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Mezzoscopic grain refinement and improved mechanical properties of titanium materials by hydrogen treatments
ht. J. Hydrogen Energy, Vol. 22, No. 213,pp. 145-150, 1997 Copyright 0 1997International Association for Hydrogen Energy Else&r ScienceLtd Printed in Great Britain. All rights reserved PII: SO360-3199(96)00165-6 0360-3199/97 $17.00+0.00
Pergamon
MEZZOSCOPIC GRAIN REFINEMENT AND IMPROVED MECHANICAL PROPERTIES OF TITANIUM MATERIALS BY HYDROGEN TREATMENTS HIROFUMI Department
of Mechanical
YOSHIMURA
Engineering, Fukuyama University
3-1, Gakuen-cho,
Fukuyama-city,
729-02, Japan
(Received for publication 26 June 1996) Abstract-The aim of this investigation is to analyse the relationship between the formation of mezzoscopic grain structures and the improvement of the tensile, fatigue properties of Ti-GAldV alloy by hydrogen treatments. As a result, it was found that the proof stress of materials with a grain size of until 1 pm increased linearly in proportion to the grain size to the - l/2 power. In particular, the mezzoscopic materials with a grain size of 1-3 pm show a high degree of elongation considering their high strength. Even for fatigue strength, which is similar to the tensile strength, a high value is shown. Very fine voids were observed directly below the fracture surface of tensile tested specimen with mezzoscopic structures. The fractured surface of the fatigue tested specimen shows irregularities equivalent to mezzoscopic grain sizes, illustrating the arrest of crack propagation. Copyright 0 1997 International Association for Hydrogen Energy
1. INTRODUCTION
A structure with a grain size in this range recently has been called “mezzoscopic structure” and its materials are mezzoscopic materials [l]. The study of these mezzoscopic materials will be a top priority in our future research. The author and others have advanced research in this area, keeping in mind the following three concepts:
1.1. Background Future research on properties and characteristics of metallic polygonal materials used mainly for structural applications will be divided into two areas: (1) Research intended to achieve extremely high performance or to create new functions of materials; (2) Research on properties and characteristics of materials in manufacturing processes with extreme merit. The word “extreme” has been used because society no longer accepts materials unless their performance far surpasses normal expectations and they offer significant improvements over existing materials. The aim of this research is to achieve high performance and create new functions of the materials and to study the relationships between microstructures and their mechanical properties. In order to achieve high performance and create new functions, the grain size is one factor that can be controlled without changing the base composition of materials. Until now, various studies on grain size have been made, but only recently have conventional fine grains of 5-10 pm given way to ultra-fine grains of 1 or half ,nm. There are two reasons for this; one is the expectation that such fine grains will improve performance, the other is that there is currently no method of practically and easily obtaining such grain sizes (l-3 pm). Thus, the performance of materials having ultra-fine grain microstructures has not been sufficiently investigated.
1. Produce the grain size of matrix the same size as precipitates. 2. Add properties through post-treatments without changing the chemical composition of materials. 3. One method is the utilization of gas element (hydrogen, oxygen and nitrogen). First, such mezzoscopic structures were apparent in conventional general materials in the past, for instance, in the case of steel materials. In matrices having grain size of 10-20 pm, there were many precipitates of size 1 pm or less, which may be as small as several nanometers. In addition, high performance materials have a greater tendency to form fine precipitates. Therefore the precipitates themselves have truly mezzoscopic size. In other words, an attempt has been made to utilize precipitates in the intermediate stage of the formation of mezzoscopic grains. Regarding point 2, research for new material development began with the controlled addition of various elements. A good example is the development of high strength steels, which began around 1950. Today, there are many kinds of high strength steel. Since steel materials contain an extremely small amount of added elements, 145
146
H. YOSHIMURA
they can be easily recycled. However, reseparation of titanium alloys is more difficult because of their high melting points, and because of many types and high content of alloying elements. Thus, the number of multielement type materials cannot keep increasing. From now on, we must consider methods for improving performance without changing the chemical composition. It is very difficult to produce a grain size of 1 pm or less by means of standard metallurgical treatments. However, this becomes possible when hydrogen atoms are used, since they have high diffusion rate and can move easily to any place, uniformly precipitate as fine hydrides. Fine hydrides can effectively act as the nucleation sites of recrystallization. Point 3 is based on the grain refinement principle involving hydrogen treatment in the previous papers [2,3]. In the case of titanium allows, the method consists of hydrogenation, hot working, heat treatment and dehydrogenation. Finally, through dehydrogenation treatment, it is possible to create grains of similar sizes to the precipitates-that is mezzoscopic structures.
