CN108913854B

CN108913854B - Gradient nanostructure with excellent comprehensive high-cycle and low-cycle fatigue performance

Info

Publication number: CN108913854B
Application number: CN201811038663.7A
Authority: CN
Inventors: 卢磊; 龙建周; 潘庆松; 陶乃镕
Original assignee: Institute of Metal Research of CAS
Current assignee: Institute of Metal Research of CAS
Priority date: 2018-09-06
Filing date: 2018-09-06
Publication date: 2020-08-14
Anticipated expiration: 2038-09-06
Also published as: CN108913854A

Abstract

The invention discloses a gradient nanostructure with excellent comprehensive high-cycle and low-cycle fatigue performance, and belongs to the technical field of metal material fatigue performance strengthening. Specifically, a gradient nano structure is introduced on the surface of the metal material by applying surface plastic processing, and the microstructure of the gradient nano structure is a nano crystal structure on the outermost layer, an ultra-fine crystal/deformation twin crystal structure on a subsurface layer and an original coarse crystal structure of a core part. Wherein the gradient nanostructure layer has an overall thickness greater than 50 microns and between 50 and 300 microns. Compared with a uniform coarse-grain structure with the same components, the stress control high-cycle fatigue limit of a pure Cu sample with a surface gradient nano structure is improved by 2 times, and the high-cycle fatigue life is improved by more than 15 times; and the strain control low cycle fatigue life is improved by 1 time compared with the common coarse-grained sample. The surface gradient nano-structure metal material obtained by the invention can improve the high cycle and low cycle fatigue performance of the metal material at the same time.

Description

Gradient nanostructure with excellent comprehensive high-cycle and low-cycle fatigue performance

Technical Field

The invention relates to the technical field of metal material fatigue property strengthening, in particular to a gradient nano structure with excellent comprehensive high-cycle and low-cycle fatigue properties.

Background

In practical application, most metal engineering components are in service under alternating load (the stress amplitude is far less than the yield strength of the material), namely, the metal engineering components are in a high-cycle fatigue stage (the fatigue life is more than 104 cycles), and parts of the engineering components, such as shafts and connecting rod parts with constantly changed positions of holes or gaps or section shapes, are in a low-cycle fatigue stage (the fatigue life is less than 104 cycles) due to stress/strain concentration. Statistics show that about 90% of failure and damage accidents are all caused by fatigue failure, so that huge social and economic losses and a large number of human injuries and deaths are caused. Therefore, the excellent high-cycle and low-cycle fatigue performance is important for ensuring the safe service of the engineering component.

Studies have shown that the stress controlled high cycle fatigue performance (e.g. fatigue limit) of a metallic material depends mainly on its strength: the strength is improved, the fatigue crack initiation resistance is increased, and the fatigue limit is improved. Strain-controlled low cycle fatigue properties (e.g., fatigue life) are primarily related to material plasticity. The better the plasticity is, the larger plastic deformation can be accumulated, the fatigue crack propagation rate is reduced, and the improvement of the low cycle fatigue life is facilitated. However, the strength and plasticity of metal materials often have a significant "inverted" relationship, and it is difficult to achieve both high strength and good plasticity. For example, most of the existing engineering metal materials are coarse-grained structures, have good plasticity but low strength, and have excellent low-cycle fatigue life but poor high-cycle fatigue performance.

Due to the increasing energy consumption and environmental pollution in recent years, engineering machinery is continuously developing towards high speed, heavy load, energy conservation and environmental protection, which puts increasing demands on the fatigue performance and safety and reliability of metal materials. The lower strength and high cycle fatigue performance of the traditional engineering coarse grain metal severely limit the application of the traditional engineering coarse grain metal in a harsher working condition environment.

