Janet Hurst

Glenn Research Center, Cleveland, Ohio

Materials and Structures Research for Gas 

Turbine Applications Within the NASA 

Subsonic Fixed Wing Project

NASA/TM—2011-216747

September 2011

GT2010–23177
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Janet Hurst

Glenn Research Center, Cleveland, Ohio

Materials and Structures Research for Gas 

Turbine Applications Within the NASA 

Subsonic Fixed Wing Project

NASA/TM—2011-216747

September 2011

GT2010–23177

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Turbo Expo 2010

sponsored by the American Society of Mechanical Engineers (ASME)

Glasgow, Scotland, United Kingdom, June 14–18, 2010
Acknowledgments

I would like to acknowledge the numerous contributors to this overview–Dr. Michael Nathal, Dr. Rebecca MacKay, 

Dr. Ron Noebe, Dr. Margaret Proctor, Dr. Cheryl Bowman, Dr. Jeff Eldridge, Dr. Robert Miller, Dr. Dongming Zhu, 

Dr. George Stefko, Dr. Kirsten Duffy, Dr. James Min, Dr. Ben Choi, Dr. Gerald Brown, Dr. Tom Cable, and others.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Level of Review: This material has been technically reviewed by technical management. 

This report is a formal draft or working 

paper, intended to solicit comments and 

ideas from a technical peer group.

This report contains preliminary findings, 

subject to revision as analysis proceeds.

Available electronically at http://www.sti.nasa.gov
NASA/TM—2011-216747 1

Materials and Structures Research for Gas Turbine Applications

Within the NASA Subsonic Fixed Wing Project

Janet Hurst

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

In an era of both declining NASA budgets and demanding

space goals, the NASA Aeronautics Research Mission 

Directorate has elected to address foundational research 

problems for aeronautics. To this end, the Subsonic Fixed 

Wing (SFW) Project, within the Fundamental Aeronautics 

Program, has selected challenging goals which anticipate an 

increasing emphasis on aviation’s impact upon the global 

issue of environmental responsibility. These SFW project 

goals are greatly reduced noise, reduced emissions and 

reduced fuel consumption. Specific goals, selected by a 

combination of systems analysis, experience and industry 

input, are generational in approach, addressing 25 to 30 years 

of technology development. Successful implementation of 

these demanding goals will require development of new 

materials and structural approaches within gas turbine 

propulsion technology.

The Materials and Structures discipline, within the SFW 

project, comprise cross-cutting technologies ranging from 

basic investigations to component validation in laboratory 

environments. Material advances are teamed with innovative 

designs in a multidisciplinary approach with the resulting 

technology advances directed to promote the goals of reduced 

noise and emissions along with improved performance. For 

propulsion needs, these technologies have been grouped into 

three basic categories. These are 1) improved hot section 

materials for hotter engines with minimal cooling 

requirements to promote reduced NOx and fuel burn. Among 

the technologies of interest have been new alloy compositions 

and coatings which are demonstrating improved capabilities 

for this purpose. Also of interest for propulsion applications 

are 2) lightweight and multifunctional systems which will 

allow reduced fuel burn via weight reduction in the engine and 

its surrounding structure. An example of this technology is 

High Temperature Shape Memory Alloys (HTSMA) for 

actuation applications requiring large displacements and low 

frequencies such as chevrons and variable area nozzles for 

high bypass ratio engines. The final area under investigation is 

3) More Electric aircraft, which at this time is focused 

primarily on Turboelectric technology as a revolutionary 

approach to the entire SFW design space. Major challenges to 

be addressed in this field are improved cryocoolers and 

superconducting materials. 

A brief overview is presented of the current materials and 

structures research focused upon propulsion applications within 

NASA Subsonic Fixed Wing Project. As such, it does not 

comprise the entirety of Materials and Structure research for gas 

turbine engines at NASA. Several other projects also include 

research of this type to address their specific project goals.

Introduction

The objective of the Subsonic Fixed Wing Project (SFW), 

one of four flight regime based projects within the NASA 

Fundamental Aeronautics Program, is to conduct discipline 

based foundational research to address environmental 

challenges and improve the performance of subsonic aircraft 

(Ref. 1). The aggressive goals of the SFW project are shown 

in Figure 1 and define a design space with large reductions in 

noise, NOx, fuel burn and reduced runway length. A 

generational approach is used by the SFW project with current 

generation aircraft referred to as “N” and so forth from nearterm concepts or N+1, mid-term ideas designated as N+2, to 

