Chapter 5 Deformation and recrystallization
合集下载
相关主题
- 1、下载文档前请自行甄别文档内容的完整性，平台不提供额外的编辑、内容补充、找答案等附加服务。
- 2、"仅部分预览"的文档,不可在线预览部分如存在完整性等问题,可反馈申请退款(可完整预览的文档不适用该条件!)。
- 3、如文档侵犯您的权益，请联系客服反馈,我们会尽快为您处理(人工客服工作时间：9:00-18:30)。
Chapter 5: Deformation and recrystallization
These properties and structures may revert back to the precold-worked states by appropriate heat treatment. Such restoration results from two different processes that occur at elevated temperatures: recovery and recrystallization, which may be followed by grain growth.
Figure 5.26 (a )Sub-grain aggragation; (b) nuclei growth; (c)bulge nucleation.
(c) Recrystallization Dynamics
Recrystallization dynamics is determined by nucleation rate N and growth rate G. Here, provided nucleation is uniform, nuclei is spherical, temperature and Ń and G are constant，the volume fraction R of recrystallizaiton can be expressed as:
under this condition.
Microstructure evolvement of cold-worked metal during heating
recovery recrystallization grain growth
Heat temperature or keep time
Photomicrographs showing recrystallization and grain growth process of brass during annealing
Leabharlann Baidu
(a) cold-worked deform 38 ％（b） 3 s at 580 º C
（c）4 s at 580 º C（d）8 s
Note：The temperature of recrystallization is not a physical constant. It will be changed with different material, and it influenced by the cold deformation Extent and original grain size of the same material.
Recovery Dynamics
Recovery is the initial stage of microstructure deformation of cold deformation metal when annealing. The recovery driving force is stored strain energy. Yield strength recovery rate R: R＝（m-
Table 5.2 Recrystallization temperatures of some metals
(The temperature at which recrystallization just reaches completion in 1 h)
4. Several influence factors
The nucleation mechanism：
(a) Bulge nucleation: For small-deformed (<20%) metal, recrystallization almost occurs in this way.
The bulge nuleation process shown in Figure 5.25.
r）/(m- 0);
where m, r, 0 are yield strength after deformation, recovery and complete annealing, respectively.
3 Recrystallization
Recrystallization: Even after recovery is complete, the grains are still in a relatively high strain energy state. The formation of a new set of strain-free grains within a previously coldworked material; normally an annealing heat treatment is necessary.
1. Microstructure and properties of coldworked metal during heating
a). Microstructure evolvement
With heat temperature is elevated, cold-worked metal will experience: recovery, recrystallization and grain growth Recovery: forming substructure and properties changes in the stage before no-distortion grain growth. Recrystallization: in this stage, the deformed grain gradually
a) The amount of prior cold work
Increasing the percentage of cold work enhances the rate of recrystallization, with the result that the recrystallization temperature is lowered, and approaches a constant or limiting value at high deformations.
substituted by new no-distortion equiaxed grains.
Grain growth: in this stage, grain continues to grow after recrystallization. New grains aggregated to a steady size
Figure 5.25
(b) Subgrain Nucleation
For large-deformed metals, recrystallization nuclei is formed by subgrain. The mechanisms:
Sub-grain combination mechanism in largedeformed and high stacking fault energy of metals. Sub-grain migration mechanism in largedeformed and low stacking fault energy of metals.
Bulge nucleation needs energy: Es≧2γ/L; Es－stored energy per volume of colddeformed grain; γ－surface energy of grain bondary; 2L－ length of grain boudary.
t1～t2 recovery, to keep original shape t2～t3 recrystallization, deformed grain converts into equiaxed grains t3～t4 grain growth, grain size rapidly changes
（e）15 min at 580 º C（f）10 min at 700 º C
Properties of cold-worked metal during heating
2 Recovery
During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature. There is some reduction in the number of dislocations, and dislocation configurations (similar to that shown in Figure 5.24) are produced having low strain energies.
R＝1－exp(-ŃG3t4/3)
Johnson-Mehl Equation
(d) Recrystallization Temperature
Concept：The recrystallization behavior of a particular metal alloy is sometimes specified in terms of a recrystallization temperature, the temperature at which recrystallization just reaches completion in 1 h. Measurement： （1）metallographic method （2）rigidity method
(b) The purity of the alloy
Recrystallization proceeds more rapidly in pure metals than in alloys.
During recrystallization, grain-boundary motion occurs as the new grain nuclei form and then grow. It is believed that impurity atoms preferentially segregate at and interact with these recrystallized grain boundaries so as to diminish their mobilities; this results in a decrease of the recrystallization rate and raises the recrystallization temperature.
