IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Weld metals of AA 2219 (Al-6%Cu), welded with 2319 filler, exhibit yield strength values of about 40% compared to their base metal counterparts. This study is an attempt to improve the yield strength of 2219 welds by using fillers containing scandium. Pitting corrosion behavior of the welds has also been studied. Welds were made with Al-Cu, Al-Cu-Sc, Al-Mg, and Al-Mg-Sc fillers. Manual AC-TIG process has been employed to weld 8.5mm thick 2219 -T87 alloy plates. Tensile and corrosion behavior of welds was studied and results were substantiated by metallography. Metallographic studies showed extensive grain refinement in welds made of scandium containing fillers. Presence of scandium, improved the resistance to cracking in case of welds made of Al-Mg fillers. Improvement in mechanical properties has been observed due to the presence of scandium and the rise in percent elongation was significant. Crack free welds with significant improvement in mechanical properties were obtained with a weld metal composition having small amounts of Mg and Sc. Scandium addition did not alter the pitting corrosion resistance of the weld metals. Magnesium containing weld metals were found to be inferior in terms of pitting corrosion resistance.

Department of Metallurgical and Materials Engineering

Transactions of the Indian Institute of Metals

Gas tungsten arc welded AA 2219 alloy using scandium containing fillers - Mechanical and corrosion behavior

The sliding wear behaviour of nickel-based Colmonoy-6 and Anval B60 hardfacing alloys were investigated at room temperature under contact stresses of 1 and 3.5 MPa for sliding distances upto 9 km. Commercial, Colmonoy 6 bare rods were deposited by gas tungsten arc welding (GTAW) and Anval B60 powder by plasma transferred arc welding (PTAW) processes respectively on stainless steel substrate. The hardness measurements across the interface revealed that, over 2.5 mm from the interface, the hardness along the deposits, were found to be 606 and 625 HV for Anval B60 and Colmonoy 6 respectively for single pass. XRD studies showed the presence of borides and carbides along with γ-nickel matrix for both deposits. Pin-on-disc wear tests showed the weight loss gradually increased with increase in sliding distance but steeply with increase in contact stress in both cases. Increase in coefficient of friction with increase in contact stress was also observed on both the deposits. Copyright © 2004 by Society for Advancement of Heat Treatment & Surface Engineering (SAHTSE).

GENERAL

Proceedings of the International Conference on Advances in Surface Treatment: Research and Applications, ASTRA

Some studies on sliding wear behaviour of Ni-base hardfacing alloys

The wear behaviour of a sintered Fe-based alloy reinforced with mechanically alloyed intermetallic has been studied using pin-on-disk wear testing machine in dry condition. A sintered iron alloy made by powder metallurgy was used as matrix. The intermetallics Fe3Al particles prepared by mechanically alloying (15 hours) have been used as reinforcement (5-10%). The processing of the composites includes mixing and cold compacting followed by sintering at 1120°C in Nitrogen atmosphere. The presence and distribution of intermetallic Fe3́Al phase on the surfaces were identified with optical microscopy and further confirmed by XRD. The influence of mechanically alloyed Fe3Al intermetallics additions on the wear behaviour was studied with different sliding conditions. The experimental results indicate that improvement in wear properties in MMCs compared to plain sintered steel as standard material. Wear tracks were characterised by SEM techniques. Studies reveal that unreinforced sintered steel shows lower wear resistance, while composites with mechanically alloyed Fe3Al are the most wear resistant. Copyright © 2004 by Society for Advancement of Heat Treatment &amp; Surface Engineering (SAHTSE).

Wear behaviour of PM Fe-based composite reinforced with mechanically alloyed Fe3Al intermetallics particles

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX-4 at temperatures between 950 and 1100°C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01-1>, and {111}<01-1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549. and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.

Materialwissenschaft und Werkstofftechnik

Shear creep deformation of the super alloy single crystal cmsx-4 at high temperatures and low stresses

Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.

