Metals at High Temperature: Their Behavior and the Assessment of Properties
(November 2008)
Matthew Donachie and Stephen Donachie
The following information is abstracted principally from An Engineer’s Guide to Selection of Metals for Structural
Design (Dr. Matthew J. Donachie, Rensselaer at Hartford ), a chapter in Eschbach’s Handbook, Myer Kutz, editor, J.
Wiley & Sons, in press, November 2008. Other information is from Engineer’s Guide to the Selection of
Superalloys for Design (Dr. Matthew J. Donachie, Rensselaer at Hartford and Dr. Stephen J. Donachie, Special
Metals [retired – now consultant] ), a chapter in Handbook of Mechanical Engineering, Myer Kutz, ed., J. Wiley &
Sons, NY, NY, 2005. Some comments are not in the Handbook Chapters.
Note: All figures/tables attached are as they exist in the original source publications, they’re redrawn for handbooks.
Properties of Metals and Alloys
General
Metals are crystalline and, in the solid state, the atoms of a metal or alloy may arrange themselves in various
crystallographic groupings, often occurring as cubic structures. Some crystal structures tend to be associated with better
property characteristics than others. Crystalline structure normally is a characteristic of the major (base) alloy element in an
alloy. In addition to basic crystal structure, crystalline aggregates of atoms have orientation relationships in space. As
noted, crystalline aggregates are called grains and, in an alloy, there are usually many grains with random orientation
directions within each manufactured article. Metal alloy articles with multiple random grain directions are known as
polycrystalline (frequently referred to as having equiaxed grain structure), often having roughly equal dimensions in all
directions. However, columnar shaped grains (with one relatively longer axis) are common in polycrystalline cast products.
Properties
Properties of alloys may be mechanical, physical or chemical. Mechanical behavior encompasses the strength properties
determined in appropriate tests and tend to be the more important properties for selection of alloys for structural
applications. Some mechanical properties of alloys are: tensile or compressive strength (i.e. onset of plasticity … yielding
strength, fracture strength under normal stress loading, etc.), fatigue strength (i.e. cycles to first crack or fracture, or stress
for a specific number of failure cycles), modulus of elasticity, etc. which are measured primarily at lower to intermediate
temperatures of operation. In addition high temperature mechanical properties such as creep or creep-rupture strength,
fatigue strength, thermally induced fatigue capability, etc. are of concern but primarily at temperatures above about 0.5 of
the absolute melting point of an alloy. At high temperatures, mechanical properties may be influenced by sequences of
cyclic loading and time, as well as frequency of load application, in addition to temperature of testing.
Physical properties include: heat capacity, electrical conductivity, thermal conductivity, magnetic properties, etc. Chemical
properties may be of most concern in environmental attack when alloys for structural applications are considered. Intrinsic
chemical properties of alloys are modified (as are mechanical properties) by chemical changes during alloy invention and
by heat treatment of alloys during processing. In addition, surface chemical properties frequently are modified by surface
changes induced, for example, by coatings.
While the physical properties to a major extent depend upon the chemistry of the alloy, the mechanical properties are
dependent not only on the chemistry of an alloy but more importantly on the microstructure. Microstructure is assumed to
mean grain shape, orientation, grain size, etc. and the presence, absence, type and location of new crystal phases in the
alloy. The mechanical properties result from the interaction of imperfections such as dislocations with the microstructural
components.
Structure of Metals
The peripheral surface of a grain is called a grain boundary. Aggregates of atoms without grain boundaries are rarely
created in nature. However, alloys without internal grain boundaries (i.e. single crystals) or alloys with aligned boundaries
(i.e. columnar grained structures) can be produced in some systems by appropriate manufacturing techniques. The
introduction of different atom types, new crystal phases, and/or the manipulation of grain boundaries, plus the use of stored
energy of deformation, enables inhibition of the movement of the imperfections (e.g. dislocations) that enable deformation
to occur.
It is quite important for the engineer selecting alloys to have a realistic understanding of the strengthening process in alloys
as the mechanical properties of alloys can be modified considerably by processing to manipulate the strengthening level
achieved. The energy of mechanical deformation (working) can be stored in an alloy (work hardening), producing higher
strengths than the unworked (soft, annealed, etc.) alloy. Alloys such as steels (ferrous alloys) can have increased strengths
produced by the introduction of fine structure from transformations of crystal lattices. This fine structure usually results
from the formation of non-equilibrium martensite phases (in preference to the expected equilibrium phases). The fine
structure produced by the martensitic reaction acts to restrict deformation by dislocation movements. This type of
hardening often produces very high strengths (as in iron alloys) but much reduced ductility. Subsequent heat treatment
(tempering) normally is used to enhance ductility at the expense of strength in transformation hardened alloys. Industrial
use of martensite to strengthen alloys is most prevalent with iron-base alloys (containing carbon) or in titanium alloys.