(a) Recrystallization Process：
Forming nucleus The driving force to produce this new grain structure is the difference in internal energy between the strained and unstrained material. The new grains form as very small nuclei and grow until they completely consume the parent material, processes that involve short-range diffusion.
Figure 5.27 The variation of recrystallization temperature with percent cold work for iron. For deformations less than the critical (about 5%CW), recrystallization will not occur.
These properties and structures may revert back to the precold-worked states by appropriate heat treatment. Such restoration results from two different processes that occur at elevated temperatures: recovery and recrystallization, which may be followed by grain growth.
Figure 5.26 (a )Sub-grain aggragation; (b) nuclei growth; (c)bulge nucleation.
(c) Recrystallization Dynamics
Recrystallization dynamics is determined by nucleation rate N and growth rate G. Here, provided nucleation is uniform, nuclei is spherical, temperature and Ń and G are constant，the volume fraction R of recrystallizaiton can be expressed as:
under this condition.
Microstructure evolvement of cold-worked metal during heating
recovery recrystallization grain growth
Heat temperature or keep time
Photomicrographs showing recrystallization and grain growth process of brass during annealing
Leabharlann Baidu
(a) cold-worked deform 38 ％（b） 3 s at 580 º C
（c）4 s at 580 º C（d）8 s
Note：The temperature of recrystallization is not a physical constant. It will be changed with different material, and it influenced by the cold deformation Extent and original grain size of the same material.
Recovery Dynamics
Recovery is the initial stage of microstructure deformation of cold deformation metal when annealing. The recovery driving force is stored strain energy. Yield strength recovery rate R: R＝（m-
Table 5.2 Recrystallization temperatures of some metals
(The temperature at which recrystallization just reaches completion in 1 h)
4. Several influence factors
The nucleation mechanism：
(a) Bulge nucleation: For small-deformed (<20%) metal, recrystallization almost occurs in this way.
The bulge nuleation process shown in Figure 5.25.
r）/(m- 0);
where m, r, 0 are yield strength after deformation, recovery and complete annealing, respectively.
3 Recrystallization
Recrystallization: Even after recovery is complete, the grains are still in a relatively high strain energy state. The formation of a new set of strain-free grains within a previously coldworked material; normally an annealing heat treatment is necessary.
1. Microstructure and properties of coldworked metal during heating
a). Microstructure evolvement
With heat temperature is elevated, cold-worked metal will experience: recovery, recrystallization and grain growth Recovery: forming substructure and properties changes in the stage before no-distortion grain growth. Recrystallization: in this stage, the deformed grain gradually
a) The amount of prior cold work
Increasing the percentage of cold work enhances the rate of recrystallization, with the result that the recrystallization temperature is lowered, and approaches a constant or limiting value at high deformations.
substituted by new no-distortion equiaxed grains.
Grain growth: in this stage, grain continues to grow after recrystallization. New grains aggregated to a steady size
Figure 5.25
(b) Subgrain Nucleation
For large-deformed metals, recrystallization nuclei is formed by subgrain. The mechanisms:
Sub-grain combination mechanism in largedeformed and high stacking fault energy of metals. Sub-grain migration mechanism in largedeformed and low stacking fault energy of metals.
Bulge nucleation needs energy: Es≧2γ/L; Es－stored energy per volume of colddeformed grain; γ－surface energy of grain bondary; 2L－ length of grain boudary.
t1～t2 recovery, to keep original shape t2～t3 recrystallization, deformed grain converts into equiaxed grains t3～t4 grain growth, grain size rapidly changes
（e）15 min at 580 º C（f）10 min at 700 º C
Properties of cold-worked metal during heating
2 Recovery
During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature. There is some reduction in the number of dislocations, and dislocation configurations (similar to that shown in Figure 5.24) are produced having low strain energies.
R＝1－exp(-ŃG3t4/3)
Johnson-Mehl Equation
(d) Recrystallization Temperature
Concept：The recrystallization behavior of a particular metal alloy is sometimes specified in terms of a recrystallization temperature, the temperature at which recrystallization just reaches completion in 1 h. Measurement： （1）metallographic method （2）rigidity method
(b) The purity of the alloy
Recrystallization proceeds more rapidly in pure metals than in alloys.
During recrystallization, grain-boundary motion occurs as the new grain nuclei form and then grow. It is believed that impurity atoms preferentially segregate at and interact with these recrystallized grain boundaries so as to diminish their mobilities; this results in a decrease of the recrystallization rate and raises the recrystallization temperature.
(a) Recrystallization Process：
Forming nucleus The driving force to produce this new grain structure is the difference in internal energy between the strained and unstrained material. The new grains form as very small nuclei and grow until they completely consume the parent material, processes that involve short-range diffusion.
Figure 5.27 The variation of recrystallization temperature with percent cold work for iron. For deformations less than the critical (about 5%CW), recrystallization will not occur.