Investigation on high temperature deformation mechanism and strengthening of V-4Cr-4Ti alloys V-4Cr-4Ti合金の高温変形機構と高強度化に関する研究
Investigation on high temperature deformation mechanism and strengthening of V-4Cr-4Ti alloys
Strengthening methods for materials have been developed for steels and other alloys. These methods include work hardening, precipitation hardening, nanoparticle-dispersion strengthening, and grain-refining strengthening. In this study, analysis of low-activation V-4Cr-4Ti alloy indicated that work hardening followed by precipitation hardening is more effective than work hardening after precipitation hardening for improving creep resistance at high temperature and high stress, due to the pinning of dislocation glide. Nanoparticle-dispersion strengthening exhibited more attractive features to strengthen this alloy. Grain-size refining enhanced hardness and tensile strength; however, creep resistance was not sufficiently improved. Based on these results, the guiding principle for further improving creep strength was obtained with a proposal regarding the control of microstructures, such as grain size and precipitate/nanoparticle distribution.Research on structural materials for the blanket is an important part of fusion research. Recently, low-activation V-4Cr-4Ti alloy has been identified as an attractive candidate for the self-cooled liquid lithium blanket, which will be operated at 700oC in fusion reactors. In order to achieve improved energy-conversion efficiency by higher temperature operation, the V-4Cr-4Ti alloy needs to be strengthened further. To achieve effective strengthening, the high-temperature deformation mechanism of the V-4Cr-4Ti alloy must be investigated. In the present study, the starting idea for strengthening V-4Cr-4Ti alloy is to increase its yield stress in order to resist dislocation motion and thus resist deformation of the material. Various strengthening mechanisms have been introduced for combinations seeking to improve high-temperature strength. (1) Work hardening combined with precipitation hardening and (2) nanoparticle-dispersion strengthening coupled with grain-size refining are the two combined strengthening methods in the present study. Combination (1) is applied to the reference V-4Cr-4Ti alloy plate, NIFS-HEAT-2, using thermo-mechanical treatment (TMT). Combination (2) has been carried out by fabricating a new V-4Cr-4Ti alloy via mechanical alloying (MA). In the MA process, Y is introduced as a scavenger for gaseous impurities. TiC, SiC, and Ti3SiC2 are used as dispersion particles.For the combination of work hardening with precipitation hardening, the order of work hardening after precipitation hardening is designated as Solution Annealing +Aging +Cold Working (SAACW), and the order of work hardening followed by precipitation hardening is designated as Solution Annealing +Cold Working +Aging (SACWA). A standard (STD) heat treatment is introduced as the reference. Mechanical test results indicate that work hardening combined with precipitation hardening in either order can lead to a remarkable increase in yield stress of the V-4Cr-4Ti alloy (NIFS-HEAT-2) at room temperature (RT), 700oC, and 750oC. However, they differ in creep characteristics. SAACW and STD behave similarly, while SACWA behaves differently. At stresses less than 180MPa, the creep rate of SACWA is higher than that of SAACW and STD. In contrast, at stresses exceeding 180MPa, SACWA has a lower creep rate than SAACW and STD. Creep-activation energy indicates that SACWA has a dislocation climbing creep mechanism in the whole testing stress region. SAACW and STD share the dislocation climbing creep mechanism mainly at stresses below 180MPa, then transition to the dislocation-glide creep mechanism at stresses exceeding 180MPa. Microstructural analysis indicates that dislocations in SAACW and STD after creep deformation are mainly a/2<111> type, while crept SACWA has a higher number density of dislocations in a/2<111> type and possibly in a<100> type which was introduced by the prior cold working. It is assumed that a<100> type dislocations in SAACW are not strongly decorated by the precipitates because the dislocations were introduced after the precipitation, and they mostly disappeared soon after creep deformation starts. In contrast, SACWA has precipitates distributed along the dislocations introduced by the prior cold working because those dislocations supply the precipitation sites. The interaction of dislocations with precipitates in SACWA is strong enough to resist the coarsening of precipitates as well as a decrease in possible type a<100> dislocation density. Accordingly, the special interaction of dislocations with the precipitates along them draws a