On the premise of not changing the material components, the strength and hardness of the polycrystalline material can be greatly improved by thinning the grain size of the polycrystalline material to a nanometer level, but the improvement of the fatigue resistance of the polycrystalline material cannot be guaranteed. Experimental results show that the high cycle fatigue performance of stress control of a pure Cu sample with an ultra-fine grain structure (the grain size is in a submicron order) is superior to that of a coarse grain sample, the low cycle fatigue life of strain control is obviously shorter than that of a coarse grain material, and continuous cycle softening occurs. For a pure Ni sample with a nano-crystalline grain structure, the crack propagation resistance of the sample in a stress-controlled fatigue test is obviously lower than that of a sample with an ultrafine-crystalline structure, which shows that the fatigue resistance of the sample is obviously deteriorated when grains are refined to a nano-scale. The irreconcilable 'inversion' relationship between the high cycle fatigue performance and the low cycle fatigue performance of traditional coarse-grained and nano-structured metals has become a key bottleneck problem restricting the safe service of metal components.

Disclosure of Invention

In order to solve the problem of inversion of high-cycle and low-cycle fatigue properties of a metal material with a uniform structure in the prior art, the invention aims to provide a gradient nano structure with excellent comprehensive high-cycle and low-cycle fatigue properties, wherein the structure inhibits fatigue crack initiation through surface nano crystals and inhibits crack propagation through core coarse crystals, and meanwhile, the structure has excellent high-cycle and low-cycle fatigue properties.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a gradient nanostructure with excellent high-cycle and low-cycle fatigue performance is prepared by mechanically treating the surface of a metal material to make coarse crystals on the surface generate plastic deformation and form the gradient nanostructure: the formed gradient nano structure enables the high cycle fatigue and the low cycle fatigue of the metal material to be synchronously improved, and the thickness of the gradient nano structure is more than 50 micrometers.

After the surface of the metal material is mechanically treated, the surface is nanocrystalline, the core part is original coarse grain, a transition region between the surface and the core part is a deformation structure (superfine grain/deformation twin crystal structure), and the grain size of the metal material is in gradient transition from the nano scale of the outermost surface to the micron scale of the core part.

The metal material is copper or 304 stainless steel; the microhardness of the gradient nano structure is changed from high to low in a continuous gradient from outside to inside in a direction vertical to the surface of the metal material.

The thickness of the gradient nanostructure (nanocrystal layer + deformation structure layer) is 50-300 microns.

The surface mechanical treatment system is adopted to carry out surface plastic deformation on the metal material, the surface mechanical treatment system comprises a treatment cutter and a cooling system, the cutter head part of the cutter is made of hard alloy balls, the hard alloy balls are made of WC-Co alloy materials, and the diameter of each hard alloy ball is 4-10 mm. For pure copper samples, liquid nitrogen atmosphere cooling is adopted in the treatment process to reduce the temperature rise of the sample surface in the treatment process. For low-stacking fault energy alloys such as 304 stainless steel, the surface mechanical treatment process does not require cooling.

The surface mechanical treatment process comprises the following steps: the metal material rotating member rotates along the self axial direction, the hard alloy ball of the processing cutter is contacted with the surface of the metal material and pressed into the surface of the metal material to a certain depth, and then the hard alloy ball moves from one end of the workpiece to the other end along the surface of the metal material rotating member to finish one pass processing; after the above process is repeated for a plurality of times, a gradient nano-structure layer is formed on the surface of the metal material; in the surface mechanical treatment process, the rotating speed of the metal material rotating member is 200-800r/min, the feeding speed of the treatment cutter along the axial direction of the metal material rotating member is 40-80mm/min, the pressing depth of the hard alloy ball cutter head on the material surface in each treatment pass is 20-60 mu m, and the processing passes are 1-12.

The pure Cu sample with the gradient nanostructure has the following properties: stretching at room temperature, wherein the yield strength of the material is twice that of the uniform coarse-grained material, and the uniform elongation is the same as that of the coarse-grained material; the high-cycle fatigue life of the stress control is more than 15 times of that of the coarse-grained material; the strain control low-cycle fatigue life is 2 times of that of a coarse-grained material, and cyclic softening does not occur in the cyclic deformation process.