Metal Strengthening
Dislocation movement at lower temperatures not only is reduced by fine structure but also is reduced by smaller grain
sizes. Ultimate tensile strength is not particularly affected by grain size changes although ductility effects are noticed. As
operating temperatures increase, finer grain size material usually becomes less strong than coarse grained material.
Increased temperatures and time at high temperatures may cause recrystallization (new grain formation and subsequent
loss of stored energy) and/or grain growth. Consequently, work hardening to increase strength has upper limits of
application since the high strength benefits of (cold) work may be lost with time at higher operating temperatures.
Some alloys derive their strength from solid-solution hardeners (elements dissolved in the basis metal) as well as from
secondary precipitates that form in the matrix (principal) phase and produce precipitation (age) hardening from the
dispersed particles. Precipitates may be phases or they may be zones of enrichment in one of the alloy elements present. In
lower temperature-use alloys, such as aluminum, age hardening may be from zones or phases. In higher temperature-use
alloys, precipitated phases are the norm. Principal strengthening precipitate phases vary with alloy but in nickel-base and
iron-nickel-base superalloys they are ordered particles of gamma prime, eta and/or gamma double prime. Phases such as
carbides may provide limited direct strengthening (e.g., through dispersion hardening) or, more commonly, indirectly (e.g.,
by stabilizing grain boundaries against movement at high temperature). In some instances, ceramic (silica, yttria, etc.) or
other phases may be dispersed mechanically instead of metallurgically in a matrix to produce dispersion hardening. Thus
the hardening of alloys can consist of:
 Solid solution hardening
 Work hardening
 Dispersion/precipitate hardening (sometimes with ordered precipitates)
 Transformation (martensite) hardening
 Grain size/morphology (shape/orientation) hardening
The results of hardening vary by alloy and by the temperature of testing. Improvements in lower temperature short time
strength may not necessarily be transferred to high temperature ranges of operation. Control of grain structure can
significantly influence mechanical properties. The extent to which the secondary phases contribute directly to
strengthening depends on the alloy and its processing. It should be noted that improper distributions of phases such as
carbides and precipitate phases can be detrimental to properties. Work hardening is an important characteristic of alloys
since work hardening may be used to strengthen an alloy (Figure 5) but also may contribute to the need for intermediate
heating (in-process annealing) during forging/forming of an article to prevent cracking of the article or damage to
fabrication equipment.
Metals at High Temperatures
General
While material strengths at low temperatures are usually not a function of time, at high temperatures the time of load
application becomes very significant for mechanical properties. Concurrently, environmental attack by oxygen and/or other
elements at high temperatures accelerates the conversion of some of the metal atoms to oxides or other compounds.
Environmental attack proceeds much more rapidly at high temperatures than at room or lower temperatures.
Environmental considerations can vary significantly with different types of environment and different temperatures, in
some instances, showing a greater rate of attack at one temperature than the attack of the same alloy at a higher
temperature.
As in the case of lower temperatures, presence of notches and/or cyclic conditions may cause premature failure of a
component. Thus, in addition to evaluation of short time tensile (e.g. ultimate, yield strengths) properties, creep-rupture
properties, fatigue and thermal-mechanical fatigue (smooth/notched) strengths, crack propagation characteristics,
toughness behavior and possible environmental considerations should be checked.
Mechanical Behavior
In the case of short time tensile properties (yield strength, ultimate strength), the mechanical behavior of alloys at higher
temperatures is similar to that at room temperature, but with alloys becoming weaker as the temperature increases.
However, when steady loads below the normal yield or ultimate strength (determined in short time tests) are applied for
prolonged times at higher temperatures, the situation is different. Figure 4 illustrates the way in which most alloys respond
to steady extended-time loads at high temperatures.
Owing to the higher temperature, a time-dependent extension (creep) is noticed under load. If the alloy is exposed for a
long time, the alloy eventually fractures (ruptures). The degradation process is called creep or, in the event of failure,
creep-rupture (sometimes stress-rupture) and alloys for elevated temperature use are selected on their ability to resist creep
and creep-rupture failure. Data for superalloys frequently are provided as the stress which can be sustained for a fixed time
(e.g. 100 hr rupture) vs. the temperature. Figure 9 shows such a plot with ranges of expected performance for various
superalloy families.
One of the contributory aspects to elevated temperature failure is that alloys tend to come apart at the grain boundaries
when tested for long times above about 0.5 of their absolute melting temperature. Thus, fine grained alloys which are
usually favored for lower temperature applications may not be the best materials for creep-rupture limited applications at
high temperatures. Elimination or reorientation/alignment of grain boundaries is sometimes a key factor in maximizing the
higher temperature life of an alloy.
