CN103331449B - Ultra-fine Grained/micron crystal block body iron material of the two size distribution of a kind of super-high-plasticity and preparation method thereof - Google Patents

Abstract

The invention discloses an ultra-fine-grained/micro-crystalline bulk iron material with a dual-scale distribution of ultra-high plasticity. Crystal needle-like α-Fe is a reinforcing phase, with excellent comprehensive mechanical properties and strong plastic deformation ability, and has a good application prospect. The invention also discloses a preparation method of ultra-fine-grained/micro-crystalline bulk iron material with ultra-high plasticity and double-scale distribution. First, the initial high-purity iron powder is milled into nano-crystalline powder by high-energy balls, and then rapidly sintered by a discharge plasma sintering system. The sintering temperature is Ts: 1253K≤Ts≤1335K, sintering time: 14~26min, sintering pressure: 40~500MPa. The invention has the advantages of simple method, convenient operation, controllable grain size, high yield, raw material saving and near-final forming.

Description

A kind of superfine-grained/micro-crystalline bulk iron material with ultra-high plasticity and double-scale distribution and its preparation method

technical field

The invention relates to an ultrafine crystal/microcrystalline metal material, in particular to an ultrafine crystal/microcrystalline bulk iron material with ultrahigh plasticity and double-scale distribution and a preparation method thereof.

Background technique

Iron and iron alloys have the characteristics of cheap price, abundant resources, excellent performance and easy realization of large-scale production, so they have become the most widely used materials at present. However, the rapid development of modern industry has put forward higher and higher requirements for the comprehensive mechanical properties of iron and iron alloys. It has become an important research topic for material workers to obtain high comprehensive mechanical properties by suitable preparation methods. Fine-grain strengthening is an important method to improve strength and plasticity. Micro-grain, ultra-fine-grain and nano-grain can be obtained by means of grain refinement, which doubles the strength of the material compared with traditional cast coarse-grained materials. At present, there are mainly the following methods for preparing ultrafine grain iron materials: (1) using the method of equal channel angular extrusion to refine grains through large plastic deformation to obtain ultrafine grain bulk iron materials with low strength and high plasticity. The grain size is 200-400nm, the compressive fracture strength is 800MPa under true stress-strain conditions, and the plastic strain is 30% (Gertsman V.Y., Birringer R., Valiev R.Z., et al., Scripta Metallμrgica et Materialia, 1994,30(2) :229-234); (2) Using cold pressing and hot pressing at low temperature (683K-863K) to obtain high-strength and low-plasticity ultra-fine-grained or nano-crystalline bulk iron materials. When the sintering temperature is 863K, an ultra-fine-grain bulk iron material is obtained, the grain size is 268nm, the compressive fracture strength is 1600MPa under true stress-strain conditions, and the plastic strain is 12%; when the sintering temperature is 683K, a nanocrystalline block is obtained Iron material, the grain size is 138nm, the compressive fracture strength under true stress-strain conditions is 2500MPa, and the plastic strain is 6% (Jia D., Ramesh K.T., Ma E..Acta Materialia, 2003,51(12):3495-3509 ); (3) Using spark plasma sintering technology, a high-strength and high-plastic dual-scale nanocrystalline α-Fe/microcrystalline α-Fe block was prepared by step-by-step pressing at a sintering temperature of 993K Bulk iron material (among them, the content of microcrystalline α-Fe is very small), the compressive fracture strength is as high as 2249MPa under the condition of true stress and strain, and the plastic strain is 40% (SrinivasaraoB., Oh-ishi K., Ohkμbo T., et al.Scripta Mater., 2008, 58:759–762).

However, when the metal grain size is refined to the nanometer level, although the strength of the material increases exponentially, the plasticity decreases significantly. As shown by the properties of nanocrystalline iron obtained in the experiment by Jia D. et al., although nanocrystalline iron has a strength as high as 2500 MPa, the plastic strain is only 6%. In addition to the method of grain refinement and toughening, according to the classical theory that structure determines performance, the preparation of composite materials with different sizes and shapes is expected to improve the plasticity of materials and obtain good comprehensive mechanical properties. The nanocrystalline and microcrystalline double-scale iron materials obtained by Srinivasarao B. not only have a fracture strength as high as 2249MPa, but also have a plastic strain of 40%. It can be seen that exploring the preparation method of ultrafine-grained materials with dual-scale or multi-scale distribution is of great significance for improving the comprehensive mechanical properties of metal materials.

