UNIVERSITI TEKNIKAL MALAYSIA MELAKA

AN INVESTIGATION OF THE EFFECT OF VARIOUS HEAT

TREATMENT PROCESSES ON MICROSTRUCTURE AND

STRESS CORROSION CRACKING (SCC) OF ALUMINIUM

ALLOY 7075

This report submitted in accordance with requirement of the Universiti Teknikal

Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials)

by

LENG SIONG CHENG

B050710018

FACULTY OF MANUFACTURING ENGINEERING

2011

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: An Investigation of the Effect of Various Heat Treatment Processes on Microstructure and Stress Corrosion Cracking (SCC) of Aluminium Alloy 7075

SESI PENGAJIAN: 2010/11 Semester 2 Saya LENG SIONG CHENG mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja dengan izin penulis. 3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan

pertukaran antara institusi pengajian tinggi.

4. **Sila tandakan (√)

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam

AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan

oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

PT 292, Jln Kotaville Indah 9,

Tmn Kotaville Indah, Wakaf Bharu,

16250 Tumpat, Kelantan.

Tarikh: _________________________

Disahkan oleh:

PENYELIA PSM

Tarikh: _______________________

** Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai

SULIT atau TERHAD.

DECLARATION

I hereby, declared this report entitled “An Investigation of the Effect of Various

Heat Treatment Processes on Microstructure and Stress Corrosion Cracking (SCC)

of Aluminium Alloy 7075” is the results of my own research except as cited in

references.

Signature : ………………………………………….

Author’s Name : LENG SIONG CHENG

Date : 18 MAY 2011

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM

as a partial fulfillment of the requirements for the Degree in Bachelor of

Manufacturing Engineering (Engineering Materials). The member of the

supervisory committee is as follow:

………………………………

Supervisor

i

ABSTRAK

Kajian menyeluruh telah dilakukan terhadap aloi aluminium 7075 kerana

keunggulannya dalam sifat-sifat mekanikal hasil daripada process penguatan

penuaan serta kegunaannya yang meluas dalam struktur kapal terbang. Walaupun

dengan kaedah T6, aloi tersebut mampu mencapai kekuatan bahannya yang tinggi,

tetapi ketahanannya terhadap SCC adalah rendah. Dengan kaedah T73, ketahanan

AA-7075 terhadap SCC dalam aloi ini mampu ditingkatkan , tetapi kekuatan

bahannya tidak mampu diperlihara seperti dalam kaedah T6. Kaedah retrogression

dan re-aging (RRA) pula dikatakan mampu meningkatkan ketahanan bahan terhadap

SCC seperti dalam kaedah T73, tetapi dalam masa sama memelihara kekuatan bahan

tersebut seperti dalam kaedah T6. Kajian dimulakan dengan pemeriksaan bahan

spesimen menggunakan analisa Arc Spark. Spesimen-spesimen ini kemudiannya

melalui pelbagai kaedah rawatan haba, iaitu T6, T73 dan RRA, sebelum diuji dalam

ujian DTSCC. Ujian DTSCC dijalankan berdasarkan ASTM G49-85 dan ASTM

G139-05, dengan pendedahan bahan ke dalam persekitaran mengakis, iaitu dalam 3.5%

larutan natrium klorida selama 5 hari bagi spesimen yang dirawat dengan kaedah

rawatan haba yang berlainan. Ujian terikan dijalankan menggunakan mesin ujian

universal (UTM) sebelum mikrostrukturnya diperhati di bawah mikroskop imbasan

elektron (SEM). Kajian menunjukkan spesimen T6 mengandungi kerentanan SCC

yang tinggi; manakala T73 mampu memberi ketahanan pemerakahan kakisan stress

yang tinggi tetapi tidak mampu memberi kekuatan bahan seperti T6. RRA mampu

menghasilkan mikrostruktur campuran T6 dan T73, yang mana ia mampu memberi

sifat-sifat gabungan daripada T6 dan T73.

