US20190040500A1

US20190040500A1 - Precipitation strengthened high-entropy superalloy

Info

Publication number: US20190040500A1
Application number: US15/713,845
Authority: US
Inventors: An-chou Yeh; Te-Kang Tsao
Original assignee: National Tsing Hua University NTHU
Current assignee: National Tsing Hua University NTHU
Priority date: 2017-08-02
Filing date: 2017-09-25
Publication date: 2019-02-07
Also published as: TW201910530A; TWI670377B

Abstract

The present invention discloses a new alloy design of precipitation strengthened high entropy superalloy (HESA), which is composed of at least one principal element, a plurality of base element, and at least one precipitation strengthening element for controlling the elemental segregation between the high-entropy matrix and ordered precipitate. Through the addition of the precipitation strengthening element, while substituting the same amount of the principle element, not only the ordering energy and the volume fraction of strengthening precipitates, but also the mechanical strength of the alloy can be apparently elevated. Therefore, this newly-developed precipitation strengthened HESA can further improve the thermal capability and mechanical properties from the previously proposed high-entropy alloys.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the technology field of superalloys, and more particularly to a precipitation strengthened high-entropy superalloy with significant volume fraction of ordered precipitate.

2. Description of the Prior Art

Superalloy has been a highly economic material for high temperature application due to their outstanding high temperature properties. For example, they can exhibit good mechanical strength such as high creep and fatigue strength till temperatures above 600° C. In addition, they possess good resistance against environmental corrosion and oxidation. Therefore, superalloys have been widely used in aerospace industry, energy industry and electronic industry, etc.
Conventional superalloys can be categorized into Iron-Nickel based superalloy, Cobalt based superalloy and Nickel based superalloy. Among these, Nickel based superalloy is mainly composed of Nickel (Ni) as the principal element with a specific range of weight percent from 30 wt % to 50 wt %. Some strengthening element like Al, Co, Cr, Ti, or Nb may added into the Nickel matrix. Furthermore, in order to improve the high temperature fatigue and creep strength, the refractory elements such as Mo, Ta, W, Re, or Ru would usually be added for solution strengthening. However, the alloying of refractory elements not only lowered the surface and phase stability of Nickel based superalloys significantly, but also limited their applications due to growing cost for manufacturing.
Accordingly, a new alloy design of high entropy superalloys has been disclosed by literature 1. Literature 1, written by Tsao et al., is entitled with “High Temperature Oxidation and Corrosion Properties of High Entropy Superalloys” and published on Entropy, 18(2),62, doi: 10.3390/e18020062. Differing from the common alloy design of Ni-based superalloys, Tsao et al. has optimized conventional superalloy by elevating the mixing entropy of the alloy to high entropy level. Particularly, Tsao et al. defined that a high entropy superalloy must contain at least one principal element with a first element content of at least 35 at % and at least one principal strengthening element with a second element content of over 5 at %, thus leads to the mixing entropy (ΔS_mix) of the alloy equal to or over 1.5 R. Therefore, the so-called “high entropy superalloy” (HESA) can benefit from the high entropy strengthening effects and show the advantages in lower density and lower cost of materials due to less refractory additions. In addition, the experimental results have indicated that HESA possess good microstructure stability, hot corrosion and oxidation resistance, as well as high hardness, tensile and creep strength at elevated temperatures.
The present invention relates to the manufacturing of HESA are disclosed by literature 1. Please refer to the following Table (1); the samples are named MA-1, MA-2, HA-1, and HA-2, and all of them contain the ordered precipitates (L1₂γ′ phase). Furthermore, the mixing entropy of the samples are in a range from mid [1.32R (MA-1) and 1.46R (MA-2)] to high [1.55R (HA-1) and 1.60R (HA-2)] entropy level.

TABLE 1

	Ni	Al	Co	Cr	Fe	Ti		γ′
	(at	(at	(at	(at	(at	(at	ΔS_mix	solvus
Sample	%)	%)	%)	%)	%)	%)	(R)	(° C.)

