Study on notch tensile properties of Fe-Mn-Si shape memory alloy

In order to investigate the effect of stress concentration on the mechanical properties of Fe-Mn-Si shape memory alloy, Fe17Mn5Si10Cr5Ni shape memory alloy was selected as the research object in this study. Specimens with different notch types and sizes were designed, and uniaxial tensile tests were conducted on the notched specimens at room temperature. The effects of notch parameters on stress–strain curve, the notch tensile strength, notch sensitivity, and fracture mode were studied. The results indicate that Fe-Mn-Si shape memory alloy is sensitive to notches. V-shaped notch specimens with the same notch radius exhibit stronger ductility and larger fracture strain than U-shaped notch specimens. The tensile strength of the notch increases with the decrease of the notch radius. The stress concentration caused by the notch limits the plastic deformation ability of the material, resulting in significantly lower ductility of the notched specimen compared to the smooth specimen. The presence of notches does not change the tensile fracture type of the alloy, which is ductile fracture. Compared with Q235 steel, Fe-Mn-Si shape memory alloy has better ductility. The stress–strain curve of Q235 steel shows a clear yield plateau and slight hardening behavior, while the yield plateau of Fe-Mn-Si shape memory alloy is not obvious.


Introduction
Fe-Mn-Si shape memory alloy (SMA) has relatively low cost [1], much higher fatigue life than ordinary materials, excellent low cycle fatigue performance at high strain levels [2], high damping characteristics [3], lower yield strength, and shape memory effect [4,5].These excellent properties provide broad application prospects for Fe-Mn-Si SMA.In recent years, Fe-Mn-Si SMA has been widely used in the fields of mechanical and civil construction, such as connectors, special supports [6,7], prestressed steel reinforcement structures, and shock absorption devices [8,9].
The reason why Fe-Mn-Si SMA initially attracted the attention of researchers was its shape memory effect.Researchers have conducted extensive research on Fe-Mn-Si SMAs, improving their shape memory effects by adding trace elements such as C, N, V, Nb [10][11][12] and using thermal mechanical training [13][14][15][16][17] methods.However, there are not many research results that can truly be applied to practical engineering [18,19].In recent years, with the application of Fe-Mn-Si SMA in the field of civil engineering and construction, researchers have found that it has excellent low cycle fatigue performance [20].The study of the mechanical properties of Fe-Mn-Si SMA has become a new hot direction.
Metal materials often cause defects or damage during processing, such as manufacturing defects, microcracks, and notches.In addition, geometric discontinuities such as notches are inevitably present in the manufacturing and use of structural components [21].These defects can induce stress concentration and triaxial stress state [22,23], thereby affecting the mechanical properties of the material.Such structural components are prone to damage and failure in practical applications.Researchers have been committed to improving the design and details of key components to reduce the risk of component failure.However, some components are difficult to improve the overall performance of the structure through design and optimization of geometric shapes.In order to prevent component damage, it is necessary to study the failure mechanism of materials with defects.U-shaped and V-shaped notches are common in engineering applications [24].When dealing with notch problems in engineering practice, it is usually to make relevant assumptions and simplify the treatment of notches of different sizes and shapes.This assumption and simplification will lead to significant differences between the results and the actual situation of the notch, and it is necessary for us to conduct more in-depth research on different types and sizes of notches.In addition, notched specimens are also an effective tool for evaluating the influence of stress concentration on the mechanical properties of materials [25].Therefore, studying the mechanical properties of Fe-Mn-Si SMA notched specimens is of great significance for material design and safety assessment.
In order to investigate the effect of notches on the mechanical properties of Fe-Mn-Si SMA, specimens with different types and sizes of notches were designed and subjected to uniaxial tensile tests at room temperature.The effects of notch parameters on stress-strain curve, notch tensile strength, notch sensitivity, and fracture type were analyzed.

