Niobium-Cobalt Intermetallic Alloy Useful for High Temperature Applications

The invention relates to niobium-cobalt (Nb—Co) alloys having a microstructure useful for high-temperature applications in the fields of aerospace and combustion engine technology.

More restrictive exhaust emissions laws for both jet and diesel/natural gas engines have driven changes in engine designs. For jet engine application, one of the approaches to meet the emission regulations is to continue the development of advanced gas turbine propulsion technologies and improve jet engine efficiency. For diesel/natural gas engines the new design includes the need for high-pressure electronic fuel injection systems. Higher temperature and pressure working condition is involved in new jet and diesel/natural gas engine designs.

Hydrogen propulsion concept for powering aircraft may enable the aviation industry to achieve net zero COemission goals in the near future. Hydrogen internal combustion engine (ICE) is one of the approaches to substantially reduce combustion emission compared to diesel/natural gas engine for on-road and off-road application. To adopt hydrogen fuel for jet or ICE application, different combustion behaviors are expected compared to kerosene fuel for jet engine and diesel/natural gas fuel for ICE. To ensure hydrogen fuel jet engine and ICE to achieve designed performance, new alloy/material with good corrosion and hot corrosion resistance along with high temperature strength is needed for making some of the engine components.

Diesel engines built according to the new designs use higher combustion pressures, higher operating temperatures and less lubrication than previous designs. Components of the new designs, including valve seat inserts (VSI), have experienced significantly higher wear rates. Exhaust and intake valve seat inserts and valves, for example, must be able to withstand a high number of valve impact events and combustion events with minimal wear (e.g., abrasive, adhesive and corrosive wear). This has motivated a shift in materials selection toward materials that offer improved wear resistance relative to the valve seat insert materials that have traditionally been used by the diesel and natural gas engine industry.

Various investigations have been made in the past with the goal of improving the high-temperature performance of superalloy compositions.

In view of the service conditions for aerospace/engine parts exposed to high temperature environments, there is a need for alloys having better hot temperature corrosion, abrasive wear and/or impact resistance.

In an embodiment, a cobalt-niobium intermetallic alloy comprises, in weight percent, 35 to 80 wt. % Co, 10 to 45 wt. %, Nb, a total of at least 70 wt. % Co plus Nb, a total of 5 to 30 wt. % Cr, Fe, Ni and Si with up to 10 wt. % Cr, up to 10 wt. % Fe, up to 12 wt. % Ni and up to 3 wt. % Si, balance up to 1.5 wt. % total other elements.