Grain refinement of @ phase
I
+ Subdivide j3 grain by a’ transformation
I
9
(HI (a>
Uniform distribution of dislocations by a’ transformation
of dislocations
by
1.2. Principle of producing mezzoscopic grains Titanium can absorb large amounts of hydrogen. For example, even in the experiments, the content of absorption was approximately 1% by mass. When this is converted into an atomic percentage, it is approximately 50 at. %. In other words, there is approximately one hydrogen atom for each Ti atom. This is the state in which the material is fully saturated with hydrogen. Figure 1 shows the schematic illustration of the treatment. First, p transus temperature decreases with increasing hydrogen contents. By p treatment at a temperature lower than normal and after making smaller P-phase grains, the martensite (clhl) contributes to subdividing of the p phase grain, containing hydrogen as super-saturated solid solution and introducing uniformly distributed dislocations (a). Then, the following procedures are conducted in order to make as many recrystallization sites as possible. Dislocation cell structures are formed at the matrix by deformation and hydrides precipitate through aging at a comparatively low temperature. High density dislocations are introduced at the hydrides precipitates in the matrix. The site of recrystallization is made through processing as shown in (b). When dehydrogenation is performed in final stage, grains of almost the same size as the hydrides are formed in the matrix through processing as shown in (c). In the stage of(b), nanoscale phenomenon is utilized and then during the fine-grain formation process of the matrix from the hydrides in the stage of (c), the mezzoscale controlling is conducted. Our research as based on the above concept was conducted using titanium alloy and hydrogen as gas atoms and mezzascopic materials with grain size of l-3 pm were obtained. Subsequently, the tensile and fatigue properties of mezzoscopic materials were investigated.
Famatin of dslccatien cell structures by hot working I
Uniform formation nucleation for recrystallization
of
J
(b)
Using
nano
scale
phenomenon
-3
cc>
Fig. 1. Schematic illustration of mezzoscopic grain refinement of titanium alloys by hydrogen treatment.
2. EXPERIMENTAL
PROCEDURES
CI+ fi type Ti-6A 14’ alloys were used as the materials in this experiment. These materials were treated by exactly the same method as in the experiment described in the previous paper [3]. Normal material that had undergone hot rolling and annealing was used, and three types of treatment of hydrogenated materials were conducted. [H: 0.4-1.0 mass% (hereinafter, only the percentages will be indicated)]. These treatments were those that had undergone aging treatment of heating at 773 K for 28.8 ks (Treat-
IMPROVED
MECHANICAL
PROPERTIES
ment A), those that were hot rolled with a reduction of 85% at 1023 K (Treatment B) and those that underwent similar hot rolling without hydrogen treatment. In the final stage, the dehydrogenation treatment at 973 K was performed in a vacuum. The residual hydrogen content after the final treatment was S-10 ppm. Specimens for tensile and fatigue tests were prepared in longitudinal (L) and transverse (T) directions relative to the rolling direction. The individual dimensions were as follows: diameter of 6.25 mm 4 and length of 25.0 mm for the tensile tests and diameter of 12 mm 4 and length of 210 mm for the fatigue tests. Tensile and fatigue tests were conducted on these specimens,and the deformation mode of the fractured parts were observed by optical and electron microscopes (SEM).
