparing samples (3mm long×4mm wide×36mm high) of Al2O3-MgAlON composites and sintered at 1500°C for 2 h in N2 atmosphere, samples’ cracks were carved by a Vickers hardometer’s pressing head on the center of the sample surface (4 mm×36 mm). Subsequently, the cracks were healed at 1000°C-1550°C for 6 h respectively. Effects of healing temperature on sample’s strength, crack healing dynamics and its molecular dynamics simulation were investigated. The results suggested that: the optimum range of cracks healing temperature was 1300°C-1550°C, and the healing process accelerated at 1300°C, meanwhile, the strength of samples increased significantly. Cracks completely healing finished at 1550°C. The dynamics equation of crack healing was lnν = -Q/kT+lnC. Through characterizing the crack healing rate with the recovering rate of sample’s strength, the diffusion activation energy Q = 4.264 × 10-30 J•K-1 and diffusion constant C=7.359 were claimed. The result of the molecular dynamics simulation suggested that cracks healing process was caused by diffusion could be divided into five stages: passivation of crack tips, formation of salient island, crack shrinkage, generation of secondary crack, and complete healing.