"Excitingly, the fracture toughness of low oxygen titanium surpasses all commercially pure titanium and titanium alloys reported so far, and even more than most metal materials." Professor Han Weizhong of Xi'an Jiaotong University said.
Figure | Han Weizhong (Source: Han Weizhong)
Recently, he and his team have successfully broken through the extreme performance of titanium and titanium alloys, reducing the oxygen impurity content of commercially pure titanium from 0.14wt.% to 0.02wt.%, and improving its fracture toughness from 117MPa∙m^1/2 to 255MPa∙m^1/2.
Through this study, they also revealed for the first time the ultra-high intrinsic fracture toughness of titanium, breaking the traditional belief that the fracture toughness of titanium and titanium alloys is less than 130MPa∙m^1/2, proving that low oxygen titanium is one of the most ductile metal materials currently known.
Overall, this achievement brings important enlightenment for the design of high-strength and high-toughness titanium alloys.
At present, in order to promote the application of titanium alloys under certain safety-critical load conditions in the aerospace field, people have adopted the design idea of controlling oxygen content in titanium alloys, which can not only improve the fracture toughness of titanium alloys, but also commercialize related products.
For example, damage-tolerant Ti-6Al-4V (TC4 DT) alloys and ultra-low clearance (ELI) Ti-6Al-4V alloys have been widely used.
However, for the current damage tolerance titanium alloy, the oxygen content in it is still at a high level, resulting in its fracture toughness is still limited to less than 130MPa∙m1/2.
In order to further improve the application range of classic titanium alloys such as Ti-6Al-4V, it is necessary to improve their service safety. Subsequently, by further reducing the content of oxygen impurities, the fracture toughness of the titanium alloy will be improved by leaps and bounds.
In fact, almost all close-packed hexagonal structural metals, including titanium, zirconium, magnesium, zinc, etc., which are widely used at present, have <C+A> dislocations that are difficult to activate, or have poor mobility.
This results in much lower plasticity and fracture toughness than most face-centered cubic metals, limiting their range of applications.
Therefore, a large number of deformed twins can be activated through the alloying design scheme in the future.
Then, the twin boundaries are used to promote the intensive priming of <C+A> dislocations, so as to significantly improve the mechanical properties of close-packed hexagonal metals.
（来源：Advanced Materials）
It is expected to greatly improve the deformation ability of close-packed hexagonal metals
Han Weizhong said that he and his team have been concerned about the effect of dissolved oxygen on the properties of metal materials for many years, and a few years ago they had studied the oxygen embrittlement mechanism of the fifth group of refractory metals vanadium, niobium and tantalum.
It is found that the dissolved oxygen in metal materials is easy to combine with the vacancies generated by dislocation movement during the deformation process to form an oxygen-vacancy complex.
The oxygen-vacancy complex can strongly nail the dislocation and promote the formation of deformed micropores, which can lead to the oxygen embrittlement phenomenon of the fifth group of refractory metals.
In order to achieve a good transformation of the embrittlement phenomenon of dissolved oxygen, the research group invented a technology called gradient oxygen infiltration on metal surfaces.
In other words, by treating the metal that is easy to absorb oxygen in a high-temperature oxygen atmosphere, the oxygen diffuses from the surface of the metal to the inside of the metal.
During this time, a gradient of oxygen concentration from the surface to the inside of the metal is formed, which causes the surface of the metal to harden while the core of the metal remains tough.
In this way, it is possible to obtain a metal material that combines high strength and high toughness, and at the same time improves the surface wear resistance of the metal.
In fact, metal surface oxygenation technology is similar to carburizing technology and nitriding technology, which is a new surface strengthening technology for metal materials.
When the high-purity titanium was treated and stretched to deform using the metal surface gradient oxygenation technique, they found that the deformation characteristics of the outer gradient oxygen zone of the sample and the hypoxic zone of the core showed a very significant difference.
Specifically, a large number of deformed twins will be generated in the hypoxic zone of the core, while the gradient permeable zone near the surface will not form deformed twins due to the relatively high oxygen content.
