阅读说明：本技术 增材制造的具有超高屈服强度和高塑性的亚稳态β钛合金 (Metastable beta titanium alloy with ultrahigh yield strength and high plasticity manufactured by additive manufacturing ) 是由 邱春雷 陈旭 孙鹏越 于 2021-09-26 设计创作，主要内容包括：本发明提供一种增材制造的具有超高屈服强度和高塑性的亚稳态β钛合金,该钛合金的Md值为2.34-2.37,Bo值为2.78-2.79；该钛合金的合金元素为Mo、Cr、Co、C,其中Mo、Cr、Co三种合金元素的质量百分比之和超过12％；该合金的屈服强度大于1000MPa,延伸率大于10％。本发明增材制造的亚稳态β钛合金具有超高的屈服强度,良好的塑性,促使该新型亚稳态β钛合金作为轻质高强的结构材料在航空航天、生物医用、国防军工等领域有望得到广泛的应用,增材制造方法克服了传统亚稳态β钛合金研发工艺途径的缺点,缩短了研发周期,降低了研发成本。(The invention provides a metastable beta titanium alloy with ultrahigh yield strength and high plasticity, which is manufactured by additive manufacturing, wherein the Md value of the titanium alloy is 2.34-2.37, and the Bo value is 2.78-2.79; the titanium alloy comprises the alloying elements of Mo, Cr, Co and C, wherein the sum of the mass percentages of the three alloying elements of Mo, Cr and Co exceeds 12%; the yield strength of the alloy is more than 1000MPa, and the elongation is more than 10%. The metastable beta titanium alloy manufactured by the additive has ultrahigh yield strength and good plasticity, so that the novel metastable beta titanium alloy is expected to be widely applied in the fields of aerospace, biomedicine, national defense military industry and the like as a light high-strength structural material, the additive manufacturing method overcomes the defects of the traditional metastable beta titanium alloy research and development process approach, the research and development period is shortened, and the research and development cost is reduced.)

1. An additively manufactured metastable beta titanium alloy with ultra-high yield strength and high plasticity, characterized in that it has an Md value of 2.34-2.37 and a Bo value of 2.78-2.79; the titanium alloy comprises the alloying elements of Mo, Cr, Co and C, wherein the sum of the mass percentages of the three alloying elements of Mo, Cr and Co exceeds 12%; the yield strength of the alloy is more than 1000MPa, and the elongation is more than 10%.

2. The metastable beta titanium alloy according to claim 1, characterized in that the titanium alloy has a composition, in mass%: 5-7% of Mo, 4.5-6.5% of Cr, 0.5-1.5% of Co, 0.05-0.15% of C, and the balance of Ti and inevitable impurities.

3. The metastable beta titanium alloy according to claim 1, characterized in that the titanium alloy has a composition, in mass%: ti-6Mo-5.5Cr-1 Co-0.1C.

4. A method of making the additive manufactured metastable beta titanium alloy with ultra-high yield strength and high plasticity according to claim 1, comprising the steps of:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder;

(2) adding high-melting-point pure molybdenum powder and carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill;

(3) preparing a sample of the mixed powder subjected to ball milling by using an additive manufacturing method to obtain a block sample;

(4) the bulk samples were subjected to solution heat treatment and water quenching.

5. The method for preparing the metastable beta titanium alloy according to claim 4, wherein in the step (1), the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 μm, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 5-10 hours; in the step (2), the grain diameter of the high-melting point pure molybdenum powder and the carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ball powder ratio is 4:1, the ball diameter is 5 mm, the rotating speed is 400-700rpm, and the ball milling time is 15-40 minutes.

6. The method of producing a metastable beta-titanium alloy according to claim 4, wherein, in step (2), the carbide is chromium carbide, molybdenum carbide or titanium carbide.

7. The method of preparing a metastable beta-titanium alloy according to claim 4, wherein, in step (3), the additive manufacturing method comprises selective laser melting, selective electron beam melting, or laser direct deposition.

8. The method for preparing the metastable beta-titanium alloy according to claim 4, wherein in the step (4), the solution heat treatment has the following process parameters: the heating rate is 5-15 ℃/min, the solid solution temperature is 800-.

9. The method for preparing the metastable beta-titanium alloy according to claim 4, wherein in the step (4), after the solution heat treatment is performed on the bulk sample, the bulk sample is immediately put into clean water or saline water for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500 ℃/s.

