Development of damage in metallic materials

Jiang

2012

JMP

A molecular dynamics model has been developed to investigate the evolution of the internal crack of nano scale during heating or compressive loading in BCC Fe. The initial configuration does not contain any pre-existing dislocations. In the case of heating, temperature shows a significant effect on crack evolution and the critical temperature at which the crack healing becomes possible is 673 K. In the case of compressive loading, the crack can be healed at 40 K at a loading rate 0.025 × 1018 Pa·m<sup>1/2</sup>/s in 6 × 10<sup>-12 </sup>s. The diffusion of Fe atoms into the crack area results in the healing process. However, dislocations and voids appear during healing and their positions change continuously

Section: Introductionmentioning

confidence: 99%

Evolution of Internal Crack in BCC Fe under Compressive Loading

Jiang

2012

JMP

“…4 , r 5 and r 6 are 1.180000, 1.150000, 1.080000, 0.990000, 0.930000 and 0.866025 respectively. R 1 represents the cut-off radii of V and R 2 represents the cut-off radii of Φ, which are 1.300000 and 1.200000 respectively [31].…”

Section:  mentioning

confidence: 98%

“…The damage evolution in metals was analysed [4], and many heat treatment methods can be used to reduce the number of defects in metals after a long-term operation at high temperatures [5]. By TEM observation, a crack in α-Fe was found to be healed completely when the temperature was increased to a critical value of 1073K [6].…”

Section: Introductionmentioning

confidence: 99%

Modelling of the evolution of crack of nanoscale in iron

Jiang

Computational Materials Science

2013

Metal owns the ability of self-healing to some extent, and the ability of the internal crack healing is most desirable for improving the reliability of metal. A molecular dynamics simulation has been further developed to investigate the evolution of a nanoscale crack in body centred cubic Fe crystal under the conditions of heating or compressive pressure. When system temperature drops, the evolution of the crack that was at elevated temperature has been studied for the first time. N-body potential according to the embedded atom method has been adopted. The original nanoscale crack is expressed by removing some atoms in the centre of the cell, and the minimum vertical distance between the atoms on the top and bottom crack surfaces has been defined as Dm for assessing the process of crack evolution. The results show that a crack healing process can be accelerated significantly with an increase of temperature. When the system temperature decreases, Dm of the crack that was in healing process does not change significantly but fluctuates in a narrow range. This means that the crack healing is the result of Fe atoms diffusing into the crack area but not the thermal stress incurred in the simulation cell at elevated temperature. The pre-compressive pressure under the condition of both biaxial and uniaxial loadings can help promote the crack healing significantly and results in more uniform distribution of defects after healing. AbstractMetals own the ability of self-healing to some extent, and the ability of the internal crack healing is most desirable for improving the reliability of metals. A molecular dynamics simulation has been further developed to investigate the evolution of a nano scale crack in body centred cubic Fe crystal under the conditions of heating or compressive pressure. When system temperature drops, the evolution of the crack that was at elevated temperature has been studied for the first time. N-body potential according to the embedded atom method has been adopted. The original nano scale crack is expressed by removing some atoms in the centre of the cell, and the minimum vertical distance between the atoms on the top and bottom crack surfaces has been defined as D m for assessing the process of crack evolution. The results show that a crack healing process can be accelerated significantly with an increase of temperature. When the system temperature decreases, D m of the crack that was in healing process does not change significantly but fluctuates in a narrow range.This means that the crack healing is the result of Fe atoms diffusing into the crack area but not the thermal stress incurred in the simulation cell at elevated temperature. The pre compressive pressure under the condition of both biaxial and uniaxial loadings can help promote the crack healing significantly and results in more uniform distribution of defects after healing.

“…From a view of dissipation structure of physics, Zhou et al [3] indicated that the essence of crack healing in materials is an exchanging process of substance and energy for an open system. Kumanin et al analyzed the damage evolution in metals, and indicated that many types of heat treatment methods can be used to decrease the amount of defects in metals after a long-term operation at high temperatures [4,5]. Furthermore, hot plastic deformation could improve the healing of internal crack in some metal structures, for example, in heavy forgings [6].…”

Section: Introductionmentioning

confidence: 99%

An Analysis on the Inhomogeneous Microstructure in Crack Healing Area

Tieu

et al. 2004

KEM

Three hypo-eutectoid steels were chosen to study the inhomogenoues microstructure in crack healing area. Internal cracks were made in 20, 20MnMo and 45 steel samples. The precracks in 20 and 20MnMo steel can mostly be healed under the conditions of hot plastic deformation in which the reduction of hot pressing were 35.6% and 44.9% respectively, and the healing area is a ferrite band. The process of crack healing in crack tips in 45 steel could be finished in a very short time during upsetting, but there exists inhomogeneity in healing area which include more ferrite, fine grain size and many micro-voids. The reason why ferrite aggregated in healing area is that large deformation and high strain energy at high temperature and low strain rate around crack produce pro-eutectoid ferrite grain precipitating in hypo-eutectoid steels. Due to numerous voids restraining the migration of grain boundaries, the grain growth in a healing area becomes a slow process in the crack tips in 45 steel. This results in the grain size in crack tips healing area much finer than that in the steel matrix in 45 steel even after a long time of heat treatment at high temperature.