RU2617729C1 - Recording method of phase transfer in the material - Google Patents

Abstract

FIELD: testing equipment.

SUBSTANCE: claimed recording method of phase transfer in the material, under the pressure and temperature influence on it, wherein the pressure on the material is created by the gaseous medium effect, and phase transfer recording is performed according to gaseous medium pressure deviation caused by the material volume change. Thus, use the gaseous medium, which is inert respectly to the testing material.

EFFECT: improved accuracy and sensitivity of the phase transition recording, simplicity and compactness of the equipment, as well as the ability to determine phase transitions at high pressures and temperatures effect and achieve low inertia of the measurement system.

2 cl, 4 dwg

Description

The invention relates to the field of metallurgy and physico-chemical analysis of substances, in particular to a method for determining the occurrence of phase transitions in metals and alloys. In general, the method can be applied to investigate the presence and characteristics of phase transitions in any materials.

Historically, the first instruments were lever (mechanical) dilatometers, in which a small change in sample size through a system of levers caused a greatly increased displacement of the arrow relative to the scale (Dilatometer // Brockhaus and Efron Encyclopedic Dictionary. In 86 volumes. St. Petersburg. 1890-1907). At present, dilatometers based on optical-mechanical, capacitive, induction, interference, X-ray, or radio-resonance methods for determining body volume changes in studies of phase transitions are widely used (Dilatometry // Big Russian Encyclopedia. In 30 volumes. Volume 8. - M. : Big Russian Encyclopedia, 2007.S. 748-749). These types of dilatometers do not allow working with samples of materials under high pressure.

There are other methods for determining the occurrence of phase transitions. The prototype, as the closest in technical essence, is taken to be a method of detecting phase transitions in materials when exposed to pressure and temperature, where the phase transition is recorded by changing the temperature of the sample from the material being studied due to changes in the properties of the sample with a constant or changing heat flux flowing through punches and sample. (AS USSR No. 1371198 A1, G01N 25/02, publ. 04/15/1994. VV Schennikov. Method for recording a phase transition).

However, this method has the following disadvantages. In the case of low strength properties of the material under study at high pressures, it is necessary to use additional equipment, which allows you to save the shape of the sample from the material to be studied (prevents severe deformations, as well as squeezing the material into the gaps between the tool and punches). Selected in the prototype method of creating the necessary pressure is associated with the use of bulky equipment. A change in temperature in this method is a consequence of a change in the thermal conductivity of the material during the phase transition, which, taking into account the thermal inertness of the system and the error of temperature measurement, can give a low sensitivity of this method.

The objective of the present invention is to improve the accuracy and sensitivity of the registration of the phase transition in the material while simplifying the method.

When using the invention, the following technical result is achieved:

- increases the accuracy and sensitivity of the registration of the phase transition;

- it becomes possible to use relatively simple, compact and cheap equipment;

- it is possible to determine the characteristics of phase transitions under uniform comprehensive compression at high pressures and at high temperatures, which increases the accuracy of registration;

- A small inertia of the measurement system is achieved.

To solve this problem and achieve a technical result, a method for detecting a phase transition in a material when exposed to pressure and temperature is proposed, in which, according to the invention, the pressure on the material is created by the action of a gaseous medium, and the phase transition is recorded by the deviation of the pressure of the gaseous medium caused by volume of material. In this case, it is necessary to use a gaseous medium inert with respect to the test material.

The essence of the invention is as follows. A sample of the test material is placed inside a high-strength research cell, into which, after sealing, gas is supplied under pressure. Further, the cell is uniformly heated at speeds that provide small temperature gradients over the volume of the sample under study, while the gas pressure inside the cell increases according to the equation of state for the gas used. The gas pressure increases mainly due to an increase in gas temperature and only slightly due to a change in the volume of the internal cavity due to the thermal expansion of the sample material and the cell material. When the temperature and pressure of the phase transition are reached, the sample material changes its volume, which results in an additional change in the gas pressure inside the cell. In the case of increasing the volume of the sample, the free volume of the cell decreases and the pressure increases, and in the case of a decrease in the volume of the sample, the free volume of the cell increases and the pressure decreases. Such a change in pressure is also described by the equation of state of a given gas, but depends on a change in the volume occupied by the gas. Since the heating is slow during the phase transition, the contribution of the temperature increase to the pressure change is insignificant (less than the pressure change from the volume change) and can be estimated using the equation of state. After a phase transition occurs, the volume of the sample stabilizes, and with further slow uniform heating of the cell with the sample, a change in gas pressure will again occur mainly due to a change in temperature. As a result, pressure and temperature curves are recorded over time, from which, knowing the equation of state of the gas used, it is possible to isolate the pressure change associated with the change in gas temperature, pressure change due to a change in the volume occupied by the gas due to thermal expansion of the sample material and the cell material, and pressure due to a significant change in the volume of the sample during the phase transition. This allows us, in the future, after processing the data, to indicate the temperature and pressure of the onset of the phase transition, the rate of occurrence of the phase transition, the change in the volume of the material during the phase transition.

To determine the phase transition from the obtained data, we construct a graph of the dependence of pressure on temperature P (T). Under ordinary conditions, an increase in gas pressure is proportional to an increase in temperature, i.e. the P (T) curve has an almost linear dependence (the slope changes slightly with pressure, due to changes in gas compressibility). Any significant deviations from this dependence indicate changes in the material of the sample. By differentiating the curve P (T) with respect to temperature, i.e. rearranging it in the coordinates dP / dT, it is more accurate to determine the temperature of the beginning and end of the phase transition, as well as the kinetics of the phase transition. An example of the P (T) and dP / dT curves for searching for a phase transition is shown in FIG. 4. The peak corresponding to the phase transition is clearly pronounced on the differentiated curve.