ii

ABSTRACT

Aluminium alloy 7075 has been widely studied, due to its excellent mechanical

properties developed by age hardening and their extensive uses in the aircraft

structure. Although T6 tempered alloys are known to possess high mechanical

strength, however it has poor resistance to SCC. The method of T73 tempering is

applied to overcome the SCC problem, but the strength of the material is sacrificed

due to over-aging. On the other hand, retrogression and re-aging is done as the

method is claimed able to improve SCC resistance of the alloy but retaining the high

mechanical strength as of T6 tempered alloy. Material specimens are checked and

determined before heat treatments by the means of arc spark analysis. Specimens

which are heat treated with the three different methods, namely T6, T73 and RRA,

are then subjected to DTSCC test according to ASTM G49-85 for test preparation

and ASTM G139-05 as the test model. The SCC test parameter includes the exposure

of material specimens to 3.5% sodium chloride solution as the corrosive environment,

with duration of 5days after the each heat treatment process. Tensile test is done by

the means of Universal Tensile Machine (UTM) according to ASTM E8-04, follows

by microstructure observation under scanning electron microscope (SEM). The

study shows that T6 specimen possesses of high susceptibility to SCC, whereas T73

tempering is able to lower the SCC susceptibility but its strength is lowered in the

same time. RRA is capable to produce the microstructure of mixture of both T6 and

T73 specimens, where provides it the combined properties of both T6 and T73.

iii

ACKNOWLEDGEMENT

I would like to express my utmost gratitude to Dr. Mohd Warikh Abd Rashid for his

all time guidance and supervision upon the execution of my project as project

supervisor. I would also like to thank Mr. Ng Guan Yao (Alans) and Mr. Miron

Gakim for their greatest support and guidance as senior and mentor, my team mates:

Nor Nadiah Abd Hamid, Noorfazidatul Fariha Mustaffa, Nur Fawwaz Asri and

Haris Fahaza Ghazali, and the helpful laboratory technicians Mr. Azhar Shah and

Mr. Shafarizat. Credits also go to my graduated seniors Mr. Luei Hong Keat, and

Ms Tan Kae Shin for providing me some useful references for my project.

iv

DEDICATION

To my beloved father Mr. Leng Kok Oar and mother Mdm. Teoh Ling

Siau, my beloved eldest sister Pn. Nur Atiqah Shuhaily Leng and family,

my beloved second sister Mdm Leng Siek Ping, brother-in-law Mr. Low

Ah Kian, my lovely niece Celine Low, and the new-born nephew

Leonard Low, my beloved brother Mr. Leng Siong Fatt and family, my

respected Dr Mohd Warikh Abd Rashid, Mr. Ng Guan Yao and Mr.

