D.S. Sarma

On the variation of mechanical properties with solute content in Cu–Ti alloys

Abstract The variation of mechanical properties and electrical conductivity of Cu–Ti alloys of four compositions, viz. Cu–1.5 wt%Ti, Cu–2.7 wt%Ti, Cu–4.5 wt%Ti, and Cu–5.4 wt%Ti, have been studied in solution treated (ST), solution treated+peak aged (ST+PA), and solution treated+cold worked+peak aged (ST+CW+PA) conditions. In the ST condition, Ti is found to be a potential solid solution strengthener of copper showing greater effect than other elements like Zn, Ni, Al, Si, Be, and Sn. Solid solution strengthening in Cu–Ti alloys is attributed to the interaction of titanium atoms with screw dislocations and the effective interaction is more due to modulus mismatch than size misfit. Further, a marked change in the linear variation of tensile strength and elongation with Ti content is observed at about 4.0 wt%Ti beyond which, tensile strength increases sharply while elongation decreases further, which is attributed to fine scale precipitation formed during quenching of Cu–4.5 Ti and Cu–5.4 Ti alloys. On the other hand, hardness and tensile properties increase linearly up to 5.4 wt%Ti in the peak aged condition with or without prior cold work, due to uniform precipitation of Cu 4 Ti, β l phase in all the four alloys. The increase in yield and tensile strengths due to solid solution strengthening, cold work, and precipitation have been determined quantitatively in ST+CW+PA alloys. While electrical conductivity is less, the mechanical properties of Cu–Ti alloys are comparable with those of commercial Cu–Be alloys.

Effect of tempering temperatures on the mechanical properties and microstructures of HSLA-100 type copper-bearing steels

Abstract Two copper-bearing high-strength low-alloy (HSLA) steels with chemistry similar to HSLA-100, were made on a laboratory scale, one in an air induction (100 kg) furnace and the other in a vacuum induction (50 kg) furnace. The ingots cast were hot-rolled to 25 mm thick plates which were subsequently austenitized and tempered at different temperatures (400–700°C) for 1 h. Evaluation of mechanical properties and microstructure of as-quenched and tempered plates revealed that substantial improvement in strength (YS-1024 and 1025 MPa; UTS-1079 and 1111 MPa for steels 1 and 2) occurred at the expense of impact toughness on tempering at 500°C owing to profuse Cu precipitation in the matrix. With increase in tempering temperature however, the notch toughness improved considerably, reaching peak values of 53 and 123 Joules (J) at −85°C for steels 1 and 2 at 650 and 700°C tempering temperatures, respectively. The partially recovered matrix and the coarsened Cu precipitates in this temperature range presumably enhanced dislocation movement and notch toughness.

Effect of prior cold work on mechanical properties, electrical conductivity and microstructure of aged Cu-Ti alloys

The mechanical properties, electrical conductivity and microstructure of Cu-2.7wt%Ti and Cu-5.4wt%Ti alloys have been studied in different conditions employing hardness and resistivity measurements, tensile tests and optical, scanning and transmission electron microscopy. Ageing of undeformed as well as cold worked alloys raises their hardness, strength and electrical conductivity. The hardness increased from 120 VHN for solution treated Cu-2.7Ti to 455 VHN for ST + cold worked + peak aged Cu-5.4Ti alloy. While tensile stength increased from 430 to 1450 MPa, the ductility (elongation) decreased from 36 to 1.5%. A maximum conductivity of 25% International Annealed Copper Standard (IACS) for Cu-2.7Ti and 14.5% IACS for Cu-5.4Ti is obtained with the present treatments. Peak strength was obtained when the solution treated alloys are aged at 450°C for 16 hours due to precipitation of ordered, metastable and coherent β′, Cu4Ti phase having body centred tetragonal (bct) structure. While mechanical properties of Cu-Ti alloys are comparable, electrical conductivity is less than that of commercial Cu-Be-Co alloys.