Technical Field

The invention relates to the technical field of titanium alloy preparation, in particular to a metastable beta titanium alloy prepared based on an additive manufacturing method.

Background

Metastable beta titanium alloys are of great interest because of their high tensile strength and high plasticity, excellent corrosion resistance, low density, low modulus of elasticity, and good biocompatibility. The common metastable beta titanium alloys developed at present, such as Ti-12Mo and Ti-9Mo-6W, etc., all exhibit high tensile strength, high strain hardening rate (up to 2GPa) and excellent plasticity (elongation up to 35%). However, such alloys tend to exhibit very low yield strengths (< 500MPa), which greatly limits the range of applications for such alloys. In recent years, while the yield strength of metastable beta titanium alloys has been improved by compositional design, the yield strength of Ti-10Cr, Ti-15Mo, Ti-10Mo-1Fe, Ti-7Mo-3Cr, Ti-6Cr-4Mo-2Al-2Sn-1Zr can reach 500-700 MPa. The yield strength of individual metastable beta titanium alloys such as Ti-4Mo-3Cr-1Fe reaches 870 MPa. Generally, however, the yield strength of metastable beta titanium alloys is generally lower than that of alpha and alpha + beta titanium alloys, the latter typically being above 900 MPa. The yield strength of the alloy is even lower than that of other high-strength structural materials such as high-temperature alloy. This limits the application prospects of metastable beta titanium alloy as structural material.

In addition, the preparation and development of the metastable beta titanium alloy are based on the traditional manufacturing process such as alloy smelting, casting, forging/rolling, solution treatment and water quenching. The preparation process has the advantages of multiple working procedures, complex flow and long period for determining each process parameter, and the development of the novel metastable beta titanium alloy by using the process has the advantages of long development period, high cost, large energy consumption and serious pollution, and is difficult to quickly discover and develop the novel high-performance metastable beta titanium alloy.

Most metastable beta titanium alloys, such as Ti-12Mo and Ti-9Mo-6W, have low yield strengths primarily due to insufficient beta phase stability of the alloy, resulting in deformation-induced martensitic transformation (beta → alpha ") and deformation-induced twinning in the early stages of tensile deformation. The former can happen in the stress range of 190-. Through the design of alloy components, the stability of beta phase in the metastable beta titanium alloy can be improved, the deformation mechanism of the alloy is promoted to change so as to improve the yield strength of the material, for example, Ti-10Cr, Ti-15Mo and Ti-10Mo-1Fe mainly take twinning and dislocation slippage as main components, the transformation of beta → alpha' martensite is inhibited, the yield strength of the alloy is improved to a certain extent, but the plasticity is reduced. The recently developed Ti-7Mo-3Cr, Ti-4Mo-3Cr-1Fe and Ti-6Cr-4Mo-2Al-2Sn-1Zr mainly take twinning deformation as the main part, and the twin deformation of the {332} <113> and {112} <111> systems is simultaneously initiated, so that the alloy not only shows higher yield strength, but also has good plasticity and high strain hardening rate. However, it should be noted that the yield strength of the metastable beta titanium alloy developed above is not high enough to enable the metastable beta titanium alloy to be popularized and applied to light high-strength structures with high bearing capacity.

Disclosure of Invention

Aiming at the problems, the invention provides the metastable beta titanium alloy with ultrahigh yield strength and high plasticity, which is prepared by adopting an additive manufacturing method, and has the advantages of wide application, high research and development efficiency and low research and development cost.

The technical effects are realized by the following technical scheme:

an additively manufactured metastable beta titanium alloy having ultra-high yield strength and high plasticity, the titanium alloy having an Md value of 2.34-2.37 and a Bo value of 2.78-2.79; the titanium alloy comprises the alloying elements of Mo, Cr, Co and C, wherein the sum of the mass percentages of the three alloying elements of Mo, Cr and Co exceeds 12%; the yield strength of the alloy is more than 1000 MPa.

Preferably, the titanium alloy comprises, by mass, 5 to 7% of Mo, 4.5 to 6.5% of Cr, 0.5 to 1.5% of Co, 0.05 to 0.15% of C, and the balance Ti and unavoidable impurities.

Preferably, the titanium alloy comprises the following components in percentage by mass: ti-6Mo-5.5Cr-1 Co-0.1C.

A preparation method of the metastable beta titanium alloy with ultrahigh yield strength and high plasticity comprises the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder;

(2) adding high-melting-point pure molybdenum powder and carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill;

(3) preparing a sample of the mixed powder subjected to ball milling by using an additive manufacturing method to obtain a block sample;

(4) the bulk samples were subjected to solution heat treatment and water quenching.