the basic research ideas which will enable new N+3 aircraft 

concepts in 30 years or more. The achievement of these goals 

requires a multidisciplinary effort to investigate new materials, 

structural concepts and technologies. Within the Materials and 

Structures discipline are cross-cutting technologies with 

overlapping applications within each of the four flight regime 

based FA projects. Technologies of interest are divided among 

the four projects, with considerable cooperation existing 

between those projects at the discipline level. For 

incorporation in the SFW portfolio, three criterions must be 

addressed. The first being whether the technology enables a 

significant, achievable impact on the goals of SFW. The 

second one is whether the technology fits best within SFW or 

would be better aligned with the goals of another project. The 

third criterion is whether there are sufficient resources 

available to make reasonable progress. Technologies from 

very basic fundamental research to nearer term concepts are

included. Additionally, vigorous supplementation of in-house 

capabilities is accomplished via periodic NASA Research 

Announcements (NRA). Four NRA rounds have been pursued 

thus far, one each year of the SFW project with significant 

university involvement. Additionally, the Small Business 

Innovation Research program (SBIR) has usefully augmented 

the research portfolio.

The critical materials and structures technology needs for 

subsonic flight regime applications have been identified as 1) 

improved hot section materials, 2) improved lightweight 

structures which employ improved and/or multifunctional 

materials with an integrated design solution and 3) more 

electric aircraft to meet future consumer and industry needs.
NASA/TM—2011-216747 2

Figure 1.—Subsonics Fixed Wing Project (SFW) System Level 

Metrics (Courtesy Ruben Delrosario, Principal Investigator).

Nomenclature

AFRL Air Force Research Laboratory

CMC Ceramic Matrix Composites

EBC Environmental Barrier Coating

ERA Environmentally Responsible Aviation Project

FA Fundamental Aeronautics

HTSMA High Temperature Shape Memory Alloys 

IPHTET Integrated High Performance Turbine Engine

Technology

LDS Low Density Superalloy

NASA National Aeronautics and Space Administration

GRC NASA Glenn Research Center

NOx Nitrogen and oxygen containing gases, very 

reactive 

NRA NASA Research Announcement

OPR Overall Pressure Ratio

PM Powder Metallurgy

TBC Thermal Barrier Coating

SAA Space Act Agreement

SBIR Small Business Innovation Research

SFC Specific Fuel Consumption

SFW Subsonic Fixed Wing Project within the NASA 

Fundamental Aeronautics Program

SOFC Solid Oxide Fuel Cell

T3 Turbine Inlet Temperature 

Hot Section Materials Research and 

Design

Historically there has been a clear trend of increasingly 

higher operating temperatures within commercial class engines

(Ref. 2). This trend is likely to continue with modern engine 

designs incorporating higher overall pressure ratios (OPR) 

and reduced film cooling and so driving the need for hot section 

materials to higher capabilities to provide both improved 

specific fuel consumption (SFC) and reduced NOx emissions 

To achieve these dual goals, new materials are needed as well as

innovative designs. Improvements to hot section performance 

are being addressed by each of the FA programs. Additionally, 

the new Environmentally Responsible Aircraft project (ERA) is 

targeting improved NOx engines. At this time SFW is

addressing several technologies for improved hot section

performance these range potentially near term applications to 

basic research. Some of these to be discussed here include

turbine seals, high temperature alloys and coatings, advanced 

hybrid disk concepts, multifunctional diagnostic coatings and 

high temperature nanotubes. Ceramic matrix composites (CMC) 

are also clearly of interest for advanced hot section components

(Ref. 3), however CMC technology is adequately addressed by 

the other NASA projects flight regime based projects.

High Temperature Seals

Advanced gas turbine engine seals represent an opportunity 

to provide a significant reduction in specific fuel consumption

(sfc) relative to current state-of-the-art (SOA) labyrinth seals.

This reduced fuel burn is applicable across N, N+1, and N+2 

aircraft platforms, previously shown in Figure 1. The 

commonly used labyrinth seals require clearance to avoid 

rubbing contact. Clearance must account for a range of engine 

conditions with centrifugal growth of the rotor, geometry 

changes due to thermal expansion, and dynamic shaft motion 

all occurring in response to mission profile. Abradable 

materials can be used to reduce this clearance, but once they 

are abraded, the gap remains. It has been shown that 

contacting brush or finger seals can result in half the leakage 

rate of conventional labyrinth seals (Refs. 4 to 7). Also studies 

by engine companies (Refs. 8 and 9) have shown that a 

50 percent reduction in leakage will yield a 2 to 3 percent

reduction in sfc. But a major drawback of these advanced seals 

is that shorter life results from rubbing wear of the contacting

seal. SFW researchers are evaluating non-contacting 

compliant seals such as non-contacting brush/finger seals. 

