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MME445: Lecture 13

Common engineering materials: 8. Nickel and its alloys

A. K. M. B. Rashid Professor, Department of MME

BUET, Dhaka

Learning Objectives

Knowledge &

Understanding Knowledge about properties and uses of different nickel alloys

Skills & Abilities Ability to know the character and potential use of nickel alloys

Values & Attitudes Aware of the importance and limitation of using nickel alloys

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Outline of the lecture …

Properties of nickel

Physical metallurgy

Commercially pure nickel

Nickel base alloys

Single crystal castings of nickel-base superalloys

Introduction

Nickel and nickel-base alloys are

vitally important to modern industry

because of their ability to withstand

a wide variety of severe operating

conditions involving corrosive

environments, high temperatures,

high stresses, and combination of

these factors.

Nickel and its alloys, like stainless

steels, offer a wide range of corrosion

resistance.

However, nickel can accommodate

larger amount of alloying elements –

mainly Cr, Mo, and W – in solid solution

that iron.

Therefore, nickel based alloys in

general can be used in more severe

environments than stainless steels.

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Typical properties

Silvery shiny appearance

High toughness and ductility

Good high and low temperature strength

High oxidation resistance

Good corrosion resistance

Ferro-magnetic

Crystal structure FCC

Atomic number 28

Atomic weight (g/mol) 58.71

Density (g/cc) 8.89

Melting point (C) 1455

Boiling point (C) 2913

Disadvantages

Relatively high cost

Not mixed with cheap alloying elements

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About 60% of nickel production is used as alloying elements in stainless and

nickel-alloy steels, and in other elements such as copper, cobalt, chromium, etc.

Most of the remainders is used for high-nickel alloys and for electroplating

Typical applications

Nickel and nickel alloys are used for

a wide variety of applications, the

majority of which involve corrosion

resistance and/or heat resistance

chemical plant, heat exchanger, reaction

furnace, rotary kiln, turbine blades.

A number of other applications for Ni alloys

involve the unique physical properties of

special-purpose nickel-base or high-nickel

alloys. These include:

• low-expansion alloys

• electrical resistance alloys

• soft magnetic alloys

• shape memory alloys

• Aircraft gas turbines: disks, combustion

chambers, bolts, casings, shafts, exhaust

systems, cases, blades, vanes, burner cans,

afterburners, thrust reversers

• Steam turbine power plants: bolts, blades,

stack gas reheaters.

• Reciprocating engines: turbochargers,

exhaust valves, hot plugs, valve seat inserts

• Metal processing: hot-work tools and dies

• Medical applications: dentistry uses,

prosthetic devices

• Space vehicles: aerodynamically heated

skins, rocket engine parts

• Heat-treating equipment: trays, fixtures,

conveyor belts, baskets, fans, furnace mufflers

• Nuclear power systems: control rod drive

mechanisms, valve stems, springs, ducting

• Chemical and petrochemical industries:

bolts, fans, valves, reaction vessels, piping,

pumps

• Pollution control equipment: scrubbers,

flue gas desulfurization equipment (liners,

fans, stack gas reheaters, ducting)

• Metals processing mills: ovens, after-

burners, exhaust fans

• Coal gasification and liquefaction systems:

heat exchangers, reheaters, piping

• Pulp and paper mills: tubing, doctor blades,

bleaching circuit equipment, scrubbers

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Physical Metallurgy

Nickel is a versatile element; alloys with most metals.

Complete solid solubility exists between nickel and copper.

Wide solubility ranges between iron, chromium, and nickel

possibility of many alloy combinations

The FCC structure of the nickel matrix (g) can be strengthened by

① solid-solution hardening

② carbide precipitation, or

③ precipitation hardening.

Co, Fe, Cr, Mo, W, V, Ti, and Al are all solid solution hardeners in nickel.

These elements differ with nickel in atomic diameter from 1 to 13 %.

Above 0.6Tm, which is the range of high-temperature creep, strengthening is

diffusion dependent and large, slow diffusing elements such as Mo and W

are the most effective hardeners.

Solid solution hardening

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Nickel is not a carbide former; other elements present in nickel form carbides.