According to various exemplary embodiments, (a) the alloy includes at least 5 wt. % Cr, at least 0.1 wt. % Fe, at least 0.01 wt. % Ni and/or at least 0.5 wt. % Si; (b) the alloy includes at least 5 wt. % Cr, at least 0.01 wt. % Ni, at least 0.1 wt. % Fe and at least 0.5 wt. % Si; (c) the alloy includes up to 1.5 wt. % total of other elements including up to 0.25 wt. % C, up to 0.1 wt. % Mn, up to 0.2 wt. % Mo, up to 0.1 wt. % P, up to 0.1 wt. % S, up to 0.15 wt. % N, up to 0.1 wt. % V, up to 0.1 wt. % Ti, up to 0.1 wt. % Al, up to 0.1 wt. % Hf, up to 0.1 wt. % Zr, up to 0.1 wt. % Ta, up to 0.1 wt. % W and up to 0.05 wt. % B; (d) the alloy includes B in an amount of 0.001 to 0.05 wt. % B; (e) the alloy includes 0.01 to 0.25 wt. % C, 0.01 to 0.1 wt. % Mn, 1 to 3 wt. % Si, 0.01 to 5 wt. % Ni, 5 to 8.5 wt. % Cr, 0.01 to 0.2% Mo, 35 to 80 wt. % Co, 0.1 to 6 wt. % Fe, and 10 to 45 wt. % Nb; (f) the alloy has a cast structure in which an intradendritic region includes NbCoand/or NbCointermetallic phases; (g) the alloy has a cast structure in which an interdendritic region includes a mixture of NbCoand NbCointermetallic phases; (h) the alloy has a cast structure of only the NbCointermetallic phase; (i) the alloy has a cast structure of only the NbCointermetallic phase; (j) the alloy includes 5 to 10 wt. % Cr, 0.01 to 5 wt. % Ni, 0.1 to 6 wt. % Fe and 1 to 3 wt. % Si; (k) the alloy includes 5 to 10 wt. % Cr and 0.01 to 5 wt. % Ni; (l) the alloy includes 5 to 10 wt. % Cr and 0.1 to 6 wt. % Fe; (m) the alloy includes 5 to 10 wt. % Cr and 1 to 3 wt. % Si; (n) the alloy includes 0.01 to 5 wt. % Ni and 0.1 to 6% Fe; (o) the alloy includes 0.01 to 5 wt. % Ni and 1 to 3 wt. % Si; (p) the alloy includes 0.1 to 6 wt. % Fe and 1 to 3 wt. % Si; (q); the alloy can be an engine part such as a valve seat insert useful in an internal combustion engine or an aerospace engine part; (r) the alloy has been melted and cast; and/or(s) the valve seat insert includes 0.01 to 0.25 wt. % C, 0.01 to 0.1 wt. % Mn, 1 to 3 wt. % Si, 0.01 to 5 wt. % Ni, 5 to 8.5 wt. % Cr, 0.01 to 0.2 wt. % Mo, 35 to 80 wt. % Co, 0.01 to 6 wt. % Fe, and 10 to 45 wt. % Nb, balance up to 1.5 wt. % total any other elements.

A cobalt-niobium alloy system (referred to herein as Jonesite 21) can be characterized as a fully intermetallic phase superalloy for high temperature aerospace or automotive applications such as gas turbine, jet engine, and internal combustion (IC) engine valve train component applications such as valve seat insert (VSI) applications.

A general alloy design concept for Jonesite 21 is an alloy composition which can be solidified into NbCoand/or NbCophases as illustrated with the binary phase diagram of Nb and Co shown in.

Jonesite 21 can include one or more intermetallic phases depending on the ratio of cobalt (Co) to niobium (Nb). In an embodiment, a casting of Jonesite 21 can have a microstructure with 100 vol. % of NbCoor 100 vol. % of NbCoor various ratios of these two intermetallic phases depending on the Co and Nb contents. In addition, chromium (Cr), iron (Fe), nickel (Ni) and/or silicon (Si) can be included in the alloy system as alloying elements to control the size and distribution of solidification substructure in Jonesite 21.

An embodiment of the Jonesite 21 alloy system can be illustrated with reference to, Nb—Co binary phase diagram (in weight percent). The compositional range of Jonesite 21 in the binary phase diagram is from 37.4 wt. % Co to 57.4 wt. % Co without considering the other alloying elements (Cr, Si, Fe, Ni, etc.). As shown in the phase diagram, at 37.4 wt. % Co the solidified structure will be 100% NbCowhereas at 57.4 wt. % Co the solidified structure will be 100% NbCo. At 45.5 wt. % Co the solidified structure will be 50 vol. % NbCoand 50 vol. % of NbCo. For Co contents between 37.4 and 45.5 wt. %, the solidified structure will be a mixture of NbCoand NbCowith the amount of NbCovarying between 100 vol. % to 50 vol. % as the amount of Co increases from 37.4 wt. % to 45.5 wt. %. Likewise, the solidified structure will be a mixture of NbCoand NbCowith the amount of NbCovarying between 50 vol. % to 0 vol. % as the amount of Co increases from 45.5 wt. % to 57.4 wt. %.

At the fully eutectic point, cobalt content is 45.5 wt. %. Thus, the Co to Nb ratio (wt. %) is about 0.83. Depending upon whether the Co to Nb ratio is on the left or right side of the fully eutectic point (45.5 wt. % Co), the initial solidification phase can be NbCoor NbCo. For example, if more NbCophase is preferred, then the specific alloy should have a Co to Nb ratio (wt. %) between 0.83 to 1.35. Likewise, if more NbCophase is preferred, then the specific alloy should have a Co to Nb ratio (wt. %) between 0.6 to 0.83.