MA-1	58.1	10.1	13.8	6.3	4.9	6.8	1.32	1234
MA-2	50.5	8.9	17.2	9.2	8.2	6.0	1.46	1146
HA-1	42.8	7.8	20.5	12.2	11.5	5.2	1.55	1087
HA-2	35.1	6.6	23.9	15.2	14.8	4.4	1.60	1013

In spite of the fact that the increase of mixing entropy can result in the high entropy strengthening effect, Table (1) indicates that the solvus temperature of γ′ phase would decrease with increasing mixing entropy of the alloy, which markedly limits the high-temperature applications of HESA. FIG. 1 shows a graph for describing the relationship of mixing entropy versus ordering energy and solvus temperature of γ′ phase. From FIG. 1, it can be understood that the ordering energy of γ′ phase would be reduced due to the more random atomic arrangement in the ordered lattice, and thus lowers the thermal stability.
In summary, literature 1 has revealed that high entropy superalloy (HESA) contains one principal element with at least 35 at % element content and multi principal strengthening elements with element content exceeding 5 at %, as well as a mixing entropy of the alloy equals to or greater than 1.5R. With the strengthening from high entropy effects, HESA can exhibit good high temperature properties and better cost-performance to conventional superalloys. However, the thermal stability of the ordered precipitate would be lowered due to the increase of mixing entropy.
From the above descriptions, it is critical to find a method for remaining good high temperature stability while increasing the mixing entropy of the alloy. As a result, present invention regards to the new precipitation strengthened high-entropy superalloy has made great efforts to solve this issue.

SUMMARY OF THE INVENTION

The goal of present invention is to provide a new alloy design for precipitation strengthened high-entropy superalloy (HESA). This precipitation strengthened HESA is composed of at least one principal element, a plurality of base element, and at least one precipitation strengthening element for controlling the elemental segregation between high entropy matrix and ordered precipitate. Therefore, not only the ordering energy and volume fraction of strengthening precipitate can be apparently elevated, but also the high entropy matrix can be further strengthened, which leads to their outstanding high-temperature mechanical properties as well as the well-balanced thermal stability.
The present invention provides an embodiment for the precipitation strengthened high-entropy superalloy, comprising:

at least one principal element with a first element content of at least 35 at %;
a plurality of base elements for forming a high-entropy matrix (ΔS_mix≥1.5 R) with the principal element, wherein each of the base elements has a second element content of at least 5 at %; and
at least one strengthening element with a third element content of at least 0.1 at %, for forming an ordered precipitate with the principal element and/or the base elements;
wherein the high-entropy matrix and ordered precipitate have a first volume fraction and a second volume fraction, respectively; and the thermal stability of the precipitation strengthened high-entropy superalloy would be enhanced through simultaneously increasing the third element content and decreasing the first element content by an element segregating amount.

In the embodiment of precipitation strengthened high-entropy superalloy, the first volume fraction is in a range from 20% to 90%, while the second volume fraction of ordered precipitate is in a range between 10% and 80%.
In the embodiment of the precipitation strengthened high-entropy superalloy, the element segregating amount is at least 0.5 at %; moreover, the second mixing entropy value would be lowered after increasing the third element content by substituting the element segregating amount of the first element content of the principal element.
The embodiment of the precipitation strengthened high-entropy superalloy can further contain at least one precious metal element as a fourth element content within less than 15 wt %.
The embodiment of the precipitation strengthened high-entropy superalloy further comprises at least one grain boundary strengthening element with a fifth element content less than 15 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be better understood with the aid of following illustrations, wherein:

FIG. 1 shows a graph for describing the relationship of mixing entropy versus ordering energy and solvus temperature of γ′ phase;

FIG. 2 shows the SEM-BEI (back scattered electron) images of sample A and B;

FIG. 3 shows the atomic arrangement of γ′ phase before and after elemental segregation modulation;

FIG. 4 shows the graph for describing the relationship of mixing entropy versus ordering energy and solvus temperature of γ′ phase;

FIG. 5 shows the bar graph of Vickers hardness of HESA samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To clearly describe precipitation strengthened high-entropy superalloy according to the present invention, embodiments of the present invention will be described in detail and refer to the attached drawings hereinafter.

First Embodiment

In the present invention, a high-entropy superalloy (HESA) is developed based one a mixing entropy equal and designed to have a mixing entropy value equal to or greater than 1.5R. Generally, the HESA is composed of at least one principal element, a plurality of base elements and at least one precipitation strengthening element. It needs to particularly explain that, the principal element has a first element content of at least 35 at %, the base element has a second element content of at least 5 at %, and the partition strengthening element has a third element content of at least 0.1 at %. By such alloy design, a high-entropy matrix phase constituted by the principal element and the base elements is produced in the HESA; moreover, the third element constitutes an ordered precipitate together with the principal element and/or the base elements for strengthening the mechanical characteristics of the HESA. Thus, in the HESA, the high-entropy matrix has a first volume fraction in a range from 20% to 90%, and the ordered precipitate has a second volume fraction in a range between 10% and 80%. For better understanding of the elemental constituents of the HESA, some descriptions for the principal element, the base element and the partition strengthening element are listed in the following Table (2).

