JP6587174B2 - High toughness magnesium-based alloy extender and method for producing the same - Google Patents

Description

  The present invention relates to a magnesium (Mg) -based alloy stretch material excellent in fracture toughness and a method for producing the same.

  Mg alloys are attracting attention as next-generation lightweight metal materials. When used as a structural member, in order to ensure the safety and reliability of the material, it is desired to develop a material that is difficult to break, that is, exhibits high fracture toughness (hereinafter, toughness). However, the toughness of the Mg alloy does not show an excellent value as compared with other metal materials. Specifically, the toughness of the Mg alloy is about half that of the Al alloy. The low toughness of Mg is due to its crystal structure (hexagonal crystal). Near room temperature, the difference in critical shear stress: CRSS between the bottom surface and the non-bottom surface (for example, the column surface) is large, and the slip system is poor. Therefore, in order to continue the plastic deformation, it is necessary to form a deformation twin. However, as in the toughness test, when a minute crack exists, the interface between the deformed twin and the parent phase serves as a crack propagation path, and the presence of the twin is known to promote embrittlement.

  In order to solve these problems, a technique for suppressing the formation of deformation twins, that is, a microstructure control method is often used. For example, in Non-Patent Documents 1 and 2, the formation of deformation twins is suppressed and the toughness is improved by reducing the crystal grain size. It has also been pointed out that plastic deformation is activated and non-bottom slip is activated in the vicinity of the grain boundary of Mg (Non-Patent Document 3). In particular, it has been reported that when the crystal grain size is 10 μm or less, the region where plastic deformation is activated acts not only in the vicinity of the crystal grain boundary but also in the entire region within the crystal grain. Therefore, an Mg alloy having a fine grain size does not require a deformation twin that works to complement plastic deformation, and the frequency of deformation twin formation is drastically reduced. On the other hand, when looking at the application of Mg alloy to large-scale members, creating a large-sized material having a uniform fine structure with an average crystal grain size of 10 μm or less and no distribution (variation) in crystal grain size is an issue. It is left.

  Contrary to the above method, Patent Document 1 discloses an Mg alloy that utilizes deformation twinning and is excellent in mechanical properties (low cycle fatigue properties). In fatigue tests in which tensile and compressive stresses are repeatedly applied, the yield anisotropy of tensile and compressive stresses is reduced by exhibiting Detwinning behavior for deformation twins introduced by pre-strain. In addition, fatigue characteristics are improved. However, in a toughness test that does not involve repeated stress application, there is no occurrence of Dewinning behavior in which deformation twins disappear. Therefore, it is clear that the deformation twins that become the crack propagation path remain in the material and the toughness is low. Further, in the pre-strain introduction method disclosed in Patent Document 1, only the deformation rate is set to 1 to 15%, and the ratio of the deformation twin interface existing in the base material is unknown. Further, after pre-strain introduction, no heat treatment has been performed for the purpose of segregating solute elements at the twin interface. Therefore, the deformation twin interface is energetically unstable, becomes a crack propagation path, and the effect of improving toughness cannot be expected.

Special table 2012-533681 gazette

H. Somekawa et al., Scripta Mater, 53 (2005) p1059. H. Somekawa et al., J. Mater. Res, 22 (2007) p2592. J. Koike et al., Acta Mater, 51 (2003) p2055.

  In the present invention, an Mg-X-based binary alloy extending material having an average grain size of the Mg matrix of 5 μm or more and 500 μm or less is applied to the entire interface length by 0.001 or more, 0. An object of the present invention is to provide an Mg-based alloy extension material having superior toughness compared to a pure Mg extension material by introducing a deformation twin having an interface length of 5 or less and then performing a heat treatment. .

A first aspect of the present invention is an Mg—X base binary alloy extender containing X as an additive element, wherein the components of the Mg—X base binary alloy extender are solid with respect to Mg and Mg. Additive element with solubility limit: from X and inevitable ingredients,
The additive element is any one of Al, Ag, Ca, Mn, Sn, Zn, Gd, and Y,
The addition amount of the additive element is 0.003 mol% or more and 80% or less of the maximum solid solution amount, and the maximum solid solution amount of each solute element with respect to Mg is Al: 11.5 mol%, Ag: 3.8 mol% Ca: 0.8 mol%, Mn: 1.0 mol%, Sn: 3.3 mol%, Zn: 2.4 mol%, Gd: 4.5 mol%, Y: 3.4 mol%,
An average crystal grain size of the stretch material is 5 μm or more and 500 μm or less, and deformation twins having a deformation twin interface length of 0.001 or more and 0.5 or less with respect to the total interface length exist in the crystal grains. and it is,
An ASTM-based toughness value J has an excellent toughness value J as compared with 175 [J / cm ² ] of a pure Mg extension material.