In an embodiment, Jonesite 21 has a total Co plus Nb content of at least 70 wt. % and 5 to 30 wt. % total Cr, Ni, Fe and Si with up to 10 wt. % Cr, up to 10 wt. % Fe, up to 12 wt. % Ni and up to 3 wt. % Si. In another embodiment, Jonesite 21 can have a Co to Nb ratio of 0.6 to 1.35.

Jonesite 21 can include one or more other elements which enhance properties of the alloy. For example, such elements include effective amounts of chromium (Cr), silicon (Si), iron (Fe), nickel (Ni), and boron (B). In addition, Jonesite 21 can be free of intentional additions of other elements such as carbon (C), phosphorus (P), sulfur(S), nitrogen (N), oxygen (O), manganese (Mn), molybdenum (Mo), vanadium (V), aluminum (Al), titanium (Ti), hafnium (Hf), zirconium (Zr), tantalum (Ta), and tungsten (W). Depending on the purity of alloying elements, some elements can be present in incidental/impurity total amounts of 1.5 wt. % or less. For Nb, Cr, Ni, Fe and Si, it is preferred to use 99%+ pure Nb, Cr, Ni, Fe, and Si additions. However, an addition of ferrochromium can be used as the Cr/Fe addition in which case the ferrochromium typically has a mixture of about 75 wt. % Cr and 25 wt. % Fe. Thus, if ferrochromium is used to provide a Cr content of 5-10 wt. % Cr, Fe would be present in an amount of about 1.25 to 2.5 wt. %.

In an embodiment, Jonesite 21 includes 5 to 30 wt. % total Cr, Si, Fe and/or Ni and less than 1.5 wt. % total C, B, Mn, Mo, P, S, N and other impurities including V, Ti, Al, Hf, Zr, Ta and W. For example, Jonesite 21 can include up to 0.25 wt. % C, up to 0.05 wt. % B, up to 0.1 wt. % Mn, up to 0.2 wt. % Mo, up to 0.025 wt. % P, up to 0.025 wt. % S, up to 0.15 wt. % N, up to 0.1 wt. % V, up to 0.1 wt. % Al, up to 0.1 wt. % Hf, up to 0.1 wt. % Zr, up to 0.1 wt. % Ta and up to 0.1 wt. % W.

In an embodiment, Jonesite 21 can be free of B or include up to 0.05 wt. % B. In an example, B can be added in an amount effective for grain refinement such as in a range of 0.001 to 0.005 wt. % B or 0.005 to 0.01 wt. % B.

In an embodiment, Jonesite 21 can include Cr in an amount effective to refine the microstructure. Preferably, Cr is added in an amount of 5 to 10 wt. %. In an example, Cr is added in a range of 5 to 6 wt. % Cr, 6 to 7 wt. % Cr or 7 to 8 wt. % Cr or 8 to 8.5 wt. % Cr.

In an embodiment, Jonesite 21 can include Fe in an amount effective to reduce the cost of the alloy additions. Preferably, Fe is added in an amount of up to 10 wt. %, such as 1 to 6 wt. % Fe. In an example, Fe can be added in a range of 1 to 2 wt. % Fe, 2 to 3 wt. % Fe, 3 to 4 wt. % Fe, 4 to 5 wt. % Fe or 5 to 5.5 wt. % Fe.

In an embodiment, Jonesite 21 can include Ni in an amount effective to reduce the cost of alloy additions. Preferably, Ni is added in an amount of up to 12 wt. %. In an example, Ni can be added in a range of 0.01 to 1 wt. % Ni, 1 to 2 wt. % Ni, 2 to 3 wt. % Ni, 3 to 4 wt. % Ni or 4 to 5 wt. % Ni.

In an embodiment, Jonesite 21 can include Si in an amount effective to promote castability. Preferably, Si is added in an amount of up to 3 wt. %. In an example, Si can be added in a range of 1 to 3 wt. % Si such as 1 to 2 wt. % Si or 2 to 3 wt. % Si.

