The Mechanical Properties of the Mo-0.5Ti and Mo-0.1Zr Alloys at Room Temperature and High Temperature Annealing

Microstructure of the Ti- or Zr-microalloyed molybdenum alloys

Figure 2 shows the SEM photos of the molybdenum and the Ti- or Zr-microalloyed alloys respectively. As showed in Figure 2(a) and (b), lots of micropores and black granular secondary phases can be found in these microalloyed alloys sintered at high temperatures. These pores disappeared and the second-phase particles would be fractured after the sample being rolled. This can be explained by the theory of interactions between dislocations and impurities [4–6]. An elastic stress field exists around the dislocations and impurities in the microalloyed alloys, where oxides formed and distributed in the biggest dislocation accumulation area of the matrix. Compared with the microalloyed alloys (see Figure 1(a) and (b)), the micrograph of pure molybdenum (see Figure 1(c)) appears a lot of sintered pores but no black second phases.

Figure 2:

SEM images of molybdenum, Mo-0.5Ti and Mo-0.1Zr: (a) Mo-0.5Ti; (b) Mo-0.1Zr; (c) Mo.

Figure 3(a) is the micrograph of the Ti-microalloyed molybdenum alloy coupled with the EDS spectrum for the second-phase particles. The spectrum shows that the main compositions of the black particles are oxygen and Ti elements. According to the relevant references [2], the big black oxides are likely to become a source of crack propagation latterly during processing, resulting in a reducing of fracture toughness. With regard to the Ti-microalloyed molybdenum alloy, highly active Ti atoms are formed via the dehydrogenation of TiH₂. After that, these Ti atoms are easy to react with oxygen to form second-phase oxides, which were confirmed to be Ti₂O₃ via the diffraction pattern (Figure 3(b)). These particles play an important role in the strengthening of Mo-0.5Ti alloy, just like other dispersion strengthening phases, as they can effectively obstruct the movement of dislocations and consume the dislocation energy [2, 7, 8].

Figure 3:

SEM image of Mo-0.5Ti and EDS patterns of the second phase.

Similarly, the Zr element added to the Molybdenum alloy can also form the corresponding oxides and thus can have a strengthening effect [9]. From the SEM image and the related diffraction pattern in Figure 4, it can be found that the ZrO₂ particles are formed and dispersedly distributed on grain boundaries, which were confirmed to be ZrO₂ via the diffraction pattern (Figure 4(b)). Moreover, there is an observable refinement effect in some grains after adding Zr element, as evidenced by the appearance of smaller grains (3~5 µm) compared to the other ones (12~14 µm). As an important factor to affect the plasticity of alloys, the grain fineness is appropriately controlled. Has a great significance on its applications? It has been shown that by virtue of the existence of second-phase particles, on one hand, the grain boundary movement can be effectively obstructed during the alloy deformation.

Figure 4:

SEM image of Mo-0.1Zr and EDS patterns of the second phase.

Tensile strength and elongation of annealed Mo-0.5Ti and Mo-0.1Zr alloys at RT

Figure 5 shows the tensile strength and the elongation of the Ti- and Zr- microalloyed molybdenum alloys and pure molybdenum at RT. It can be seen from the figure that the tensile strength of these two kinds of microalloyed alloys is higher than that of pure molybdenum while their plasticity is lower. The Mo-0.5Ti alloy has a higher tensile strength and a lower elongation in comparison with the Mo-0.1Zr alloy. This is because after the addition of TiH₂ in molybdenum, highly active Ti atoms generating from TiH₂ dehydrogenation are easy to combine with oxygen to generate second-phase oxides [10–12]. The dispersive distribution of second-phase particles (Figure 3) can effectively increase the strength of the matrix. After adding ZrH₂ to Molybdenum alloy, there are tiny ZrO₂ particles as well. Although the Zr element is more likely than Ti to solve into the matrix, the solution amount of Zr into Mo is smaller. Obvious diffusion occurs only when the temperature is above 1,300 °C, so that the newly generated Zr atoms are mostly been oxidized before their effective solution to Mo matrix. Besides, the amount of Zr addition itself and the distribution is not even, resulting in that most of Zr atoms exist in a form of zirconia while only a small portion of Zr atoms can dissolve into the matrix [2, 13, 14]. Moreover, the second phase formed by adding Zr element has a certain refinement effect to the grains as showed in Figure 4.

Figure 5:

Tensile strength and the elongation of molybdenum, Mo-0.5Ti and Mo-0.1Zr at room temperature.

