CN102103148B - Scanning electron microscope in-situ observation sample stage for stress corrosion cracking of metal materials - Google Patents

Abstract

The invention belongs to the field of material environment fracture research, and is a sample platform for in-situ observation of stress corrosion fracture by a scanning electron microscope (SEM), which can realize the SEM in-situ observation of stress corrosion fracture of metal materials in a liquid medium. The sample table is designed according to the space size of the SEM sample chamber, and the micro-tensile sample can be loaded on the sample and maintained in a constant displacement state. The sample table and the constant displacement sample are put into the liquid medium to cause stress corrosion cracking, and then taken out of the medium and put into the SEM sample chamber, the continuous process of stress corrosion crack expansion can be observed and recorded. At the same time, based on the SEM image of stress corrosion fracture, the crack growth rate and crack tip opening displacement can be accurately calculated. The invention solves the problem that the SEM cannot observe the stress corrosion fracture of the material in the liquid medium, and is an effective tool for studying the microscopic mechanism of the stress corrosion fracture of the material.

Description

Scanning electron microscope in-situ observation sample stage for stress corrosion cracking of metal materials

technical field

The invention belongs to the field of material environment fracture research, and relates to a scanning electron microscope sample stage, which is used for in-situ observation and recording of stress corrosion fracture of metal materials in a liquid medium.

technical background

The study of material fracture is an important topic in material development. The in situ observation of material fracture by transmission electron microscope (TEM) and scanning electron microscope (SEM) can directly observe the microscopic continuous process of material fracture under the electron microscope. It provides the most direct evidence at the microscopic level for the analysis of the fracture mechanism, so it is the most powerful method for mechanism analysis.

In the early 1980s, Kobayashir and Ohr first used the transmission electron microscope in situ tension (TEM in situ tension) method to study the fracture behavior of Mo, W, Cu, and Al, and directly observed and recorded the dislocation at the crack tip under TEM. The dynamic process of movement, formation (DFZ Dislocation Free Zone), dislocation backfilling and microcrack propagation. Since then, Pestman and Hosson have used this method to study the interaction of slip dislocations and small-angle grain boundaries in Ni3Al crystals, and found that the deformation of superlattice intrinsic stacking faults (SISF) leads to the formation of scissors on slip dislocation lines , which is completely consistent with the computer simulation results. Zielinski, Lii and Gerberich studied the crack tip emission dislocation of Fe-2wt.%Si crystal, and found that the number of crack tip emission dislocations has a definite relationship with the length of DFZ and the opening displacement of crack tip. Recently, G. Wilde and his researchers conducted TEM in-situ observations on the deformation and fracture process of the strips of palladium and its compounds after rapid cooling. The results showed that the tensile fracture of nanocrystals showed the characteristics of intergranular fracture and crack The deformation twins at the tip become the propagation paths of intergranular cracks.

The success of TEM-in-situ stretching method has promoted the wide application and development of SEM in-situ observation. Chang-Young Sona, et al. applied the SEM in situ method to study the fracture process of Cu-based alloy sintered alloys containing precipitated phase particles, and found that cracks nucleate and propagate in the amorphous matrix, and the crack tip is passivated, deflected and bonded in front of the particles. bridge, identifying the important role of composite particles. XinhuaWu et al. used SEM in situ tension to study the fracture mechanism of TiAl alloy. Under SEM, it was observed that cracks were formed in the oxygen-rich region in the region of about 100nm, and the phenomenon of oxidative embrittlement of the alloy was discovered.

In situ observations by electron microscopy are also used for environmental fracture studies of materials. The Birnbaum research group performed in-situ stretching in an ultra-high-voltage electron microscope with an environmental chamber, studied the influence of hydrogen on the movement and fracture behavior of dislocations such as Fe and Ni, and found that isolated dislocations and dislocation entanglements under hydrogen pressure The speed of the knot movement is significantly increased. Since the liquid environment cannot be realized in the electron microscope, Gu, Zhang, Chu et al. designed a constant displacement loading platform to realize the TEM in-situ observation of stress corrosion cracking, and observed that the crack tip of the Cu alloy in the liquid medium emits dislocations and dislocations Dynamic process of motion and crack nucleation.

SEM in-situ observation of stress corrosion cracking in liquid medium has not been reported yet. Due to the limitation of the vacuum environment of the sample chamber in the electron microscope, it is an insurmountable problem to add a measuring device and a liquid environment in the sample chamber of the electron microscope. This directly limits the application of the SEM in situ stretching method.