In an embodiment, Jonesite 21 can include up to 0.25 wt. % C. In an example, C can be added in a range of 0.01 to 0.15 wt. % C or 0.15 to 0.25 wt. % C.

In an embodiment, Jonesite 21 can include up to 0.1 wt. % Mn. In an example, Mn can be added in a range of 0.01 to 0.06 wt. % Mn.

In an embodiment, Jonesite 21 can include up to 0.2 wt. % Mo. In an example, Mo can be added in a range of 0.01 to 0.1 wt. % Mo or 0.1 to 0.2 wt. % Mo.

In an embodiment, Jonesite 21 can include up to 0.15 wt. % N. In an example, N can be added in a range of 0.01 to 0.1 wt. % N or 0.1 to 0.15 wt. % N.

In an embodiment, Jonesite 21 can include up to 0.1 wt. % Al. In an example, Al can be added in a range of 0.001 to 0.1 wt. % Al.

In an embodiment, Jonesite 21 can include incidental amounts of refractory elements V, Ti, Hf, Zr, Ta and W in a total amount of up to 0.1 wt. % each. For example, Jonesite 21 can include up to 0.1 wt. % V, 0.1 wt. % Ti, up to 0.1 wt. % Hf, up to 0.1 wt. % Zr, up to 0.1 wt. % Ta and up to 0.1 wt. % W.

Exemplary compositions of Jonesite 21 are set forth in Table 1.

In the above Table 1, the Jonesite 21 alloys have incidental amounts of other elements including phosphorus (P) and sulfur(S) with a maximum of 0.025% P and a maximum of 0.025% S.

Properties of Jonesite 21 are compared to two other LE Jones alloys called Jonesite 19 and J513. In Table 2, alloy heats 9H20XA, 2D27XA, 2J12XA and 9H26XA correspond to Jonesite 19 and alloy heat 9K19XA corresponds to J513.

In the above Table 2, the Jonesite 19 and J513 alloys have incidental amounts of other elements including phosphorus (P) and sulfur(S) with a maximum of 0.025% P and a maximum of 0.025% S.

show a general microstructure and microstructural distribution in Jonesite 21 under 200×, 1000×, and 3000× magnification backscattered electron images, respectively. It is clearly shown that under normal casting solidification conditions, the intermetallic phases exhibited very uniform distribution, at large, in cellular dendritic morphology. In the interdendritic regions, finer lamella eutectic phases of NbCoand NbCocan be identified.shows an SEM image marked with three locations where EDS analysis was performed representing matrix intermetallic phase (spot 1-intradendritic region), interdendritic region with lamella eutectic phases (spot 2-intradendritic region), and darker particle phase (spot 3-dark particle in interdendritic region). The detailed EDS analysis results for the three spots can be found in, respectively.

For an alloy with a fixed Nb to Co ratio, e.g. 53.0 at. % Co and 47.0 at. % Nb, based on the Nb—Co binary phase diagram and using the lever rule, the % of each intermetallic phase can be estimated. Here, |53.0 at. %-56.8 at. % (eutectic point)|=3.8 at. %; 3.8 at. %/(56.8 at. %-49.5 at. %)=3.8 at. %/7.3 at. %-0.521. Thus, the alloy contains approximately 52 at. % of NbCoand 48 at. % of fully eutectic (56.8 at. % Co) phases. The fully eutectic phases contain, |56.8 at. %-67 at. %|/(67 at. %-49.5 at. %)=0.583, approximately 58 at. % of NbCoand 42 at. % of NbCo. Thus, the alloy can have a mixture of about 79.9 at. % of NbCoand about 20.1 at. % of NbCo.

shows elemental dot mapping in Jonesite 21 showing major alloying elemental distribution primarily related to three phases namely, Nb—Co intermetallic, Co—Nb intermetallic and Co—Cr—Nb phase.

is a graph of hot hardness comparing Jonesite 21 to other LE Jones alloys Jonesite 19 and J513. Hot hardness values for measurements at various temperatures are set forth in the following Table 3 in which sample heat 2K29XA corresponds to Jonesite 21, sample heat 2D27XA corresponds to Jonesite 19 and sample heat 9K19XA corresponds to J513.