BY HYDROGEN
1050 2 4
-1000
: z -t;f 950 B Ii 900 s d 850
0.0
0.2
0.4
Grain Fig. 3. Relationship
3. EXPERIMENTAL
147
TREATMENTS
0.6
size-~
0.8
1.0
1.2
(pm-~)
between proof stress and grain size.
RESULTS
From the various treatments described above, five types of materials with grain sizes of 1, 3, 6, 8 and 16 pm were obtained. The examples of microstructures are shown in Fig. 2. Grain sizes of less than 6 pm were obtained through hydrogen treatment. Materials with grain sizes of l-3 pm resulting from hydrogen treatment are what are called the “mezzoscopic” materials. Little unisotropy of the tensile properties in the L and T directions for materials of various grain sizes was observed for the material of treatment A, but the difference between the two directions was 7fk-80 MPa for the materials of treatment B. Thus, the average of the two
values was used to express the tensile properties of materials. The relationship between proof stress and grain size can be as shown in Fig. 3. The proof stress increases linearly with decreasing the grain size until a grain size of approximately 1 pm, in accordance with the so-called Hall-Petch’s formula. The relationship between proof stress and elongation is shown in Fig. 4. The proof stress increases and the elongation tends to decreaseuntil the grain size of 3 pm. In material with a so-called mezzoscopic structure of grain size l-3 pm, the elongation is also high considering its high strength.
Fig. 2. Structures of titanium alloys by various treatments.
148
H. YOSHIMURA
-;;;1050(
tures. The microstructures of the central part of the tested specimen are shown in Fig. 6. In comparison with the coarse grained materials, the mezzoscopic materials show a “zig-zag” fracture pattern. In addition, the voids are extremely fine along the grain boundary. This is the characteristic of mezzoscopic materials. The fatigue characteristics in relation to the grain size are shown in Fig. 7. Similarly to the tensile properties, the fatigue strength values parallel to L and T directions were averaged. As the grain decreasesin size, the fatigue limit will improve similarly to the tensile properties. The fractured mode of Elongation (%) the coarse and mezzoscopic materials are shown in Fig. Fig. 4. Relationshipbetweenproof stressand elongation. 8. For coarse grained material, the fracture surface is relatively straight and the fracture crack advances For instance, at a grain size (d) of 6 pm, the proof smoothly. In addition, striations caused by the fatigue stress is 910 MPa and the elongation is 12.6%, whereas generally found on fractured surfacescan be seen.On the at grain size (d) of 2 to 1 pm, the proof stress is 1000 other hand, for mezzoscopic materials, the fracture route MPa and the elongation is nearly 13%. In this way, is zigzag. Furthermore, the fractured surface shows fine improvement of elongation is clearly observed. This may irregularities of a size similar to that of the grain and it be considered a characteristic of materials with a mez- indicates the arrest of crack propagation. zoscopic structure. Next, an explanation of the fracture state of the tensile 4. DISCUSSION test specimen will be given. Regarding the shape of the fracture surface, the edgeof the tested specimen of coarse Concepts concerning mezzoscopic structure formation and mezzoscopic grained materials is like the tip of a for the purpose of creating high performance and new knife blade. The optical microscopic structures near the functions of materials are as follows: central part of fractured specimensare shown in Fig. 5. For coarse grained materials with grain size of 16 pm, 1. Grains should be of the samesizeas the precipitates. the shape of the fracture part shows no sharp edges,and 2. A multi-element system should be adopted. a mode of small apparent elongation. In the vicinity of 3. Hydrogen should be utilized effectively. the fracture surface, large number of voids are observed. Based on this concept, it was found that materials with On the other hand, the fracture part of the mezzoscopic material shows a rather sharp edge indicating a mode of mezzoscopic structures having grain sizesof l-3 pm, high flexible elongation. Very fine voids were observed directly strength and ductility could easily be obtained. In the following, the relationship between morphology below the fracture surface. Similar modes can also be observed in the SEM struc- and mechanical properties is discussed. I
ins)
ezzoscopic grai
Fig. 5. Optical micrographsof fractureparts obtainedfrom tensiletestedspecimens.
