Modified Heat Treatment Research Articles

Wire + Arc Additively Manufactured Inconel 718: Effect of post-deposition heat treatments on microstructure and tensile properties

Wire + Arc Additive Manufacturing (WAAM) can be used to create large free-form components out of specialist materials such as nickel-base superalloys. Inconel (IN) 718 is well suited for the WAAM process due to its excellent weldability. However, during deposition, WAAM IN718 is susceptible to micro-segregation, leading to undesirable Laves phase formation in the interdendritic regions. Further, the WAAM process encourages columnar grain growth and the development of a strong fibre texture, leading to anisotropy in grain structure. This unfavourable microstructure can be addressed through specialised post-deposition homogenisation heat treatments. A new modified heat treatment was found to be effective in dissolving Laves phase, whereas a standard treatment precipitated δ phase. Tensile test results revealed that Laves and δ phases lead to low ductility when present in a precipitation-hardened matrix. The modified heat treatment also reduced the anisotropy in grain structure, leading to almost isotropic elevated temperature tensile properties, which meet minimum specifications for conventional cast but not for wrought material. Specialised post-deposition heat treatments, which address the unique microstructure of WAAM IN718, are crucial to achieving optimal mechanical properties.

Mitigation of anisotropic fatigue in nickel alloy 718 manufactured via selective laser melting

The tension-tension fatigue behavior of heat treated selective laser melted (SLM) nickel alloy 718 (IN718) tensile specimens was tested and compared to as-built specimens. In addition to the industry standard IN718 heat treatment, a modified heat treatment was developed and tested in order to mitigate the anisotropic mechanical properties inherent to SLM materials. Electron backscatter diffraction verified that the modified heat treatment significantly affected the microstructure in all build planes. Three different print orientations were studied to determine their effect on the fatigue behavior. Both heat treatments improved the fatigue life of the specimens, although neither surpassed the fatigue life of wrought specimens. The modified heat treatment reduced the effect of the print orientation on the fatigue life. Surface roughness estimates made by laser scanning microscopy revealed a threshold between roughness controlled fatigue life behavior and print orientation fatigue life dependence. Fracture surface analysis indicated that the print orientation was the greatest factor that influences crack initiation and propagation.

Improved self-pierce rivet performance through intentional decarburization

Two significant self-pierce rivet (SPR) performance metrics are column strength, to be able to pierce materials without buckling, and the ability of the rivet to flare without the formation of fractures. The ability to flare is required to facilitate interlock, which provides joint strength. A test method to evaluate these properties through mechanical flaring of the rivet over a conical die was developed. The test method was utilized to assess performance improvements gained through the incorporation of a decarburized ferrite surface layer that was produced through a modified quench and temper process. Mechanical flaring studies performed using digital image correlation techniques indicated a significant improvement in flaring performance with minimal compromise in column strength when the modified heat treatment process was applied to the current SPR alloy, 10B37. The improvements in flaring capability, while retaining sufficient column strength to pierce the material, have been evaluated in riveting trials for cases where the mechanical properties of the rivet have limited the joint development and joint commonization. Intentionally decarburized rivets were used to join two demanding joint configurations: press-hardened steel to AA6111 and a joint of aluminum sheets with a longer than optimal rivet. In each case, rivets were free of cracking and buckling, and the rivet interlock exceeded the minimum to qualify the joints for application.

Stainless steel valves with enhanced performance through microstructure optimization

Compressor valves are made of hardened and tempered martensitic steels. The main design criterion for the material selection is the fatigue performance of the material under bending loads. In some cases impact loads and corrosive atmospheres additionally act on the part. For the first time, the microstructure of the most commonly used stainless steel and its influence on the properties relevant for flapper valves is presented and described in this paper. It is demonstrated how the tensile properties of a martensitic stainless steel can be enhanced by tailoring the microstructure. Electron back scatter diffraction method is carried out to explain the changes in monotonic mechanical properties. Through a modified heat treatment the martensite microstructure is refined resulting in an increase of yield and ultimate tensile strength and at the same time a significant increase of elongation.