As can be seen from the hot hardness data in Table 3, Jonesite 21 has a higher hot hardness than Jonesite 19 but like Jonesite 19, the hot hardness of Jonesite 21 is sustained at high temperatures compared to J513.is a graph of hot hardness of Jonesite 21 compared to Jonesite 19.

In hardness and radial crush tests (8.33×ft-lbf) versus tempering temperature, Jonesite 21 heats 2K16XA (#1), 2K29XA (#2) and 3B22XA (#3) were compared and the results are listed below in Table 4.

The hardness data from Table 4 is illustrated in the graph of hardness versus tempering temperature shown inand the radial crush data is illustrated in the graph of compressive yield strength (CYS) versus tempering temperature shown in. The heats tested are 2K16XA (data points shown as circles), 2K29XA (data points shown as diamonds) and 3B22XA (data points shown as triangles). The alloys tested were hardened at 1700° F. and tempered for 3 hours. Sustainable bulk hardness was exhibited in Jonesite 21 from ambient to greater than 800° C. There was a general trend that an increase in tempering temperature increased the radial crush toughness within a range from ambient through 816° C.

Thermal conductivity data is listed below in Table 5 and illustrated in the graph shown inin which Jonesite 21 corresponds to heats 9H11XA and 2K29XA and Jonesite 19 corresponds to heats 9H26XA and 2J12XA.

Thermal conductivity data of Jonesite 21 shows the influence of Nb to Co ratio. For Heat 9H11XA the Nb:Co ratio is 11.56 wt. %/76.4 wt. %=0.151 and for Heat 2K29XA the Nb:Co ratio is 26.69 wt. %/56.8 wt. %=0.469. Accordingly, the thermal conductivity data shows lower thermal conductivity for alloys with a higher Nb:Co ratio. Low thermal conductivity of Jonesite 21 at lower temperature can be beneficial to some engineering applications; for instance, potential application as surface coating material for hydrogen storage or transportation equipment.

HBJ(60) hardness measurement is a bulk hardness scale testing method. Not like a microhardness test such as Vickers hardness, HBJ hardness has, at large, a good correlation with compressive yield strength for metals and alloys. The hardness indenter applied for HBJ(60) is ⅛″ spherical in shape and made of aluminum oxide type ceramic material. Compared to Vickers hardness that the indenter is made of diamond, a higher maximum testing temperature can be applied for HBJ(60) testing method than Vickers hardness.

is a graph showing a bulk hardness comparison among Jonesite 21, Jonesite 19, and J513. The heat composition of Jonesite 21, Jonesite 19 and J513 adopted for the comparative study can be summarized in Table 6. The hardness range measured within a temperature range from ambient to 900° C. Evidently, under 500° C., J513 possessed higher bulk hardness than Jonesite 21 and Jonesite 19. When the test temperature was greater than 500° C., the bulk hardness of J513 was lower than Jonesite 21 and Jonesite 19. The testing results have manifested that Jonesite 19 and Jonesite 21 possess a higher elevated temperature capability than J513.

The thermal expansion behavior (dilatometry investigation) of Jonesite 21 (3E11XA) was conducted using Linseis dilatometer Model L75HS1000C. The test specimen dimension was ¼″ in diameter and 1″ long. Three specimen heating rates, 1°/min, 3°/min, and 6°/min were applied. It was revealed that thermal expansion coefficient of Jonesite 21 was very similar to common martensitic type of steels but lower than conventional high temperature nickel- or cobalt-based alloys.

A linear behavior of Jonesite 21 was observed when heating rate of 1°/min and 3°/min was applied, respectively. Evidently, no bulk solid phase transformation occurred within the temperature range from ambient to 999° C. As the heating temperature rate increased to 6°/min, only a slight change of thermal expansion rate as a function of temperature was observed. The results of the dilatometry investigation are shown in.