Numerical simulation of interference connection
The alloy material used in the experiment is Fe17Mn5Si10Cr5Ni shape memory alloy, which is made by smelting industrial pure iron, manganese, silicon, chromium, and nickel in a vacuum induction furnace according to the elemental composition ratio.During smelting, wait for the raw materials to melt, keep them warm for 30 min to ensure even composition, and then proceed with metal casting.After annealing at 1200 °C for 24 h, remove the cap and outer skin of the ingot, and forge it at 1200 °C to produce Ф 80 mm bar material.The material composition is shown in table 1.In order to investigate the influence of notch type and notch size on the tensile properties of Fe-Mn-Si SMA, different specimens were designed in this study, as shown in figure 1.The notch parameters of all notched specimens are shown in table 2. Fe-Mn-Si SMAs were processed into specimens with different types and sizes of notches and subjected to uniaxial tensile testing at room temperature.The specimens were divided into smooth specimens and notched specimens.The stress-strain curves of different specimens were obtained through  uniaxial tensile tests, and the failure process of the specimens were observed.Smooth specimens are used to obtain the tensile performance parameters of materials, while notched specimens are used to study the influence of stress concentration effects of different notches on the tensile properties of materials.The specimens were processed by wire cutting.In order to eliminate the influence of processing on the surface composition and stress state of the specimens, the specimens were subjected to a solid solution treatment at 1050 °C for 1 h in a vacuum resistance furnace.The tensile tests were conducted on the CRIMS SDS-100 electro-hydraulic servo dynamic and static testing machine, with displacement loading and a loading speed set at 0.05 mm s −1 .The tests were conducted at room temperature.In addition, in order to compare the mechanical properties of Fe-Mn-Si SMA with commonly used building structural steel, Q235 steel was used as the comparative material, and the two materials were processed into V-shaped notch specimens of the same size and different angles for analysis.After the tests were completed, the fracture morphology of the specimens were analyzed using field emission scanning electron microscopy.

Analysis of stress-strain curve
By conducting uniaxial tensile tests on smooth and notched specimens, stress-strain curves of different specimens were obtained, as shown in figure 2. Figure 2 shows the stress-strain curves of V-shaped notch specimens with different notch radii when the opening angle is 60°.The Fe-Mn-Si SMA notched specimens all fractured at the notch, and the necking phenomenon were not significant.At the beginning stage of the tensile test, the parent phase (austenite phase) inside the material undergoes normal elastic deformation, and the stressstrain curve of materials is mainly linear.As the tensile test continues, the stress-strain relationship exhibits a smooth upward curve with a clear non-linear relationship.There is no obvious yield plateau in the stress-strain relationship after the stress level exceeds the proportional limit, because the parent phase inside the material undergoes stress induced martensitic transformation under the action of stress.The third stage is plastic deformation until fracture.The monotonic tensile stress-strain curve generally exhibits a three-stage characteristic.The stress-strain curves of the notched specimen and the smooth specimen exhibit the same variation pattern, which is consistent with the stress-strain curve characteristics of Fe-SMA [26].Meanwhile, there are also some differences between the two.The notched specimen forms stress concentration at the notch, resulting in significantly higher stress at the notch than the smooth part.The notch can induce stress induced martensitic transformation earlier, allowing the specimen to enter the yield stage earlier.During the tensile process of smooth specimens, the combination of plastic deformation and deformation strengthening of the material causes the internal stress of the material to propagate and redistribute, exhibiting a relatively uniform tensile curve.However, the presence of notches limits the plastic deformation ability of the material at the root of the notch, and plastic deformation cannot be fully carried out, resulting in a rapid increase in internal stress in the material.The presence of notches leads to tensile fracture of the specimen within a smaller strain range.The Table 2. Notch parameters of the specimen.