This is either a bane or a blessing to the designer of alloys. An understanding of

carbide class and its morphology is critical for alloy design.

Carbide strengthening

The alloy chemistry, its prior processing history, and the heat treatment given to the

material influence carbide precipitation and ultimately performance of the alloy.

• MC – large blocky carbide, randomly distributed,

not desired

• M6C – blocky; formed in grain boundaries (can

be used to control grain size), or precipitated in a

Widmanstaitten pattern throughout the grain

(impair ductility and rupture life)

• M7C3 – form intergranularly; beneficial if

precipitated as discrete particles, or cause

embrittlement if they agglomerate and form

continuous grain-boundary films.

• M23C6 – form as grain-boundary precipitates;

influential in enhancing rupture properties

Most common carbides forms: MC, M6C, M7C3, and M23C6.

Precipitation hardening

The precipitation of g’, Ni3(AI,Ti) in a high-

nickel matrix provides significant

strengthening to the material.

This unique intermetallic phase has a FCC

structure similar to that of the matrix and a

lattice constant having 1% or less

mismatch in the lattice constant with the g

matrix. This close matching allows low

surface energy and long time stability.

Precipitation of the g’ from the

supersaturated matrix yields an increase

in strength with increasing precipitation

temperature, up to the overaging or

coarsening temperature.

The amount of g’ formed is a function of the

hardener content of the alloy. Al, Ti, Nb, and

Ta are strong g’ formers.

Effective strengthening by g’ decreases above

about 0.6Tm as the particles coarsen.

The g’ phase can transform to other (Ni3X)

precipitates if the alloy is supersaturated in Ti,

Nb, or Ta (indicated as X) and modify

mechanical properties.

The phases precipitated are functions of alloy

chemistry and the heat treatment given the

material prior to service or the

temperature/time exposure of in-service

application.

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① Commercially pure nickel

② Nickel-copper alloys (Monels)

③ Nickel-chromium alloys

④ Nickel-iron and Nickel-chromium-iron alloys

⑤ Nickel-base superalloys

Classification of Nickel Alloys

Commercially pure nickel

High purity nickel contains at least 99 % Ni

Commercially pure nickels also have Co and such impurities

like Mn, Fe, Si, and Cu to enhance specific properties.

These alloys are non-heat treatable and may be hardened by

cold work.

Some alloys containing Al and Ti are heat treatable and may

be strengthened by precipitation hardening

Microstructure consists of solid solution g phase in annealed

condition Cold drawn Nickel 200

annealed at 829 C

Typical properties

Good mechanical properties and retains its

strength at elevated temperature

Excellent resistance to most corrosive

environment

Applications

Food processing equipment

Electrical and electronic parts

Caustic handling equipment

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Alloy Major Mechanical Principal Typical Designation Composition Properties Characteristics Applications Pure nickel 99.99 Ni (min) TS = 46 ksi E = 30% A nickel 99.40 Ni+Co TS = 70 ksi Used where strength in combination Chemical and soap E = 40% of corrosion and oxidation resistance industry HBN = 100 are required D nickel 95.00 Ni+Co TS = 75 ksi Improve resistance to atmospheric Spark plug electrode, 4.75 Mn E = 40% attack at high temperature; ignition tube HBN = 140 strength greater than A nickel Duranickel 93.90 Ni+Co TS = 100 ksi High strength in combination with spring for laundry clip, 4.5 Al, 0.45 Ti E = 40% excellent corrosion resistance; jewelry parts, optical 0.55 Si HBN = 160 age hardenable frames; instrument parts Parmanickel 98.65 Ni+Co TS = 105 ksi Strength and corrosion resistance Used in place of duranickel 0.45 Ti, E = 45% similar to duranickel; good electrical where good conductivity 0.35 Mg HBN = 160 and thermal conductivity; and magnetic properties age hardenable are required

Nickel - copper alloys (Monels)

Ni and Cu form complete solid solution

Ni-Cu alloy (a.k.a. Monels) contains 29-33 %

Cu as the major alloying element

Ni-Cu alloys containing Al and Ti ( K Monel)

is heat treatable and may be strengthened by

precipitation hardening

Microstructure of cold drawn Monel R405 and

annealed at 829 C, showing solid solution

phase of Ni-Cu with sulphide stringers (black)