IMPROVED MECHANICAL
PROPERTIES BY HYDROGEN TREATMENTS
149
Fig. 6. SEM micrographs of fracture parts obtained from tensile tested specimens.
B (d:lam) G It 6,
---__----___----_-
0”
B’ (16&
Number
of cycles
to failure
C+
(Nf)
Fig. 7. Fatigue limit of specimenswith mezzoscopic and coarse grains.
4.1. Morphology Based on the concept of producing crystal grain of the same size as the precipitates, it was made clear that a fine grain formation of approximately 1 pm was formed. Furthermore, it is believed that, in principle, an even smaller grain size is possible. Takaki [4] has reported that submicron grains can be obtained through thermomechanical treatment in Fe-Cr-Ni-Mo alloy, but the grains are formed in the matrix by the same variant of martensitic transformation. Consequently, only proof stress lower than the values estimated from the relationship of Hall-Petch’s formula can be obtained. It may be said that this is a drawback of utilizing martensitic transformation. In this research, the fine grains of the matrix are formed from the fine hydrides. However, thus far, submicron grain sizes have not been attained yet. This is a problem that will have to be dealt with in the future. Although it will be referred to later, if precipitation of the hydrides accompanies martensitic trans-
formation, it may be impossible to attain submicron or less grain sizes for increasing proof stress efficiently. Treatments intended to precipitate hydrides from the hydrogen solute matrix utilize the so-called nanoscale phenomena, which include diffusion precipitation and dislocation. It may be said that treatment intended to form ultra-fine grain of uniform size from precipitates of less than 1 pm utilize the mezzoscale control. The true character of the hydrides must be clarified, since they plan an important role in the treatment. Basic studies are currently being conducted on their crystal structure, and many characteristics have already been clarified. However, there is one more area that attracts our attention: the question of whether the hydrides are martensitic. In other words, are precipitating hydrides accompanied by martensitic transformation? The reason for our concern is that both are very similar. Hydrides are characterized by (1) a needle-like phase with surface relief, (2) habit plane, (3) “midrib’, (4) volumetric expansion and tetragonality, (5) the introduction of high density dislocation into the inside structure by shear deformation and (6) orientation relationship. All of these characteristics are almost identical to those of martensite. This is an interesting point for future study. 4.2. Mechanical properties The characteristics of the tensile and fatigue properties of the mezzoscopic materials have high strength. They also begin to show a high and uniform elongation. The fine and dispersed voids are formed at the time of fracture. It may be said that mezzoscopic materials have a tendency toward superplasticity even at room temperature. 5. SUMMARY Using Ti-6A1-4V alloys, investigations were conducted on tensile, fatigue properties, and the fracture
150
H. YOSHIMURA
Fig. 8. Optical micrographs (a) and SEM fractographs (b) of fatigue tested specimens. modes accompanying these properties for materials with grain sizes of l-16 pm, including mezzoscopic structures through hydrogen treatment. The characteristics of mezzoscopic materials with grain sizes of l-3 pm based on hydrogen treatment are summarized as follows:
given their high strength. It may be said that mezzoscopic materials have tendency towards superplasticity even at room temperatures.
1. For grain size of 1-3 pm, 0.2% proof stress increases proportionately to the grain size (d) to the - l/2 power (Fig. 3). 2. The elongation is comparatively high considering its high strength (Fig. 4.) 3. The fatigue strength is also high (Fig. 7). 4. The fine voids are dispersedly formed near the fractures (Figs 5,6,8).
1. Osamura, K., Design of Mezzoscale Materials, Nanoscale Materials, ed. Japan Institute of Metals, 1993,p. 43. 2. Yoshimura, H., Kimura, K., Hayashi, M., Ishii, M. and Takamura, J. Journal of Japan Institute of Metals, 1991,55, 1375(in Japanese). 3. Yoshimura, H., Kimura, K., Hayashi, M., Ishii, M., Hanamura, T. and Takamura, J., Journal of Japan Institute of Metals, 1994,35(4), 266. 4. Takaki, S. and Tokunaga, Y., Innovation Stainless Steel. Florence, Italy, 1993.