Failure analysis of an aluminum extrusion aircraft wing component

Purpose The purpose of this paper is to deal with the failure analysis of a fractured spar stiffener, extruded from 7075-T6 aluminum alloy, which was found in the central wing, trailing edge structure of a military transport aircraft. The previous loading history and the dominant environmental factors (corrosive and humid atmosphere, water entrapment, etc.) suggest corrosion and fatigue as the principal failure modes, synergistically acting on the wing component. Design/methodology/approach This study presents the failure analysis concentrated on finding evidence of failure mechanisms and plausible root-cause(s) of the fractured spar stiffener. Chemical analysis, stereo and scanning electron microscopy, as well as finite element analysis employed as the main analytical tools for material characterization and failure investigation. Findings The overall evaluation of the findings suggest that the failure caused by a synergy of two mechanisms; a crack initiated in the longitudinal, extrusion direction by an environmentally assisted corrosion attack, then propagated by the superimposed transverse stress field, branched/deflected due to a low crack driving force and extended in a transverse path through a high cycle fatigue process. Finally, the complete fracture occurred as fast fracture, resulted by a ductile overload. Originality/value This paper deals with an industrial damage case study, providing analysis and modeling from structural engineering standpoint. The aforementioned findings concerning the fractured aircraft component allow gaining a deeper knowledge about the mechanisms of crack initiation and propagation which, in turn, can produce a valuable feedback to design, inspection and maintenance procedures. This includes a modified heat treatment from T6 to T73 temper for the redesigned component.

Effect of Heat Treatment on the Mechanical Properties of Copper-Beryllium Alloy (C17200)

Owing to high elastic modulus and good strength, copper beryllium alloys are widely used in many engineering applications. The addition of beryllium to copper makes the alloy respond to aging treatment and thus develops very high strength. Conventional heat treatment cycles are available for copper-beryllium to obtain peak ageing hardness condition. Present study has focused on developing a heat treatment cycle to obtain synergetic combination of moderate strength and good toughness for the C17200 copper-beryllium alloy. Ageing curves have been generated for varying temperature and time. Detailed mechanical properties (hardness, impact, tensile) evaluation at room temperature and sub-zero temperatures have been carried out for the selected samples. Modified heat treatment cycle resulted in higher toughness with adequate strength. Optical microscope (OM) and transmission electron microscope (TEM) analysis were carried out to understand the precipitation behavior. Also, measurements of coefficient of thermal expansion (CTE) and thermal conductivity were carried out on the aged samples.

Improvement of the Ductility of Press-Hardened Plane Sheets through a Modified Heat Treatment

Tailored press-hardening processes are used to reduce both production costs and component weight. The aim of these development methods is to generate regions zones in the component with both high and low tensile strengths. The B-pillar, for instance, needs high tensile strength in the region of the roof frame to prevent deformation. However, the connection to the body should have lower tensile strength to absorb the energy of a crash.Regarding the production process for tailored welded blanks, the tailored press-hardening processes for monolithic sheets need no joining operation. As an addition to recent publications, this paper presents a modified tailored press-hardening process, with a modified time-temperature process. Starting from the required tailored material properties of the part, with a sheet thickness of 1.5 mm, research has been done on the process window and process design.This contribution concentrates on modifications to the time-temperature profile. After heating the hot-dip galvanized, heat-treatable 22MnB5 steel above its austenitic temperature, the aim is to adjust the material’s mechanical properties within the cooling process.Based on the continuous TTT diagram, the cooling rate has an impact on the material’s mechanical material properties. Different proportions of constituents such as Bainite, Ferrite or Perlite are created by varying the cooling rate. Furthermore, during an intermediate stage in the cooling-down period, the holding temperature has an even stronger effect on the material’s microstructural composition and the corresponding mechanical properties. The rate of the transformation process changes, depending on the intermediate temperature. The third parameter investigated is the holding time at this intermediate temperature. As the holding time is increased a transformation, progressing from austenite to other constituents, can be observed.The results of this parametric study could be transferred to a prototype environment.

Evaluation of hardness and surface quality of different wood species as function of heat treatment