Notch type
presence of notches reduces the mechanical properties of materials through stress concentration effects.The smooth specimen exhibits a larger nonlinear region, and the presence of notches limits the plastic deformation of the specimen, resulting in a smaller nonlinear region of the notched specimen.The increase in notch size significantly reduces the integrity of the material.Compared with smooth specimens, the slope of stress-strain curves of specimens with different notch radii has increased, especially in the nonlinear stage.There is a significant difference in fracture strain, with the maximum fracture strain occurring at the maximum notch radius.The greater the fracture strain, the better the ductility of the specimen.When the notch radius is 0.5 mm, the influence of the opening angle of the V-shaped notch specimen on the stress-strain curve is shown in figure 3.As shown in the figure, the stress-strain curves of notched specimens with different opening angles show a consistent trend.
When the notch size is the same, the stress-strain curves of specimens with different notch types are shown in figure 4. As shown in the figure, when the notch radius is 0.5 mm, there is no significant difference in ductility between the two specimens.The V-shaped notch specimen exhibits stronger ductility than the U-shaped notch specimen when the notch radius is 1.0 and 1.5 mm.The fracture strain of V-shaped notched specimens with the same gauge length is greater than that of U-shaped notches.Due to the presence of notches, the plastic deformation ability of the material at the root of the notch is limited, and plastic deformation cannot be fully carried out.The internal stress of the material increases rapidly, and fracture occurs earlier than smooth  specimens.Compared with V-shaped notches, U-shaped notches with the same radius have greater limitations on the plastic deformation ability of the material at the root of the notch, resulting in earlier fracture.
In order to compare the mechanical properties of Fe-Mn-Si SMA with commonly used building structural steel, Q235 steel was used as the comparative material, and the two materials were processed into V-shaped notch specimens of the same size and different angles for analysis.Figure 5 shows the stress-strain curves of Q235 steel and Fe-Mn-Si SMA with different opening angles.The fracture strain of Fe-Mn-Si SMA specimens at various opening angles is greater than that of Q235 steel.Compared with Q235 steel, Fe-Mn-Si SMA has better ductility.The stress-strain curves of the notched specimens of both materials exhibit linear behavior during the initial loading stage, and as loading continues, the stress-strain curves exhibit nonlinear growth.The stressstrain curve of Q235 steel shows a clear yield plateau and slight hardening behavior, while the yield plateau of Fe-Mn-Si SMA is not obvious.