Preferably, in the step (1), the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 5-10 hours; in the step (2), the grain diameter of the high-melting point pure molybdenum powder and the chromium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ball powder ratio is 4:1, the ball diameter is 5 mm, the rotating speed is 400-700rpm, and the ball milling time is 15-40 minutes.

Preferably, in the step (2), the carbide is chromium carbide, molybdenum carbide or titanium carbide.

Preferably, in step (3), the additive manufacturing method includes selective laser melting, selective electron beam melting, or laser direct deposition.

Preferably, in the step (4), the process parameters of the solution heat treatment are as follows: the heating rate is 5-15 ℃/min, the solid solution temperature is 800-.

Preferably, in the step (4), after the solid solution heat treatment is carried out on the bulk sample, the bulk sample is immediately put into clean water or saline water for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500-.

The invention has the beneficial effects that:

the invention designs the components of the high-performance metastable beta titanium alloy based on the additive manufacturing method, and prepares the metastable beta titanium alloy with ultrahigh yield strength by additive manufacturing, solid solution and quenching treatment, wherein the yield strength of the titanium alloy reaches above 1060MPa, which is improved by 200-500MPa compared with the yield strength of most of the prior metastable beta titanium alloys, thereby greatly widening the application prospect of the metastable beta titanium alloy as a high-performance structural material. The addition of a small amount of carbon not only enables a sample to form carbide particles along a grain boundary and promotes the refinement of grains, but also effectively hinders dislocation movement of the carbide particles near the grain boundary in the deformation process and contributes to the improvement of strength. And the formation of carbides absorbs a large amount of oxygen from the matrix, contributing to the improvement of the stability of the beta matrix. The segregation of Mo, Cr and Co in the matrix and the precipitation of high-density superfine omega precipitate phase from the matrix further improve the chemical stability of the matrix, promote the deformation mechanism of the beta matrix to be mainly dislocation slip, and contribute to the improvement of the yield strength. Meanwhile, the solid solution treatment promotes the carbide to be distributed along the grain boundary, and the dislocation can freely move in the crystal grain, so that the material not only obtains ultrahigh yield strength, but also obtains good plasticity, and the elongation of the titanium alloy is more than 10%.

According to the invention, the high-performance metastable beta titanium alloy can be efficiently prepared by using the additive manufacturing, solid solution and quenching approaches, compared with the traditional preparation process, the research and development and preparation periods of the alloy are greatly shortened, and the preparation process is greatly simplified. The combination of the advanced alloy design concepts greatly improves the discovery and development efficiency of novel high-performance metastable beta titanium alloy. In addition, the invention utilizes the existing powder mixing technology to rapidly mix different simple substance powder and compound powder according to the requirements and proportions, so that various alloy powder are uniformly distributed, and the total amount of the powder required for preparing the block sample is smaller (less than or equal to 4 kilograms). Therefore, the material consumption of the novel high-performance metastable beta titanium alloy in the development process can be greatly saved, and the development cost of the novel alloy is saved. The development and preparation route is environment-friendly because the laser additive manufacturing process does not generate any carbon dioxide and other harmful gases, and a large number of thermo-mechanical processing and machining procedures are not needed subsequently.

Drawings

FIG. 1 is a scanning electron micrograph of powder particles of example 1 of the present invention.

FIG. 2 is a view showing the microstructure of example 1 of the present invention.

FIG. 3 is a transmission electron micrograph and diffraction spots of example 1 of the present invention.

FIG. 4 is a graph of tensile engineering stress-strain curves of alloy prepared by the laser exposure time under 400W power in example 1 of the present invention.

FIG. 5 is a transmission electron micrograph showing the interaction of internal dislocations with carbide particles and the distribution thereof in a matrix after tensile deformation of a sample according to example 1 of the present invention.