These non-contacting finger seals holds promise to achieve 

low leakage rates and long life capability in subsonic gas 

turbine engines. A GRC designed non-contacting brush/finger 

turbine seal (Ref. 10) is shown in Figure 2. Verification and 

refinement of design methodology using experimental data 

and analysis is on-going with testing up to 800 °C and 

pressures up to 1.7 MPa differential (250 psid). The goal of 

the current effort is to both validate design and analysis 

methodology, and to demonstrate leakage that is no more than 

50 percent of SOA labyrinth seals and twice the life of

contacting brush or finger seals at subsonic engine conditions.

This work is conducted in the GRC High Temperature, High 

Speed Turbine Seal Test Facility, seen in Figure 3. Details 

regarding this facility have been reported elsewhere (Ref. 11).

CORNERS OF 

THE TRADE

SPACE

N+1 (2015)

GENERATION

Conventional 

Configurations

(Tube and Wing)

N+2 (2020)

GENERATION

Unconventional 

Configurations

(Hybrid Wing)

N+3 (2030)

GENERATION

Advanced Aircraft 

Concepts relative to 

user-defined reference 

Noise -32 dB -42 dB -71 dB

(cum below Stage4) 

LTO NOx

Emissions -60% -75% better than -75%

(below CAEP/6)

Performance:

Aircraft Fuel Burn -33% -40% better than -70%

Performance: exploit

Field Length -33% -50% metroplex concepts
NASA/TM—2011-216747 3

Advanced High Temperature Alloys

Metallurgurical engineers continue to make great 

improvements to alloy compositions and processing as engine 

temperature needs have increased. While there is undoubtedly 

a limit to the ultimate temperature which can be achieved with 

alloys, increases in upper use temperatures continue. Some 

applications, such as rotating components, are likely to require 

metal parts for the foreseeable future. Advanced alloy 

development continues to be a priority for the SFW project.

Within SFW, the alloy design space is characterized and 

optimized for cyclic oxidation, creep resistance, hot salt 

corrosion and microstructural stability. A series of nickelbased superalloys is under investigation for airfoil

applications. The baseline composition in this series, known as 

LDS for low density superalloy, has been reported elsewhere 

(Refs. 12 and 13). These compositions possess excellent creep 

resistance and at the same time offer densities lower by 5 to 

10 percent than the current generation of alloys. Figure 4 

(Ref. 13) is a Larsen-Miller plot which illustrates the 

improved creep behavior of the baseline alloy (LDS,) as well 

as a further optimized composition, relative to state-of-the-art 

superalloys. These improvements have resulted in an 

additional 19 °C (35° F) increase in temperature capability and 

a 56 °C (100 °F) increase over commercial blade alloys. 

Efforts to transfer this technology to industry are underway 

through a NASA Space Act Agreement (SAA) with 

Honeywell Corporation.

Advanced Hybrid Disk Concepts

Improved turbine disks are another critical technology to 

enable higher temperature engine operation. Design 

considerations push for turbine inlet temperatures, T3, of 

760 °C (1400 °F) to 816 °C (1500 °F). While high temperature 

CMCs are being considered for other engine components, the 

high damage tolerance required by the turbine disk will likely 

negate the use of intermetallics or CMCs, at least for many 

years. Yet use of even advanced 2nd and 3rd generation powder 

metallurgy metals such as Rene 88 (Ref. 14) and ME3 

(Ref. 15), confine T3 to no more than 1300 °F due to creep and 

corrosion limitations. The need for higher use temperatures 

has prompted efforts to develop dual alloy concepts comprised

of a powder metallurgy (PM) disk bonded to a single crystal 

superalloy rim, shown in Figure 5. By placing the joint 

between the two metals such that the PM alloy remains at no 

more than 704 °C (1300 °F), the single crystal rim alloy may 

operate to 760 °C (1400 °F) or more. The improved LDS 

single crystal alloy compositions developed for blades are 

candidates for this application as well. 

Significant advances have been made in understanding the 

ultimate temperature capability of 3rd generation powder 

metallurgy (PM) disk alloys. Also single crystal alloys with 

the necessary creep resistance have been identified, an 

example is shown in Figure 6. Additional work is on-going to 

Figure 2.—GRC non-contacting brush/finger turbine seal

(Ref. 10).

Figure 3.—GRC High Temperature, High Speed Turbine Seal 

Test Facility (Ref. 11).

Figure 4.—Larsen-Miller plot of creep behavior of 

improved creep of SFW Developed alloys 

compared to commercial blade alloys (Ref. 13).
NASA/TM—2011-216747 4

Figure 5.—Schematic of a hybrid disk 

section. (Courtesy of Michael Nathal, 

GRC.)

Figure 6.—LDS alloys have creep resistance required for 

816 °C (1500 °F) turbine disk rim temperatures (Ref 13).