Kwan Wai Loon, and my beloved girl-friend Ms Chee Chew Yen…

v

TABLE OF CONTENTS

PAGE

Abstrak i

Abstract ii

Acknowledgement iii

Dedication iv

Table of Contents v

List of Tables viii

List of Figures ix

List of Abbreviations xii

1. INTRODUCTION

1

1.1 Project Background 1

1.2 Problem Statements 2

1.3 Objectives and Aims 5

1.4 Scope of Project 5

2. LITERATURE REVIEW

6

2.1 Aluminium Alloy 7075 6

2.1.1 Aluminium Alloys Designation for Wrought Alloys 8

2.1.2 Temper Designation of Wrought Aluminium Alloys 9

2.1.3 T- designation for Aluminium Alloys 7075 11

2.2 Heat Treatment of Aluminium Alloy 7075 15

2.2.1 Solution Heat Treatment 16

2.2.2 Quenching 16

2.2.3 Precipitation Hardening 17

2.2.4 Heat Treatment to Overcome the SCC Problems in

AA-7075

19

2.2.5 T6 and T7 Tempering 20

vi

2.2.6 Retrogression and Re-aging 21

2.27 Microstructure of Aluminium Alloy 7075 22

2.3 Stress Corrosion Cracking (SCC) 26

2.3.1 Mechanism of SCC 27

2.3.2 SCC Occurrence and Environments 29

2.3.3 Environment Causing SCC 30

2.3.4 The Effect of Electrode Potential 31

2.3.5 Alloy Dependence 32

2.3.6 Stress Effect 33

2.3.7 Stress Corrosion Cracking Problem in Aluminium

Alloy 7075

34

2.3.8 Stress Corrosion Testing 35

3. METHODOLOGY

37

3.1 Introduction 37

3.2 Raw Material Specification 39

3.3 Raw Material Characterisation 40

3.4 Heat Treatment Processes 40

3.4.1 T6 Tempering 41

3.4.2 T73 Tempering 43

3.4.3 Retrogression and Re-Aging (RRA) 46

3.5 Direct Tensile Stress Corrosion Cracking Test (DTSCC) 48

3.6 Tensile Test 50

3.7 Hardness Test 50

3.8 Microstructure Observation 51

4. RESULTS AND DISCUSSIONS 53

4.1 Material Characterisation 53

4.2 Microstructure Observation 54

4.3 Tensile Test 58

4.4 Hardness Test 62

vii

5. CONCLUSION AND RECOMMENDATIONS 64

5.1 Conclusions 64

5.2 Recommendations 65

REFERENCE 66

APPENDIX A 68

APPENDIX B 69

APPENDIX C 70

APPENDIX D 73

APPENDIX E 74

APPENDIX F 75

APPENDIX G 81

viii

LIST OF TABLES

PAGE

Table 2.1 Aluminium Alloys Code Designation 9

Table 2.2 Temper Designations 10

Table 2.3 List of T-temper Designations 12

Table 2.4 Results of Mechanical Tests Done on AA -7075 which are

Heat Treated via Tempering Methods T6, T73 and RRA

20

Table 2.5 Examples of SCC Environments 29

Table 3.1 Specification of the Test Specimen (ASTM E8-04) 39

Table 3.2 Mixture composition of Keller’s etchant used 52

ix

LIST OF FIGURES

PAGE

Figure 2.1 Example of microstructure of 7075-T6 Aluminium Alloy 15

Figure 2.2 Phase diagram of aluminium-copper binary system 23

Figure 2.3 TEM bright field micrograph of T6 tempered aluminium

alloy 7075

24

Figure 2.4 TEM bright field micrograph of T7 tempered aluminium

alloy 7075

25

Figure 2.5 TEM bright field micrograph of RRA aluminium alloy

7075

25

Figure 3.1 Flow chart of project execution 38

Figure 3.2 The Geometry of the Specimen (ASTM E8-04) 39

Figure 3.3 Image of the furnace used 40

Figure 3.4 Temperature- time graph for the solution heat treatment

process

41

Figure 3.5 Temperature-time graph for aging process 42

x

Figure 3.6 Temperature-time graph for T6 tempering 42

Figure 3.7 Temperature-time graph for T73 over-aging (Stage 1) 43

Figure 3.8 Temperature-time graph for over-aging (Stage 2) 44

Figure 3.9 Temperature-time graph for T73 tempering 45

Figure 3.10 Temperature-time graph for retrogression 46

Figure 3.11 Temperature-time graph for RRA 47

Figure 3.12 Universal Testing Machine Used 48

Figure 3.13(a) DTSCC test setup 49

Figure 3.13(b) DTSCC test setup, where the NaCl solution is contained

in the green rubber tube, with the specimen soaking in the

solution

49

Figure 3.14 Image of hardness testing machine used 50

Figure 3.15 Image scanning electron microscope used 51

Figure 4.1 Microstructure of heat treated of aluminium alloy 7075 57

Figure 4.2 Tensile strength of non corroded heat treated aluminium

alloys 7075

59

xi

Figure 4.3 Tensile strength of corroded heat treated aluminium alloy

7075. 60

Figure 4.4 Comparison of tensile strength between the corroded and

non corroded specimens. 61

Figure 4.5 Reduction of tensile strength of heat treated aluminium

alloy 7075 62

Figure 4.6 Hardness results for heat treated specimens of aluminium

alloy 7075 63

xii

LIST OF ABREVIATION

Al - Aluminium

AA - Aluminium alloy

AH - Age hardening

ASM - American Society of Materials

ASTM - American Standard for Testing of Materials

BCC - Body-centre cubic

FCC - Face-centre cubic

HT - Heat treatment

Nd-YAG - Neodymium-doped Yttrium Aluminium Garnet

PH - Precipitation hardening

PHT - Precipitation heat treatment

RRA - Retrogression and re-aging

SCC - Stress Corrosion Cracking

SEM - Scanning Electron Microscope