Mezzoscopic materials have relatively high elongation
REFERENCES
Pergamon
MEZZOSCOPIC GRAIN REFINEMENT AND IMPROVED MECHANICAL PROPERTIES OF TITANIUM MATERIALS BY HYDROGEN TREATMENTS HIROFUMI Department
of Mechanical
YOSHIMURA
Engineering, Fukuyama University
3-1, Gakuen-cho,
Fukuyama-city,
729-02, Japan
(Received for publication 26 June 1996) Abstract-The aim of this investigation is to analyse the relationship between the formation of mezzoscopic grain structures and the improvement of the tensile, fatigue properties of Ti-GAldV alloy by hydrogen treatments. As a result, it was found that the proof stress of materials with a grain size of until 1 pm increased linearly in proportion to the grain size to the - l/2 power. In particular, the mezzoscopic materials with a grain size of 1-3 pm show a high degree of elongation considering their high strength. Even for fatigue strength, which is similar to the tensile strength, a high value is shown. Very fine voids were observed directly below the fracture surface of tensile tested specimen with mezzoscopic structures. The fractured surface of the fatigue tested specimen shows irregularities equivalent to mezzoscopic grain sizes, illustrating the arrest of crack propagation. Copyright 0 1997 International Association for Hydrogen Energy
1. INTRODUCTION
A structure with a grain size in this range recently has been called “mezzoscopic structure” and its materials are mezzoscopic materials [l]. The study of these mezzoscopic materials will be a top priority in our future research. The author and others have advanced research in this area, keeping in mind the following three concepts:
1.1. Background Future research on properties and characteristics of metallic polygonal materials used mainly for structural applications will be divided into two areas: (1) Research intended to achieve extremely high performance or to create new functions of materials; (2) Research on properties and characteristics of materials in manufacturing processes with extreme merit. The word “extreme” has been used because society no longer accepts materials unless their performance far surpasses normal expectations and they offer significant improvements over existing materials. The aim of this research is to achieve high performance and create new functions of the materials and to study the relationships between microstructures and their mechanical properties. In order to achieve high performance and create new functions, the grain size is one factor that can be controlled without changing the base composition of materials. Until now, various studies on grain size have been made, but only recently have conventional fine grains of 5-10 pm given way to ultra-fine grains of 1 or half ,nm. There are two reasons for this; one is the expectation that such fine grains will improve performance, the other is that there is currently no method of practically and easily obtaining such grain sizes (l-3 pm). Thus, the performance of materials having ultra-fine grain microstructures has not been sufficiently investigated.
1. Produce the grain size of matrix the same size as precipitates. 2. Add properties through post-treatments without changing the chemical composition of materials. 3. One method is the utilization of gas element (hydrogen, oxygen and nitrogen). First, such mezzoscopic structures were apparent in conventional general materials in the past, for instance, in the case of steel materials. In matrices having grain size of 10-20 pm, there were many precipitates of size 1 pm or less, which may be as small as several nanometers. In addition, high performance materials have a greater tendency to form fine precipitates. Therefore the precipitates themselves have truly mezzoscopic size. In other words, an attempt has been made to utilize precipitates in the intermediate stage of the formation of mezzoscopic grains. Regarding point 2, research for new material development began with the controlled addition of various elements. A good example is the development of high strength steels, which began around 1950. Today, there are many kinds of high strength steel. Since steel materials contain an extremely small amount of added elements, 145
146
H. YOSHIMURA
they can be easily recycled. However, reseparation of titanium alloys is more difficult because of their high melting points, and because of many types and high content of alloying elements. Thus, the number of multielement type materials cannot keep increasing. From now on, we must consider methods for improving performance without changing the chemical composition. It is very difficult to produce a grain size of 1 pm or less by means of standard metallurgical treatments. However, this becomes possible when hydrogen atoms are used, since they have high diffusion rate and can move easily to any place, uniformly precipitate as fine hydrides. Fine hydrides can effectively act as the nucleation sites of recrystallization. Point 3 is based on the grain refinement principle involving hydrogen treatment in the previous papers [2,3]. In the case of titanium allows, the method consists of hydrogenation, hot working, heat treatment and dehydrogenation. Finally, through dehydrogenation treatment, it is possible to create grains of similar sizes to the precipitates-that is mezzoscopic structures.