2nd of this invention is a manufacturing method of the Mg-X base binary system twin introduction | transduction alloy extension material of invention 1, Comprising: After casting and solution treatment, in the temperature range of 50 degreeC or more and 500 degrees C or less, After an extension process for imparting a strain of 0.01 or more and 4 or less, in addition, in a temperature range of −50 ° C. or more and 300 ° C. or less, 0.001 or more, and imparting a strain of 0.3 or less, and 100 ° C. or more, Provided is a method for producing a Mg-X-based binary twinned alloy-stretched material subjected to twinning introduction processing combined with heat treatment at 500 ° C. or lower.

  When using an Mg alloy as a structural member, a material that is difficult to break, that is, exhibits high toughness, is obtained in order to ensure the safety and reliability of the material.

The schematic diagram regarding a grain boundary and a deformation | transformation twin interface. Manufacturing process diagram until twinning process. The photograph which observed the fine structure of the Mg-Ag heat processing material with the optical microscope. The photograph which observed the fine structure of Mg-Ag twin material by the electron beam backscattering diffraction method. A load-displacement curve obtained by a three-point bending test for obtaining toughness.

A grain boundary, a deformation twin, and a deformation twin interface will be described with reference to the schematic diagram shown in FIG. A crystal grain boundary surrounds one crystal grain (referred to as G in FIG. 1), and is roughly classified into a large-angle grain boundary and a small-angle grain boundary depending on the difference in crystal orientation difference angle between adjacent crystal grains. That is, a crystal grain boundary having a crystal orientation difference angle of 15 ° or more is defined as a large-angle grain boundary, and a crystal grain boundary having a crystal orientation difference angle of less than 15 ° is defined as a small-angle grain boundary. On the other hand, deformation twins are often observed in the deformation structure of Mg alloys.

It is a deformed twin, formed in a crystal grain (FIG. 1: expressed as DT), the deformed twin interface has a crystal orientation difference angle of 86 °, and the crystal orientation between the parent phase and the deformed twin is a mirror image Indicates. Further, the deformation twin interface (referred to as T in FIG. 1) is defined as the boundary between the deformation twin and the Mg matrix.

The “ratio of the deformation twin interface length to the total interface length: F” for obtaining the effect of the present invention is the sum of the grain boundary lengths composed of the large-angle grain boundaries and the small-angle grain boundaries, and the twin-interface length. Of the total interface length expressed as the sum of

When defined as, it is preferable that the value of F is 0.001 or more and 0.50 or less, more preferably 0.01 or more and 0.40 or less, and 0.05 or more and 0.30 or less. More preferably it is.

  In order to introduce deformation twins into crystal grains, it is necessary to impart strain to the material. The value of F in Equation 1 depends on the amount of strain applied, and it is necessary to apply a large strain in order to introduce deformation twins at high density in the crystal grains. However, the deformation is localized as the applied strain increases. Therefore, when the value of F exceeds 0.50, cracks and cracks are likely to occur, and it is difficult to create a sound material. On the other hand, when the value of F is less than 0.001, it is not possible to expect a value larger than the toughness of pure Mg because the content of deformation twins is very small. Note that the lengths of the crystal grain boundaries and the deformation twin interface may be measured using an optical microscope, an electron beam backscatter diffraction method (EBSD), or the like. Further, the value of F may be obtained by a point arithmetic method in which a prescribed lattice line is drawn and the number of crossings with the grain boundary / twin interface is counted.

  In the examples, in order to evaluate the influence of the solute element on the toughness, an example of F = 0.2 is shown. However, if the value of F is in the range of 0.001 or more and 0.50 or less, the effect of the invention is the same.

  The crystal grain size of the Mg matrix is preferably 5 μm or more and 500 μm or less, more preferably 10 μm or more and 100 μm or less, and further preferably 15 μm or more and 50 μm or less. When the crystal grain size exceeds 500 μm, it is difficult to maintain the strength (high strength) of the material and it is impossible to use it as a member. When the crystal grain size in the Mg matrix is finer than 5 μm, plastic deformation is activated at the crystal grain boundary and non-bottom dislocation movement easily occurs, so that it is difficult to introduce a deformed twin into the crystal grain.