SHT - Solution heat treatment

SSRT - Slow Strain Rate Test

UTM - Universal Testing Machine

XRD - X-Ray Diffractometer

1

CHAPTER 1

INTRODUCTION

1.1 Project Background

Aluminum alloy 7075 is a well known type of aluminum alloy used as structural

materials of aerospace, transportation, and sports, where the need of lightweight and

high strength are needed. Aluminum alloy 7075 is one of the 7000-series aluminum

alloys, which captured its reputation in the aeronautical industries due to their

attractive comprehensive properties, such as low density, high strength, ductility,

toughness and resistance to fatigue (Li et. al, 2007). However, like other 7000-series

aluminium alloy, aluminium alloy 7075 is sensitive to localized corrosion such as

intergranular corrosion, exfoliation corrosion and stress corrosion. Further

enhancement is necessary in order to extend its further applications.

Li et. al (2007) and Reda et al. (2008) stated that the corrosion resistance is

modifiable via the means of heat treatment. Most of the researchers are favorable to

study the effect on corrosion resistance of the aluminum alloy 7075 by comparing the

other heat treatment processes to the T6 tempering of aluminum alloy 7075.

Aluminum alloy 7075-T6 possesses high strength, but it is highly subjected to

localized corrosion. Some heat treatment processes such as T73, T74, and T76 are

developed to increase their corrosion resistance, especially T73 tempering, which is

develop to enhance the resistance of the aluminum alloy 7075 against stress

2

corrosion. Besides of T73, another process named retrogression and re-aging or RRA

is another heat treatment process which is well known to enhance the corrosion

resistance of aluminum alloy.

This project is done to study the effect of various heat treatment processes on the

microstructure and stress corrosion cracking of aluminum alloy 7075. Heat treatment

is applied on the material specimens to enhance their corrosion resistance against the

stress corrosion cracking. Two different types of heat treatment processes, namely

T73 tempering and RRA are applied on the aluminum alloy 7075, with T6 tempering

as the reference. Direct tensile stress corrosion cracking test is done on the specimens,

where later tensile test is done to determine the residual stress of the material after it

undergone DTSCC. Besides, the tensile strength of the materials after DTSCC is

then compared to the tensile strength of the materials which are not corroded under

DTSCC, at which the difference of strength, indicates the occurrence of SCC within

the materials. This cause of such properties is explained by relating with the

microstructure appearance of specimens.

1.2 Problem Statement

In modern day transportations, as well as aerospace applications, the demand for

lighter materials that possess good mechanical properties have become the direction

for researches and developments, with the hope to find the best possible material for

the structural components that suits the all requirements. Besides the aspect of light

weight and mechanical properties, one other important concern in the structural

components for these applications is their susceptibility to corrosion, especially when

the materials are alloys.

The problem of corrosion is not a fresh issue in metals and alloys, and many

solutions had been proposed to overcome corrosion problem. Among all corrosion

failures, stress corrosion cracking can be said as one of the important failure cause.

Its occurrences are mostly undetectable or not apparent at the first place. Stress

3

corrosions cracks are mostly microcracks, where the cracks normally occur at the

grain level. This phenomenon happens in all alloys, depending on the environment

and stress level, at which the effect and consequence can be catastrophic compared to

mechanical cracking. The corrosion takes place on the surfaces of the material,

forming surface discontinuities that eventually becomes the stress raiser or notch to

crack propagation at microstructural level, but the mechanism of crack propagation

in such phenomenon is not merely caused by the atomic dislocation due to the stress,

but it is also caused by the chemical attack on the crack tip, causing inter-granular

cracks along the grain boundaries of the material. As the cracking happens within the

grain boundaries, it is invisible to naked eyes. These inter-granular cracks will then

become a mechanical crack when the crack growth achieves a certain crack size that

is quite visible to naked eyes. SCC can happen in also all type of environment, but

the corrosion rate maybe faster is the material is subjected to its susceptible

environment.