The gas used in research, it is desirable to choose from the conditions

- chemical inertness in relation to the studied material and structural materials used in the construction of the cell;

- low solubility and low diffusion coefficient in materials;

- the required value of gas compressibility in the used range of pressures and temperatures;

- the presence of a sufficiently accurate equation of state of the gas in the used range of pressures and temperatures, or accurate experimental data on the compressibility of the gas.

In the inventive method, the generated gas pressure affects the sample material from all sides, therefore, the strength characteristics of the studied material are not important. Modern equipment for creating gas pressure up to several thousand atmospheres is quite compact (the use of thermocompressors, gas generators). The registration of phase transitions in this invention is based on the registration of changes in the pressure of the gaseous medium, where the main measurement error is determined by the pressure sensor used (provided that the cell is tight, small deformations of the internal cavity of the cell under high pressure) and the error of the used equation of state (less than 0.5% for gases widely used in science and technology). The inertia of such a registration system is extremely small.

Ha figure 1 shows one of the possible structural schemes of the research cell.

The figure 2 shows one of the possible gas circuits of the installation for recording phase transitions in the material.

Figure 3 shows a typical graph of pressure and temperature over time, indicating which processes determine the pressure change. The graph is obtained by mathematical modeling of processes.

The figure 4 shows the experimental graph of the dependence of pressure on temperature and the curve obtained by differentiation.

In the drawings, the following notation is used.

In FIG. 1: 1 - fitting for gas supply; 2 - heater; 3 - thermal insulation; 4 - cover; 5 - place for the sample from the studied material; 6 - place of installation of the thermocouple; 7 - consolidation; 8 - base; 9 - threaded connection.

In FIG. 2: 10 - a gas source with a heater; 11 - research cell in which the sample is placed; 12 - valve gas source; 13 - valve research cell; 14 - gas outlet communication valve (to a vacuum post and to the atmosphere); 15 - communication of the removal of gas into the atmosphere; 16 - sensor for monitoring gas pressure in the gas source; 17 is a sensor for monitoring gas pressure in the research cell.

In FIG. 3: 18 - fragments of the pressure graph, where the pressure increase is mainly caused by the temperature increase; 19 is a fragment of a pressure graph, where the pressure increase is mainly caused by a change in sample volume due to a phase transition; 20 is a graph of temperature change over time; 21 is a graph of pressure versus time.

Using the method is as follows.

Inside the base 8, a thermocouple or a platinum temperature sensor with thermal grease is installed in the installation site of the thermocouple 6 (Fig. 1) to improve thermal conductivity. The base 8 is attached to the working surface. A seal 7 is installed on the base 8. The test sample is installed on the base 8 so that after assembly it is in the cavity between the cover 4 and the base 8 (place for sample 5). Then the cover 4 is screwed (threaded connection 9) relative to the base 8 to a sharp increase in effort. Heater 2 and heat insulation 3 are installed on the lid 4. The assembled research cell 11 (Fig. 2) is connected to the valve 13 of the gas supply unit by means of a fitting 1 (Fig. 1).

The experiment begins with evacuation (if necessary, remove air or other gas medium in which the cell was assembled) and check the tightness of the collected cell (necessary for further correctness of the results). Various methods can be used to check for leaks, in particular applying gas pressure and holding for a relatively long time. In this case, a drop in gas pressure by the sensor 17 indicates a leak. After checking the tightness in cell 11 (Fig. 2), conditions corresponding to the beginning of the experiment are created, i.e. gas is supplied under the necessary pressure from the source 10 through valves 12 and 13 to the research cell 11, the valve 13 is closed, and the cell 11 with the sample is heated by the heater 2 (Fig. 1) to the desired temperature (to the temperature of the region where a phase transition is assumed). The gas pressure in the research cell 11 is controlled by a pressure sensor 17. Next, a slow uniform heating of the cell 11 with the sample in the proposed region of the phase transition is ensured. In this case, the gas pressure and temperature are recorded (see Fig. 3). The change in gas pressure from temperature corresponds to more gentle sections 18 of the pressure graph, and in the phase transition region, the plot of graph 19 is steeper. Comparison of the graphs of changes in time of temperature 20 and pressure 21 allows you to determine the temperature and pressure at which the phase transition occurs, as well as the duration in time. After the experiment is completed, cell 11 is cooled, gas is vented through valves 13 and 14 into the atmosphere via communication 15, and the sample is extracted. The obtained experimental data (Fig. 4) confirm the results of mathematical modeling (Fig. 3), where the pressure change depending on temperature and the change in the free volume of the cell during the phase transition of the sample material for the inventive method of recording phase transitions are modeled.

This method allows you to study phase transitions in various substances and materials (in gallium, cerium, tin, strontium, lanthanum, etc., as well as in various alloys) at pressures of up to several thousand atmospheres and temperatures up to 500-600 degrees Celsius (when used in special steel cell structures). The temperature range can be expanded using heat-resistant structural materials, which allows this method to study phase transitions at high temperatures (in titanium, zirconium, etc.).

The advantages of this method of determining phase diagrams are the relative simplicity and compactness of the equipment, its low cost compared to equipment for a number of dilatometric methods. The most important advantage of the proposed method is to increase the accuracy and sensitivity of recording a phase transition, the ability to determine phase transitions when exposed to high pressures and temperatures on the material, the uniformity of the pressure on the sample from the material being studied, and the low inertia of the measurement system.

Claims (2)

1. A method of recording a phase transition in a material when exposed to pressure and temperature, characterized in that the pressure on the material is created by the action of a gaseous medium, and registration of a phase transition is carried out by the deviation of the pressure of the gaseous medium caused by a change in the volume of the material. 2. The method according to p. 1, characterized in that they use a gaseous medium inert with respect to the test material.

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