CN107099717B - The technology of preparing of crystal boundary auto purification tungsten magnesium alloy - Google Patents

Abstract

The invention provides a preparation technology of grain boundary self-purifying tungsten-magnesium alloy, which uses metal magnesium powder and tungsten powder to mix, and adds magnesium powder with appropriate particle size and mass percentage to the tungsten powder, and performs step-by-step processing under vacuum conditions. For pressure sintering, the first stage is to heat up to 400°C-800°C for 1-30 minutes, then the second stage is to heat up, and the temperature is to be sintered at 1200°C-1600°C for 1-60 minutes, and then the third stage is to heat up to 1600°C-1900°C Sintering for 1-30min to obtain tungsten-magnesium alloy. The invention utilizes the fluidity of magnesium and the pores in the initial state of the mixed powder to realize the microscopic flow of magnesium in the tungsten matrix, absorb the contacted oxygen elements, purify the local harmful oxygen elements, and combine with the inherent oxygen elements in the matrix. The combination of oxygen elements forms a stable and dispersed reinforcing phase, eliminates the weakening effect of oxygen elements on the tungsten grain boundaries, and forms a stable and tough tungsten-magnesium alloy, which significantly improves the mechanical properties of the tungsten-magnesium alloy block and can be widely used in high-temperature industrial fields.

Description

Preparation technology of grain boundary self-purifying tungsten-magnesium alloy

technical field

The invention belongs to the field of tungsten-magnesium alloy materials, and in particular relates to a preparation technology of grain boundary self-purifying tungsten-magnesium alloy.

Background technique

Tungsten is a metal material widely used in various industrial fields. It has the characteristics of high density, high melting point, high thermal conductivity, and low thermal expansion coefficient. It has unique applications in military industry, nuclear industry, and aerospace. Under these application conditions, tungsten needs to be frequently subjected to coupled external fields, such as high temperature, radiation and thermal shock, etc. Under these working conditions, tungsten materials are prone to failure, mainly manifested as brittle fracture under external force and regeneration under thermal effect. Crystal embrittlement. For tungsten whose crystal structure is body-centered cubic, its grain boundary strength is poor, and it is a place where fractures easily occur. Under the action of external load, intergranular fractures often occur. Therefore, for tungsten materials, purifying grain boundaries and strengthening grain boundaries are effective means to toughen tungsten materials. The second phase dispersion strengthening is the most effective strengthening and toughening technology for tungsten. There are two main types of dispersion strengthening particles: metal carbides (such as titanium carbide, zirconium carbide, hafnium carbide, etc.) and rare earth oxides (yttrium oxide, lanthanum oxide, thorium oxide, etc.). For example, in the document "The Effect of Adding Trace TiC on the Properties and Microstructure of Tungsten" (Chinese Journal of Nonferrous Metals, 2015(1):80-85), adding 1% titanium carbide powder to tungsten by mechanical alloying method, Its sintered strength increased from 260MPa to 401MPa, and the fracture mode changed from intergranular fracture of pure tungsten to "intergranular fracture + transgranular fracture". Similarly, in the literature "The effect of La_2O_3 on the sintering properties of ultra-fine tungsten composite powder and the microstructure of tungsten alloy" (Powder Metallurgy Materials Science and Engineering, 2014(3):439-445), by adding 0.7% oxide to tungsten Lanthanum, the flexural strength of sintered state increased from 213MPa to 548MPa. The above studies have shown that the addition of ultrafine ceramic particle powder can refine the grains of the tungsten matrix, thereby improving the strength and toughness. However, carbides and oxides used as dispersed phases may form and cause the content of carbon and oxygen elements in tungsten to increase. As pointed out in the literature "Effect of Impurities on the Brittleness of Tungsten Grain Boundary" (Rare Metal Materials and Engineering, (1986) 41-45), these elements segregated at the grain boundary affect the properties of the tungsten matrix, and these elements segregated at the grain boundary It will significantly reduce the grain boundary strength; at the same time, the rare earth oxide has a low melting point, and it will melt and evaporate at high temperature, leaving pits and causing a series of microscopic cracks, which is not conducive to the maintenance of high temperature strength.