Sadhana - Academy Proceedings in Engineering Sciences

Rafting in single crystal nickel-base superalloys - An overview

The present study investigates the kinetics of the widening of γ-channels in the γ/γ′-microstructure of the single crystal superalloy CMSX-4 during high temperature and low stress creep using scanning and transmission electron microscopy (TEM) in combination with quantitative image analysis. γ-channel widening is an important process because it affects dislocation plasticity in the γ-channels which is one key element of the overall creep process in single crystal super alloys. We compare the kinetics of rafting in &lt;001&gt;-tension and in 011 &lt;011̄&gt;-shear creep testing at a constant maximum principal stress σ1 = 50 MPa at 1080 °C. γ-channel widths follow log-normal-distributions. After subtracting effects due to the change of morphology associated with rafting, the increase of the mean value of γ-channel widths is governed by a parabolic rate law and there is no significant difference between uniaxial &lt;001&gt;-tension and 011 &lt;011̄&gt;-shear creep testing. This suggests that γ-channel widening is controlled by a multiatom diffusion process through the channels and that for multiaxial loading, the normal stress components perpendicular to the γ/γ′-interfaces are important. © 2001 Elsevier Science B.V. All rights reserved.

World University root node

Materials Science and Engineering A

Influence of stress state on the kinetics of γ-channel widening during high temperature and low stress creep of the single crystal superalloy CMSX-4

This work describes a metallographic method of determining the γ-channel width in single crystal super alloys. Using a line intersection method, the distribution of channel widths is determined from TEM and SEM pictures, and appropriate mean values are evaluated. The quantitative procedure aims at measuring the increase of γ-channel width as a result of diffusion through the channel. Widening effects taking place as a result of morphological changes (cube → plates) are not taken into account. It is shown that the widening of the γ-channels follows a parabolic rate law. The method described in this article, in which a FE-SEM is used, yields better results than an older method for which the micrographs were obtained using an LaB6-SEM. This is not only due to the type of SEM used; other parameters have also been optimised (e.g. the etching time) in the new method. The older method produced data for parabolic rate constants which, according to the results obtained with the new method, are about 20% too high.

Praktische Metallographie/Practical Metallography

A quantitative metallographic study of rafting and γ-channel widening during creep in single crystal nickel base super alloys [Eine quantitative metallographische studie zur flossbildung und zur kanalverbreiterung beim kriechen einkristalliner nickel-basis-superlegierungen]

The plain and fretting fatigue behavior was studied of a γ-base TiAl intermetallics at 675 °C in air. The TiAl material was produced by the XDTM process and had a nominal composition of Ti-47Al-2Nb-2Mn+0.8vol%TiB2. The contact material was Inconel 718. The reduction of fatigue strength by fretting was very small (around 20%) compared to metallic materials (30 to approximately 60%). The plain and fretting fatigue S-N curves were flat, which resembled the behavior of ceramic materials. The flat S-N curves are corresponding to the very steep crack growth curve reported for TiAl-based materials containing TiB2. A fretting fatigue crack nucleated in the very late stage of fatigue life (later than 0.75 Nf), while it is known to nucleate in the very early stage of fatigue life (less than few percent of Nf) for steels. The fatigue lifetimes of this material were suggested to be determined by the cycles needed for nucleation of a crack rather than by crack propagation.

ASTM Special Technical Publication

High temperature fretting fatigue behavior in an XDTM γ-base TiAl

Scripta Materialia

On the influence of stress state on rafting in the single crystal superalloy CMSX-6 under conditions of high temperature and low stress creep

Creep crack growth behavior of type 308 Cb austenitic stainless steel and its weldment has been studied in the temperature range of 875 to 1075 K under plane stress condition. The creep crack growth (CCG) rate has been correlated with parameters like stress intensity factor, K, net section stress and energy rate line integral C*. Among these parameters C* appears to describe well the CCG rate in both the cases of base metal and weldment. The rupture time tr of the cracked material has been found to be related to the energy rate line integral C*s in the steady-state condition, given by a relation of the type C*s · tr = constant. © 1991 by ASME.