A factor in mechanical behavior not often recognized is that the static modulus (e.g. Young’s modulus – E) is affected by
increases in temperature. This should be obvious from the above discussion about creep. The dynamic modulus of alloys
shows a decrease with increasing temperatures. However, dependent on rate of load application, as test temperatures
increase, static moduli determined by measurements from a (short time) tensile test tend to gradually fall below moduli
determined by dynamic means. Since moduli are used in design and may affect predictions of life and durability, every
effort should be made to determine the dynamic, not just static, moduli of alloys for high temperature applications. Table
10 shows dynamic moduli of cast superalloys for illustration.
Moduli of cast alloys also may be affected dramatically by the orientation of grains or by use of single crystals. Thus, a
columnar grain nickel-base superalloy will show a dynamic modulus parallel to the growth direction of the columnar
grains that is around 18 x 106 psi (124.1 GPa) at 70oF (21 oC). This is much higher than the polycrystalline (random
orientation) value of around 30 x 106 psi (206.8 GPa). A more accurate method of depicting moduli would be to use the
appropriate single crystal elastic constants (three needed) of an alloy but these are rarely available.
Cyclically applied loads that cause failure (fatigue) at lower temperatures also cause failures in shorter times (lesser
cycles) at high temperatures. For example, Figure 3 shows schematically how the cyclic resistance is degraded at high
temperatures when the locus of failure is plotted as stress vs. applied cycles (S-N) of load. From the S-N curves shown,
it should be clear that there is not necessarily an endurance limit for metals and alloys at high temperatures.
Cyclic loads can be induced not only by mechanical loads in a structure, but also by thermal changes. The combination
of thermally induced and mechanically induced loads leads to failure in thermal-mechanical fatigue (TMF). TMF
failures occur in a relatively low number of cycles. Thus TMF is a low cycle fatigue (LCF) process (less than about 105
cycles). LCF can be induced by high repeated mechanical loads while lower stress repeated mechanical loads lead to
fatigue failure in a high number of cycles (HCF – greater than 106 cycles). Dependent on application, LCF failures in
structures can be either mechanically-induced or TMF-type. In airfoils for the hot section of gas turbines, TMF is a
major concern. In highly mechanically loaded parts such as gas turbine disks, mechanically induced LCF is the major
concern. HCF normally is not a problem with alloys unless a design error occurs and a component is subjected to a
high frequency vibration that forces rapid accumulation of fatigue cycles. Although S-N plots are common, the
designer should be aware that data for LCF and TMF behavior frequently are gathered as plastic strain (εp or delta εp)
vs. applied cycles. The final component application will determine the preferred method of depicting fatigue behavior.
While life under cyclic load (S-N behavior) is a common criterion for design, resistance to crack propagation is an
increasingly desired property. Thus, the crack growth rate (da/dn) vs. a fracture toughness parameter is required. The
toughness parameter in this instance is the stress intensity factor (K) range (delta K) over an incremental distance which a
crack has grown. A plot of the resultant type (da/dn vs. delta K) is shown schematically and for several superalloys in
Figure 10.
Impact of Materials’ Data Validity on Selection
There ought to be one basic rule for alloy selection and that is: “Do not believe all you see or hear” (from your fellow
employees, or from persons purveying new alloys, coatings or processes). The most common type of statement from
alloy/process developers with whom one talks (the “sellers”) at the start of new alloy (or process) evaluation is: “This is
our average product/property.” Translated, this statement should probably be treated by the alloy selectors (the “buyers” in
this instance) as meaning: “It’s the best we have gotten!” One needs to be skeptical and inquisitive about statements from
the seller!
One should be especially wary of selective use of data. There was a situation where an alloy (never used in production)
was claimed to have property levels up to 150 oF (94 oC) better than any other similar alloy then existent. Unfortunately,
when a laboratory program was run at a different laboratory (Lab 2) than the laboratory (Lab 1) of alloy invention, results
were troublingly low. On inquiry to the “inventor of the alloy,” the question was raised as to how he got such good results.
The answer? The inventor only cut out good material to test.
At that point, it was reported to the manager of Lab 2 that, with careful work in the laboratory, one might get a “100 oF
(62.5 oC)” improvement over the best existing alloy but never 150 oF (94 oC). And, it was said that, in a production shop,
the best improvement might be no better than 50 oF (31 oC). The latter projection was proven true when maximum alloy
improvement turned out in production to be only 47 oF (29.4 oC).
Engineers have been known to discard data that they don’t like when it is not as good as what they expect (and
occasionally get). There need to be valid reasons for excluding data. When much data (properties, results of processing,
etc.) gets discarded, the discarded data often come back to haunt the user of the alloy/production process. Dig into the data
base for anything you use in your alloy selection. Assure yourself that there are no hidden items which may appear again in
production.