Effect of Ti additions on the electrical resistivity of copper

Abstract Electrical resistivity of Cu Ti alloys containing 1.5, 2.7, 4.5 and 5.4 wt.% Ti, has been determined from the resistance values measured using Kelvins Bridge apparatus at room temperature. The resistivity in solution-treated alloys increases with Ti content linearly up to about 4.0 wt.% Ti, beyond which it decreases with further additions of Ti. However, in peak-aged condition, the resistivity continues to increase linearly up to 5.4 wt.% Ti without showing any decrease. Nordheims rule of resistivity is followed up to approximately 4.0 wt.% Ti in the solution-treated alloys. Further, Nordheims rule modified with the incorporation of the law of mixtures for two-phase systems ( ρ t = ρ m ν fm + ρ p ν fp ) is obeyed right up to 5.4 wt.% Ti in the peak-aged alloys. The difference in behaviour is attributed to the fine scale precipitation formed during quenching in solution-treated Cu—4.5Ti and Cu—5.4Ti alloys, as revealed by transmission electron microscopy (TEM). The contribution to the total resistivity by β ′-Cu 4 Ti precipitate and prior cold deformation is considerable in deformed and peak-aged Cu Ti alloys.

Steady state creep behaviour of NiAl hardened austenitic steel

Abstract Creep behaviour of a Fe–Ni–Cr–Al alloy, hardened by ordered NiAl precipitates, has been studied over a temperature range 823–923 K at stresses ranging from 150 to 500 MPa. The behaviour following a stress dip or stress increment and the primary transient indicate that creep is recovery controlled. The stress dependence of steady state or minimum creep rate, covering almost five orders of magnitude, can be represented by a power-law with two distinct regimes of creep deformation. In the high stress regime, creep data reveal apparent stress exponents of 10–12 and an apparent activation energy of 282 kJ mol −1 . On the other hand, the low stress regime is characterized by lower values of stress exponents (6–7) as well as activation energy (165 kJ mol −1 ). Creep data in the high stress regime is rationalized by invoking the concept of threshold stress. Activation energy for creep in both the regimes thus derived is equal to that for core or pipe diffusion of γ -iron (159 kJ mol −1 ). The transition in stress exponents can be attributed to a change in dislocation by-pass mechanism from Orowan bowing in the high stress regime to general climb over the precipitates in the low stress regime. Creep data obtained on the alloy subjected to different ageing conditions provide indirect evidence for the suggested creep mechanisms.

Effect of carbon on structure and properties of FeAl based intermetallic alloy

The benefits of carbon addition achieved in iron aluminides based on Fe3Al, could not be achieved in the case of iron aluminides based on FeAl because of the precipitation of soft graphite phase as against hard Fe3AlC0.5 at high concentration of carbon. This study also indicates that the strength of iron aluminides containing carbon appears to be determined by the volume fraction of Fe3AlC0.5 precipitates in these alloys.

Effect of prior cold work on mechanical properties and structure of an age-hardened Cu–1.5wt% Ti alloy

The effects of prior cold work on hardness, tensile properties, electrical conductivity and microstructure of an aged Cu–1.5 wt% Ti alloy have been studied by employing hardness and resistivity measurements, tensile tests and scanning and transmission electron microscopy. The hardness increased from 80 VHN in the solution-treated condition, to 210 VHN on peak ageing and 280 VHN with prior cold work followed by ageing. While a similar trend has been observed in yield and tensile strengths, the ductility (percentage elongation) decreased from 45% to 9%. The electrical conductivity of the alloy also increased up to 26% International annealed copper standard upon ageing the cold-worked alloy. Maximum strengthening of the alloy was associated with the precipitation of metastable, coherent and ordered Cu4 Ti, β′ phase having body-centred tetragonal structure. The differences in the properties and microstructural evolution between low and high titanium alloys (for example, the absence of composition modulations and deformation twins in Cu–1.5 Ti alloy, while they are present in Cu–4.5 Ti alloy) have been discussed. Prior cold work did not change the fracture mode of microvoid coalescence.