The objective of this study was to evaluate the effect of heat treatment on surface roughness and hardness of four wood species, namely black alder (Alnus glutinosa L.), red oak (Quercus falcata Michx.), Southern pine (Pinus taeda L.) and yellow poplar (Liriodendron tulipifera). Samples were exposed to heat treatment schedules having two temperature and exposure levels of 120°C and 190°C for 3 and 6h, respectively. Average hardness value of red oak samples exposed to a temperature of 190°C for 6h was 41.7% lower than that recorded before the heat treatment. Temperature of 190°C produced 7.9% lower hardness values for black alder with the increased exposure time from 3h to 6h. No significant differences were found between same type of Southern pine and yellow poplar specimens before and after the heat treatment in terms of their hardness values. Among the four species considered in this study red oak having the most porous anatomical structure showed the roughest surface. An improvement in surface quality (Ra) with 7.46% with extending exposure time from 3h to 6h at the temperature level of 190°C was noticed. However all four types of wood species kept in the oven at 190° for 6h presented smoother surface quality. It was found that increased temperature from 120°C to 190°C for both exposure times showed significant differences from the surface quality of nontreated samples at 95% confidence level. The anatomical structure of samples was also observed by scanning electron microscope (SEM) and some damage of the cell wall was determined due to heat treatment. The findings of this study demonstrated that heat treatment resulted in adverse effect on hardness characteristics of the samples. It appears that strength losses can be limited through alternative modified heat treatment techniques. On the other hand, surface quality of the samples from all species was enhanced as a result of heat treatment. Therefore such heat treatment would be considered to improve surface quality of the sample for furniture applications where smooth surfaces are ideal adding potential value on wood material to be used more effectively in furniture manufacturing.

High Temperature Tensile Behavior of Directionally Solidified MAR-M247 Superalloy

In this study, two heat treatment schemes were proposed to study the high temperature mechanical behavior of directionally solidified MAR-M247 superalloy. Two withdraw rates, namely, 60 and 180 mm/h were used to produce directionally solidified MAR-M247 specimens by the Bridgeman type furnace. Standard heat treatment (HT1) procedures are solution treatment at 1230°C for 2 h/ArC, then first aging at 980°C for 5 h/AC and followed by second aging at 870°C for 20 h/AC. Modified heat treatment (HT2) is solution treatment at 1260°C for 3 h/ArC and first aging at 980°C for 6 h/AC, then the same second aging procedure. Uneven size, shape and fusion-alike of gamma prime precipitates are observed after full HT1 scheme, whereas even size but fine gamma prime precipitates are observed in HT2 specimen. All three tensile properties (elongation to failure, ultimate tensile strength and yield strength) of HT2 specimens are higher than these of HT1 specimens either at room temperature 25°C or at high temperature 982°C for both withdraw rates. Uneven distribution of the γ′ precipitates attributes to the initial fracture of HT1 specimens.

Alloy Design for Semi Solid Metal Forming

Semi-solid metal forming is more than 40 years old but its full potential to near net shape form high strength aluminium alloys has been realised only to a limited degree. Alloys developed for traditional manufacturing processes were initially used but it became apparent that alloys specific to SSM forming needed to be developed. The main alloy development criteria revolved around SSM processing temperature, solid fraction (fs) versus temperature sensitivity and age hardening potential. This methodology while sound does not fully address the unique processing behaviour of SSM forming. By its very nature SSM requires the controlled solidification of a part of the melt before forming. From basic solidification fundamentals this results in the enrichment of the remaining liquid with alloying elements. During the forming process segregation of liquid phase essentially produces a component with very different compositions in the regions where the liquid solidifies last. From recent work completed on a wide range of standard alloy systems it has become apparent that this segregation effect has a significant impact on aging behaviour and strength. Low melting point structures formed in the these regions result in localised melting in the grain boundary region and along areas of gross liquid segregation during solution heat treatment, contributing to the poor mechanical properties. Although this behaviour can be addressed using modified heat treatment, this cannot be applied to all current alloy systems. Alloy design for SSM forming must take these phenomena into account in order to develop and or specify aluminium alloys with acceptable mechanical properties.

Microstructure and stress-rupture property of an experimental single crystal Ni-base superalloy with different heat treatments

The influence of heat treatment on the microstructure and stress-rupture property at 1,100 °C/140 MPa was investigated in a 5.0 wt% Re containing experimental single crystal Ni-base superalloy. The results indicate that the γ′ morphology is nearly cuboidal in the dendrite core after conventional heat treatment. The lattice misfit of alloy becomes more negative after modified heat treatment and results in more cuboidal γ′ precipitates than that after conventional heat treatment. The increased stress-rupture life after modified heat treatment is attributed to higher γ′ volume fraction, more negative lattice misfit, well-rafted structure, and narrower γ channel width.