This shows that the oxygen content has a great influence on the deformation twinning tendency of titanium, that is, when the oxygen content is low, the deformation twinning is more likely to occur.
（来源：Acta Materialia）
According to the research group, in recent years, they have also been studying the tough and brittle transformation of metal materials.
As for the sudden decrease in the plastic deformation ability of the body-centered cubic metal after the temperature decreases, they focused on the microscopic mechanism behind it.
The results show that the toughness and brittleness of body-centered cubic metal is closely related to the efficiency of dislocation sources. That is, the determining factor is whether enough movable dislocations can be generated in time to coordinate the deformation in the metal material at the time of deformation.
At the same time, for the dislocation source efficiency, it is determined by the relative motion capacity of the screw dislocation and the edge dislocation.
At the tough-brittle transition temperature, the screw dislocation has poor mobility, while the blade dislocation is prone to sliding, resulting in the limitation of the movement capacity of the entire dislocation line.
More importantly, in the case of such uncoordinated movements, the dislocation line cannot be transformed into an efficient dislocation source, so it is difficult to achieve effective self-proliferation of the dislocation.
In this case, because the number of movable dislocations that can be coordinated and deformed is too small, once the ambient temperature is lowered below a certain temperature, the body-centered cubic metal will undergo a sudden tough-brittle transformation.
It can be seen that the toughness and brittleness of metal materials are closely related to the relative motion ability of screw dislocation and blade dislocation.
Based on this discovery, the research group has found a new idea in regulating the deformation ability of close-packed hexagonal metals.
According to the researchers, close-packed hexagonal metals such as titanium, zirconium and magnesium have low lattice symmetry.
In general, cylindrical or base <a>slip is easier to start, while cone <C+A> slip is more difficult to start.
This will make the close-packed hexagonal metal unable to meet the Taylor-von Mises criterion, and eventually lead to the weakness of the overall deformation ability of the close-packed hexagonal metal.
In fact, the dislocation slip of the cone <C+A> is difficult to start because of its own reasons. Since <C+A> edge dislocation is easily broken down to the cylinder and base surface, the articulation is poor.
However, <C+A> screw dislocations are relatively easier to slide, so in close-packed hexagonal metals, long straight <C+A> edge dislocations and very short <C+A> screw dislocations are usually observed.
This suggests that the articulation of the two dislocations varies greatly, while the dislocations that are difficult to slide are usually retained. This is also similar to the dislocation of long straight screws often seen in brittle body-centered cubic metals.
For the blade component and the spiral component of the cone <C+A> dislocation, there is a big difference in their mobility, so their self-proliferation ability is very weak, which will lead to the lack of sufficient cone <C+A> dislocation coordination <c>axis deformation during deformation.
In order to increase the number of <C+A> dislocations, a strategy can be adopted to increase the number of <C+A> dislocation sources.
It is worth noting that the team happened to find in previous research that the twin grain boundaries in zirconium can emit < dislocations > C+A. Based on this, they promoted the deformation of the twins by reducing the oxygen content.
At this time, a large number of twin boundaries can be used as dislocation sources, which can excite high-density <a>and <C+A > slips, which is expected to greatly improve the deformation ability of close-packed hexagonal metals.
（来源：Advanced Materials）
The fracture toughness value is stable at 255MPa∙m1/2
In fact, at the beginning of the study, they did not focus on the effect of oxygen content in titanium on fracture toughness, but focused on the relationship between the microstructure and fracture toughness of pure zirconium with close-packed hexagonal structure.
After discovering the important influence of oxygen on the close-packed hexagonal metal twins, the research group began to try to use low-oxygen titanium as a model material to study its deformation mechanism and fracture toughness.
During the test, they encountered a situation that they had never encountered before: the crack propagation rate of the low-oxygen titanium sample was very slow during the loading process compared to commercial pure titanium.