Effect of solution treatment temperature on microstructure and mechanical properties of hot isostatically pressed superalloy Inconel* 718

Abstract Hot isostatically pressed superalloy Inconel 718 heat treated as per the standard schedule of aerospace material specification (AMS) 5662 (solution treatment at 980°C/1 h/water quenching followed by two step aging at 720°C/8 h/ furnace cooling to 620°C/8 h/air cooling to room temperature) has shown yield strength (YS) and ultimate tensile strength (UTS) comparable to that of wrought heat treated alloy, but it had poor ductility and inferior stress rupture properties. To find an appropriate heat treatment to improve these properties, the influence of solution treatment temperature on the structure and properties of the alloy has been studied. The results show that although YS and UTS decrease with increasing solution treatment temperature, considerable improvement occurs in the ductility of the unaged alloy. For the aged alloy, an increase in solution treatment temperature resulted in only a marginal improvement in strength, but the 650°C ductility and stress rupture life were improved significantly. This has been attributed to the dissolution of MC carbides and disruption of prior particle boundaries (PPBs) at higher solution treatment temperatures that had led to enhanced bonding between the particles. Fractography studies revealed a transgranular ductile mode of fracture for the unaged condition for all solution treatment temperatures. In contrast, aged alloys subjected to lower solution treatment temperatures have shown decohesion of particle boundaries leading to an intergranular mode of fracture, while alloys solution treated at temperatures greater than 1250°C revealed a transgranular ductile mode of fracture suggestive of improved ductility. The improvement of high temperature properties obtained by higher solution treatment temperature makes it possible to explore the near net shape capability of HIP technology to its maximum potential, when adopted for producing alloy 718 components.

Age hardening studies in a Cu–4.5Ti–0.5Co alloy

Abstract Age hardening in a Cu–4.5Ti–0.5Co alloy has been studied at different aging temperatures and times. It has been observed that this alloy exhibits considerable age hardening with hardness increasing from 225 H V to a peak value of 320 H V on aging. Yield strength increases from 360 to 710 MPa and tensile strength from 610 to 890 MPa on aging the solution treated alloy for peak strength. The electrical conductivity of the alloy is found to be 4 and 8% International Annealed Copper Standard (IACS) in solution treated and peak aged conditions, respectively. Addition of cobalt to Cu–4.5Ti alloy reduces the aging temperature and time for attaining peak hardness. Ordered, metastable and coherent Cu 4 Ti (β l ) precipitate is found to be responsible for maximum strengthening of the alloy. Interestingly, absence of equilibrium precipitate Cu 3 Ti and presence of Cu 4 Ti phase have been noticed in the overaged condition. The absence of Cu 3 Ti is attributed to the addition of cobalt. In addition, intermetallic phases of Ti and Co like Ti 2 Co and TiCo have been observed in solution treated, peak aged and overaged conditions. Cold work prior to aging enhances the hardness, strength and electrical conductivity of the alloy. For example, 90% cold work followed by aging at 400°C for 1 h increases the hardness from 320 to 430 H V ; yield and tensile strengths, from 710 to 1185 and 890 to 1350 MPa, respectively, and electrical conductivity, marginally by 1% IACS. While mechanical properties are comparable, electrical conductivity of Cu–4.5Ti–0.5Co is less than that of the binary Cu–4.5Ti alloy in the solution treated as well as peak aged conditions.

Austenite precipitation during tempering in 16Cr-2Ni martensitic stainless steels

The 16Cr-2Ni steel when quenched from austenitizing temperature of 1,323K results in the formation of a complex microstructure consisting of the inherited {delta}-ferrite, martensite and retained austenite with a few undissolved M{sub 23}C{sub 6} carbides. There do not appear to be many reports on tempering behavior of 16Cr-2Ni steel through microstructural characterization using transmission electron microscopy. A comprehensive study is under progress to examine the structure-fracture-property relationship on 16Cr-2Ni steel and the microstructural changes that occur on tempering the steel are dealt with in this paper.