The method of preparing dispersion-strengthened tungsten powder mostly adopts mechanical alloying, as reported in the document "A Preparation Method of Nano-yttrium Oxide Dispersion-strengthened Tungsten Alloy" (China.CN201310123415.3[P].2015-04-08), the dispersion The particles and tungsten powder are mixed into a ball mill for long-term high-energy ball milling, and the second-phase particles are combined with the tungsten powder through physical effects such as mechanical collision and grinding. There is also a precipitation coating method, such as the report in the document "A Preparation Method for Submicron Near-Spherical Tungsten Powder" (China.CN201010581902.0[P].2012-06-06), where soluble tungsten salt and dispersed particles are configured By adjusting the PH value of the solution, tungstic acid precipitates are generated, and these precipitates coat the second phase particles suspended in the solution to obtain a core-shell structure, which is then reduced by calcination to obtain dispersion-strengthened tungsten powder. For mechanical alloying, it needs to consume a lot of energy. During the mixing process, impurities are easily introduced due to the violent movement of the tank body and the grinding ball. Moreover, the preparation of dispersed tungsten powder by this method takes a long period and is economical. The precipitation coating method can obtain doped tungsten powder with good dispersion and uniform quality, but because this method requires a large amount of acid and alkali to prepare and control the pH value of the solution for precipitation coating reaction, it is an unfriendly method for the environment. In summary, we can see that the traditional dispersion strengthening is to introduce the second phase into the tungsten matrix from the initial state. This introduction process is easy to introduce impurity elements, and the added second phase is usually stable ceramic particles or oxides. substances, and cannot react with the oxygen contained in the powder preparation process. Moreover, the method of adding and the technological process are complicated, which makes the production process of tungsten materials longer and the cost higher.

Magnesium is an active metal element that easily reacts with oxygen and other elements. In the mature iron and steel industry, magnesium, as a beneficial additive element, has already proved its effect of purifying the matrix and reducing the oxygen content of steel. Magnesium has a relatively low melting point of 648°C, while magnesium oxide has a melting point of 2852°C. Magnesium oxide is a high-temperature refractory material with good performance. In addition, at high temperature, magnesium oxide and tungsten can react to form magnesium tungstate. The formation of magnesium tungstate can increase the contact between the second phase particles and the matrix. Bonding force, curing oxygen elements, strengthening the matrix.

Contents of the invention

The present invention is aimed at the above-mentioned deficiencies, and proposes to use metal magnesium powder and tungsten powder to mix, by adding appropriate particle size and mass percentage magnesium powder to the tungsten powder, and utilizing the staged sintering process, to promote the flow of magnesium in the tungsten matrix, and to combine with the tungsten matrix. The inherent oxygen elements are combined to form a stable and dispersed reinforcement phase, which eliminates the weakening effect of oxygen elements on the tungsten grain boundaries and forms a stable and tough tungsten-magnesium alloy. The preparation method is simple and effective, and the prepared tungsten-magnesium alloy block has significantly improved mechanical properties, and can be widely used in high-temperature industrial fields.

The realization scheme of the present invention is: selecting magnesium powder and tungsten powder with suitable particle size——carrying out mechanical mixing under protective atmosphere——carrying out segmental sintering process for the mixed powder——obtaining tungsten-magnesium alloy block. Concrete preparation process steps are as follows:

Prepare magnesium powder and tungsten powder, mix them mechanically under a protective atmosphere, then put the mixed powder into a mold, and carry out staged pressure sintering under vacuum conditions. Firstly, heat up to 400℃-800℃ for sintering in the first stage. -30min, followed by the second stage of temperature rise, sintering at 1200°C-1600°C for 1-60min, and then the third stage of temperature rise, and sintering at 1600°C-1900°C for 1-30min to obtain a tungsten-magnesium alloy.

The preparation of magnesium powder and tungsten powder is to add magnesium powder with a mass percentage of 0.1%-2% to the tungsten powder, and the particle size of the magnesium powder is less than 1 micron.