The present invention designs and manufactures a SEM in-situ observation sample platform for stress corrosion fracture by referring to the achievements of previous work, which can realize the SEM in-situ observation of stress corrosion fracture in a material in a liquid environment.

Contents of the invention

The invention is a SEM in-situ observation sample platform for metal stress corrosion fracture, which realizes the in-situ observation of metal stress corrosion fracture in a liquid medium in a limited space of a scanning electron microscope and in a vacuum environment. It provides an effective tool for the study of fracture mechanism in metal environment. The solution of the present invention to technical problem is as follows.

Design the size of the sample table (the present invention) according to the spatial structure of the SEM sample chamber, and the stress corrosion fracture sample is loaded on the sample table, which can be loaded to cause pre-cracks to occur in the sample, measure the stress value, and maintain the constant displacement state of the sample. The sample table and the sample are immersed in the liquid corrosion medium as a whole, and after a certain period of time, the sample table and the sample are taken out from the liquid medium, put into the SEM for observation, and the continuous process of crack growth under stress corrosion conditions is recorded. Record the photos to calculate the crack growth rate and crack tip opening displacement. .

Specifically, the present invention provides a sample stage for in-situ observation of stress corrosion cracking of metal materials by a scanning electron microscope. The base of the sample table is composed of two half cylinders at the front and rear, and the relative position is fixed by pins. A Φ4 threaded hole is opened on the top of the front and rear cylinders for fixing experiments; in addition, a Φ4 thread is opened on the front of the front cylinder. Through holes, screw in M4 screws for loading.

The stress corrosion samples are fixed on the sample stage by M4 screws. The screws screwed into the front base of the sample stage act on the rear base, and the front and rear bases realize relative displacement, thereby realizing the loading of the fixed stress corrosion specimen. Immerse the loaded sample together with the sample table in the liquid corrosive medium to cause stress corrosion cracking, take it out at regular intervals, conduct SEM observation, and record the crack growth process.

The stress corrosion specimens were stretched on a micro-tensile testing machine to obtain the load-displacement curves of the specimens. The displacement is measured from the SEM image of the stress corrosion specimen, and the stress field intensity factor at the crack tip is calculated according to the load-displacement curve for calibration.

The outstanding features and beneficial effects of the present invention are: ① realize in-situ observation and recording of the dynamic process of stress corrosion cracking of metal materials in liquid medium in scanning electron microscope; ② can accurately calculate the crack tip tension of stress corrosion cracking Opening displacement and crack growth rate.

Description of drawings

Fig. 1 is the front view of the mechanical design drawing of the present invention (scanning electron microscope in-situ observation sample stage for stress corrosion cracking of metal materials).

Fig. 2 is a top view of the mechanical design drawing of the present invention.

Fig. 3 is the physical photograph of the present invention. In Fig. 3, 1-screw, 2-front base, 3-rear base, 4-sample, 5-sample fixing screw, 6-pin.

Fig. 4 is a photograph of the present invention installed in a KYKY-2800 scanning electron microscope. Mark 1 refers to the object of the present invention.

Fig. 5 is an in-situ SEM image of stress corrosion cracking of U75V steel in 1.0% HCl aqueous solution recorded by using the present invention in Example 1. Figure 5(a) is the morphology image of the prefabricated crack in the state of constant displacement; Figure 5(b) is the morphology image of the pre-crack maintained in 1.0% HCl aqueous solution for 20 hours; Figure 5(c) is the morphology image maintained for 40 hours image; Figure 5(d) is the topographic image kept for 60 hours.

Fig. 6 is an in-situ SEM image of stress corrosion cracking of H62 brass in 0.5M NH ₄ OH aqueous solution recorded by using the present invention in Example 2. Figure 6(a) is the morphology image of the prefabricated crack in the state of constant displacement; Figure 6(b) is the morphology image of the pre-crack maintained in 0.5M NH ₄ OH aqueous solution for 5 minutes.

Fig. 7 is the SEM in-situ image of stress corrosion cracking of 7075-T6 aging aluminum alloy recorded in 0.5M NaCl aqueous solution by using the present invention in Example 3; Fig. 7(a) is the stress corrosion of crack in 0.5M NaCl aqueous solution for 1.0h Fracture like. Figure 7(b) is the crack growth morphology after 2.0h.

Detailed ways

(1) Micro-tensile specimens of processing experimental materials;

(2) Load the sample on the in-situ observation sample platform, load the sample to form micro-cracks, maintain a constant displacement state, and place it for 24 hours;

(3) Put the loading platform into the SEM sample chamber, observe and record the initial microcracks;

(4) Take out the sample table, immerse it in the corrosive medium, and keep it for a certain period of time; the length of time depends on the nature of the material, the type and concentration of the medium, and the temperature.