Figure 7.—Three-dimensional strain mapping of a PM/SX joint 

tensile specimen showing most of the deformation occurs in 

the single crystal region (Ref. 13).

understand long term corrosion and oxidation behavior of this 

alloy family. Compositions must be optimized for a balance of 

properties within the alloy/property behavior maps. Also, 

promising brazing schemes and compositions for the 

PM/single crystal joint have been identified. Understanding 

the properties of this joined region will be critical to predict 

behavior of a component. By utilization of 3–D strain 

mapping during tensile testing, it has been shown that most of 

the deformation occurred within the single crystal, shown in 

Figure 7 (Ref. 13). Currently, an optimized high temperature 

coating scheme is being developed for this alloy/application 

system. Potentially a hybrid disk with CMCs may be 

considered, although thermal expansion mismatch would 

cause considerable stress at the joint.

The hybrid disk concept has been pursued in the past by 

DOD programs such as IHPET with NASA involvement.

SFW GRC researchers are collaborating with AFRL to further 

the hybrid disk concept to enable to achieve a 100 to 200 °F 

increase in T3 temperature. This effort relies on synergy 

between the advanced alloy work done within NASA’s SFW, 

Supersonics and Aging Aircraft projects. 

High Temperature Coatings

To further improve high temperature performance of stateof-the-art alloys, thermal barrier coatings must be developed 

in tandem with those alloys to optimize specifically targeted 

applications. Within SFW, coatings are actively under 

development are for the advanced superalloy compositions 

discussed previously for both blades and disks. As advanced 

engine designs push materials to increased temperatures, these 

coatings become critically important. An additionally 

pervasive issue for coating technology has been the potential 

premature loss of coating through spallation or erosion.

Evaluation of the condition of the coatings is done by 

inspection of the part, often a difficult and expensive 

procedure. An innovative solution to this problem may be the 

application of luminescent coatings, described below, which 

provide both thermal protection and diagnostic health 

monitoring of the coating system. An additional

multifunctional coating scheme also pursued within SFW has 

been thermal barrier coatings which also provide vibration 

damping to engine component. 

Delamination and Temperature Sensing 

Luminescence-Based Diagnostics for TBCs

Thermal barrier coatings (TBCs) have been used to provide 

thermal protection for many turbine engine components.

However, TBCs fail through spallation, leading to conditions 

which may damage engine performance or safety. Inspection 

of the TBC coating with nondestructive diagnostic tools could 

assess the progression of cracks within the coating (Refs. 16 to 

18). This crack progression causes spallation which will lead 

to eventual TBC failure, as shown in Figure 8. Non-contacting 
NASA/TM—2011-216747 5

temperature measurement of the substrate would also be 

useful, as then prediction of when the thermal function of the 

TBC was no longer effective would be possible. By careful 

doping of the TBC with luminescent sublayers, it has been 

found that crack progression can be monitored from an early 

stage, by near-infrared and upconversion luminescence 

imaging, before delamination can be detected visually, shown

in Figure 9. Detection is possible because the buried 

delamination cracks are highly reflective and cause a relative 

increase in luminescent emission from delaminated areas.

Prior to this near-infrared and upconversion luminescence

imaging, the SOA techniques all had drawbacks which made 

implementation in an engine unlikely. Eddy current (Ref. 19) 

and impedance measurements (Refs. 20 and 21) are unsuitable 

for use in an engine, while techniques using optical probes had 

limitations in depth probing (Refs. 22 and 23). Pyrometry

(Ref. 24) also suffered from uncertainty as emissivity 

measurements change with time at temperature. Luminescent

subcoatings can also be used to monitor substrate temperature 

or surface temperature, depending on the location of the 

dopant (Ref. 25). It is important note that TBC life has not 

been affected by incorporation of luminecensing dopants

(Ref. 26). Additionally, the incorporation of the dopants does 

not require difficult or cumbersome processing techniques.

Dopants may be incorporated into commonly used coating 

deposition techniques such as plasma spraying (Ref. 26). 

Techniques such as this will be needed as engine operating 

temperatures are pushed to higher operating temperatures to 

monitor the condition of protective coatings. Plans are in place 

to demonstrate this technology in an engine test.

Damping Coatings

Another multifunctional capability of TBCs is the potential 

to damp vibrational stress within hot section components 

while also providing oxidation and erosion resistance. This 

coating scheme would enable thinner blades, or other 

component, while also reducing high cycle fatigue damage. A 

unique high temperature, high frequency, high force test 

capability was developed and demonstrated (Ref. 27), capable 

of testing at temperature up to 1230 °C, shown in Figure 10.

Metallic, ceramic and cermet coatings have been applied by 

both air plasma spraying and high velocity oxy-fuel (HVOF).

An example of the initial results is shown in Figure 11. Up to 

a fourfold improvement in damping at 800 °C was found 

among the investigated coatings (Ref. 27).