The tungsten powder is commercially pure tungsten powder.

The preparation process is carried out under the protection of argon atmosphere.

The mechanical mixing under a protective atmosphere is to add the prepared powder into the mixing tank, and carry out mechanical mixing under the protection of an argon atmosphere. The mixing time is between 0.5h and 2h according to the different addition amount.

The sintering pressure of the segmented pressure sintering is selected from 1-100 MPa, and the sintering pressure of each segment is kept constant.

The beneficial effect of the present invention is that the first stage of low-temperature sintering can realize the microscopic flow of magnesium in the tungsten matrix by utilizing the fluidity of magnesium and the pores in the initial state of the mixed powder, absorbing the contacted oxygen element, and reacting to form A part of stable magnesium oxide purifies the local harmful oxygen elements; the second stage of medium-temperature sintering densifies the matrix and fixes the dispersed particles. The particles are wrapped into the inside of the grain to form dispersed particles; the third stage of high-temperature sintering, under the combined action of high temperature, vacuum and sintering pressure, makes part of the magnesium oxide in the matrix react with the tungsten matrix to generate a certain amount of tungsten Magnesium acid can increase the binding force between the dispersed particles and the matrix.

The analysis of the prepared tungsten-magnesium alloy shows that there are amorphous dispersions on the grain boundaries and fine dispersion particles inside the grains. Its mechanical performance test shows that the bending strength and hardness are significantly improved. At the same time, this production process does not require a long-term and high-energy-consuming mixing process, and the performance difference between powder batches is small, which can realize economical and efficient production.

Description of drawings

Figure 1 is a photo of the sintered structure of 0.1% tungsten-magnesium alloy.

Figure 2 is a photo of the sintered structure of 0.5% tungsten-magnesium alloy.

Figure 3 is a photo of the sintered structure of 1% tungsten-magnesium alloy.

Figure 4a is the energy spectrum analysis of intragranular dispersoids in tungsten-magnesium alloy.

Figure 4b is an energy spectrum analysis of grain boundary dispersants in tungsten-magnesium alloys.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawings and examples.

Example 1:

A tungsten-magnesium alloy with a content of 0.1% magnesium was prepared by the above method, and its powder morphology, sintered morphology were observed, mechanical properties were tested and fracture morphology was observed, as shown in Figure 1.

1. Mix 0.02g of magnesium powder with a diameter of 50nm and 19.98g of pure tungsten powder with a diameter of 3μm in a glove box. Put it into a blender and perform mechanical mixing for 0.5h under the protection of argon.

2. Put the mixed powder into a graphite mold with a diameter of 20mm and a height of 40mm for segmental sintering. The sintering pressure is 50MPa. The first stage: heat preservation at 700°C for 3 minutes, the second stage: heat up to 1400°C, heat preservation for 5 minutes, the third stage: heat up to 1600°C, heat preservation for 1 minute, and obtain tungsten-magnesium alloy.

Example 2:

A tungsten-magnesium alloy with a content of 0.5% magnesium was prepared by the above method, and its powder morphology, sintered morphology were observed, mechanical properties were tested and fracture morphology was observed, as shown in Figure 2.

1. Mix 0.1g of magnesium powder with a diameter of 50nm and 19.9g of pure tungsten powder with a diameter of 3μm in a glove box. Put it into a blender and perform mechanical mixing for 0.6h under the protection of argon.

2. Put the mixed powder into a graphite mold with a diameter of 20mm and a height of 40mm for segmental sintering. The sintering pressure is 55MPa. The first stage: heat preservation at 750°C for 3 minutes, the second stage: heat up to 1450°C, heat preservation for 5 minutes, the third stage: heat up to 1650°C, heat preservation for 1 minute, and obtain tungsten-magnesium alloy.

Example 3:

A tungsten-magnesium alloy with a content of 1% magnesium was prepared by the above method, and the powder morphology and sintered morphology were observed, and mechanical property tests and fracture morphology were observed, as shown in Figure 3.