resisting capability over the threshold stress for dislocation glide. As a result, SACWA maintains dislocation climbing throughout the examined stresses.SACWA and STD have also been evaluated in Li exposure. The present study uses static liquid Li at 650oC for 250hr exposure.The stabilization of precipitates by dislocations in SACWA is sustained in this Li exposure. Both SACWA and STD are strengthened by Li exposure due to the trapping of impurities such as C and N from liquid Li. However, O transfer from V matrix to Li softens the alloy. Such mass transfers are due to the chemical affinity of those impurities for Li competing with V, Cr, and Ti. It is evident that the hardened ranges are equivalent to the diffusion ranges for C and N. Strengthening due to C and especially N mass transfers can decrease the creep rate of V-4Cr-4Ti alloy because of the increased number density of precipitates. However, the contaminants are considered to be able to induce just limited hardening near the surface area of a blanket wall. The diffusion range of N, for example, at 100,000hr is estimated as 1.55mm, smaller than the typical blanket wall thickness (5mm).The second combination, nanoparticle-dispersion strengthening with grain-size refining via MA, has been successfully achieved. In the present study, MA was conducted for the candidate V-4Cr-4Ti system for the first time. Strengthening increases with increasing MA time. After the parametric survey, 40hr is determined as the minimum process time required for MA of V-4Cr-4Ti alloy with Y and TiC additions. This is mainly determined by the dissolution of Ti. The MA process study found that the dissolution of Y into the V matrix is faster than that of Cr, and the dissolution of Cr is faster than that of Ti. Different dissolution capabilities are also found for the carbides. In detail, TiC behaves similarly to SiC, and they dissolve faster than Ti3SiC2 into the V matrix.Hardness and tensile strength are increased by long-term MA because of more complete solid-solution hardening, improved nanoparticle-dispersion strengthening, and enhanced grain-refining strengthening. Transmission electron microscope (TEM) observation indicates that the V-4Cr-4Ti alloy has high number densities of TiN and Y2O3 nanoparticles. These two species of nanoparticles are considered the main strengthening agents in improving nanoparticle-dispersion strengthening. Annealing at 1200oC for 1hr indicates that Y2O3 has higher thermal stability than TiN. The grain size of mechanically alloyed V-4Cr-4Ti-1.5Y-0.3TiC alloy is 0.37m after annealing, which is much smaller than that of NIFS-HEAT-2 in STD state (17.8m). These ultra-fine grains can account for the strengthening, due to grain-size refining.Refined grains generally degrade creep resistance due to grain boundary deformation. The creep resistance of the mechanically alloyed V-4Cr-4Ti-1.5Y-0.3TiC alloy is degraded at 280MPa, compared with that of NIFS-HEAT-2 in either STD or SACWA state. Creep deformation in this stress region may be dominated by grain-boundary deformation. However, in a creep test at 100MPa, the creep rate of the mechanically alloyed V-4Cr-4Ti-1.5Y-0.3TiC alloy is improved to 1/9 times lower level than that of NIFS-HEAT-2 in SACWA states. The high number density of nanoparticles in the grain interior is believed to effectively resist the dislocation motion at 100MPa. Based on the above discussions, the stabilization of precipitates by dislocations in SACWA is the assumed reason for enhanced resistance to dislocation glide creep at stress above 180MPa. These dislocations in SACWA also increase dislocation climbing creep at lower stress (e.g., 100MPa, which is the assumed design stress for blanket structures). Therefore, introducing more precipitates without high density of dislocations is desirable. When strengthening V-4Cr-4Ti alloy via MA, collisions between the alloy powder and milling balls cause dissolutions of alloying elements and carbides for solid-solution hardening, along with nanoparticle-dispersion strengthening and refined grains. These refined grains may cause grain boundary deformation to the alloy and degrade its creep resistance under high stress. In such a situation, the high number density of thermally stable nanoparticles without coexisting refined grains is preferable. In conclusion, we successfully strengthened the V-4Cr-4Ti alloy using various mechanisms and enhanced creep resistance by combined strengthening. We also developed a guiding principle for strengthening V-4Cr-4Ti alloy for high temperature applications. This principle is to introduce high number densities of precipitates or nanoparticles as well as low number densities of dislocations and fewer grain boundaries.