High Temperature Nanotubes 

In partnership with the Supersonics project, high 

temperature nanotubes are investigated for applications which 

are too harsh for carbon nanotubes (CNT). CNT are limited to 

approximately 500 °C in air while Boron Nitride Nanotubes 

(BNNT) and Silicon Carbide nanotubes (SiCNT) are stable to

over 1000 °C, as shown in Figures 12(a) (BNNT), and (b)

Figure 8.—Furnace test showing progression of 

delaminations to TBC failure as imaged by 

upconversion luminescence.

Figure 9.—A small delaminated region is detected 

on a plasma spray coated disk specimen.

Figure 10.—Laser Heated Cantilever Beam 

Vibration Test Rig, GRC.

Figure 11.—Improved damping of a coated single 

crystal superalloy.

Localized Delamination

NIR Luminescence

1 cm
NASA/TM—2011-216747 6

(SiCNT). While CNT reinforced nanocomposites may be 

useful for some low temperature uses, the superior

temperature and oxidative stability of BNNT make it of 

interest for engine applications. NASA GRC has a robust

capability for producing BNNT (Ref. 28), capable of 

producing both bulk nanotubes and in-situ coating on 

substrates. Many substrates have been successfully coated 

include superalloys, carbon, silicon carbide, silicon, 

superalloys and various foam structures, an example is shown 

in Figure 13. As a structural reinforcement within composites, 

the GRC BNNT has successfully demonstrated improved 

mechanical properties within glass compositions (Ref. 29) and 

also recent work within aluminum. Additional applications 

include thermal management and high temperature electronics.

Silicon Carbide nanotubes (SiCNT) are synthesized by 

converting the outer wall layers of carbon nanotubes to silicon 

carbide via a chemical templating procedure (Refs. 30 and 31). 

Several grams of silicon carbide nanotubes can be synthesized 

each day. The process generally results in dual layer 

nanotubes, with an inner core of C surrounded by an outer 

layer of SiC, much like a coaxial cable. Samples are 65 to 

70 percent SiC with balance being carbon. Temperature 

stability of SiCNT, determined by thermogravimetric data in 

Figure 12(a), shows that weight loss occurs up to 800 °C, as 

carbon is removed from the sample by oxidation. Slight 

weight gain occurs above 1000 °C as oxidation reactions begin 

to form glassy phases. SiCNTs are being incorporated into 

environmental barrier coatings (EBC), TBCs and composites.

An NASA NRA with Brown University compliments inhouse experimental work by providing valuable modeling and 

experimental insight into the mechanical behavior of ceramic 

nanocomposites. One significant finding which has influenced 

the direction of in-house work highlighted the strength 

advantage of multi-wall nanotubes over single wall nanotubes 

when used as structural reinforcement for composites

(Ref. 32).

Lightweight and Multifunctional Engine 

Structures

Metal Foams

Engine and airframe manufacturers alike have made great 

strides in recent years in weight reduction for both engine and 

airframe by incorporating new composites and materials. As 

fan cases and by-pass ducts are large structures relative to 

other engine components, weight reductions here are an 

excellent target. One method of weight reduction is to use 

multifunctional materials. To provide for engine containment, 

structure must be strong and lightweight, and also capable of 

containing impact events such as blade out. Various material 

systems composed of polymer, nanocomposite or metals, 

could be utilized in sandwich structures. One potentially 

useful material for lightweight sandwich or other structures 

may be metal foams. Metal foams, as shown in Figure 14(a),

(a)

(b)

Figure 12.—(a) Thermogravimetric data demonstrates the 

thermal stability of BNNT in air at temperature up to 

1200 °C. Photo of BNNT following heat treatment in air to 

1200 °C. (b) Thermogravimetric data demonstrates the 

thermal stability SiCNT in air at temperature up to 1200 °C. 

Photo of SiCNT following heat treatment in air to 1200 °C.

Figure 13.—BNNT deposited on superalloy foam.

Inset—coated foam at a lower magnification.

0

10

20

30

40

50

60

70

80

90

100

110

0 200 400 600 800 1000 1200

Mass Change (%)

Temperature (C)

TGA Data in Air

MWCNT

SiCNT 

Improved 

conversion
NASA/TM—2011-216747 7

could supply structural strength and acoustic damping for 

engine related structures such as the fan case. Polymer foams 

have already been widely used for impact resistance and sound 

absorption. Potentially, through the use of metal foams, these 

functions could be extended into hotter sections of the engine

as well. Coupon level testing was carried out on metal foams 

and demonstrated sound absorption (Ref. 33). Simulated tip 

rub engine testing with Williams International has also 

recently been completed (Ref. 33) and demonstrated graceful 

degradation and minimum tangential loading (Fig. 14(b)).

To understand, model, and eventually optimize foam 

structures and properties, characterization of non-idealized 

foams is necessary. A methodology to quantify metal foams 

based on strut and cell face dimensions has been developed 

(Ref. 34). Next, understanding of how microstructural features 

affect pertinent properties needs development.