Typical properties

Mechanical properties higher than brasses

and bronzes but lower than alloy steels

Good toughness and fatigue strength over a

range of temperature

Good formability and weldability, but poor

machinability

Excellent corrosion resistance to acid, alkalis,

brines, waters, food products, and atmosphere

Reduced price

Applications

Values, pumps, marine fixtures and fasteners

Chemical processing equipment

Oil-well drill collars and instruments

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K Monel

Addition of ~3% Al make it age hardenable

Non-magnetic, corrosion resistant material

with extra strength and hardness

Uses: marine pump shaft, springs, aircraft

instruments, ball bearings, safety tools

H and S Monel

Contains 3% and 4% Si, respectively; casting alloy

High strength, pressure tightness, corrosion resistance

H Monel, containing less silicon has better machinability

Uses: valve seats, pump liners, impellers

Constantan

Contains 45% Ni and 55% Cu

Highest electrical resistivity

Uses: thermocouple

Alloy Principal composition Condition TS, ksi 0.2PS, ksi %E in 2” BHN

Monel 66.15 Ni+Co, 31.3 Cu, 1.3 Fe Annealed 75 35 40 125

K-500 Monel 65.25 Ni+Co, 29.6 Cu, 2.75 Al Annealed 100 45 40 155

H Monel 63.0 Ni+Co, 30.5 Cu, 3.2 Si As-cast 115 70 10 265

Constantan 44-46 Ni, Bal. Cu Annealed

Cold-worked

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100

Electrical resistivity

= 49 mOhm-cm

Ni-Cu alloys

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Nickel - chromium alloys

Cr forms solid solution with Ni up to ~30% at RT,

resulting high corrosion resistance

Alloys containing 80Ni-20Cr compositions (Chromel

A, Nichrome V) and 60Ni-16Cr-24Fe compositions

(Chromel C, Nichrome) are used as electric

heating elements

The alloy forms nickel-rich single phase solid solution

having annealing twins.

Properties

High electrical resistance

High corrosion resistance at high temperature

High strength and workability

Applications

Heat exchanger tubing

Heaters for electric furnace, cookers,

kettles, immersion-heaters, hair-dryers,

toasters, etc.

Resistivity is ~108 mohm-cm

Change in electrical resistivity is

not constant with temperature

Value depends on heat treatment;

annealing improves resistivity

Nickel-iron and Nickel-chromium-iron alloys

Optical micrograph of Inconel 901

after precipitation hardening

SEM micrograph of Inconel 718

after exposure at 705 C/6,048 h

Fe added to replace some of Ni

lower cost

lower properties (as compared with nickel base superalloy)

used at lower temperatures

Ni-Fe alloys contains 25-45%Ni and 15-60%Fe

Higher Ni content increases operating temp (up to 815 C)

due to improved stability but more costly

microstructure consists of austenitic FCC matrix

can be strengthened by solid solution strengthening

(Mo, Cr), and precipitation hardening (Ti, Nb, Al) by

forming intermetallic phases

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Non-heat-treatable Ni-Cr-Fe alloys

The major alloying elements of these alloys (15-22% Cr and up to 46% Fe) form single phase solid solution with nickel

May be hardened by cold working

Alloys are identified according to trade names: Inconel, Incoloy, and Hastelloy

Good mechanical strength and high resistant to creep combined with excellent corrosion resistance to chloride-ion solution, sulphur compounds and other organic and inorganic compounds

Good machinability, weldability and workability

Used for furnace and heat treating equipment (nitriding container, carburizing boxes, retorts).