In summary, the highest plasticity of ultrafine-grained iron materials prepared by various methods can reach 40% (ScriptaMater., 2008, 58:759–762), but the experimental sintering temperature used by Srinivasarao B. et al. The isomerization transition temperature (1185K) is below, and the step-by-step pressurization process is complicated, and it is not easy to densify the finished product.

Contents of the invention

In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the object of the present invention is to provide a method for preparing an ultrafine-grained/micro-crystalline bulk iron material with a dual-scale distribution of ultrahigh plasticity, and the prepared ultra-fine-crystalline/micro-crystalline bulk iron material The iron material has the advantages of uniform microstructure and grain, nearly fully dense, ultra-high plasticity, and dual-scale distribution.

Another object of the present invention is to provide the ultra-fine-grained/micro-crystalline bulk iron material with high plasticity obtained by the above-mentioned preparation method.

The object of the present invention is achieved through the following technical solutions:

A method for preparing an ultra-fine-grained/micro-crystalline bulk iron material with ultra-high plasticity and dual-scale distribution, comprising the following steps:

(1) Preparation of nanocrystalline iron powder by high-energy ball milling: under argon protection, place pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of 8-12nm is obtained;

(2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put the nanocrystalline iron powder obtained in step (1) into a mold, and use spark plasma sintering under the protection of argon to obtain ultra-high plasticity. Ultrafine-grained/micro-crystalline bulk iron material with plastic dual-scale distribution, wherein the rapid sintering process conditions are as follows:

Sintering equipment: spark plasma sintering system;

Sintering current type: pulse current;

Sintering temperature Ts: 1253K≤Ts≤1335K;

Sintering time: 14~26min;

Sintering pressure: 40~500MPa.

In the spark plasma sintering described in step (2), the heating rate is 54-235K/min, and the holding time is controlled at 0-10 min.

In the spark plasma sintering described in step (2), the sintering pressure is 40-50 MPa when a graphite mold is used, and the sintering pressure is 50-500 MPa when a tungsten carbide mold is used.

The ultra-fine-grained/micro-crystalline bulk iron material with high plasticity and dual-scale distribution obtained by the above-mentioned preparation method has a microstructure with blocky micro-crystalline α-Fe as the matrix phase, ultra-fine equiaxed α-Fe and ultra-fine Crystal needle α-Fe is the strengthening phase.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

(1) The dual-scale distribution of ultra-fine-grained/micro-crystalline bulk iron material prepared by the present invention has the advantages of uniform microstructure grains, nearly fully dense, ultra-high plasticity, excellent comprehensive mechanical properties, and its room temperature compression fracture strength and fracture strength The strain reaches 734MPa and more than 58%, respectively, especially in terms of plasticity, it is far superior to other bulk iron materials.

(2) The preparation method of the ultra-fine-grained/micro-crystalline bulk iron material with dual-scale distribution of the present invention has the advantages of simple processing, convenient operation, high yield, saving raw materials and near-final forming; at the same time, the size of the formed material is relatively large , the internal interface of the material is clean and its grain size is controllable.

(3) The preparation method of the ultra-fine-grained/micro-crystalline bulk iron material with dual-scale distribution of the present invention, when the heating rate is between 54-235K/min, and the holding time is controlled within 0-10min, the holding time and heating The change of the rate has no obvious influence on the plasticity, and the consistency of the product is good.

(4) The preparation method of the dual-scale distribution ultra-fine crystal/microcrystalline bulk iron material of the present invention, the present invention can prepare materials with a larger size and a diameter greater than 20mm, which can basically meet the application requirements as a new type of structural material , has broad application prospects.

Description of drawings

FIG. 1 is a scanning electron microscope image of the ultrafine-grained/micro-crystalline bulk iron material with ultrahigh plasticity and dual-scale distribution prepared in Example 1.

Fig. 2 is the room temperature compression true stress-strain curve of the ultrafine-grained/micro-crystalline bulk iron material with ultrahigh plasticity and dual-scale distribution prepared in Example 1.

Detailed ways

The present invention will be described in further detail below in conjunction with the examples, but the embodiments of the present invention are not limited thereto.