Many issues with metal foams remain. A serious drawback 

of this material is weight. Additionally metal foams are not 

manufactured for the purpose of sound absorption or strength. 

These attributes along with fatigue tolerance, and density have 

not been optimized. There are many manufacturers of metal 

foam so processing approaches and resultant materials vary 

considerably. Additionally, few high temperature superalloy 

compositions are available. 

Shape Memory Alloys

Aircraft engines of the future are moving toward real time 

adaptable systems rather than static engine configurations.

These adaptive structures respond to changing performance 

demands by altering shape, position or geometry as necessary 

to achieve fuel burn, emission reduction and /or performance 

benefits. But work performing structures, such as motors, 

actuators, solenoids, hydraulics and pneumatics, represent 

additional weight and may therefore negate actual fuel savings 

from the performance enhancement which they enabled. To 

avoid this, new solid-state devices may allow adaptive 

structures for engine applications to also achieve a weight 

decrease. These actuators would likely be used in devices 

where traditional hydraulic systems would not be possible. To 

meet these needs, SFW is developing two types of smart 

materials, high temperature shape memory alloys (HTSMA), 

and high temperature adaptive piezoelectric devices.

Shape memory alloys (SMA), with their high energy 

density, will be useful for large stroke and low frequency 

applications. There are many potential engine applications 

starting with simple replacement of conventional actuators, 

but in addition there are adaptive chevrons (Ref. 35), variable 

area exhaust nozzles (Ref. 36), acoustic liners, shape changing 

blades and vanes (Ref. 37), flow control devices, adaptive 

inlets (Ref. 38), seals, and others. Yet while these applications 

have been demonstrated, the commercial SMAs or hydraulic

systems that were used were only a placeholder for HTSMAs.

Currently available SMAs, typically NiTi, are usable to only 

70 °C, insufficient for practical situations. SFW has targeted

the development of high temperature shape memory alloys 

(HTSMA) for use to 200 °C. Additional compositions are 

being developed for much higher temperature applications, up 

to 500 °C, by the FA Supersonics Project. A HTSMA driven 

variable area exhaust nozzle demo for a high by-pass ratio 

engine is a currently unfunded goal within SFW.

(a)

(b)

Figure 14.—(a) Typical metal foam. (b) Post engine 

tested metal foam arc segments

Figure 15.—Idealized behavior of a HTSMA (Ref. 43).

Temperature (oC)

Strain (%)

Permanent Deformation

Heating Curve

Cooling Curve Mf

As

Ms Af

Transformation

Strain
NASA/TM—2011-216747 8

There are at least four requirements (courtesy Ron Noebe, 

GRC) for successful HTSMA development for actuators: 1) 

Martensitic finish temperature (Mf) of at least 80 °C. This is 

necessary to prevent premature actuation in harsh 

environments. Commercial SMA materials do not meet this 

requirement. 2) Dimensional stability during thermal cycling 

for long-life and durability. This requires little or no 

irrecoverable strain. 3) Large recoverable transformation 

strain for large stroke and high work output. 4) Narrow 

hysteresis for fast actuation. These qualities are shown in the 

idealized behavior of a HTSMA shown in Figure 15. 

Several new NiTiPd-X compositions have been 

documented (Refs. 39 to 43) to meet most of the above 

requirements. These compositions are superior to NiTi alloys 

in both higher transformation temperatures and dimensional 

stability. On-going research is striving for additional 

improvements in dimensional stability. The hysteresis 

behavior of some of the compositions in this family is shown 

in Figure 16 (Ref. 44).

In another alloy system, NiTiHf, very promising results 

were also achieved (Ref. 44). This system has been reported to 

have poor performance with low Mf temperatures, wide 

hysteresis, poor dimensional stability, poor ductility and 

difficult processing. However, by incorporating nanodispersion strengthening, alloys with outstanding properties 

including high Mf temperatures were obtained, shown in

Figure 17.

Collaboration with industrial partners as well as other 

government facilities and universities, is a desired outcome for 

most efforts within SFW. A clear success story exists in the 

collaboration within the SMA actuator community. The 

CASMART organization is comprised of primary and 

secondary materials producers to end users, along with alloy 

specialists, and modelers. They come together to advanced 

SMA technology, developing alloy compositions and 

processing, developing test standards and alloy certification 

standards, by creating models and methodologies, and by 

promoting system-level developments. This group was formed 

in 2007 by Boeing, GRC, LaRC, and Texas A&M and has 

grown to nine members (shown in Fig. 18), each with their 

own unique role. Additionally, GRC researchers have worked 

through the NRA process to find productive partnership with 

researchers and students at the Texas Engineering Experiment 

Station, Texas A&M University, who have performed 

complimentary work in both processing and modeling under a 

cooperative agreement.