Detailed Description

In order to obtain the ultra-high yield strength metastable beta titanium alloy, firstly, the alloy is subjected to component design, the alloy is ensured to contain higher beta stability elements Mo, Cr and Co, the Md (average d orbital level) value and the Bo (covalent bond and strength of titanium and alloy elements) value of the total components of the alloy are respectively 2.34-2.37 and 2.78-2.79, and the two ranges are positioned on a boundary line of Md-Bo graphic dislocation slip and a twinning deformation zone, so that the beta phase of the alloy is ensured to have high stability, and the martensite phase transformation at the initial deformation stage is prevented. Since the metal material produced by additive manufacturing is also an as-cast structure in nature, it is necessary to ensure that the material produced has as fine grains as possible to achieve high yield strength without subsequent thermomechanical processing. For this reason, when selecting the alloy elements, elements with high grain growth restriction factors in titanium, such as Co and C, are specially selected to promote the grain refinement and the transformation of columnar crystal orientation equiaxed crystals of the alloy in the process of additive manufacturing solidification. The addition of C is also expected to promote the formation of carbide particles to further strengthen the alloy. Specifically, the mass percent of the three elements of Mo, Cr and Co exceeds 12%. The yield strength of the high yield strength titanium alloy thus obtained is greater than 1000 MPa.

Preferably, the titanium alloy comprises, by mass, 5-7% of Mo, 4.5-6.5% of Cr, 0.5-1.5% of Co, 0.05-0.15% of C, and the balance of Ti and unavoidable impurities.

More preferably, the nominal composition of the titanium alloy is Ti-6Mo-5.5Cr-1Co-0.1C (wt.%).

The metastable beta titanium alloy with ultrahigh yield strength and high plasticity is prepared by the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder;

(2) adding high-melting-point pure molybdenum powder and carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill;

(3) preparing a sample of the mixed powder subjected to ball milling by using an additive manufacturing method to obtain a block sample;

(4) the bulk samples were subjected to solution heat treatment and water quenching.

Wherein, in the step (1), the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 5-10 hours; in the step (2), the grain diameter of the high-melting point pure molybdenum powder and the chromium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ball powder ratio is 4:1, the ball diameter is 5 mm, the rotating speed is 400-700rpm, and the ball milling time is 15-40 minutes. Ti, Co and Cr simple substance metal powders with similar melting points are preferentially mixed by a three-dimensional swing powder mixer in the form of the same particle size range (15-53 microns). Then, high-melting-point Mo metal powder particles and compound particles such as carbide (C is mainly added to the alloy in the form of carbide such as chromium carbide, molybdenum carbide or titanium carbide) are mixed with the high-particle-diameter elementary powder particles in a short time and high efficiency by using a horizontal high-energy ball mill, wherein the low-particle-diameter elementary powder particles are 1-3 microns. The mixing time is usually less than 1 hour to ensure that the powder particles with small particle size are uniformly assembled and distributed on the surface of the powder particles with large size, and simultaneously keep the nearly spherical shape of the powder particles with large size, which helps to keep good fluidity of the powder particles in the laser additive manufacturing process.

Additive manufacturing methods include selective laser melting, selective electron beam melting, or laser direct deposition.

Because the metal material manufactured by the additive is usually subjected to orientation, rapid solidification and rapid cooling, a high-density dislocation structure and large residual stress are easily formed in the material, and in order to eliminate internal stress and dislocation, the alloy is subjected to solution heat treatment and water quenching after the additive manufacturing. In the step (4), the technological parameters of the solution heat treatment are as follows: the heating rate is 5-15 ℃/min, the solid solution temperature is 800-; the technological parameters of water quenching are as follows: immediately putting the material into clear water or saline water for cooling after the solution treatment until the temperature reaches room temperature, wherein the quenching cooling rate is 500-.

The additive manufacturing of ultra-high yield strength and high plasticity metastable beta titanium alloys and methods of making the same according to the present invention are further illustrated by the following specific examples.

Example 1

The Md value of the titanium alloy is 2.36, and the Bo value is 2.78; the titanium alloy comprises the following components in percentage by mass: ti-6% Mo-5.5% Cr-1.0% Co-0.1% C.

The preparation method comprises the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder; the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 6 hours;

(2) adding high-melting-point pure molybdenum powder and chromium carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill; the grain diameter of the high-melting point pure molybdenum powder and the chromium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ratio of the ball powder to the powder is 4:1, the diameter of the ball is 5 mm, the rotating speed is 600rpm, and the ball milling time is 30 minutes.

(3) Preparing a sample of the ball-milled mixed powder by using a selective laser melting method to obtain a block sample;

(4) carrying out solid solution heat treatment and water quenching on a block sample, wherein the technological parameters of the solid solution heat treatment are as follows: the heating rate is 10 ℃/min, the solid solution temperature is 850 ℃, and the solid solution heat preservation time is 40 min; the technological parameters of water quenching are as follows: after the solid solution treatment, the material is immediately put into clean water for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500-.