Heat-treatable Ni-Cr-Fe alloys

These alloys contains 15-22% Cr and up to 33% Fe as the major alloying elements

May be strengthened by precipitation hardening due to the presence of additional alloying elements: Al, Be, Ti, Si

Trade names of some alloys: Nimonic, Inconel X-750, Udimet, Waspaloy, Rene, Astroloy

Very high mechanical strength and high resistant to creep at temperatures up to 815 C combined with good corrosion and oxidation resistance

Used for making gas turbine components, parts of nuclear steam generators, hot working tools, exhaust valves for IC engines

Alloy Principal composition Condition TS, MPa 0.2PS, MPa %E

Inconel Ni 72% min, Cr 14-17%, Fe 6-10% Annealed 655 310 45

Incoloy 800 Ni 30-35%, Cr 19-23%, Fe 39.5% min,

Al 0.15-0.60%, Ti 0.15-0.6% Annealed 600 275 45

Incoloy 800HT Ni 30-35%, Cr 19-23%, Fe 39.5% min,

Al 0.15-0.60%, Ti 0.15-0.6% Annealed 560 250 45

Hastelloy X Ni 45-50%, Cr 20.5-23%, Fe 6-10%,

Mo 8-10%, W 0.2-1.0%, Co 0.5-2.5% Annealed 765 380 44

Non-heat-treatable Ni-Cr-Fe alloys

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Alloy Principal composition Condition TS, MPa 0.2PS, MPa %E

Nimonic 80A Ni 69% min, Cr 18-21%, Fe 3% max,

Al 1.0-1.8%, Ti 1.8-2.7% Precip. hard. 1250 780 30

Nimonic 115

Ni 54% min, Cr 14-16%, Fe 1% max,

Al 4.5-5.5%, Ti 3.5-4.5%,

Mo 3-5%, Co 13-15.5%

Precip. hard. 1300 850 25

Inconel X-750

Ni 70% min, Cr 14-17%, Fe 5-9%,

Al 0.4-1.0%, Ti 2.25-2.75%,

Mo 8-10%, Nb 0.7-1.2%

Precip. hard. 1250 850 30

Waspaloy

Cr 18-21%, Fe 2% max, Al 1.0-1.5%,

Ti 2.6-3.25%, Mo 3.5-5%, Co 12-15.5%,

Ni balance

Precip. hard. 1250 850 30

Rene 41

Cr 18-20%, Fe 5% max, Al 1.4-1.6%,

Ti 3.0-3.3%, Mo 9-10.5%, Co 10-12%,

Ni balance

Precip. hard. 1420 1062 14

Heat-treatable Ni-Cr-Fe alloys

Nickel based superalloys

High temperature heat-resistance

alloys, which can retain high

strengths at elevated temperatures

Three types of Ni superalloys

① nickel base

② nickel-iron base, and

③ cobalt base containing nickel

Alloys contain high Cr with

Ti and Al to from precipitates, and

additions of Mo, Co, Nb, Zr, B, Fe.

Complex microstructures

Properties

Heat resistant and high strength

at high temperature (760-980 C)

Good corrosion resistance

Good oxidation resistance

Applications

Aircrafts, space vehicles, rocket engines

Industrial gas turbines, high temperature

applications

Nuclear reactors, submarines

Steam power plants, petrochemical

equipment

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These superalloys are “super” because of the g’ precipitation strengthening effect

g’ is an intermetallic compound (Ni3Al) with ordered fcc structure

Amazing property of superalloys: they become stronger at higher temperature

Ni atoms

Ni atoms

Al atoms

fcc Ni g

matrix

ordered fcc g’ ppt.

(cuboid in shape)

Turbine blades in a jet engine experience:

• Mechanical forces – (1) creep, (2) fatigue, (3) thermomechanical fatigue

• High temperature environment – (1) oxidation, (2) hot corrosion

Turbine blades

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Turbine blade heat treatment

As-cast dendritic microstructure

precipitation hardening

(solution treatment + ageing)

precipitation hardened

g’ in g matrix

g’ g

The major phases present in the nickel-base superalloys:

• g (gamma) phase – the continuous matrix of FCC austenite

• g’ (gamma prime) phase – the major precipitate phase (more cubic shape)

• carbides – various types, mainly M23C6 and MC (M = metal)

Note: GB carbides affect high-temp strength, ductility, creep

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Single-crystal castings of nickel-base superalloys

A major increase in strength and

temperature capability of superalloy

casting can be obtained with the

introduction of columnar-grained and

single crystal casting

equiaxed

crystal

directionally

solidified

columnar crystals

single

crystal

property comparison between polycrystal,

columnar crystal and single crystal

Next Class

MME445: Lecture 14

Introduction to CES EduPack 2016