Adaptive Piezoelectric Technology

While SMAs are appropriate for applications which require 

large displacements and low frequencies, applications 

requiring small displacements and high frequencies, may 

benefit from piezoelectric devices. Piezoelectrics may be 

appropriate for applications such as active fuel modulation, 

and active vibration damping. The integrated structural 

Figure 16.—Hysteresis plots for NiTiPd-X compositions

(Ref. 43). 

Figure 17.—Promising hysteresis properties in the NiTiHf 

system. Inset—Microstructure of nanodispersion 

strengthened NiTiHf alloy.

approach envisioned for an advanced term hybrid wing 

aircraft utilizing embedded engines will likely require 

vibration suppression (Ref. 45). An effort to develop high 

temperature piezo-ceramic compositions exists at GRC, 

funded by the FA Supersonics Project. Leveraging this effort, 

SFW is developing the controls and methodology to utilize 

these new materials. Active damping technology enables the 

reduction of hazardous vibration and associated dynamic 

stresses, thereby increasing life of a component. To assure 

sufficient fatigue life, designers often must increase structure 

and thereby add weight and volume. As an example problem, 

the Adaptive Structures team is working to develop shunted 

piezoelectric damping and/or active control with piezoelectric 

actuators to reduce fan and compressor blade vibrations. This 

NiTiHf

Temperature (oC)

0 50 100 150 200 250 300 350

True Strain (%) 0

1

2

3

4

5

500 MPa (15.15 J/cm3

)

400 MPa (11.09 J/cm3

)

300 MPa (7.26 J/cm3

)

200 MPa (3.84 J/cm3

)

100 MPa (1.24 J/cm3

)
NASA/TM—2011-216747 9

will permit thinner and more highly loaded blades. This will 

also allow distortion tolerant fan blades for embedded engines 

which are of interest for N+2 and N+3 concepts where 

boundary layer ingestion may occur (Ref. 45). More near 

term, thinner blades will enable reduced fuel burn through 

weight reduction and reduced noise as well.

Significant resonant stress reduction in flat plates (Refs. 46 

and 47) and also on a fan blade was demonstrated with both 

passive shunt circuits and an active control system (Refs. 47 

and 48). A representative result is shown in Figure 19. Testing 

has also been performed under centrifugal loading to ascertain 

material and circuit integrity (Ref. 49). The next phase will 

test embedded piezoelectric devices within scaled composite 

blades in the Dynamic Spin Rig. Electrical power will be 

transferred to the rotating blades using wireless telemetry.

Preliminary runs employing a lightweight inductive power 

supply have been promising.

More Electric Propulsion

Without doubt, the future of aviation will be more electric. 

While an all-electric commercial aircraft may be considerably 

in the future, there are other options. An exciting N+3 concept 

under investigation in SFW is turboelectric distributed 

propulsion. Also, current civil aviation has increasing 

electrical demands from instrumentation to passenger laptops. 

Powering these requirements constitutes a drain on available 

power from the engine. Solid oxide fuel cells (SOFC) are one 

method of reducing this demand on engines. Of additional 

interest for the SFW project is lightweight motors, super 

capacitors and nanotechnology.

Turboelectric Distributed Propulsion

Turboelectric distributed propulsion technology is a 

fundamentally unique approach to potentially meet future 

propulsion requirements. While this approach can be

developed for tube-and-wing configured aircraft, when 

coupled with a hybrid wing aircraft, significant benefits are 

expected (Ref. 50). The potential N+3 future aircraft would 

address every corner of the SFW design space (Fig. 1). Some 

of these benefits are listed in Figure 20. This concept enables 

integrated engine and aircraft architectures that are predicted 

to reduce both drag and fuel burn from the airframe and allow 

for lower fuel burn and emissions from the propulsion system. 

This idea relies on power distributed electrically from turbinedriven generators to motors that drive the multiple propulsive 

fans, see Figure 21. The turboelectric propulsion concept has 

been addressed in detail elsewhere (Refs. 50 to 52) and was 

the subject of a recent NASA sponsored workshop (Ref. 51).

Representatives from three NASA centers, Air Force, industry

and universities were present and took part in developing 

technology roadmaps. Recent advances in both cryocooler 

technologies and superconducting materials have generated 

renewed interest in this approach. The distributed turboelectric

propulsion system requires significant weight reduction of 

cryogenic and superconducting components for this concept to 

be competitive (Ref. 50) with more conventional approaches.