Grain refinement of @ phase
I
+ Subdivide j3 grain by a’ transformation
I
9
(HI (a>
Uniform distribution of dislocations by a’ transformation
of dislocations
by
1.2. Principle of producing mezzoscopic grains Titanium can absorb large amounts of hydrogen. For example, even in the experiments, the content of absorption was approximately 1% by mass. When this is converted into an atomic percentage, it is approximately 50 at. %. In other words, there is approximately one hydrogen atom for each Ti atom. This is the state in which the material is fully saturated with hydrogen. Figure 1 shows the schematic illustration of the treatment. First, p transus temperature decreases with increasing hydrogen contents. By p treatment at a temperature lower than normal and after making smaller P-phase grains, the martensite (clhl) contributes to subdividing of the p phase grain, containing hydrogen as super-saturated solid solution and introducing uniformly distributed dislocations (a). Then, the following procedures are conducted in order to make as many recrystallization sites as possible. Dislocation cell structures are formed at the matrix by deformation and hydrides precipitate through aging at a comparatively low temperature. High density dislocations are introduced at the hydrides precipitates in the matrix. The site of recrystallization is made through processing as shown in (b). When dehydrogenation is performed in final stage, grains of almost the same size as the hydrides are formed in the matrix through processing as shown in (c). In the stage of(b), nanoscale phenomenon is utilized and then during the fine-grain formation process of the matrix from the hydrides in the stage of (c), the mezzoscale controlling is conducted. Our research as based on the above concept was conducted using titanium alloy and hydrogen as gas atoms and mezzascopic materials with grain size of l-3 pm were obtained. Subsequently, the tensile and fatigue properties of mezzoscopic materials were investigated.
Famatin of dslccatien cell structures by hot working I
Uniform formation nucleation for recrystallization
of
J
(b)
Using
nano
scale
phenomenon
-3
cc>
Fig. 1. Schematic illustration of mezzoscopic grain refinement of titanium alloys by hydrogen treatment.
2. EXPERIMENTAL
PROCEDURES
CI+ fi type Ti-6A 14’ alloys were used as the materials in this experiment. These materials were treated by exactly the same method as in the experiment described in the previous paper [3]. Normal material that had undergone hot rolling and annealing was used, and three types of treatment of hydrogenated materials were conducted. [H: 0.4-1.0 mass% (hereinafter, only the percentages will be indicated)]. These treatments were those that had undergone aging treatment of heating at 773 K for 28.8 ks (Treat-
IMPROVED
MECHANICAL
PROPERTIES
ment A), those that were hot rolled with a reduction of 85% at 1023 K (Treatment B) and those that underwent similar hot rolling without hydrogen treatment. In the final stage, the dehydrogenation treatment at 973 K was performed in a vacuum. The residual hydrogen content after the final treatment was S-10 ppm. Specimens for tensile and fatigue tests were prepared in longitudinal (L) and transverse (T) directions relative to the rolling direction. The individual dimensions were as follows: diameter of 6.25 mm 4 and length of 25.0 mm for the tensile tests and diameter of 12 mm 4 and length of 210 mm for the fatigue tests. Tensile and fatigue tests were conducted on these specimens,and the deformation mode of the fractured parts were observed by optical and electron microscopes (SEM).
BY HYDROGEN
1050 2 4
-1000
: z -t;f 950 B Ii 900 s d 850
0.0
0.2
0.4
Grain Fig. 3. Relationship
3. EXPERIMENTAL
147
TREATMENTS
0.6
size-~
0.8
1.0
1.2
(pm-~)
between proof stress and grain size.