Journal of Pressure Vessel Technology, Transactions of the ASME

An analysis of high-temperature crack growth in type 308 Cb stainless steel and its weldment

Experimental investigations have been carried out to study the creep crack growth in types 316, 308 Cb, and 304 L stainless steel in the temperature range of 873-1073 K under plane stress condition. Testings have been carried out with both the base metal and the welded composite joints, because such joints are commonly used in nuclear power industries. Among the various parameters tried to correlate the creep crack growth, the energy rate line integral has been found to give the best description of the crack growth rate. The steady-state energy rate line integral has been found to correlate well with the rupture time. Based on this observation, life estimations are presented for thin components containing various initial defect sizes. © 1991 by ASME.

Journal of Engineering Materials and Technology, Transactions of the ASME

Life estimation of cracked stainless steel components under creep conditions

The creep crack growth behaviour of type 316 stainless steel and its weldment in the temperature range 600$DGC to 800°C has been studied under plane stress conditions. The creep crack growth (CCG) rate bears a relation with the load point deflection (LPD) rate independent of the load. The parameters stress intensity factor, K, and the energy rate line integral C*, have been correlated with the CCG rate. At 600°C the crack growth takes place along the interface between austenite and the delta ferrite. At 700° and 800°C sigma phase formation is dominant and crack growth is along the sigma phase and austenite.

High temperature technology

Creep crack growth in type 316 stainless steel and its weldment

Experimental investigations have been carried out on the creep crack growth behaviour in types 316, 308 Cb and 304 L stainless steels under a plane stress condition. The crack initiation starts at the second half of the steady state deformation of the material in all the cases. It has been observed that the rupture time (tr) bears an inverse relation to the steady state energy rate line integral, C*, in the form of C*str = R where Cs* is that value of the energy rate line integral in the steady state condition and R is a constant. © 1990.

Relation between rupture time and steady state rate integral

Creep life is mainly determined by the growth of voids until a critical stage is reached when final failure takes place. Based on metallurgical considerations, there are many models for the growth of voids in creep. However, in many operating conditions the components are subjected to both hot corrosion and creep, which accelerate the failure process. In the present analysis a model is proposed for life prediction under the interacting conditions of creep and hot corrosion based on the fracture mechanics parameter, the energy rate line integral C*. This model appears to describe experimental data on Type 304 stainless steel well.

High Temperature Technology

A model of creep rupture life under hot corrosion

Type 308 stainless steel containing niobium is often used in high temperature applications. Welding is normally resorted to for joining purposes in such components. The crack growth behaviour in the weldment of 308 S.S. has been studied in the temperature range of 600 to 800°C. The stress intensity factor, the COD approach and the energy rate line integral have been tried to correlate the crack growth rate. Crack growth takes places along the interphase between the austenite and the primary arm of the delta ferrite at 600°C. At higher temperatures sigma phase becomes more dominant in the crack growth process. © 1989.

Engineering Fracture Mechanics

High temperature crack growth in austenitic weld metal

Defining a creep-brittle materialsas one in which the load point deflection rate is a function of the crack propagation rate only and is independent of the load or crack length, the applicability of C* to correlate the crack growth rate is discussed in this letter. © 1987.

Materials Science and Engineering

Crack growth in creep-brittle materials

Investigations have been carried out to study crack growth behaviour in type 304 stainless steel under constant-load creep conditions in the temperature range 550-650°C. At 550 and 600°C the load point deflection rate bears a unique relation to the creep crack growth rate, independent of the load. However, the results at 650°C show that the relation is load dependent. The parameters, stress intensity factor and net section stress, do not describe the crack growth rate well and there is wide scatter. The energy rate line integral C* appears to be applicable at 550 and 600°C but not at 650°C where it gives a systematic segregation of data. On the basis of a phenomenological approach a new parameter is derived which appears to correlate well with the creep crack growth at all the three temperatures investigated. This approach also provides an index which can differentiate between creep brittle and creep ductile materials. © 1987.