  A manufacturing process for obtaining the microstructure is shown in FIG. 2 and described below. After melting, after solution treatment to eliminate casting segregation and dendrite structure, strain is applied in a temperature range of 50 ° C. or more and 500 ° C. or less to produce an extended Mg alloy (hereinafter referred to as extension processing). ). The purpose of the extension process is (1) to control the shape of the material to plate, rod, and lump, and (2) to set the Mg matrix phase size (crystal grain size) to 5 μm or more and 500 μm or less. is there. The processing temperature during the extension processing is preferably 50 ° C. or more and 500 ° C. or less, more preferably 75 ° C. or more and 450 ° C. or less, and further preferably 100 ° C. or more and 400 ° C. or less. When it is less than 50 ° C., cracks and cracks occur during processing, and a sound material cannot be created. When the temperature exceeds 500 ° C., the crystal grains become coarse, so that a predetermined crystal grain size cannot be obtained. Of course, if the Mg alloy subjected to solution treatment after melting is within a predetermined crystal grain size, there is no need to perform extension processing. Moreover, the strain imparting method may be any method as long as it is a plastic working method capable of imparting the strain mentioned in rolling, extrusion, and forging. The strain to be applied is preferably 0.01 or more and 4 or less, more preferably 0.05 or more and 3.5 or less, and further preferably 0.1 or more and 3.0 or less. When a strain exceeding 4 is applied, dislocations introduced into the Mg matrix by the extension process are saturated. For this reason, the effect of crystal grain size refinement cannot be obtained.

  In addition, although it is the heat processing after the extending | stretching process of FIG. 2, if the crystal grain size of the extended Mg alloy produced by the extending process is 5 micrometers or more and 500 micrometers or less, it is not necessary to implement heat processing. However, as shown in this embodiment, when it is desired to control to a predetermined crystal grain size (20 μm in the embodiment), this step may be performed.

  Next, strain is applied to the extended Mg alloy produced by the extension processing in a temperature range of −50 ° C. or higher and 300 ° C. or lower (hereinafter referred to as twinning introduction processing). The purpose of the twinning process is to introduce deformation twins into the crystal grains. The applied strain is preferably 0.001 or more and 0.30 or less, more preferably 0.005 or more and 0.25 or less, and still more preferably 0.01 or more and 0.20 or less. If it is larger than 0.30, a large amount of deformation twins are introduced into the crystal grains, so that the deformation is localized and a sound material cannot be created. The temperature during twinning is preferably −50 ° C. or higher and 300 ° C. or lower, more preferably −25 ° C. or higher and 200 ° C. or lower, and further preferably 0 ° C. or higher and 100 ° C. or lower. When the temperature exceeds 300 ° C., the difference in CRSS between the bottom surface and the non-bottom dislocation is reduced, and the non-bottom dislocation motion is easily activated. Therefore, it is difficult to introduce deformation twins. On the other hand, when the temperature is lower than −50 ° C., the work is dangerous because it is extremely low temperature, which is unsuitable for practical use. As a method used for twinning introduction processing, any method can be adopted as long as it is a plastic working method capable of imparting compressive strain represented by extrusion, rolling, and forging.

  After the twinning process, heat treatment is performed to segregate solute elements at the deformation twin interface and stabilize the energy of the deformation twin interface. The heat treatment temperature is preferably 100 ° C. or higher and 500 ° C. or lower, more preferably 125 ° C. or higher and 450 ° C. or lower, and further preferably 150 ° C. or higher and 400 ° C. or lower. When the heat treatment temperature is less than 100 ° C., the diffusion rate of the solute atoms becomes slow, so the time required for the solute elements to segregate at the twin interface becomes long, which is not desirable from an industrial viewpoint. When the heat treatment temperature exceeds 500 ° C., the crystal grains become coarse, so that a predetermined crystal grain size cannot be obtained. If heat treatment is not performed after twinning processing, solute elements do not segregate at the deformation twin interface. Therefore, the deformed twin interface is energetically unstable, the deformed twin interface becomes a crack propagation path, and cannot show a value larger than the toughness of pure Mg, which is the effect of the present invention.