7000 series Al alloys have been widely used as structural materials in aeronautical

and transportation purposes due to their attractive comprehensive properties, such as

low density, high strength, ductility, toughness and resistance to fatigue. Perhaps due

to its lower price and cost of manufacturing, 7000 series AA gains higher popularity

as compared to a better but more expensive material known to be titanium alloys.

7075 Al alloy is one type of the 7000 series AA. Not only it possesses the same

principle alloying element as other 7000 series AA, i.e. zinc, it is also sensitive to

localized corrosion, such as inter-granular corrosion, exfoliation corrosion and stress

corrosion cracking. (Li et al., 2007)

For about 40 years as since aluminium alloys begin to gain their wide acceptance in

the modern day applications, researches and developments in improving the

mechanical properties and the resistance to corrosion had been done vigorously. In

the studies done by Li et al. (2007) and Reda et al. (2008), the stress corrosion

resistance, in the 7000 series aluminium alloys, including aluminium alloy7075 can

be modified by heat-treatment. In the heat treatment done for such purposes, the most

common heat treatment methods for aluminium alloy 7075 would be over-aging, as

in T7 tempering. Although the aluminium alloy 7075 with T6 tempered, possesses

high strength, however their localized corrosion resistance is poor. As enhancement

4

upon this problem in T6, over-aging treatments such as T73, T76 and T74 have been

developed. However, the strength of these over-aging treated aluminium alloy 7075-

T7 is relatively poorer than T6. Retrogression and re-aging, RRA is then developed

as it produces a balance of properties for both strength and corrosion resistance for

the 7000 series Al alloys. (Reda et. al., 2008) However, the RRA treatment cannot be

used for large-section Al alloys due to its very short retrogression time.

In this investigation, by applying the three methods of T6, T73 and RRA tempering,

the microstructure of the heat treated aluminium alloy 7075 is altered. T73

tempering and RRA are done to produce the microstructure that would resist the

stress corrosion cracking, while the T6 tempering is done to produce materials with

properties referable by both T73 and RRA. It is not sufficient to compare only in

stress corrosion resistance of both heat treatment processes to T6, as in the same time,

other mechanical properties are taken into account as well. In this study, it is crucial

to identify the effect on the tensile strength and hardness of the material, besides of

looking in the performance of the material to resist stress corrosion cracking. Then,

both heat treatment processes is compared to identify which one will actually be the

best process of giving the material good stress corrosion cracking resistance and

mechanical properties. The identification of the best process is done by looking at the

formation of microstructure of both heat treatment processes compared to T6, as well

as the reduction rate of the tensile strength when subject to stress corrosion. The

reduction rate of the tensile strength will eventually provide the indication of the

occurrence of stress corrosion cracking. By identifying these measuring keys, it is

possible to answer the problem of which is the best process that would provide good

stress corrosion cracking resistance, but in the same time having good mechanical

properties.

5

1.2 Objectives and Aims

The objectives of the investigation include:

i. To relate the microstructure of the heat treated aluminium alloy

7075 with material’s stress corrosion behaviour.

ii. To study the effect of various heat treatment on the mechanical

properties of aluminium alloy 7075.

1.4 Scope of Project

The study in this project focuses on the microstructure formed within the material

after undergoing heat treatment processes, and how those microstructures would

actually affecting the stress corrosion cracking resistance of the material. In this

project as well, the DTSCC is done to simulate the stress corrosion condition on the

heat treated material, and the effect is shown in the tensile test results. The

determination of the stress corrosion cracking occurrence is determine by comparing

the tensile strength of both corroded and non corroded specimens. This study will not

include the cost of the processes and the effect of heat treatment on the size and

geometry of material.