The invention has the following advantages:

1. under the action of cyclic load, fatigue cracks usually grow on the surface of a sample, the surface layer structure of the sample is thinned to be in a nanometer size, the core part keeps a coarse crystal structure, the size of crystal grains changes from outside to inside in a gradient mode, the fatigue cracks can be effectively prevented from growing on the surface layer nanocrystalline layer due to the high strength of the surface nanocrystalline layer, the crack can be prevented from expanding on the core part due to the high plasticity of the coarse crystal structure, and the fatigue cracks can be prevented from growing and expanding simultaneously under the combined action of the two mechanisms. Therefore, the invention realizes the synchronous optimization of the high-cycle and low-cycle fatigue performance of the metal material by changing the three-dimensional microstructure of the metal material and the prepared surface gradient nanostructure.

2. The invention provides a method for preparing metal with a surface gradient nanostructure by surface plastic deformation, wherein the microstructure of the metal comprises a nanocrystalline layer, an ultrafine grain layer, a deformed coarse nanocrystalline layer and a core coarse grain structure, and the grain size and the microhardness of the metal are in gradient change in the depth direction from the surface, which is different from a uniform structure material prepared by a traditional method. Wherein the thickness of the surface gradient nanostructure layer is more than 50 microns and less than 300 microns.

3. In the prior art, the comprehensive performance of the metal material is generally improved by increasing the thickness of the deformation layer, and in the test process, the characteristics of the base material are combined, and various parameters, cutter parameters and the like in the surface mechanical treatment process are optimized, so that the comprehensive performance of the material can still be considered when the thinner deformation layer is prepared.

4. The gradient nanostructure with excellent comprehensive high-cycle and low-cycle fatigue properties proposed by the invention has been realized in pure copper materials. Under the stress control condition, the high cycle fatigue life of the surface gradient nanostructure Cu is more than 15 times of the fatigue life of the coarse grain Cu; under the strain control condition, the low cycle fatigue life is about 2 times of the fatigue life of the coarse-grained copper. The gradient nano structure can synchronously improve the high-cycle fatigue performance and the low-cycle fatigue performance of the material.

5. The invention has wide application range: the present invention relates to surface gradient nanostructures with excellent high and low cycle fatigue performance, obtainable by simple application of mechanical deformation to the surface. The gradient nano-structure can be introduced into the surfaces of workpieces such as transmission shafts, gears, bearing beams and the like serving under fatigue load by improving the traditional lathe, shot blasting or sand blasting equipment. By changing the surface mechanical deformation process, the grain size, the gradient nanostructure layer thickness and the like in the gradient structure can be flexibly controlled, so that the gradient nanostructure metal with different high-cycle and low-cycle comprehensive fatigue performance matching is obtained, the service requirement of workpieces is met, and the method has important significance for light weight of mechanical equipment, energy conservation and emission reduction.

Drawings

FIG. 1 is a microstructure scanning and transmission electron micrograph of a surface gradient nanostructured Cu #1 sample in a direction perpendicular to the surface in example 1.

FIG. 2 is a microstructure scanning and transmission electron micrograph of a surface gradient nanostructured Cu #2 sample in a direction perpendicular to the surface in example 2.

Fig. 3 is a graph of the microhardness as a function of depth from the surface for samples of surface gradient nanostructures Cu #1 and #2 of examples 1 and 2.

FIG. 4 is stress-strain curves of the surface gradient nanostructures Cu #1 and #2 and the macrocrystalline Cu uniaxial tensile engineering in examples 1 and 2.

FIG. 5 shows the relationship between the high cycle fatigue stress amplitude and the fatigue life of the surface gradient nanostructures Cu #1 and #2 and the coarse grain Cu in examples 1 and 2. For comparison, the fatigue data of ultra-fine grain Cu in the literature is also plotted in the figure.

FIG. 6 is a graph showing the fatigue ratio-tensile strength relationship between the surface gradient nanostructure Cu #1 and #2 and the uniform coarse grain Cu in examples 1 and 2. For comparison, the fatigue data of ultra-fine grain Cu in the literature is also plotted in the figure.

FIG. 7 is the relationship between the low cycle fatigue strain amplitude and the fatigue life of the surface gradient nanostructure Cu #1 and #2 and the uniform coarse grain Cu in examples 1 and 2. For comparison, the fatigue data of ultra-fine grain Cu in the literature is also plotted in the figure.

Fig. 8 is a transmission electron micrograph of the microstructure of the surface gradient nanostructure 304 stainless steel sample of example 3.