Therefore continued improvements to these technologies are 

currently pursued within SFW through a combination of 

SBIRs, contracts and in-house efforts. Recent 

accomplishments include a preliminary design of a cryocooler 

which enabled an 80 percent weight reduction while also 

meeting efficiency goals (Ref. 52). Additionally, in a SBIR 

effort, an improved low loss MgB2 superconducting wire was 

developed with fine filaments and a high resistance matrix 

(Ref. 52). Model developments for motor and generator 

concepts are currently underway. Also, an in-house high 

temperature super conducting motor has been constructed and 

will be soon tested.

Figure 18.—CASMART organization works to develop SMAs.

Figure 19.—Typical piezoelectric damping.
NASA/TM—2011-216747 10

Figure 20.—Multiple benefits may be obtained from the turboelectric propulsion approach. 

(Courtesy G. Brown, GRC).

Figure 21.—Components of turboelectric propulsion. (Courtesy G. Brown, GRC).

Superconducting transmission lines 

between generators and motors 

Turbine engine driven 

superconducting generator 

Superconducting motors 

to drive propulsive fans 

Cryocooler(s) for 

cryogenic components

Cryogenic Inverter for 

variable speed fans
NASA/TM—2011-216747 11

Figure 22.—Microstructure of the GRC all-ceramic 

SOFC.

Solid Oxide Fuel Cells (SOFC)

The auxiliary power unit (APU) provides electricity to the 

plane’s on-board systems while in flight and on the ground to 

provide for passenger convenience and comfort. Generally, 

APUs are gas-turbine driven generators and are inefficient and 

polluting. Fuel cell driven APUs would provide significant 

reduction in fuel burn, NOx and noise while the aircraft is on 

the ground. But PEM fuel cells do not perform well at high 

altitudes due to the need to control hydration of the electrolyte. 

Hybrid PEM/turbine APUs combines fuel cell ground based 

performance with a gas turbine’s high altitude advances

(Ref. 53). Potentially, the commercially PEM fuel cells could 

be replaced with less mature Solid Oxide Fuel Cell (SOFC)

technology. Advantages of SOFC fuel cells include better 

performance at altitude than PEM cells, reduced CO poisoning 

and better sulfur tolerance. However, SOFCs are heavy and so 

unsuitable for aviation. SFW has a small effort in SOFC 

technology with the goal of improved specific power density 

(power output relative to weight). New SOFC designs at GRC

have enabled improved specific power density (power output 

relative to weight) devices up to 1 kW/kg (Ref. 53). The 

unique microstructure of the GRC SOFC, shown in Figure 22, 

provides the opportunity for much lighter weight devices 

while maintaining a robust structure. An additional advantage 

of this technology is that it does not require expensive 

precious metals. The all-ceramic design has been successfully 

tested as a small stack. Future work will test larger stacks and 

evaluate durability. 

Additional Efforts

Additional work exists at a low level as an informal 

technology incubator. All are at low TRL. Topics such as 

nanocomposites, aerogels, thermoelectrics, power harvesting, 

lightweight motors, oil free bearings and others are explored. 

As the technology matures, feasibility is continually evaluated.

Some of these topics will be elevated in significance, while 

other technologies will be de-emphasized or dropped. The 

SFW Materials and Structures portfolio is flexible and as some 

technologies are found to fall short of the hoped-for impact, 

rebalancing occurs. As always within the FA Program, 

research will be supplemented by NRA awards as well as 

SBIRs

Conclusion

The Subsonics Fixed Wing Project has the clear objective 

of addressing environmental challenges for subsonic aircraft 

and improving their performance through the development of 

technologies and tools. Advances in materials and structures 

technologies are essential to meet the SFW goals for reduced 

fuel consumption, reduced noise and reduced emissions for 

future generations of aircraft. From new engine concepts 

operating at higher temperatures with less fuel consumption 

and pollution, to radically different aircraft propulsion 

systems, structures and materials advances are the enabler to 

the future of aviation.

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NASA/TM—2011-216747 13

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Materials and Structures Research for Gas Turbine Applications Within the NASA Subsonic 

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14. ABSTRACT

A brief overview is presented of the current materials and structures research geared toward propulsion applications for NASA’s Subsonic 

Fixed Wing Project one of four projects within the Fundamental Aeronautics Program of the NASA Aeronautics Research Mission 

Directorate. The Subsonic Fixed Wing (SFW) Project has selected challenging goals which anticipate an increasing emphasis on aviation’s 

impact upon the global issue of environmental responsibility. These goals are greatly reduced noise, reduced emissions and reduced fuel 

consumption and address 25 to 30 years of technology development. Successful implementation of these demanding goals will require 

development of new materials and structural approaches within gas turbine propulsion technology. The Materials and Structures discipline, 

within the SFW project, comprise cross-cutting technologies ranging from basic investigations to component validation in laboratory 

environments. Material advances are teamed with innovative designs in a multidisciplinary approach with the resulting technology advances 

directed to promote the goals of reduced noise and emissions along with improved performance. 

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