Example 1

The preparation method of the ultra-fine-grained/micro-crystalline bulk iron material with ultra-high plasticity and dual-scale distribution in this embodiment includes the following steps:

(1) Preparation of nanocrystalline iron powder by high-energy ball milling: Put pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of about 8-12nm is obtained:

The initial powder is high-purity electrolytic iron powder (99.5wt.%, particle size 38um). Put the initial iron powder and stainless steel balls together into a stainless steel ball mill jar (use an "O"-shaped sealing ring to seal between the ball mill jar and the lid. The diameters of the grinding balls are 15mm, 10mm and 6mm respectively, and their weight ratio is 1:3:1, and the weight ratio of grinding balls and powder is 10:1). In order to prevent oxidation, the ball mill tank is filled with high-purity argon for protection (99.99%, 0.5MPa). Finally, the ball mill tank filled with argon protection was placed on a QM-2SP20 planetary ball mill for high-energy ball milling (3.8s ^-1 rotational speed). During the ball milling process, stop the ball mill every 5h and cool down to room temperature, take out a certain amount of powder (about 5g) for various characterization tests of the powder, until the nanocrystalline iron powder with a grain size of about 10nm is obtained.

(2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put 8g of nanocrystalline iron powder obtained in step (1) into a graphite sintering mold with a diameter of Φ10mm, and pre-press through positive and negative graphite electrodes Nanocrystalline iron powder to 50MPa, evacuated to 10 ^-3 Pa, under the condition of argon protection, spark plasma sintering is used to obtain ultrafine grain/micron crystal bulk iron material with ultrahigh plasticity and double scale distribution, among which the rapid sintering process The conditions are as follows:

Sintering equipment: Dr.Sintering SPS-825 spark plasma sintering system

Sintering current type: pulse current

Duty ratio of pulse current: 12:2

Sintering temperature Ts: 1253K

Sintering time: heat up to 373K in 4min, then heat up to 1233K in 9min (heating rate is 97K/min), then heat up to 1253K in 1min and hold for 10min;

Sintering pressure: 50MPa;

The powder is rapidly sintered, and the pressure is always kept at 50MPa during the sintering and cooling process to obtain ultra-fine grained/micro-crystalline bulk iron materials with a diameter of Φ10mm and ultra-high plasticity with dual-scale distribution. The scanning electron microscope picture shown in Figure 1 shows that the iron material (if the diameter of the sintering mold is large, the size of the iron material is also large). The scanning electron microscope picture shown in Figure 1 shows that its microstructure is based on bulk microcrystalline α-Fe (A in the figure) as the matrix, with ultrafine equiaxed α-Fe (B in the figure) and ultrafine grain Acicular α-Fe (C in the figure) is the reinforcing phase. Further TEM analysis showed that the grain size of bulk α-Fe is 2-3 μm, the grain size of equiaxed α-Fe is 700-900 nm, and the width of needle-like α-Fe is 150-160 nm. The room temperature compression true stress-strain curve shown in Figure 2 shows that the true fracture strength and fracture strain are 743MPa and 59%, respectively.

Example 2

A method for preparing an ultrafine-grained/micro-crystalline bulk iron material with ultrahigh plasticity and dual-scale distribution in this embodiment includes the following steps:

(1) Preparation of nanocrystalline iron powder by high-energy ball milling: Put pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of about 8-12nm is obtained:

The initial powder is high-purity electrolytic iron powder (99.5wt.%, particle size 38um). Put the initial iron powder and stainless steel balls together into a stainless steel ball mill jar (use an "O"-shaped sealing ring to seal between the ball mill jar and the lid. The diameters of the grinding balls are 15mm, 10mm and 6mm respectively, and their weight ratio is 1:3:1, and the weight ratio of grinding balls and powder is 10:1). In order to prevent oxidation, the ball mill tank is filled with high-purity argon for protection (99.99%, 0.5MPa). Finally, the ball mill tank filled with argon protection was placed on a QM-2SP20 planetary ball mill for high-energy ball milling (3.8s ^-1 speed). During the ball milling process, stop the ball mill every 5h and cool down to room temperature, take out a certain amount of powder (about 5g) for various characterization tests of the powder, until the nanocrystalline iron powder with a grain size of about 10nm is obtained.

