Patent Application
20080031769
Preferred embodiments of the invention will be described in detail with reference to
The hardness of the alloy specimens was measured by a Vickers hardness tester (Model MV-1, Matsuzawa Seiki Co., Ltd.) under a load of 5 kgf, for a loading duration of 15 seconds, and at a rate of 50 μm/s. The hardness of each specimen was taken from the mean average of five random hardness measurements. Before the hardness measurements, the surface of each alloy specimen was ground flat by sequentially using #80, #180, #240, #400, #600, #800, and #1200 silicon carbide (SiC) grinding papers.
The microstructure of the specimens was observed by both optical (OM) and scanning electron microscopes (SEM, JEOL-5410). The composition was analyzed using an energy dispersive spectrometer (EDS). The specimen to be analyzed was cut with a diamond cutter. During cutting, in order to limit the effect of the heat generated from affecting the microstructure, the sample was water cooled. The cut specimens were then ground flat by sequentially using #180, #240, #400, #600, #800, #1200, #2000, and #4000 SiC grinding papers, and underwent a final polishing stage using a 3 μm diamond paste. The polished specimen was then etched using aqua regia (HNO3+3HCl) in order to facilitate observation of the microstructure.
The crystal structure of the specimens was evaluated by X-ray diffraction (XRD). The X-ray diffractometer (Rigaku ME510-FM2) employed a Cu target X-ray source operated at 30 kV, 20 mA, producing radiation with a wavelength 1.54056 Å. XRD scans were performed over a scan range of 20 to 100 degrees 2θ, at a scanning speed of 4 degrees/min.
Table 1 shows the compositions of the alloys selected for this embodiment. The specimens, which are labeled HE1 to HE11, are multi-principal-element alloys prepared by adding an appropriate amount of Al, Ti, or Mo to the Co1.5CrFeNi1.5 base alloy. The base alloy has a Vickers hardness of HV113 and an FCC crystal structure. All of the raw elemental materials have a purity of greater than 99%. Table 2 lists the basic properties of each principal element in these alloys, including their atomic weight, atomic size, melting point, boiling point, density, crystal structure, and the transition temperature of their polymorphs.
The crystal structure and hardness of the eleven alloys prepared according to Table 1 have, as shown in Table 3, promising alloy properties. The addition of Al, Mo, and Ti increases the hardness of the base Co1.5CrFeNi1.5 alloy system, which has a hardness of 113 HV5.0. It is apparent that Ti is the most effective element for enhancing the hardness, whereas Al has the least pronounced effect. Although increasing the amount of each of these elements enhances the hardness, it is found that a BCC phase develops as the second phase. Nevertheless, the principal crystal structure of the alloys is still FCC, and therefore by adding a different amount of these three elements the alloy properties can be adjusted for applications needing, for example, high-temperature strength can be obtained.
In accordance with the alloy preparation and processing flow chart, shown in
The change in hardness is given in Table 5 for the homogenized (furnace-cooled) alloy specimen (Co1.5CrFeNi1.5Ti0.5) which underwent further cold rolling for different thickness reductions of 0%, 5%, 15%, 30%, and 80%, respectively on a two-high rolling machine (Model: DBR250). The hardness of the rolled alloy specimen is enhanced with the increasing reduction. The hardness of the sample is seen to increase significantly at a thickness reduction of 30%, where the hardness of the alloy specimen is about 1.78 times higher than that of the original sample (0% reduction). Hence, the alloy displays excellent work hardening behavior. The intensity of the XRD peaks pertaining to the FCC structure of the Co1.5CrFeNi1.5Ti0.5 alloy gradually decreases with increasing reduction. This is a result of the increased lattice distortion for the worked alloy, which causes a decrease in the constructive interference of the diffracted X-ray radiation.
Table 6 presents the alloy compositions selected for this embodiment. Specimens are labeled HE12 to HE22, and are multi-principal-element alloys prepared by incorporating a suitable amount of Al, Ti, or Mo into a Co2CrFeNi2 base alloy. The hardness of the base alloy is 108 HV5.0, and it possesses an FCC crystal structure. The purity of the elemental raw materials is higher than 99%.
The crystal structure and hardness of the eleven alloys prepared in this embodiment, as given in Table 6, have, as provided in Table 7, promising alloy properties. The addition of Al, Mo, and Ti increases the hardness of the base Co2CrFeNi2 alloy system. It is apparent that Ti is the most effective element for enhancing the hardness, whereas Al has the least pronounced effect. Although increasing the amount of each of these elements enhances the hardness, it is found that a BCC phase develops as the second phase. Nevertheless, the principal crystal structure of the alloys is still FCC, and therefore by adding a different amount of these three elements, the alloy properties can be adjusted for applications needing, for example, different high-temperature strength can be obtained.