In an instance of mechanical property tests of a new wrought nickel-base superalloy to be used for a gas turbine disk, low
creep-rupture test results were routinely discarded. The alloy (as a disk) went into production and most of the first
production run began to fail the creep-rupture acceptance tests in the shop, with test values almost identical to those which
had been routinely discarded during alloy development. A modification of heat treatment was necessary to bring the alloy
creep-rupture life back to the range promised in development and much time and money was spent in the recovery
operation.
Level of Property Data
Different organizations have different ways to treat property data. Generally minimum and typical properties are desired by
design engineers. Minima can vary in definition (minus 2 sigma, minus 3 sigma deviations are used). One of the cardinal
rules of alloy property development is that enough data are never going to be generated to truly determine a statistically
valid property level over all possible property space. Data cost money! Estimates are made and may often be used in
design. If these estimates are conservative and can be justified by alloy data on the alloy of interest and similar alloys,
satisfactory designs with the selected alloy usually result.
Not all properties require minimum values. Some property values used for design are typical values. While minima may be
used in design, estimates of typical behavior of a component made in order to track its operation may require typical
property values.
Alloy Information
There is no substitute for consultation with alloy producers or manufacturers about the forms (cast, wrought, powder
metallurgy) which can be provided and the exact chemistries/properties available in alloys. It should be understood that not
all alloys are readily available as off-the-shelf items. While many thousands of alloy compositions have been evaluated,
some for over a century, only a relative handful are routinely produced. Moreover, some alloys are not available for use in
all forms and sizes. Sometimes the highest strength alloys will be useful only as powder metal products or as castings. In
many instances, specific alloy compositions may only be available at limited times in a production cycle.
Design data for alloys are not intended to be conveyed here. Design properties should be obtained from internal testing if
possible. Data from producers or other validated sources may be substituted if sufficient test data is not available in-house.
Typical properties are merely a guide for comparison. Exact alloy chemistry, article section size, heat treatment and other
processing steps must be known to generate adequate property values for design.
The alloy selector needs to be aware that processing treatments such as forging conditions, heat treatment, coatings for
corrosion protection, etc. dramatically affect properties of alloys. There are instances when different heat treatments may
be applied to the same nominal alloy composition with resultant differences in properties. It also is not unusual for the heat
treatments applied to cast parts of the same alloy composition to differ from the heat treatments for wrought products of the
same alloy. All data should be reconciled with the actual manufacturing specifications and processing conditions expected.
Alloy selectors should work with competent metallurgical engineers to establish the validity of data intended for design as
well as to specify the processing conditions that will be used for component production.
Application of design data must take into consideration the probability of components containing locally
inhomogeneous regions under some circumstances. Short time properties such as tensile strength may be little affected
by local inhomogeneities. Time- or cyclic-dependent properties at high temperatures and cyclic properties at low to
intermediate temperatures can be detrimentally affected by inhomogeneities. For wrought alloys, the probability of
occurrence of such regions (which are highly detrimental to fatigue life) is principally dependent upon the melting
method selected to produce the cast ingot for subsequent processing. For alloys, whether used as-cast or in the
wrought form, the degree of inhomogeneity and the likelihood of defects such as porosity is related to the alloy
composition, the casting technique used and the complexity of the final component. Defects may be detected by
metallurgical surface examination, radiography or sonic inspection but all methods have limitations on the defect size
that can be detected.
For sources of property data other than data from the producers (melters, forgers, etc.) of an alloy or from an alloy
selector’s own institution, one may refer to organizations such as ASM International which publish compilations of data
that may form a basis for the development of design allowables for many alloys. Many consumers of alloys actually only
use a few alloys. Within the customary user groups, data may be more plentiful for certain compositions. The extent to
which such data are available to the general public is unknown. However, even if such data were disseminated widely, the
alloy selector needs to be aware that processing treatments such as forging conditions, heat treatment, coatings for
corrosion protection, etc. dramatically affect properties of alloys. All data should be reconciled with the actual
manufacturing specifications and processing conditions expected. Alloy selectors should work with competent
metallurgical engineers to establish the validity of data intended for design as well as to specify the processing conditions
that will be used for component production.
Standards organizations such as ASTM publish information about alloys but that information does not ordinarily contain
design data. It is important to note that the same nominal alloy chemistry may have some composition modifications made
from one manufacturer or customer to another. Sometimes this extends from one country to another. Tweaking of the
casting or “wroughting” processes or the heat treatment often associated with what seem to be minor composition changes,
can caused significant variations in properties. All facets of chemistry and processing need to be considered when selecting
an alloy for an application.