(2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put 8g of nanocrystalline iron powder obtained in step (1) into a graphite sintering mold with a diameter of Φ10mm, and pre-press through positive and negative graphite electrodes Nanocrystalline iron powder to 40MPa, evacuated to 10 ^-3 Pa, under the condition of argon gas protection, using spark plasma sintering, to obtain ultra-high plasticity double-scale distribution of ultra-fine crystal / micro-crystalline bulk iron material, in which the rapid sintering process The conditions are as follows:

Sintering equipment: Dr.Sintering SPS-825 spark plasma sintering system

Sintering current type: pulse current

Duty ratio of pulse current: 12:2

Sintering temperature Ts: 1283K

Sintering time: 4min to 373K, then 9min to 1263K (heating rate 97K/min), then 1min to 1283K

Sintering pressure: 40MPa

Rapidly sinter the powder, and keep the pressure at 40MPa during the electrified sintering and cooling process to obtain ultra-fine grained/micro-crystalline bulk iron materials with a diameter of Φ10mm and ultra-high plasticity with dual-scale distribution. Microcrystalline α-Fe is the matrix phase, with ultrafine equiaxed α-Fe and ultrafine acicular α-Fe as the reinforcing phase, and the grain size of bulk α-Fe is 2-4 μm, equiaxed The grain size of the shape α-Fe is 500-700 nm, and the width of the needle-like α-Fe is 120-130 nm. The corresponding room temperature compression true stress-strain curves show that the room temperature compression fracture strength and fracture strain of the bulk iron material are 955 MPa and 58%, respectively.

Example 3

(1) Preparation of nanocrystalline iron powder by high-energy ball milling: Put pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of about 8-12nm is obtained:

The initial powder is high-purity electrolytic iron powder (99.5wt.%, particle size 38um). Put the initial iron powder and stainless steel balls together into a stainless steel ball mill jar (use an "O"-shaped sealing ring to seal between the ball mill jar and the lid. The diameters of the grinding balls are 15mm, 10mm and 6mm respectively, and their weight ratio is 1:3:1, and the weight ratio of grinding balls and powder is 10:1). In order to prevent oxidation, the ball mill tank is filled with high-purity argon for protection (99.99%, 0.5MPa). Finally, the ball mill tank filled with argon protection was placed on a QM-2SP20 planetary ball mill for high-energy ball milling (3.8s ^-1 speed). During the ball milling process, stop the ball mill every 5h and cool down to room temperature, take out a certain amount of powder (about 5g) for various characterization tests of the powder, until the nanocrystalline iron powder with a grain size of about 10nm is obtained.

(2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put 8g of nanocrystalline iron powder obtained in step (1) into a tungsten carbide sintering mold with a diameter of Φ10mm, and pass through positive and negative tungsten carbide electrodes First pre-press nanocrystalline iron powder to 200MPa, vacuumize to 10 ^-3 Pa, and use spark plasma sintering under the condition of argon protection to obtain ultra-fine grained/micro-crystalline bulk iron materials with ultra-high plasticity and dual-scale distribution. The rapid sintering process conditions are as follows:

Sintering equipment: Dr.Sintering SPS-825 spark plasma sintering system

Sintering current type: pulse current

Duty ratio of pulse current: 12:2

Sintering temperature Ts: 1253K

Sintering time: heat up to 373K in 4min, then heat up to 1233K in 16min (heating rate is 54K/min), then heat up to 1253K in 1min and hold for 5min

Sintering pressure: 200MPa

The powder is sintered rapidly, and the pressure is always kept at 200MPa during the sintering and cooling process, and the ultra-fine grain/micro-crystal bulk iron with a diameter of Φ10mm and ultra-high plasticity can be obtained, and its microstructure is in the form of a block Microcrystalline α-Fe is the matrix phase, with ultra-fine equiaxed α-Fe and ultrafine needle-like α-Fe as the reinforcing phase, among which the grain size of bulk α-Fe is 1-3 μm, equiaxed The grain size of α-Fe is 600-800 nm, and the width of acicular α-Fe is 140-150 nm. The corresponding room temperature compression true stress-strain curves show that the room temperature compression fracture strength and fracture strain of the block sample are 769MPa and 58%, respectively.