TABLE (2)

Category	Elements

Principal element	The principal elements are
	siderophile element selected from
	the group consisting of Nickel (Ni),
	Titanium (Ti), Vanadium (V),
	Chromium (Cr), Manganese (Mn),
	Iron (Fe), Cobalt (Co), and
	Platinum group element (PGE).
Base element and	Both the base element and the
partition strengthening element	partition strengthening element are
	selected from the group consisting
	of Aluminum (Al), Titanium (Ti),
	Tantalum (Ta), Niobium (Nb),
	Chromium (Cr), Manganese (Mn),
	Vanadium (V), and combination of
	the aforesaid two or more elements

It needs to emphasize that the exemplary elements listed in Table (2) does not used for limiting the manufacturing materials of the principal element, the base element and the precipitation strengthening element. Moreover, the principal element, the base element and the precipitation element constitute the HESA based on the following mixing entropy calculation.
ΔS _mix =−R(X _Aln(X _A)+X _Bln(X _B)+ . . . ) (1)
For the mixing entropy formula, X_Aand X_Brepresent the mole percent of an element A and a second mole percent of an element B, respectively. In addition, “ln( )” means a natural logarithm Particularly, the present invention further applies an elemental segregation process on the high-entropy matrix and the ordered precipitate, so as to improve the high-temperature stability and volume fraction of the ordered-phase of HESA. It is particular that an element segregating process is adopted by the present invention for converting the HESA to a precipitation strengthened high-entropy superalloy (HESA), wherein the element segregating process is carried out by increasing the third element content by an element segregating amount while substituting the same amount of the first element. According to a variety of experimental data, the addition of third element content (i.e., the element segregating amount) should be equal to or greater than 0.5 at %. Therefore, by controlling the elemental segregating process between the high-entropy matrix and ordered precipitate, not only are the ordering energy and the volume fraction of the ordered phase of the precipitation strengthened HESA apparently elevated compared to the simple HESA never been treated with the same element segregation modulation, but the high-temperature stability of the precipitation strengthened HESA is also increased. Therefore, the precipitation strengthened HESA can exhibit better high-temperature stability than previous HESAs; furthermore, good high-temperature mechanical properties of the the precipitation strengthened HESA are remained.
The experimental data in the following paragraphs will prove that a proper elemental segregation between γ phase (i.e., high-entropy matrix) and γ′ phase (i.e., ordered precipitate) can contribute to improve the high-temperature stability of an HESA. Table (3) lists the compositions of sample A and sample B, wherein sample A is the sample HA-1 provided in the above-presented Table (1). From Table (3), it is clear that the elements Ni, Co, Fe, and Cr mainly constitute the high-entropy matrix (γ phase) in sample A and sample B. On the other hand, Al and Ti forms the ordered precipitate (γ′ phase) in sample A and sample B together with the principal element (i.e., Ni) and/or those base elements (Co, Fe, Cr). For example, Ni₃(Al, Ti) γ′ is a kind of ordered structure phase. Therefore, both samples are composed of a high entropy matrix and the coherent/semi-coherent ordered precipitate structure.
Compare sample B with sample A, the content of Ti in sample B is greater than that in sample A by 2 at %. Moreover, the content of Ni in sample B is the same less than that of Ni in sample A by 2 at %. In briefly, sample B is the product by applying the elemental segregation process on sample A. Furthermore, in order to fully understand the effect of the elemental segregation process on the microstructure of the alloy, analyses on the compositions of γ and γ′ phase of both samples are listed in the following Table (4). From Table (4), it is found that the mixing entropy of γ phase in sample B rises from 1.55R of sample A to 1.59R; while the mixing entropy of γ′ phase in sample B decreases from 1.43R to 1.39R. Apparently, the results demonstrated that the atomic randomness on the ordered γ′ lattice of sample B has instead lowered.