RESULTS
From the various treatments described above, five types of materials with grain sizes of 1, 3, 6, 8 and 16 pm were obtained. The examples of microstructures are shown in Fig. 2. Grain sizes of less than 6 pm were obtained through hydrogen treatment. Materials with grain sizes of l-3 pm resulting from hydrogen treatment are what are called the “mezzoscopic” materials. Little unisotropy of the tensile properties in the L and T directions for materials of various grain sizes was observed for the material of treatment A, but the difference between the two directions was 7fk-80 MPa for the materials of treatment B. Thus, the average of the two
values was used to express the tensile properties of materials. The relationship between proof stress and grain size can be as shown in Fig. 3. The proof stress increases linearly with decreasing the grain size until a grain size of approximately 1 pm, in accordance with the so-called Hall-Petch’s formula. The relationship between proof stress and elongation is shown in Fig. 4. The proof stress increases and the elongation tends to decreaseuntil the grain size of 3 pm. In material with a so-called mezzoscopic structure of grain size l-3 pm, the elongation is also high considering its high strength.
Fig. 2. Structures of titanium alloys by various treatments.
148
H. YOSHIMURA
-;;;1050(
tures. The microstructures of the central part of the tested specimen are shown in Fig. 6. In comparison with the coarse grained materials, the mezzoscopic materials show a “zig-zag” fracture pattern. In addition, the voids are extremely fine along the grain boundary. This is the characteristic of mezzoscopic materials. The fatigue characteristics in relation to the grain size are shown in Fig. 7. Similarly to the tensile properties, the fatigue strength values parallel to L and T directions were averaged. As the grain decreasesin size, the fatigue limit will improve similarly to the tensile properties. The fractured mode of Elongation (%) the coarse and mezzoscopic materials are shown in Fig. Fig. 4. Relationshipbetweenproof stressand elongation. 8. For coarse grained material, the fracture surface is relatively straight and the fracture crack advances For instance, at a grain size (d) of 6 pm, the proof smoothly. In addition, striations caused by the fatigue stress is 910 MPa and the elongation is 12.6%, whereas generally found on fractured surfacescan be seen.On the at grain size (d) of 2 to 1 pm, the proof stress is 1000 other hand, for mezzoscopic materials, the fracture route MPa and the elongation is nearly 13%. In this way, is zigzag. Furthermore, the fractured surface shows fine improvement of elongation is clearly observed. This may irregularities of a size similar to that of the grain and it be considered a characteristic of materials with a mez- indicates the arrest of crack propagation. zoscopic structure. Next, an explanation of the fracture state of the tensile 4. DISCUSSION test specimen will be given. Regarding the shape of the fracture surface, the edgeof the tested specimen of coarse Concepts concerning mezzoscopic structure formation and mezzoscopic grained materials is like the tip of a for the purpose of creating high performance and new knife blade. The optical microscopic structures near the functions of materials are as follows: central part of fractured specimensare shown in Fig. 5. For coarse grained materials with grain size of 16 pm, 1. Grains should be of the samesizeas the precipitates. the shape of the fracture part shows no sharp edges,and 2. A multi-element system should be adopted. a mode of small apparent elongation. In the vicinity of 3. Hydrogen should be utilized effectively. the fracture surface, large number of voids are observed. Based on this concept, it was found that materials with On the other hand, the fracture part of the mezzoscopic material shows a rather sharp edge indicating a mode of mezzoscopic structures having grain sizesof l-3 pm, high flexible elongation. Very fine voids were observed directly strength and ductility could easily be obtained. In the following, the relationship between morphology below the fracture surface. Similar modes can also be observed in the SEM struc- and mechanical properties is discussed. I
ins)
ezzoscopic grai
Fig. 5. Optical micrographsof fractureparts obtainedfrom tensiletestedspecimens.
IMPROVED MECHANICAL
PROPERTIES BY HYDROGEN TREATMENTS
149
Fig. 6. SEM micrographs of fracture parts obtained from tensile tested specimens.