Example 4

The preparation method of the ultra-fine-grained/micro-crystalline bulk iron material with ultra-high plasticity and dual-scale distribution in this embodiment includes the following steps:

(1) Preparation of nanocrystalline iron powder by high-energy ball milling: Put pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of about 8-12nm is obtained:

The initial powder is high-purity electrolytic iron powder (99.5wt.%, particle size 38um). Put the initial iron powder and stainless steel balls together into a stainless steel ball mill jar (use an "O"-shaped sealing ring to seal between the ball mill jar and the lid. The diameters of the grinding balls are 15mm, 10mm and 6mm respectively, and their weight ratio is 1:3:1, and the weight ratio of grinding balls and powder is 10:1). In order to prevent oxidation, the ball mill tank is filled with high-purity argon for protection (99.99%, 0.5MPa). Finally, the ball mill tank filled with argon protection was placed on a QM-2SP20 planetary ball mill for high-energy ball milling (3.8s ^-1 speed). During the ball milling process, stop the ball mill every 5h and cool down to room temperature, take out a certain amount of powder (about 5g) for various characterization tests of the powder, until the nanocrystalline iron powder with a grain size of about 10nm is obtained.

(2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put 35g of nanocrystalline iron powder obtained in step (1) into a graphite sintering mold with a diameter of Φ20mm, and pre-press through positive and negative graphite electrodes Nanocrystalline iron powder to 500MPa, evacuated to 10 ^-3 Pa, under the condition of argon protection, adopt spark plasma sintering to obtain ultra-fine grain/micron grain bulk iron material with ultra-high plasticity and dual-scale distribution, among which the rapid sintering process The conditions are as follows:

Sintering equipment: Dr.Sintering SPS-825 spark plasma sintering system

Sintering current type: pulse current

Duty ratio of pulse current: 12:2

Sintering temperature Ts: 1335K

Sintering time: heat up to 373K in 4min, then heat up to 1315K in 4min (heating rate is 235K/min), then heat up to 1335K in 1min and hold for 10min

Sintering pressure: 500MPa

Rapidly sinter the powder, and keep the pressure at 500MPa during the electrified sintering and cooling process to obtain ultra-fine grained/micro-crystalline bulk iron with a diameter of Φ20mm and ultra-high plasticity with dual-scale distribution, and its microstructure is in the form of block Microcrystalline α-Fe is the matrix phase, and ultrafine equiaxed α-Fe and ultrafine needle-like α-Fe are the reinforcing phases. The grain size of bulk α-Fe is 3-5 μm, the grain size of equiaxed α-Fe is 400-600 nm, and the width of acicular α-Fe is 100-120 nm. The corresponding room temperature compression true stress-strain curves show that the room temperature fracture strength and fracture strain are 1025MPa and 60%, respectively.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the embodiment, and any other changes, modifications, substitutions and combinations made without departing from the spirit and principle of the present invention , simplification, all should be equivalent replacement methods, and are all included in the protection scope of the present invention.

Claims (4)

1. A method for preparing an ultrafine-grained/micro-crystalline bulk iron material with an ultrahigh plasticity double-scale distribution, characterized in that it comprises the following steps: (1) Preparation of nanocrystalline iron powder by high-energy ball milling: under argon protection, place pure iron powder in a stainless steel ball milling medium for high-energy ball milling until nanocrystalline iron powder with a grain size of 8-12nm is obtained; (2) Spark plasma sintering to prepare bulk iron materials with ultra-high plasticity and dual-scale distribution: put the nanocrystalline iron powder obtained in step (1) into a mold, and use spark plasma sintering under the protection of argon to obtain ultra-high plasticity. Ultrafine-grained/micro-crystalline bulk iron material with plastic dual-scale distribution, wherein the rapid sintering process conditions are as follows: Sintering equipment: spark plasma sintering system; Sintering current type: pulse current; Sintering temperature Ts: 1253K≤Ts≤1335K; Sintering time: 14~26min; Sintering pressure: 40~500MPa. 2. The method for preparing ultra-fine-grained/micro-crystalline bulk iron materials with ultra-high plasticity and dual-scale distribution according to claim 1, characterized in that, in the spark plasma sintering in step (2), the heating rate is 54~ 235K/min, the holding time is controlled at 0-10min. 3. The method for preparing ultra-fine-grained/micro-crystalline bulk iron materials with ultra-high plasticity and dual-scale distribution according to claim 1, characterized in that, in the spark plasma sintering described in step (2), when a graphite mold is used The sintering pressure is 40-50MPa, and the sintering pressure is 50-500MPa when a tungsten carbide mold is used. 4. The ultra-fine-grained/micro-crystalline bulk iron material with high plasticity and dual-scale distribution obtained by the preparation method described in any one of claims 1 to 3, is characterized in that its microstructure is based on massive micro-crystalline α-Fe phase, with ultra-fine equiaxed α-Fe and ultra-fine acicular α-Fe as the reinforcing phase.