Tensile strength and elongation of Mo-0.5% Ti and Mo-0.1Zr alloys annealed at different temperatures

Figure 6 shows the tensile strength and elongation of the Ti- and Zr-microalloyed molybdenum alloys and pure Mo after annealing at different temperatures. Compared with Figure 5, the tensile strength of the Ti- and Zr-microalloyed alloys and pure molybdenum decreases after annealing, which that of the Mo-0.1Zr alloy decreased seriously. After annealing, the extensibility of alloys and pure molybdenum largely increases. The annealing treatment can remove part of the internal stress in samples forming in the process of rolling, and then dynamic recovery occurs, leading to a decrease in the strength and an increase in the plasticity. As showed in Figure 6(a), the tensile strength decreases with increasing annealing temperature. In the whole annealing temperature range, RT tensile strength of Mo-0.5Ti alloy is higher than that of pure molybdenum. With the increase of annealing temperature, grains turn to be fibrous after rolling, followed by the dynamic recrystallization. Previous deformed structures replaced completely the fine equiaxed grains, thus causing a decrease in the strength. Because of the difference in recrystallization temperatures, the flow stress of pure molybdenum falls sharply after 1,000 °C and that of Mo-0.5Ti alloy declines sharply after 1,200 °C while that of Mo-0.1Zr alloy is always in a slow decrease trend [13]. It can be concluded that the deformed microstructure of pure molybdenum occurs that the oxide particles formed in the microalloyed molybdenum alloys obstruct the grain boundary migration, reduce the grain growth speed, extend the time of recrystallization process, and effectively increase the recrystallization temperature, when the test temperature reaches to the critical value of 1,000 °C. As can be seen from Figure 6(b), the elongation is generally in a declined trend as the annealing temperature increases. The extensibility of pure molybdenum in the range of 800 °C~1,000 °C has a slight increase and then sharply drops, which is attributed to the change in microstructure. At the right temperature, the dynamic recovery of grains enhances the plasticity of this alloy at first. However, with the increase of annealing temperature, the grain size grows up gradually, and the plasticity decreases sharply when the grain coarsening is serious. The extensibility of Mo-0.5Ti alloy falls fast at 1,100 °C, but is still slower than pure molybdenum, which is about to the increase in its recrystallization temperature [15, 16]. Mo-0.1Zr alloy displays good extensibility and plasticity though they present in a slow reduced extent, as the dispersion ZrO₂ particles can refine grains, and thus improve the plasticity (Figure 4).

Figure 6:

Tensile strength and the elongation of molybdenum, Mo-0.5Ti and Mo-0.1Zr after different temperatures of annealing.

Fracture analysis of Mo-0.5Ti and Mo-0.1Zr alloys after annealing

Figure 7 shows the RT tensile fracture feature of pure molybdenum after annealed at different temperatures. The fracture features of samples before and after annealing are similar at 800 °C when the grains are at the recovery stage. With the increase of annealing temperature, the fibrous texture formed from cold deformation gradually transforms into non-distorted equiaxed grains, but the continuous rising of temperature will lead to the mergence and growth of new grains. At 1,000 °C, the fracture surface is characterized by river pattern, and the mode changes from plastic fracture to brittle fracture. In the range of 1,000–1,300 °C, the grains continuously grow.

Figure 7:

Tensile fracture topography of pure molybdenum after different temperatures of annealing (a) unannealing; (b) 800 °C; (c) 1,000 °C; (d) 1,100 °C; (e) 1,200 °C; (f) 1,300 °C.

Figure 8 shows the RT tensile fracture surfaces of the Mo-0.5Ti alloy after annealed at different temperatures. There are lots of dimples on the fracture surfaces of the samples annealed at 800 °C and 1,000 °C while the fracture mode changes into a mix of plastic and cleavage fractures at 1,100 °C and with the increase of annealing temperature the incidence of cleavage fracture tends to increase at the expense of plastic fracture. Compared with pure molybdenum in Figure 8, there exists a higher temperature where the cleavage fracture mode appears, which is coincident with the increase of recrystallization temperature. Moreover, some small particles of the second phase on the fracture surface are confirmed to be Ti₂O₃ by energy spectrum. These oxides broke and were dispersedly distributed on grain boundaries when pressurized during processing, increasing tensile strength by obstructing the dislocation activity, and improving the recrystallization performance by restricting the growth of grain during annealing [10, 17, 18].

Figure 8:

Tensile fracture topography of Mo-0.5Ti alloy after different temperatures of annealing (a) unannealing (b) 800 °C (c) 1,000 °C (d) 1,100 °C (e) 1,200 °C (f) 1,300 °C.

Figure 9 shows the RT tensile fracture feature of Mo-0.1Zr after annealed at different temperatures. The fracture mode turns from plastic fracture into cleavage fracture after annealed at 1,000 °C. The heterogeneity of grain size can be directly observed on the fracture surface at 1,100 °C; that is to say that adding Zr element plays the role of grain refinement, greatly increasing the plasticity of the alloy. Fine grains can be found locally in the alloy, due to the grain refinement effect caused by oxides [19–21]. Especially in Figure 9(e), the fine microstructure leads to an increase in the plasticity of the alloy, especially higher than that of pure Mo and Mo-0.5Ti after annealing at 1,000 °C. In other words, ZrO₂ particles confirmed by EDS analysis in Figure 4(a) play an obvious refinement effect in the Zr-microalloyed alloys, significantly improving the plasticity of the alloy.

Figure 9:

Tensile fracture topography of Mo-0.1Zr alloy after different temperatures of annealing (a) unannealing (b) 800 °C (c) 1,000 °C (d) 1,100 °C (e) 1,200 °C (f) 1,300 °C.