TABLE 4

	Ni	Al	Co	Cr	Fe	Ti
	(at	(at	(at	(at	(at	(at	ΔS_mix	γ′
Sample	%)	%)	%)	%)	%)	%)	(R)	vol %

A	γ	40.1	5.8	22.2	13.9	13.7	4.3	1.55	34%
	γ′	53.4	10.2	13.4	5.7	6.5	10.8	1.43
B	γ	32.3	4.8	24.7	18.2	16.1	3.9	1.59	53%
	γ′	54.4	9.9	13.7	4.3	5.7	12.0	1.39

Herein, sample A is a kind of high entropy superalloy (HESA) produced by the disclosed HESA manufacturing method in literature 1 written by Tsao et. al. On the other hand, sample B can be called as precipitation strengthened high-entropy superalloy, which is obtained by controlling the elemental segregation between γ and γ′ phase of sample A. Please refer to FIG. 2, which shows the BEI images of sample A and B. By image analyses, it is clear that the volume fraction of γ′ in sample B rises from 34% in sample A to 53%. Please also refer to FIG. 3, which depicts the atomic arrangement on the ordered lattice of γ′ phase before and after elemental segregation modulation. Since the atomic arrangement can be less random, the ordering energy of γ′ phase in sample B would be instead enhanced. FIG. 4 describes the relationship of different mixing entropy of alloys versus ordering energy and solvus temperature of γ′ phase. It is clear that due to less randomness of atomic arrangement, the ordering energy of γ′ is instead elevated, which leads to the increase of the volume fraction and solvus temperature of γ′ phase.
Please refer to FIG. 5, which shows a bar graph of HESA samples versus Vickers hardness. The room temperature hardness of sample A can surpass that of traditional Nickel-based superalloy CM247LC. Moreover, by applying the elemental segregation process on sample A, the precipitation strengthened HESA sample B can be further improved from the conventional HESA (i.e., sample A).

Second Embodiment and Third Embodiment

The present invention also provides a second embodiment and a third embodiment for the precipitation strengthened HESA. In addition to the above-described first embodiment, the second embodiment of the precipitation strengthened HESA further comprises of at least one precious metal element as a fourth element content less than 15 wt %. On the other hand, compared to the above-described second embodiment, the third embodiment of the precipitation strengthened HESA further comprises of at least one grain boundary strengthening element as a fifth element content less than 15 wt %. Some examples for the grain boundary strengthening element and the precious metal element are listed in following Table (5).

	TABLE (5)

	Category	Elements

	Precious metal element	The precious metal element is
		selected from the group consisting
		of Tantalum (Ta), Molybdenum
		(Mo), Tungsten (W), Rhenium
		(Re), Ruthenium (Ru), Silver (Ag),
		and combination of the aforesaid
		two or more elements.
	Grain boundary	The grain boundary
	strengthening element	strengthening element is selected
		from the group consisting of
		Carbon (C), Silicon (Si), Boron
		(B), Beryllium (Be), Magnesium
		(Mg), Calcium (Ca), Hafnium (Hf),
		rare earth element, and combination
		of the aforesaid two or more
		elements.

It needs to be emphasized that the exemplary materials listed in Table (5) does are not used for limiting the manufacturing material of grain boundary strengthening element and the precious metal element. For instance, Ti is a kind of γ′ forming element for Ni₃(Al, Ti) precipitate in HESA. However, Ti can also form the TiC or Ti(C,N) on the grain boundaries of HESA. Therefore, the additions of grain boundary strengthening element and/or the precious metal element (i.e., refractory element) may further enhance the mechanical properties of the precipitation strengthened HESA.
The following Table (6) lists many examples for the second embodiment and the third embodiment. Moreover, basic information for the samples listed in Table (6) are provided in the following Table (7).

TABLE (7)

Sample	Basic information

1	HESA comprised of grain boundary strengthening element(s)
	and/or precious metal element(s).
2	Precipitation strengthened HESA by applying an element
	segregation process to the HESA of sample 1.
3	Precipitation strengthened HESA by applying an element
	segregation process to the HESA of sample 1.
4	Precipitation strengthened HESA by applying an element
	segregation process to the HESA of sample 1.
5	Precipitation strengthened HESA by applying an element
	segregation process to the HESA of sample 1.
12	HESA comprised of grain boundary strengthening element(s)
	and/or precious metal element(s).
13	Precipitation strengthened HESA by applying an element
	segregation process to the HESA of sample 12.