Effects of Mechanically Activated Milling and Calcination Process on the Phase Stability and Particle Morphology of Monoclinic Zirconia Synthesized by Hydrolysis of ZrOCl2 Solution

The purpose of this paper was to investigate the effect of a high-energy milling (HEM) process on the particle morphology and the correlation between a thermal treatment and tetragonal/monoclinic nanostructured zirconia powders obtained by a precipitation process. To eliminate chloride residue ions from hydrous zirconia, a modified washing method was used. It was found that the used washing method was effective in removing the chloride from the precipitated gel. In order to investigate the effect of a pre-milling process on the particle morphology of the precipitate, dried <TEX>$Zr(OH)_4$</TEX> was milled using a HEM machine with distilled water. The particle size of the <TEX>$Zr(OH)_4$</TEX> powder exposed to HEM reduced to 100~150 nm, whereas that of fresh <TEX>$Zr(OH)_4$</TEX> powder without a pre-milling process had a large and irregular size of 100 nm~1.5 <TEX>${\mu}m$</TEX>. Additionally, modified heat treatment process was proposed to achieve nano-sized zirconia having a pure monoclinic phase. It was evident that two-step calcining process was effective in perfectly eliminating the tetragonal phase, having a small average particle of ~100 nm with good uniformity compared to the sample calcined by a single-step process, showing a large average particle size of ~300 nm with an irregular particle shape and a broad particle size distribution. The modified method is considered to be a promising process for nano-sized zirconia having a fully monoclinic phase.

Investigating Alternative Strategies for Managing Bacterial Angular Leaf Spot in Strawberry Nursery Production

This article discusses some of the approaches we have tested for managing the bacterial pathogen Xanthomonas fragariae in infected strawberry nursery stock. X. fragariae causes angular leaf spot in strawberry and is transmitted to production fields almost exclusively through infected nursery stock. Of the methods that we have investigated over the past several years, a modified heat treatment has proven to be very effective at reducing systemic infections in propagation material, but cultivar selection affected the outcome. Surface-sterilizing treatments also had an effect on angular leaf spot. We tried procedures from dipping plants in a solution of 10% chlorine bleach to the use of UV-C radiation to reduce the severity of angular leaf spots in the field. Lastly, a sanitation-type treatment, namely removing or trimming remnant leaf and petiole tissue from nursery-trimmed plants, was found to have a significant impact on angular leaf spot. No one method completely eliminated X. fragariae from the planting stock, but there is good indication that a strategic combination of control practices that includes heat treatment should help to reduce significantly the initial amount of bacteria introduced into a field.

Saving energy by modification of Tomita heat treatment method for AISI 4340 steel

In this paper a modified up-quenching heat treatment method (Tomita method) for AISI 4340 steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal austempering and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at 800oC for 60min and followed by quenching at 430oC for the high austempering temperature to achieve the upper bainite morphology and at 360oC for the lower austempering temperature to achieve the lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below Ms (martensite start temperature) (320oC) for 120sec, followed by heating at 500oC to achieve the mixed structure of tempered martensite and upper bainite for 1000sec. The specimens selected for modified up-quenching, after austenitization were quenched to below Ms for 120sec, followed by heating at 400oC to achieve the mixed structure of tempered martensite and lower bainite for 1000sec. It is also shown that the best combination of strength and ductility can be achieved by the modified heat treatment that has been suggested in this investigation by the decrement of heat energy for the austenitization temperature, up-quenching temperature and time.

Microstructural study of fatigue and dwell fatigue crack growth behaviour of ATI 718Plus alloy