Specimens of HE13 alloy (Co2CrFeNi2Ti0.5), of Embodiment 3, were heated in an air furnace to a temperature of 1000° C. for 15 minutes, and then subjected to hot forging using a pneumatic forging machine (Model: OT-1521280). Forging was conducted at a load of 250 kg to obtain a thickness reduction of 40%. Next, the forged specimens were placed in a furnace under ambient atmosphere and homogenized at 1100° C. for 24 hours, after which they either underwent furnace cooling or water quenching. The hardness of the specimens at each stage is given in Table 8. The hardness is enhanced by about 28% after forging, then decreases after homogenization combined with furnace cooling, but is elevated slightly after homogenization combined with water quenching. This alloy is not found to suffer high-temperature softening at 1100° C., and therefore exhibits excellent high temperature performance. At each stage, only a single FCC phase can be identified from XRD analysis.
The change in hardness is given in Table 9 for the homogenized (furnace-cooled) alloy specimen (Co2CrFeNi2Ti0.5) which underwent further cold rolling for different thickness reductions of 0%, 5%, 15%, 30%, and 70%, respectively on a two-high rolling machine (Model: DBR250). The hardness of the rolled alloy specimen is increased with the increasing reduction. The hardness of the sample is seen to increase significantly at a thickness reduction of 30%, where the hardness of the alloy specimen is about 1.57 times higher than the original sample (0% reduction). Hence, the alloy displays excellent work hardening behavior. The intensity of the XRD peaks pertaining to the FCC structure of the Co2CrFeNi2Ti0.5 alloy gradually decreases with increasing reduction. This is a result of the increased lattice distortion for the worked alloy causing a decrease in the constructive interference of the diffracted X-ray radiation.
The alloy compositions selected for this embodiment are given in Table 10, with the specimens labeled HE23 to HE40. The eighteen alloys are multi-principal-element alloys prepared by incorporating a suitable amount of at least one minor element, such as Ag, B, C, Cu, Mn, Nb, Ta, Si, V, W, Y, and Zr, to a base alloy having compositions of HE1 to HE9 in Table 1, or HE12 to HE20 in Table 6. The purity of the elemental raw materials is higher than 99%.
The crystal structure and hardness of the eighteen alloys prepared according to Table 10 are given in Table 11, and convey that these alloys have promising alloy properties. The hardness varies with the addition of the minor elements, as seen in Table 11. By comparing the results of Tables 3, 7 and 11, it can be observed that, apart from Ag and Cu, the addition of other minor elements enhances the hardness. Although the addition of some of the elements increases the formation of a BCC phase, the principal phase is still FCC. Therefore, by using a suitable amount(s) or a specific type(s) of minor element(s), the alloy properties can be adjusted for applications needing, for example, different high-temperature strength can be obtained.
According to another embodiment of the invention, the as-cast Co1.5CrFeNi1.5Ti0.5 alloy specimens were hardened by a high-temperature aging process. The cast specimens were placed in a furnace and treated at temperatures of 400° C., 600° C., and 800° C. for aging times of 1, 2, 5, and 10 hours. The results are shown in Table 12. A rare high-temperature age hardening phenomenon can be observed at 800° C., especially after a duration of 5 hours, for which the hardness of the alloy increased from 378 to 513HV5.0. After 10 hours the hardness is about 1.33 times better than that of the as-cast specimen (i.e. the specimen that was not aged).
It is understood from the aforementioned description that the invented alloy system can be expressed as (Co, Cr, Fe, Ni)xMyNz, where M is at least one element selected from Al, Mo, and Ti, and N is at least one minor element selected from Ag, B, C, Cu, Mn, Nb, Ta, Si, V, W, Y, and Zr. The values of x, y and z are ≧65, 5 to 25, and 0 to 10 atomic percent, respectively. The Co and Ni elements both have contents from 20 to 35 atomic percent, and those of Cr and Fe is 12.5 to 20 atomic percent.
The alloy systems all possess a principal FCC phase. As the FCC phase has twelve independent slip systems, it is easy to slip and deform, and therefore it has good ductility. Furthermore, the strength of an FCC structure is well known to be retained to high temperatures. Thus, by the suitable adjustment of the alloy composition, the presently invented multi-principal-element alloys, with an FCC structure as the matrix, can be tailored to have different ranges of strength, and be operated at room or high-temperatures, depending on their applications. Furthermore, as the invented alloys contain at least 12.5 atomic percent of Cr, and at least 20 atomic percent of both Co and Ni, they should have improved corrosion and oxidation resistance. Thus, the alloy should, in principle, be resistant to corrosion and oxidization in high-temperature environments. In addition, since the amount of Co in the alloy is less than 35 atomic percent, the cost is relatively low compared to the present Co-based metal alloys, which have Co contents of at least 50 atomic percent. Hence, the invention utilizes the concept of multi-principal-element alloy design to develop an alloy system that is novel, inventive, cost effective, and industrially applicable.
While the invention has been described by means of examples, and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. For example, the processing steps in FIG. 1—forging, thermal homogenization, rolling, and age hardening—are for an illustrative purpose, and are therefore not only limited to the sequence described. For instance, in some cases, just one or a combination of the steps will need to be carried out. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 95127668 | Jul 2006 | TW | national |