6

CHAPTER 2

LITERATURE REVIEW

2.1 Aluminium Alloy 7075

According to Budinski et al. (2005), aluminium and its alloys are the second only to

steel in importance in our modern world. With steels, it is possible to make large

structures and tools, but aluminium alloys made large structures possible with a

lighter weight.

Aluminium alloys are essential engineering materials. The curious aspect of this

material is that it is relatively new to our world. This “commodity metal” was made

common for about 60 years. Pure aluminium was first produced in the laboratory in

1825, by the reduction of aluminium chloride, and its wide acceptance did not occur

until World War II. Nowadays, aluminium alloys are mostly used in aerospace,

marine, and transportation applications as the main material for the structural

components.

Budinski et al. (2005) stated that aluminium is a good electrical conductor; it is

ductile and can be readily cast or machined. It has a face centred cubic structure, as

do other „metallic‟ metals, such as copper, silver, nickel and gold. It is lighter than all

other engineering metals, except for magnesium and beryllium. It has the density of

about 2990kg/ . Aluminium has the conductivity of about 60% IACS. However

due to its lower density, aluminium has a higher conductivity than copper per unit

7

mass. For example, a 10-mm-diameter aluminium wire will have the same resistivity

as a 6-mm-diameter copper wire. The resistivity of the material is just the inverse of

the conductivity of the material. However, the aluminium wire is still 13% lighter

than the copper wire. This is a vital consideration for long distance power

transmission cables.

Aluminium alloys, in some extent, are regarded as corrosion-resist material.

However, the corrosion termed in the case of aluminium is regarded as atmospheric

corrosion, rather than chemical corrosion. Aluminium is still subjected to

electrochemical corrosion. (Budinski et al., 2005)

In overall, aluminium alloys are best described of having some noteworthy

advantages like

i. One-third the weight of steel.

ii. Good thermal and electrical conductivity.

iii. High strength-to-weight ratio.

iv. Can be given a hard surface by anodizing and hardcoating.

v. Most alloys weldable.

vi. Will not rust.

vii. High reflectivity.

viii. Can be die cast.

ix. Easily machined.

x. Good formability.

xi. Nonmagnetic.

xii. Nontoxic.

xiii. One-third the stiffness of steel.

Aluminium alloy 7075 is an aluminium alloy, with zinc as the primary alloying

element, as according to the designation registered by US Aluminium Association.

The composition of aluminium alloy 7075 includes 5.1-6.1% zinc, 2.1-

2.9% magnesium, 1.2-2.0% copper, and less than half a percent of silicon, iron,

manganese, titanium, chromium, and other metals.

8

It is strong, with the strength comparable to many steels, and has good fatigue

strength and average machinability, but has less resistance to chemical corrosion than

many other aluminium alloys. Its relatively high cost limits its use to applications

where cheaper alloys are not suitable. It is commonly produced in several

heat temper grades, 7075-O, 7075-T6, and 7075-T651.

The first aluminium alloy 7075 was developed by Japanese company Sumitomo

Metal in 1936. 7075 was used for the Zero fighter's air frame of the Imperial

Japanese Navy in pre-war times. Aluminium alloy 7075 is often used in transport

applications, including marine, automotive and aviation applications, due to their

high strength-to-density ratio. Its strength and light weight are also desirable in other

fields. Rock climbing equipment, bicycle components, and hang glider airframes are

commonly made from aluminium alloy 7075. One interesting use for 7075 is in the

manufacture of M16 rifles for the American military. It is also commonly used in

shafts for lacrosse sticks.

Due to its strength, high density, thermal properties and its polishability 7075 is

widely used in mould tool manufacture. This alloy has been further refined into other

7000 series alloys for this application namely 7050 and 7020.

2.1. 1 Aluminium Alloys Designation for Wrought Alloys

Aluminium alloy compositions are registered with The Aluminium Association.

However, many organizations published more specific standards for the manufacture

of aluminium alloys, including the Society of Automotive Engineers standards

organization, specifically its aerospace standards subgroups, and ASTM International.

Aluminium Association of United States had designated 4-digit code to the

aluminium alloys based on their principal alloying elements. Table 2.1 shows the

designation code and the representing principal alloying elements.