The yield strength of the prepared metastable beta titanium alloy is 1062MPa, and the elongation is more than 10%.

Example 2

The Md value of the titanium alloy is 2.36, and the Bo value is 2.78; the titanium alloy comprises the following components in percentage by mass: ti-5% Mo-6.5% Cr-0.6% Co-0.05% C.

The preparation method comprises the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder; the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 5 hours;

(2) adding high-melting-point pure molybdenum powder and titanium carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill; the grain diameter of the high-melting point pure molybdenum powder and the titanium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ball powder ratio is 4:1, the ball diameter is 5 mm, the rotating speed is 600rpm, and the ball milling time is 15 minutes.

(3) Preparing a sample of the ball-milled mixed powder by using a laser direct deposition method to obtain a block sample;

(4) carrying out solid solution heat treatment and water quenching on a block sample, wherein the technological parameters of the solid solution heat treatment are as follows: the heating rate is 5 ℃/min, the solid solution temperature is 800 ℃, and the solid solution heat preservation time is 30 min; the technological parameters of water quenching are as follows: after the solid solution treatment, the material is immediately put into clean water for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500-.

The yield strength of the prepared metastable beta titanium alloy is 1054MPa, and the elongation is more than 10 percent.

Example 3

The Md value of the titanium alloy is 2.35, and the Bo value is 2.78; the titanium alloy comprises the following components in percentage by mass: ti-7% Mo-4.5% Cr-1.5% Co-0.15% C.

The preparation method comprises the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder; the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 10 hours;

(2) adding high-melting-point pure molybdenum powder and molybdenum carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill; the grain diameter of the high-melting point pure molybdenum powder and the titanium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ratio of the ball powder to the powder is 4:1, the diameter of the ball is 5 mm, the rotating speed is 600rpm, and the ball milling time is 40 minutes.

(3) Preparing a sample of the ball-milled mixed powder by using a selective electron beam method to obtain a block sample;

(4) carrying out solid solution heat treatment and water quenching on a block sample, wherein the technological parameters of the solid solution heat treatment are as follows: the heating rate is 15 ℃/min, the solid solution temperature is 1000 ℃, and the solid solution heat preservation time is 120 min; the technological parameters of water quenching are as follows: after the solution treatment, the material is immediately put into brine for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500-800 ℃/second.

The yield strength of the prepared metastable beta titanium alloy is 1039MPa, and the elongation is more than 10 percent.

Example 4

The Md value of the titanium alloy is 2.35, and the Bo value is 2.78; the titanium alloy comprises the following components in percentage by mass: ti-6.5% Mo-6% Cr-1.5% Co-0.1% C.

The preparation method comprises the following steps:

(1) mixing spherical pure titanium powder, pure chromium powder and pure cobalt powder by using a three-dimensional swinging powder mixer to obtain mixed powder; the grain diameters of the pure titanium powder, the pure chromium powder and the pure cobalt powder are 15-53 microns, and the technological parameters of the mixed powder are as follows: the rotating speed is 28rpm, and the powder mixing time is 8 hours;

(2) adding high-melting-point pure molybdenum powder and chromium carbide powder particles into the mixed powder obtained in the step (1), and carrying out ball milling by using a horizontal high-energy ball mill; the grain diameter of the high-melting point pure molybdenum powder and the titanium carbide powder is 1-3 microns, and the technological parameters of ball milling are as follows: the ratio of the ball powder to the powder is 4:1, the diameter of the ball is 5 mm, the rotating speed is 600rpm, and the ball milling time is 40 minutes.

(3) Preparing a sample of the ball-milled mixed powder by using a laser direct deposition method to obtain a block sample;

(4) carrying out solid solution heat treatment and water quenching on a block sample, wherein the technological parameters of the solid solution heat treatment are as follows: the heating rate is 15 ℃/min, the solid solution temperature is 900 ℃, and the solid solution heat preservation time is 60 min; the technological parameters of water quenching are as follows: after the solid solution treatment, the material is immediately put into flowing clear water for cooling until the temperature reaches room temperature, and the quenching cooling rate is 500-.

The yield strength of the prepared metastable beta titanium alloy is 1057MPa, and the elongation is more than 10 percent.

The above description is only exemplary of the present invention and should not be taken as limiting the scope of the present invention, and any modifications, equivalents, improvements and the like that are within the spirit and principle of the present invention should be included in the scope of the present invention.