Analysis of notch tensile performance
The fracture location of the Fe-Mn-Si SMA notched tensile specimen is shown in figure 6.From figure 6, it can be seen that all the notched specimens fractured at the notch, and there was no obvious necking near the fracture surface of the specimens, and the deformation of the gauge length segment was uniform.The reasons are as follows: (1) The notch is the minimum net cross-sectional position of the specimen.During the tensile process, the load continuously increases, and the damage to the minimum net cross-sectional area of the specimen accumulates, ultimately penetrating the entire section, causing the specimen to fracture; (2) Due to the stress concentration effect, the stress on the notch during tension is much higher than that on the parallel section; (3) The notched specimen is equivalent to artificially specifying a fracture position on the specimen, masking the random generation and propagation of cracks during the tensile process of the specimen.The definition of notch tensile strength is: where F max is the maximum force borne by the notched specimen before fracture, A min is is the minimum original cross-sectional area at the notch.
Notch sensitivity is usually quantified using notch sensitivity ratio (NSR), which is the ratio of the tensile strength of a notched specimen to the tensile strength of a smooth specimen.The definition of notch sensitivity ratio is: where σ bH is the tensile strength of the notched specimen, σ b is the tensile strength of the smooth specimen.When the NSR is greater than 1, it indicates the expansion of plastic deformation at the notch.The larger the ratio, the greater the expansion of plastic deformation and the smaller the tendency for embrittlement.It is considered that the material is less sensitive or insensitive to the notch.When the NSR is less than 1, it indicates early fracture at the notch before significant plastic deformation propagation occurs, it is considered that the material is sensitive to the notch.The smaller the NSR, the greater the sensitivity of the notch.
The tensile strength and notch sensitivity of Fe-Mn-Si SMA notched specimens are shown in table 3. The NSR of all V-shaped notch specimens is less than 1, indicating that Fe-Mn-Si SMA is sensitive to notches.At the same opening angle (60°), the tensile strength of the V-shaped notch specimen decreases with the increase of notch radius.From a notch radius of 0.25 mm to a notch radius of 1.5 mm, its tensile strength decreases by 18.9%, as shown in figure 7(a).At the same opening angle (60°), NSR decreases with increasing notch radius, indicating that the sensitivity of the material to notches increases with increasing radius, as shown in figure 7(b).
The tensile strength and notch sensitivity of Fe-Mn-Si SMA U-shaped notch specimens are shown in figure 8.The NSR of all U-shaped notch specimens is less than 1, indicating that Fe-Mn-Si SMA is sensitive to notches.As shown in figure 8(a), with the increase of the notch radius, the tensile strength of the U-shaped notch specimen gradually decreases, decreasing by 6.9% from a notch radius of 0.5 mm to a notch radius of 2.0.As shown in figure 8(b), NSR decreases with the increase of notch radius, indicating that the sensitivity of the material to notches increases with the increase of radius.The NSR of both V-shaped and U-shaped notches is less than 1, indicating that Fe-Mn-Si SMA is sensitive to notches according to the NSR criterion.The tensile strength and NSR of the two types of notched specimens decrease with the increase of notch radius, indicating that the sensitivity of Fe-Mn-Si SMA to notches increases with the increase of radius.
The NSR of Fe-Mn-Si SMA and Q235 steel are shown in figure 9. From figure 9, it can be observed that the NSR of Q235 steel is significantly higher than that of Fe-Mn-Si SMA, indicating that Fe-Mn-Si SMA is more sensitive to notches than Q235 steel.The notched specimen forms stress concentration at the notch, resulting in significantly higher stress at the notch than the smooth part.The Fe-Mn-Si SMA notched specimen undergoes early stress induced martensitic transformation at the notch due to stress concentration, which affects the mechanical properties of the specimen.

Analysis of fracture morphology
Figure 10 shows the tensile fracture morphology of Fe-Mn-Si SMA smooth specimens.From figure 10(a), it can be seen that the tensile fracture surface of Fe-Mn-Si SMA exhibits typical plastic fracture characteristics.The overall height of the cross-section is uneven, with obvious shear lips.The crack originates from the edge and propagates radially until the specimen fractures.From figure 10(b), it can be seen that a large number of ductile  dimples are distributed on the fracture surface of the sample, indicating that the tensile fracture mode of Fe-Mn-Si SMA is ductile fracture.
The macroscopic morphology of the tensile fracture surface of Fe-Mn-Si SMA notched specimens is shown in figures 11 and 12. From the figure, it can be seen that the fracture morphology of the notched specimens is relatively rough, with uneven and undulating fracture surfaces.Shear lip structures were observed at the circumferential edges of some specimens.Compared with smooth specimens, notched specimens form stress concentration at the notch, resulting in cracks mainly originating at the root of the  notch.The cracking of the notched specimen originates from the bottom of the notch, where cracks initiate from the surrounding areas and then propagate towards the center of the specimen.From the fracture morphology of the notched specimen, it can be seen that the tensile fracture of the notched specimen is more prone to the initiation of secondary microcracks, which are related to the three-dimensional stress state of the notched specimen.
Figures 13 and 14 show the microstructure of the tensile fracture surfaces of two types of notches in Fe-Mn-Si SMA specimens.From the figure, it can be seen that there are a large number of ductile dimples distributed on the cross-section.The V-shaped and U-shaped notched specimens exhibit the same fracture mode, both of which are ductile fractures.A large number of micropores can be observed on the fracture surface of the notched specimen, which are small and shallow in depth.This is because in the early stage of stretching, the average stress of the specimen is relatively small, which is not conducive to nucleation and coalescence.With further increase in load and the presence of notches, the three-dimensional stress provides favorable conditions for the growth of micropores and the formation of tough dimples, causing the specimen to undergo plastic deformation other than martensitic transformation, ultimately leading to fracture.Therefore, there are two characteristics on the fracture surface of the specimen, namely the coexistence of micropores and dimples.As shown in figure 13, there are significant differences in the number and size of ductile dimples on the fracture surface with different notch radii.The specimen with a notch radius of 0.25 mm has the highest number of fracture dimples, while the specimen with a notch radius of 1.5 mm has the lowest number of fracture dimples.The reduction of notch radius implies an intensification of stress concentration effect.As the radius of the notch decreases, the number and size of dimples gradually increase, and the corresponding number of micropores gradually decreases.This is because when the sample is in a three-dimensional stress state, the tough dimples are formed by the aggregation of micropores.The increase in the three-dimensional stress state inside the sample provides favorable conditions for the aggregation of micropores.The resistance of microvoid coalescence is reduced, thereby connecting and forming tough pits.The formation of tough pits is also constantly consuming the number of micropores, leading to a decrease in the number of micropores.The differences in the size and quantity of dimples under different triaxial stress states are the result of the competition between the two mechanisms of microvoid coalescence and plastic deformation.As the triaxial stress state increases, the microvoid coalescence mechanism gradually becomes dominant, and the local plastic deformation of the material increases.The plastic deformation of the specimen varies with different notch radii.The presence of notches suppresses the full dislocation plastic slip during the alloy stretching process, resulting in a small degree of plastic deformation of the notched specimen.