Detailed Description

The present invention is described in detail below with reference to examples

Example 1

And (3) mechanically treating the surface of the Cu #1 sample with the surface gradient nano structure by using a common lathe to obtain the Cu #1 sample. The technological parameters of the surface mechanical treatment are selected as that the diameter of the copper bar material is 6mm, and the rotating speed is 600 r/min; the processing tool bit is a WC-Co hard alloy ball with the diameter of 6mm, the feeding speed is 40mm/min, the pressing depth of the hard alloy ball tool bit on the surface of the material in each processing pass is 40 mu m, and the processing pass is 8. The treatment temperature is that the temperature of the liquid nitrogen is 173K.

The grain size in this material exhibited a monotonically increasing gradient trend with increasing distance from the surface depth, with the average grain size gradually increasing from 42nm at the outermost layer to 21 μm at the core, as shown in fig. 1. The thickness of the surface layer nano-crystal and the superfine crystal layer is respectively 20 μm and 200 μm.

In the embodiment, the microhardness of the surface gradient nanostructure Cu #1 sample is gradually reduced from 1.9GPa to 0.8GPa with the increase of the surface depth from the material, and the surface gradient nanostructure Cu #1 sample has the characteristic of gradient change, as shown in FIG. 3.

In this example, a uniaxial tensile test was performed on a surface gradient nanostructure Cu #1 sample at room temperature, and an engineering stress-strain curve is shown in fig. 4, where the uniaxial tensile yield strength is 144MPa, the tensile strength is 248MPa, and the uniform elongation is 28%.

In this example, the surface gradient nanostructure Cu #1 sample has a stress controlled high cycle fatigue limit of 98MPa and a fatigue ratio of 0.4. when the stress amplitude is 112MPa, the high cycle fatigue life is 3 × 10⁶Cycle times are 34 times of that of uniform coarse-grained Cu, as shown in figures 5 and 6, at a plastic strain amplitude of 0.32%, the low cycle fatigue life reaches 2.6 × 10³Weekly, 2 times as large as homogeneous macrocrystalline Cu, as in fig. 7. Gradient nanostructures with improved high cycle and low cycle fatigueFatigue property.

Example 2

The difference from the embodiment 1 is that:

and (3) obtaining a surface gradient nano-structure Cu #2 sample by utilizing surface mechanical treatment. The technological parameters of the surface mechanical treatment are selected as that the diameter of the copper bar material is 6mm, and the rotating speed is 400 r/min; the processing tool bit is a WC-Co hard alloy ball with the diameter of 6mm, the feeding speed is 40mm/min, the pressing depth of the hard alloy ball tool bit on the surface of the material in each processing pass is 40 mu m, and the processing passes are 3. The treatment temperature is that the temperature of the liquid nitrogen is 173K.

Along the increase of the depth from the surface, the grain size in the material shows a monotonously increasing gradient trend, and the average grain size is gradually increased from 58nm to 21 μm, as shown in FIG. 2. The thicknesses of the surface layer nanocrystalline and the superfine crystalline layer are respectively 5 mu m and 60 mu m, and are smaller than those of the nanocrystalline and the superfine crystalline layer of the surface layer gradient nanostructure Cu #1 sample.

In the embodiment, the microhardness of the surface gradient nanostructure Cu #2 sample is gradually reduced from 1.9GPa to 0.6GPa along with the increase of the depth from the surface of the material, and the distribution of gradient change is shown as shown in FIG. 3.

In this example, a surface gradient nanostructure Cu #2 sample was subjected to a uniaxial tensile test at room temperature, and an engineering stress-strain curve is shown in fig. 4, where the uniaxial tensile yield strength is 123MPa, the tensile strength is 246MPa, and the uniform elongation is 32%.

In this example, in the surface gradient nanostructure Cu #2 sample, the stress-controlled high cycle fatigue limit reached 88MPa, the corresponding fatigue ratio was 0.36, and when the stress amplitude was 112MPa, the high cycle fatigue life was 2 × 10⁶The cycle time is 23 times of that of uniform coarse-grained copper, as shown in figures 5 and 6, the low cycle fatigue life reaches 2.1 × 10 when the plastic strain amplitude is 0.32 percent³Weekly, 1.6 times as much as the uniform macrocrystalline copper, as shown in fig. 7.