  The additive elements and addition amounts for obtaining the effects of the present invention will be described. The additive element is a solute element that dissolves in Mg in the range of 0.1 mol% or more and 10 mol% or less, and any one of Al, Ag, Ca, Mn, Sn, Zn, Gd, and Y can be selected. . The addition amount of each element is preferably 0.003 mol% or more and 80% or less of the maximum solid solution amount, more preferably 0.005 mol% or more and 75% or less of the maximum solid solution amount, 0.01 mol% or more, and the maximum solid solution amount. 70% or less of the dissolved amount is more preferable. When the addition amount is less than 0.003 mol%, the additive element does not affect plastic deformation and mechanical properties and is recognized as an impurity element. On the other hand, when it exceeds 80% of the maximum solid solution amount, the additive element is precipitated during the extension process, and an intermetallic compound is formed in the Mg matrix. Since these intermetallic compounds serve as stress concentration sites at the time of a toughness test, that is, the starting point of fracture, the toughness is lowered and the effects of the present invention cannot be obtained. In addition, although the example shows an example in which the addition amount is 0.3 mol%, of course, as long as the addition amount is within a range of 0.003 mol% or more and 80% or less of the maximum solid solution amount, than pure Mg High toughness can be obtained. Moreover, the maximum solid solution amount with respect to Mg of each solute element is Al: 11.5 mol%, Ag: 3.8 mol%, Ca: 0.8 mol%, Mn: 1.0 mol%, Sn: 3.3 mol%, Zn : 2.4 mol%, Gd: 4.5 mol%, Y: 3.4 mol%.

  Commercially available pure Mg (99.98 mass%) and additive elements were weighed so that the addition amount was 0.3 mol%, and various binary Mg alloys were melted by gravity casting. The melting temperature was 700 ° C., the dissolution holding time was 5 minutes, and casting was performed using an iron mold having a diameter of 50 mm and a height of 200 mm. After the cast material was subjected to a solution treatment at 500 ° C. for 2 hours, the element concentrations of the additive element and the inevitable component were analyzed and evaluated by ICP emission spectroscopic analysis. The results of the analysis are shown in Table 1.

  In order to perform the extension processing, the cast material after the solution treatment was processed into a cylindrical extruded billet having a diameter of 40 mm and a length of 60 mm by machining. The billet after processing is held in a container set at 175 to 370 ° C. for 30 minutes, and then extrusion is performed at an extrusion ratio of 19: 1 to extrude a plate having a plate thickness of 5 mm, a plate width of 10 mm, and a length of 500 mm or more. A material was prepared. Thereafter, heat treatment was performed to make the average crystal grain size about 20 μm. The heat treatment temperature and time (hereinafter referred to as heat treatment material) are as shown in Table 2.

  The microstructure of the binary Mg alloy was observed using an optical microscope and a scanning electron microscope / electron beam backscatter diffraction apparatus. FIG. 3 shows a typical microstructure example observed. In FIG. 3, the region surrounded by the black line is one crystal grain (Mg parent phase), and the average crystal grain size of the heat-treated Mg—Ag alloy obtained by the intercept method was 21.1 μm. The average crystal grain size of each binary Mg alloy heat treated material is summarized in Table 2. It can be seen that an average grain size of approximately 20 μm is exhibited regardless of the additive element.

  Next, a method for producing an Mg alloy in which a deformation twin is contained in the parent phase will be described. In order to introduce deformation twins, twinning processing was performed on the heat-treated material at room temperature. The strain was applied by a compression test using an Instron universal test. The strain applying direction was parallel to the extrusion direction. However, in order to control the deformation twin interface length, the compressive strain was set to 0.02 to 0.03. Thereafter, heat treatment was performed for 2 hours in a muffle furnace set at 150 ° C. in order to segregate the solute element at the twin interface. (Hereafter referred to as twin material)

The microstructure of the twin material was observed using an optical microscope and a scanning electron microscope / electron beam backscatter diffractometer. FIG. 4 shows a typical microstructure example observed. Unlike FIG. 3, in FIG. 4, the presence of a lens-like microstructure can be confirmed in the matrix as indicated by arrows. From analysis using electron backscatter diffraction (EBSD),

It was found to be a deformation twin. Further, F = 0.22 was obtained from Equation 1 using the grain boundary and the deformation twin interface length obtained by EBSD. Ratio of twin interface length to the total interface of each binary Mg alloy twin: F is as summarized in Table 2, and was about 0.20 for each binary Mg alloy.