Conclusions
In this paper, this study selected Fe17Mn5Si10Cr5Ni SMA as the research object, designed specimens with different types and sizes of notches, and conducted uniaxial tensile tests on notched specimens at room temperature.The effects of notch parameters on notch tensile strength, notch sensitivity, stress-strain curve, and fracture mode were studied.The main conclusions are as follows.
(1) The NSR of all notched specimens is less than 1, indicating that Fe-Mn-Si SMA material is sensitive to notches.The tensile strength and notch sensitivity of U-shaped and V-shaped notch specimens increase with the decrease of notch radius.V-shaped notch specimens with the same notch radius exhibit stronger ductility than U-shaped notch specimens, and the fracture strain of V-shaped notch specimens is greater than that of U-shaped notch specimens.
(2) Compared with Q235 steel, Fe-Mn-Si SMA has better ductility.The stress-strain curve of Q235 steel shows a clear yield plateau and slight hardening behavior, while the yield plateau of Fe-Mn-Si SMA is not obvious.The NSR of Q235 steel is significantly higher than that of Fe-Mn-Si SMA, indicating that Fe-Mn-Si SMA is more sensitive to notches than Q235.The Fe-Mn-Si SMA notched specimen undergoes early stress induced martensitic transformation at the notch due to stress concentration, which affects the mechanical properties of the specimen.(3) The fracture types of smooth and notched specimens have the same characteristics, and the presence of notches does not change the tensile fracture type of the alloy.The fracture type is ductile fracture.

Figure 2 .
Figure 2. Stress-strain curves of V-shaped notch specimens with different notch radii.

Figure 3 .
Figure 3. Stress-strain curves of V-shaped notch specimens at different opening angles.

Figure 4 .
Figure 4. Stress-strain curves of specimens with different types of notches.

Figure 5 .
Figure 5. Stress-strain curves of Q235 steel and Fe-Mn-Si SMA with different opening angles.

Figure 7 .
Figure 7. Effect of notch radius on the properties of V-shaped notch specimens.

Figure 8 .
Figure 8.Effect of notch radius on the properties of U-shaped notch specimens.

Figure 10 .
Figure 10.The fracture morphology of smooth specimen.

Table 1 .
Chemical composition and mass fraction of Fe-Mn-Si SMA.

Table 3 .
Notch parameters of the specimen.