Example 3

The difference from the embodiment 1 is that:

and (3) obtaining a gradient nano structure on the surface of the 304 stainless steel material by utilizing surface mechanical treatment. The technological parameters of the surface mechanical treatment are selected as that the diameter of the 304 stainless steel material is 6mm, and the rotating speed is 300 r/min; the processing tool bit is a WC-Co hard alloy ball with the diameter of 6-8mm, the feeding speed is 10mm/min, the pressing depth of the hard alloy ball tool bit on the surface of the material in each processing pass is 20 microns, and the processing passes are 6 passes. The treatment temperature was room temperature.

The grain size in this material exhibits a monotonically increasing gradient trend with increasing distance from the surface depth, with the average grain size increasing from 30nm at the outermost layer (fig. 8) to 46 μm at the core. The thickness of the surface gradient nanostructure is 400 μm.

In the embodiment, the microhardness of the stainless steel material of the surface gradient nanostructure 304 is gradually reduced from 4.7GPa on the outermost layer to 1.3GPa of coarse crystals of the core part along with the increase of the depth from the surface of the material.

In this example, the surface gradient nanostructure 304 stainless steel material was subjected to a uniaxial tensile test at room temperature, and had a uniaxial tensile yield strength of 294MPa, a tensile strength of 500MPa, and a uniform elongation of 50%.

In this example, the surface gradient nanostructure 304 stainless steel sample has a high cycle fatigue life of 1 × 10 when the stress control high cycle fatigue limit reaches 300MPa and the stress amplitude is 300MPa⁷The cycle time is 100 times of that of the uniform coarse grains 304, and the low cycle fatigue life reaches 5 × 10 when the plastic strain amplitude is 0.32 percent⁴Weekly, 1.3 times of the uniform coarse-grained copper.

Comparative example 1

The ordinary annealed coarse grain Cu (grain size about 21 μm) was stretched at room temperature, with a yield strength of 56MPa, a tensile strength of 231MPa, and a uniform elongation of 37%, as shown in the gray curve of FIG. 4. Under stress control conditions, at 10⁷High cycle fatigue limit of 50MPa at cycle time and high cycle fatigue life of 8.7 × 10 at stress amplitude of 112MPa⁴Week number, see the gray box symbol in FIG. 5, and low cycle fatigue life of 1.3 × 10 at 0.32% plastic strain amplitude under strain control³Week number (fig. 7 gray box symbol). Therefore, coarse grain Cu has a long fatigue life in strain fatigue, but has poor stress control fatigue properties.

Comparative example 2

American scientist s.r.agnewThe uniform ultra-fine grain Cu material is prepared by an equal channel angular extrusion technology, the average grain size is about 200-300nm, the purity is 99.9w.t., under the stress control condition, the ultra-fine grain Cu has excellent high-cycle fatigue performance (shown as a triangular symbol in figure 5), for example, the high-cycle fatigue limit is 80MPa, and the high-cycle fatigue life is 8.2 × 10 when the stress amplitude is 112MPa⁵The cycle, however, is short under strain-controlled conditions, e.g., at 0.32% plastic strain amplitude, the low cycle fatigue life is 1 × 10³Week (see triangle symbol in fig. 7).

Comparative example 3

Goto et al, a Japanese scientist, prepares a uniform ultrafine grain Cu material by an equal channel angular extrusion technology, the average grain size is about 295nm, the purity is 99.99 percent, the material has a high cycle fatigue limit of 78MPa, and the high cycle fatigue life is 1.6 × 10 when the stress amplitude is 112MPa⁶Once in a week, it is significantly higher than the coarse-grained structure (see triangle symbol in fig. 5). But has a shorter low cycle fatigue life when strain fatigued. At a plastic strain amplitude of 0.32%, the low cycle fatigue life was 500 cycles, see triangle symbol of fig. 7. Therefore, the high cycle and low cycle fatigue properties of ultra-fine grain Cu have a significant inverse relationship.