As a result, they revisited the research plan. Later, he decided to reveal the intrinsic fracture toughness of titanium, which led to the discovery of low oxide titanium as one of the toughest known metallic materials.
At the same time, there are two questions that must be answered:
First, how to measure a fracture toughness value that not only meets the fracture mechanics standards but can be widely recognized by peers?
Second, why does titanium low oxide have such high fracture toughness? What is the intrinsic mechanism of toughening?
In order to measure the standard fracture toughness values, they began to study the fracture toughness of low titanium oxide as a function of the thickness of the sample.
In detail, the research group prepared multiple samples with thicknesses ranging from 2.5mm to 30mm, and measured the corresponding fracture toughness values respectively, and then studied the trend of fracture toughness of the samples with thickness.
The results show that the fracture toughness value of low titanium oxide can be stable at 255MPa∙m1/2. The thickest thickness of the sample they used was 30mm, which exceeded the minimum thickness of 27mm required by the fracture mechanics standard.
(Source: Courtesy of the researcher)
In order to compare, they conducted a standard test for commercial pure titanium and found that the fracture toughness of commercial pure titanium was only 117MPa∙m1/2, which was far lower than that of low-oxygen titanium.
The biggest difference between the above two pure titanium is the different content of oxygen impurities. The oxygen content in commercially pure titanium is approximately 7 times that of low-oxygen titanium, suggesting that oxygen impurity content is the main cause of the difference in fracture toughness.
Subsequently, they further analyzed the dislocation structure characteristics in the two titanium alloys. It is found that the dislocations are mainly in commercial pure titanium<a>, and there are few dislocations > <C+A.
In contrast, the low-oxide titanium crack tip region activates a high density of <a>dislocations and initiates a large number of <C+A> dislocations that are emitted from the twin boundaries > these <C+A. This indicates that the twin boundaries are > source of dislocations <C+A.
Because of this, low titanium oxide forms a unique progressive toughening mechanism. When the oxygen content is reduced, the crack tip deformation twins of low oxygen can be activated in large quantities.
Subsequently, the twin boundaries will emit high-density <C+A> dislocations, and can significantly increase the deformation density and plastic zone size at the crack tip, which can effectively passivate the crack, and finally allow the low oxygen titanium to have ultra-high fracture toughness. This is the official end of this study.
（来源：Advanced Materials）
日前，相关论文以《通过降低氧杂质含量揭示钛的固有高断裂韧性》（Uncovering the Intrinsic High Fracture Toughness of Titanium via Lowered Oxygen Impurity Content）为题发在 Advanced Materials[1]。
Xiaowei Zou, a Ph.D. student at Xi'an Jiaotong University, is the first author, and Prof. Weizhong Han and Prof. Ma are the co-corresponding authors.
Figure | Related papers (Source: Advanced Materials)
The reviewers of this paper said that compared with high-toughness metal materials such as stainless steel, titanium usually shows lower fracture toughness, which reinforces the stereotype that the inherent toughness of close-packed hexagonal metals is poor.
However, the results of this study for low titanium oxide challenge this stereotype. It shows that the close-packed hexagonal metal has a high fracture toughness potential and is comparable to stainless steel.
Subsequently, the research group hopes to apply the strategy of low oxygen to the design and manufacture of high-strength and high-toughness titanium alloys.
For the currently widely used Ti-6Al-4V alloy, they plan to develop a new sample preparation process that will significantly reduce the oxygen impurity content in the Ti-6Al-4V alloy.
It is expected that this will help to promote a large number of start-up of crack tip deformation twins, further activate <c+a> dislocations, thereby significantly improving the fracture toughness of Ti-6Al-4V alloy, expanding its application range and improving service safety.
In addition, the alloy composition design of titanium alloy can also be carried out, and the initiation of crack tip deformation twins or <C+A> dislocations can be promoted through specific alloying elements, which can help to realize the synergistic improvement of titanium alloy strength and fracture toughness.
Resources:
1.https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202408286
Operation/Typesetting: He Chenlong