(5) Take out the sample table from the solution, wash and dry it with alcohol, put it into the SEM sample chamber, observe and record the crack growth form;

(6) Repeat steps (4)-(5) to record the continuous process of the crack under the stress corrosion state.

(7) Measure the crack growth length on the recorded photos to accurately determine the crack growth rate and the opening displacement of the paper tip.

Description: ①This implementation method is suitable for the process research of stress corrosion cracking of any metal material; ②The size of the micro-tensile sample is designed according to the size of the sample table and the strength of different materials; ③The corrosion medium and concentration of different materials can be determined according to the needs Make a selection.

Example 1

The invention records the SEM in-situ observation process of stress corrosion cracking of U75V steel in 1.0% HCl aqueous solution. The process is given by 4 SEM images: the morphology image of the prefabricated crack maintained in a constant displacement state; the morphology image maintained in a 1.0% HCl aqueous solution for 20 hours, it can be seen that the surface is corroded and the crack expands; it is maintained for 40 hours The appearance of crack growth, crack tip passivation; the appearance of 60h hours, corrosion aggravated, crack growth, microcrack connection before the crack tip. Crack propagation from 20 hours to 40 hours, measured crack tip opening displacement from 5.0 μm to 8.13 μm, an increase of 3.13 μm. That is: 6 = 3.13 μm.

Example 2

The invention records the SEM in-situ observation process of stress corrosion cracking of H62 brass in 0.5M NH ₄ OH aqueous solution. The process is given by two SEM images, which are: the image of the prefabricated crack in a constant displacement state, and the crack tip _forms a slip line; A small amount of corrosion products appeared on both sides of the crack. It was measured that the crack length extended from 116.0 μm in the initial state to 130.0 μm, an increase of 14.0 μm. Calculate the crack growth rate: da/dt=14.0/5=2.8 μm/min.

Example 3

The present invention records the SEM in-situ observation process of stress corrosion cracking of 7075-T6 aging aluminum alloy in 0.5M NaCl aqueous solution. The process is given by two SEM images, which are the stress corrosion fracture image of the crack in 0.5M NaCl aqueous solution for 1.0h, with a magnification of 30 times; the surface of the sample is corroded, and step cracks appear at the crack tip; the crack growth image after 2.0h , 200X magnification, step crack growth, showing the role of precipitation versus crack growth. The front length of the step crack extended from 179.0 μm to 320.0 μm, an increase of 141.0 μm. Calculate the crack growth rate: da/dt=141.0/60=2.33 μm/min.

Claims (9)

1. a metal material stress corrosion fracture scanning electron microscope home position observation sample bench, sample bench pedestal is comprised of former and later two semicylinders, by the fixing relative position of pin; The fixing sample of each threaded hole of opening a Φ 4 at the top of two semicylinders; The tapped through hole of Φ 4 is opened in this cylindrical front of external first half, screws in M4 screw and acts on rear portion pedestal, and former and later two pedestals are realized relative displacement, thereby realize, fixing stress corrosion sample are loaded; To load sample and immerse certain hour taking-up in interval in liquid medium together with sample bench, carry out SEM observation, the dynamic process of Crack Extension when observed and recorded metal material, in liquid medium, stress corrosion fracture occurs.

2. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein metal material comprises iron and steel, aluminium alloy, aldary and other kinds metal material.

3. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein liquid medium is alkaline aqueous solution, acidic aqueous solution, pure water.

4. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein brittle fracture, ductile rupture and the tough-fragility mixed fracture of stress corrosion fracture for occurring under stress and corrosive medium double action.

5. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein Crack Extension comprise that crackle is expanded continuously, the discontinuous expansion of crackle and the expansion of crackle zigzag.

6. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein the enlargement factor of scanning electron microscope home position observation comprises from 20 times-20000 times continuous adjustable.

7. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, its situ dynamic process is sample fracture place from the germinating of crackle, Crack Extension, to the process of final fracture.

8. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, to sample stress application, by the screw on the anterior pedestal of precession sample bench, makes sample bench two pedestals produce relative displacement and realizes.

9. metal material stress corrosion fracture scanning electron microscope home position observation sample bench as claimed in claim 1, wherein by Crack Extension image calculation crack growth rate and crack tip opening displacement, carries out according to fracturing mechanics correlation formula.

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