Comparative example 4

Uniform ultrafine-grained Cu material with the grain size of 200nm is prepared by Anxianghai and the like, which is a metal research institute of Chinese academy of sciences, by using an equal-channel angular extrusion technology, and the purity is 99.99 w.t.%. The material has a high cycle fatigue limit of 100MPa, a low cycle fatigue life of 600 cycles and low cycle fatigue cycle softening when the plastic strain amplitude is 0.32 percent, and the stress amplitude is rapidly reduced from 380MPa to 198MPa, as shown in triangular symbols of figures 5 and 7. Therefore, the poor low cycle fatigue properties of nanostructured metals severely limit their practical applications.

The result shows that the invention introduces a gradient nano structure on the surface of the metal material by applying surface plastic processing, and the microstructure of the gradient nano structure is the nano crystal structure of the outermost layer, the ultra-fine crystal structure of the subsurface layer and the original coarse crystal structure of the core part. Wherein, the whole thickness of the gradient nano-structure layer (including nano-crystal and ultra-fine crystal structures) is more than 50 microns and between 50 and 300 microns. The gradient nanostructure provided by the invention can simultaneously improve the high-cycle and low-cycle fatigue properties of a metal material.

Claims

1. A gradient nanostructure having excellent high cycle and low cycle fatigue properties, characterized by: carrying out surface mechanical treatment on the metal material to enable the surface layer coarse crystals to generate plastic deformation and form a gradient nano structure; the formed gradient nano structure enables the high cycle fatigue and the low cycle fatigue of the metal material to be synchronously improved, and the thickness of the gradient nano structure is more than 50 microns and less than or equal to 220 microns; the metal material is pure copper;

carrying out surface plastic deformation on the metal material by adopting a surface mechanical treatment system, wherein the surface mechanical treatment system comprises a treatment cutter and a cooling system, and the cutter head part of the cutter is a hard alloy ball; liquid nitrogen atmosphere cooling is adopted in the treatment process so as to reduce the temperature rise of the surface of the sample in the treatment process;

the surface mechanical treatment process comprises the following steps: the metal material rotating member rotates along the self axial direction, the hard alloy ball of the processing cutter is contacted with the surface of the metal material and pressed into the surface of the metal material to a certain depth, and then the hard alloy ball moves from one end of the workpiece to the other end along the surface of the metal material rotating member to finish one pass processing; after the above process is repeated for a plurality of times, a gradient nano-structure layer is formed on the surface of the metal material; in the surface mechanical treatment process, the rotating speed of the metal material rotating member is 400-800r/min, the feeding speed of the treatment cutter along the axial direction of the metal material rotating member is 40-80mm/min, the pressing depth of the hard alloy ball cutter head on the material surface in each treatment pass is 20-60 mu m, and the processing passes are 1-12.

2. The gradient nanostructure having excellent high cycle and low cycle fatigue properties according to claim 1, characterized in that: after the surface of the metal material is mechanically treated, the surface is nanocrystalline, the core part is original coarse crystal, a transition region between the surface and the core part is a deformed structure, and the grain size of the metal material is transited from the nano-scale gradient on the outermost surface to the micron scale of the core part.

3. The gradient nanostructure having excellent high cycle and low cycle fatigue properties according to claim 1, characterized in that: the microhardness of the gradient nano structure is changed from high to low in a continuous gradient from outside to inside in a direction vertical to the surface of the metal material.

4. The gradient nanostructure having excellent high cycle and low cycle fatigue properties according to claim 1, characterized in that: the hard alloy ball is made of WC-Co alloy, and the diameter of the hard alloy ball is 4-10 mm.

5. The gradient nanostructure having excellent high cycle and low cycle fatigue properties according to claim 1, characterized in that: after a gradient nano structure is formed on the surface layer of the metal material, stretching the metal material at room temperature, wherein the yield strength is twice of that of the uniform coarse grain Cu, and the uniform elongation is the same as that of the coarse grain Cu; the stress control high cycle fatigue life is more than 15 times of that of coarse grain Cu; the strain-controlled low cycle fatigue life is 2 times that of macrocrystalline Cu.