B (d:lam) G It 6,
---__----___----_-
0”
B’ (16&
Number
of cycles
to failure
C+
(Nf)
Fig. 7. Fatigue limit of specimenswith mezzoscopic and coarse grains.
4.1. Morphology Based on the concept of producing crystal grain of the same size as the precipitates, it was made clear that a fine grain formation of approximately 1 pm was formed. Furthermore, it is believed that, in principle, an even smaller grain size is possible. Takaki [4] has reported that submicron grains can be obtained through thermomechanical treatment in Fe-Cr-Ni-Mo alloy, but the grains are formed in the matrix by the same variant of martensitic transformation. Consequently, only proof stress lower than the values estimated from the relationship of Hall-Petch’s formula can be obtained. It may be said that this is a drawback of utilizing martensitic transformation. In this research, the fine grains of the matrix are formed from the fine hydrides. However, thus far, submicron grain sizes have not been attained yet. This is a problem that will have to be dealt with in the future. Although it will be referred to later, if precipitation of the hydrides accompanies martensitic trans-
formation, it may be impossible to attain submicron or less grain sizes for increasing proof stress efficiently. Treatments intended to precipitate hydrides from the hydrogen solute matrix utilize the so-called nanoscale phenomena, which include diffusion precipitation and dislocation. It may be said that treatment intended to form ultra-fine grain of uniform size from precipitates of less than 1 pm utilize the mezzoscale control. The true character of the hydrides must be clarified, since they plan an important role in the treatment. Basic studies are currently being conducted on their crystal structure, and many characteristics have already been clarified. However, there is one more area that attracts our attention: the question of whether the hydrides are martensitic. In other words, are precipitating hydrides accompanied by martensitic transformation? The reason for our concern is that both are very similar. Hydrides are characterized by (1) a needle-like phase with surface relief, (2) habit plane, (3) “midrib’, (4) volumetric expansion and tetragonality, (5) the introduction of high density dislocation into the inside structure by shear deformation and (6) orientation relationship. All of these characteristics are almost identical to those of martensite. This is an interesting point for future study. 4.2. Mechanical properties The characteristics of the tensile and fatigue properties of the mezzoscopic materials have high strength. They also begin to show a high and uniform elongation. The fine and dispersed voids are formed at the time of fracture. It may be said that mezzoscopic materials have a tendency toward superplasticity even at room temperature. 5. SUMMARY Using Ti-6A1-4V alloys, investigations were conducted on tensile, fatigue properties, and the fracture
150
H. YOSHIMURA
Fig. 8. Optical micrographs (a) and SEM fractographs (b) of fatigue tested specimens. modes accompanying these properties for materials with grain sizes of l-16 pm, including mezzoscopic structures through hydrogen treatment. The characteristics of mezzoscopic materials with grain sizes of l-3 pm based on hydrogen treatment are summarized as follows:
given their high strength. It may be said that mezzoscopic materials have tendency towards superplasticity even at room temperatures.
1. For grain size of 1-3 pm, 0.2% proof stress increases proportionately to the grain size (d) to the - l/2 power (Fig. 3). 2. The elongation is comparatively high considering its high strength (Fig. 4.) 3. The fatigue strength is also high (Fig. 7). 4. The fine voids are dispersedly formed near the fractures (Figs 5,6,8).
1. Osamura, K., Design of Mezzoscale Materials, Nanoscale Materials, ed. Japan Institute of Metals, 1993,p. 43. 2. Yoshimura, H., Kimura, K., Hayashi, M., Ishii, M. and Takamura, J. Journal of Japan Institute of Metals, 1991,55, 1375(in Japanese). 3. Yoshimura, H., Kimura, K., Hayashi, M., Ishii, M., Hanamura, T. and Takamura, J., Journal of Japan Institute of Metals, 1994,35(4), 266. 4. Takaki, S. and Tokunaga, Y., Innovation Stainless Steel. Florence, Italy, 1993.
Mezzoscopic materials have relatively high elongation
REFERENCES