From Table (6) and Table (7), a precipitation strengthened HESA can be obtained by applying the elemental segregation process to specific HESA with at least one grain boundary strengthening element and/or at least one precious metal element.
Again, the so-called precipitation strengthened high-entropy superalloy (HESA) is comprised of: (1) at least one principal element, a plurality of base element, and at least one precipitation strengthening element; and produced by (2) increasing the content of the third element for elemental segregating, while substituting the same amount of the principal element. Furthermore, in order to find the minimum element segregating amount, more alloy samples have been produced. The following Table (8) shows the compositions of alloy samples; moreover, basic information is also listed in the above-presented Table (7).
Based on the experimental results of Table (3), Table (4), Table (6), and Table (8), it can be assumed that the minimum amount for the elemental segregation process is 0.5 at %. That is, a specific HESA can be treated with an elemental segregation process by increasing the content of the third element by 0.5 at %, while substituting the content of the principal element by 0.5 at %, which leads to their higher thermal stability and volume fraction of γ′ phase.
Therefore, through the above descriptions, the method for producing a precipitation strengthened high-entropy superalloy has been completely introduced. In summary, the present invention includes the advantages of:
(1) The precipitation strengthened HESA is composed of at least one principal element, a plurality of base element, and at least one precipitation strengthening element. By controlling the elemental segregation between the high-entropy matrix and ordered precipitate, not only the ordering energy and the volume fraction of ordered phase, but also the hardness of the precipitation strengthened HESA are apparently elevated. Therefore, this newly-developed precipitation strengthened HESA may exhibit the outstanding mechanical strength.
(2) Moreover, experimental results have proved that, the precipitation strengthened HESA can further be comprised of at least one precious metal element and/or at least one grain boundary strengthening element by applying an elemental segregation process on HESA.
However, the embodiments are not intended to limit scope of the present invention, and all equivalent implementations or alterations within the spirit of the present invention still follow the scope of present invention.

Claims

What the claim is:

1. The precipitation strengthened high-entropy superalloy, comprising:

at least one principal element with a first element content of at least 35 at %, being a siderophile element;

a plurality of base elements for forming a high-entropy matrix with the principal element, wherein each of the base elements has a second element content of at least 5 at %; and

at least one precipitation strengthening element with a third element content of at least 0.1 at %, for forming an ordered precipitate with the principal element and/or the base elements;

wherein the high-entropy matrix and ordered precipitate have a first volume fraction and a second volume fraction, respectively; and the thermal stability of the precipitation strengthened high-entropy superalloy would be enhanced through simultaneously increasing the third element content and decreasing the first element content by an element segregating amount.

2. The precipitation strengthened high-entropy superalloy of claim 1, wherein the first volume fraction is in a range from 20% to 90%, while the second volume fraction of ordered precipitate is in a range between 10% and 80%.

3. The precipitation strengthened high-entropy superalloy of claim 1, wherein the high-entropy matrix has a first mixing entropy value equal to or greater than 1.5R, and the ordered precipitate has a second mixing entropy value less than the first mixing entropy value.

4. The precipitation strengthened high-entropy superalloy of claim 1, wherein the element segregating amount is at least 0.5 at %; moreover, the second mixing entropy value would be lowered after increasing the third element content by substituting the element segregating amount of the first element content of the principal element.

5. The precipitation strengthened high-entropy superalloy of claim 1, wherein the siderophile element is selected from the group consisting of Nickel (Ni), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co) and Platinum group element (PGE).

6. The precipitation strengthened high-entropy superalloy of claim 1, wherein both the base element and the strengthening element are selected from the group consisting of Aluminum (Al), Titanium (Ti), Tantalum (Ta), Niobium (Nb), Chromium (Cr), Manganese (Mn), Vanadium (V), and combination of the aforesaid two or more elements.

7. The precipitation strengthened high-entropy superalloy of claim 1, further comprising at least one precious metal element with a fourth element content less than 15 wt %.

8. The precipitation strengthened high-entropy superalloy of claim 7, wherein the precious metal element is selected from the group consisting of Tantalum (Ta), Molybdenum (Mo), Tungsten (W), Rhenium (Re), Ruthenium (Ru), Silver (Ag) and combination of the aforesaid two or more elements.

9. The precipitation strengthened high-entropy superalloy of claim 7, further comprising at least one grain boundary strengthening element with a fifth element content less than 15 wt %.

10. The precipitation strengthened high-entropy superalloy of claim 9, wherein the grain boundary strengthening element is selected from the group consisting of Carbon (C), Silicon (Si), Boron (B), Beryllium (Be), Magnesium (Mg), Calcium (Ca), Hafnium (Hf), rare earth element and combination of the aforesaid two or more elements.