  The toughness evaluation method based on ASTM and the result are described. A three-point bending specimen having a thickness of 3.5 mm, a width of 7 mm, and a length of 35 mm was collected from the twin material. All specimens were taken from a direction parallel to the extrusion direction, and the V-notch was cut perpendicular to the extrusion direction. In order to reduce the effect of crack tip angle, a crack was introduced using a fatigue testing machine. Thereafter, a room temperature three-point bending test was performed at an indentation speed of 0.5 mm / min. FIG. 5 shows a typical load-displacement curve obtained by a three-point bending test for obtaining toughness. These curves vary greatly depending on the additive element. For example, the Mg—Ag alloy shows a large maximum load point and indentation displacement, but the maximum load point and indentation displacement of the Mg—Pb alloy are the smallest in FIG. In the figure, a portion indicated by a broken line portion corresponds to absorbed energy: S, which is closely related to toughness. That is, the larger the value of S, the higher the toughness. The toughness: J can be determined from the linear fracture mechanics based on ASTM using the following equation.

Here, B is the plate thickness (= 3.5 mm), W is the plate width (= 7 mm), and a ₀ is the crack length. Table 2 summarizes the toughness obtained by Equation 2 above. Similar to the load-displacement curve of FIG. 5, the toughness of the twin material varies greatly depending on the type of additive element. Since the binary Mg alloy used in the examples had the same crystal grain size, F value, and added amount of solute element, the difference in toughness is considered to be due to the type of additive element. That is, it can be seen that Ag, Mn, Zn, Y, and Gd are effective additive elements for improving toughness.

[Comparative example]
As a comparative example, a commercially available pure Mg (99.98 mass%) was used, a twinning material was produced in the same procedure as the example, and a room temperature three-point bending test was performed. Test conditions and test piece shapes are the same as described above. The toughness obtained by Equation 2 is as shown in Table 2. When compared with the toughness of the binary Mg alloy twin material, the toughness of the binary Mg alloy twin material to which Li or Pb is added is smaller than J = 175 J / cm ² which is the toughness of the pure Mg twin material. These additive elements are poor in improving toughness, and the effects of the present invention cannot be obtained. On the other hand, the toughness of binary Mg alloy twins added with solute elements of Al, Ag, Ca, Mn, Zn, Y, Gd is higher than J = 175 J / cm ^2, which is the toughness of pure Mg twins. It shows a high value and it can be seen that the addition of these solute elements is extremely effective in improving toughness.

  The Mg alloy of the present invention is not only excellent in toughness but also exhibits a large absorbed energy, so that it can be applied as an impact absorbing material and a structural material including automobiles. In addition, since the twin interface contributes to strength improvement, adaptation as a biomedical implant material that requires strength and toughness is also conceivable.

G Crystal grains (or Mg matrix) surrounded by large-angle or small-angle grain boundaries
S absorbed energy (toughness)
T Deformation twin interface DT Deformation twin

Claims (5)

A Mg-X based binary alloy extension material containing X as an additive element,
The component of the Mg-X-based binary alloy extender is composed of Mg and an additive element having a solid solubility limit with respect to Mg: X and an inevitable component,
The additive element is any one of Al, Ag, Ca, Mn, Sn, Zn, Gd, and Y,
The addition amount of the additive element is 0.003 mol% or more and 80% or less of the maximum solid solution amount, and the maximum solid solution amount of each solute element with respect to Mg is Al: 11.5 mol%, Ag: 3.8 mol% Ca: 0.8 mol%, Mn: 1.0 mol%, Sn: 3.3 mol%, Zn: 2.4 mol%, Gd: 4.5 mol%, Y: 3.4 mol%,
The stretch material has an average crystal grain size of 5 μm or more and 500 μm or less,
A deformation twin having a deformation twin interface length of 0.001 or more and 0.5 or less with respect to the total interface length exists in the crystal grains ,
An Mg-X-based twin-introduced alloy-stretched material having a toughness value J based on ASTM having a toughness value J superior to 175 [J / cm ² ] of pure Mg-stretched material. It is a manufacturing method of the Mg-X base binary system twin introduction alloy extension material according to claim 1,
After the casting and solution treatment, in the temperature range of 50 ° C. or more and 500 ° C. or less, after the extension processing that imparts strain of 0.01 or more and 4 or less,
In the temperature range of −50 ° C. or more and 300 ° C. or less, a twinning process that combines a strain application of 0.001 or more and 0.3 or less and a heat treatment of 100 ° C. or more and 500 ° C. or less is performed. A method for producing an Mg-X based binary twin introduced alloy extender.   An impact absorbing material or a structural material using the Mg-X-based binary twin introduced alloy extending material according to claim 1.   An automobile using the Mg-X-based binary twin introduced alloy extending material according to claim 1.   A biomedical implant material using the Mg-X-based binary twin introduced alloy extension material according to claim 1.