1. Mix 0.2g of magnesium powder with a diameter of 50nm and 19.8g of pure tungsten powder with a diameter of 3μm in a glove box. Put it into a blender, and perform mechanical mixing for 0.7h under the protection of argon.

2. Put the mixed powder into a graphite mold with a diameter of 20mm and a height of 40mm for segmental sintering. The sintering pressure is 60MPa. The first stage: heat preservation at 800°C for 3 minutes, the second stage: heat up to 1500°C, heat preservation for 5 minutes, the third stage: heat up to 1700°C, heat preservation for 1 minute, and obtain tungsten-magnesium alloy.

The prepared sample was cut into strips of 2mm*3mm*18mm for microhardness measurement and three-point bending test respectively, and the results are as follows.

Table 1 Comparison table of mechanical properties of 0.1%Mg, 0.5%Mg, 1%Mg tungsten bulk and pure tungsten

material type Microhardness (HV) Three-point bending strength (MPa) Pure Tungsten 412 356.7 0.1% Mg Tungsten 509 724 0.5% Mg Tungsten 562 807.4 1% Mg Tungsten 584 1105.2

From the test results, the flexural strength and hardness of the tungsten-magnesium alloy prepared by adding magnesium increased significantly, and showed an increasing trend with the increase of the addition amount, which is related to the purification effect of magnesium on the grain boundary of tungsten and the strengthening effect of generating the second phase dispersion are inseparable. Observing the microstructure of tungsten-magnesium alloy, as shown in Figure 1, it can be seen that there are two forms of second phases distributed in the tungsten grain boundaries and inside the grains. There is another kind of dots wrapped inside the grains. As the magnesium content increases, as shown in Figure 2 and Figure 3, the amount of the second phase in the tungsten matrix increases significantly, but it still maintains a uniform distribution in the grain and grain boundaries. Energy spectrum analysis was carried out on the dispersed phase in the grain and on the grain boundary. As shown in Figure 4, the dispersed phase in the grain and on the grain boundary contained a certain amount of magnesium oxide and magnesium tungstate respectively, which confirmed that the tungsten-magnesium alloy was formed during the preparation process. A second phase containing magnesium oxide and magnesium tungstate was formed. It is proved that the magnesium element can solidify the oxygen element in the tungsten matrix, and the formation of magnesium tungstate strengthens the bonding force between the dispersed particles and the matrix to purify the tungsten grain boundary and form dispersion strengthening to effectively improve the flexural strength and hardness of the material.

Claims (4)

1. A preparation method for grain boundary self-purifying tungsten-magnesium alloy, characterized in that magnesium powder and tungsten powder are prepared, mechanically mixed under a protective atmosphere, then the mixed powder is packed into a mold, and then carried out under vacuum conditions. Segmented pressure sintering, the first step is to raise the temperature to 400°C-800°C for 1-30min, then proceed to the second stage of temperature rise, sintering at 1200°C-1600°C for 1-60min, and then carry out the third stage of temperature rise, at 1600°C Sintering at -1900°C for 1-30min to produce tungsten-magnesium alloy; The preparation of magnesium powder and tungsten powder is to add magnesium powder with a mass percentage of 0.1%-2% to the tungsten powder, and the particle size of the magnesium powder is less than 1 micron; The sintering pressure of the segmented pressure sintering is selected to be 50-60 MPa, and the sintering pressure of each segment is kept constant. 2. The preparation method of grain boundary self-purifying tungsten-magnesium alloy according to claim 1, characterized in that, the tungsten powder is selected from commercially pure tungsten powder. 3. The preparation method of grain boundary self-purifying tungsten-magnesium alloy according to claim 1, characterized in that, the preparation process is carried out under the protection of an argon atmosphere. 4. the preparation method of grain boundary self-purifying tungsten-magnesium alloy according to claim 1, is characterized in that, described under protective atmosphere, carry out mechanical mixing, is that the powder of preparation is added mixing tank body, in argon atmosphere protection Under mechanical mixing, the mixing time is between 0.5h and 2h.