A new superalloy, ATI 718Plus alloy (718Plus), has been developed to have a 55°C higher temperature capability over the traditionally used alloy 718, while maintaining favourable processing characteristics and intrinsic raw material costs. The fatigue and dwell fatigue behaviour of four microstructural variations of 718Plus are evaluated and compared with Waspaloy: standard heat treated condition (HT1), standard heat treated condition with thermal exposure at 732°C (1350°F) for 1000 h (HT2), fine grain condition with a modified δ phase (HT3) and fine grain overaged through modified heat treatment (HT4). Fatigue crack growth rate tests were performed at 649°C (1200°F) and 704°C (1300°F) under constant amplitude loading at 10 Hz or with a 100 s tension dwell. The results show that the steady state fatigue crack growth rates of each microstructural variation and both alloys are identical, with all of the 718Plus variations showing a clear advantage over Waspaloy in the fatigue crack threshold regime. The 718Plus variants HT1 and HT3 exhibited the highest fatigue crack growth threshold, followed closely by HT4 and finally HT2. At 649°C (1200°F), the 718Plus alloys had average threshold values ranging from 8·8 to 10·4 MPa m1/2 whereas the Waspaloy material was 6·1–7·5 MPa m1/2. At 704°C (1300°F), the advantage increased with the 718Plus alloys having average threshold values ranging from 10·1 to 11·6 MPa m1/2 compared with Waspaloy, which exhibited average threshold values of 6·6–7·7 MPa m1/2. Dwell fatigue results show that Waspaloy has better resistance to crack propagation under dwell conditions; however, optimisation of the precipitate phase (HT2) shows vast improvement in the dwell fatigue resistance of 718Plus.On a développé un nouveau superalliage, l’alliage ATI 718PlusÒ (718Plus), afin qu’il soit capable de supporter une température de 55°C plus élevée par rapport à l’alliage 718 utilisé traditionnellement, tout en maintenant des caractéristiques favorables de traitement et les coûts intrinsèques de métal brut. On évalue, et compare au Waspaloy, le comportement de fatigue et de pause-fatigue de quatre variations de microstructure du 718Plus. 1) Condition de traitement thermique standard (HT1); 2) condition de traitement thermique standard avec exposition thermique à 732°C (1350°F) de 1000 heures (HT2); 3) condition à grain fin avec une phase d modifiée (HT3); 4) grain fin avec vieillissement artificiel au moyen d’un traitement thermique modifié (HT4). On a effectué des essais de vitesse de croissance de fissure de fatigue à 649°C (1200°F) et à 704°C (1300°F) sous une charge à amplitude constante à 10 Hz ou avec une pause en traction de 100 secondes.Les résultats montrent que les vitesses de croissance de fissure de fatigue en régime permanent de chaque variation microstructurale et des deux alliages étaient identiques, alors que toutes les variations du 718Plus montraient un avantage clair sur le Waspaloy lors du régime seuil de fissure de fatigue. Les variantes HT1 et HT3 du 718Plus exhibaient le seuil de croissance de fissure de fatigue le plus élevé, suivies de près par HT4 et finalement par HT2. À 649°C (1200°F), les alliages 718Plus avaient des valeurs moyennes de seuil allant de 8·8 à 10·4 MPa-m1/2 alors que pour le matériau de Waspaloy, elles variaient de 6·1 à 7·5 MPa-m1/2. À 704°C (1300°F), l’avantage augmentait, les alliages 718Plus ayant des valeurs moyennes de seuil allant de 10·1 à 11·6 MPa-m1/2 comparés au Waspaloy, lequel exhibait des valeurs moyennes de seuil de 6·6 à 7·7 MPa-m1/2. Les résultats de pause-fatigue montrent que le Waspaloy a une meilleure résistance à la propagation de fissure sous des conditions de pause; cependant, l’optimisation de la phase de précipité (HT2) montre une vaste amélioration de la résistance à la pause-fatigue du 718Plus.

Optimising heat treatment requirements for improved toughness of V containing 3%NiCrMo steel

AbstractOne major field of application of medium carbon low alloy steels is military hardware, especially the production of mortars. In manufacturing, it is usually necessary to adhere to specific standards; however, since economic and technological requirements depend on using scrap extensively, it can be difficult to keep minor additions within the specification. Heat treatment will therefore play an important role in obtaining the required mechanical properties. The combined effect of vanadium level and heat treatment cycles on strength and toughness of type 4337V (3%NiCrMo) 'gun steel' has been investigated. Vanadium is added mainly as a grain refiner, but it is also a strong carbide former, and therefore likely to reduce toughness. Conventional heat treatment (normalising at 900°C followed by quenching from 850°C), a modified heat treatment (double normalising at 900 and 800°C, successively, before the conventional treatment), and thermomechanical treatment (quenching in oil directly from the forging...

Fatigue properties of duplex-aged Ti 38-644 metastable beta titanium alloy

Solute-rich beta titanium alloys such as Ti 38-644 (Beta- CTM) are known for an inhomogeneous precipitation of the strengthening alpha phase during aging. Consequently precipitate-free zones, referred to as “β flecks”, remain within the beta matrix when the material is solution treated above the transition temperature and subsequently aged at temperatures between 454 °C and 538 °C. Nevertheless, tensile tests performed on simplex-aged Beta-C show reasonable UTS values of about 1300MPa. However, β flecks are considered to be detrimental with respect to fatigue properties. In order to eliminate this effect, a duplex aging cycle was designed for Beta-C. Depending on the prior working history of the alloy, heat treatment parameters in terms of times and temperatures were specified, in order to optimize the microstructure with respect to a high fatigue limit at comparable monotonic strength values. The results of this study indicate clearly that duplex aging leads to a significant improvement of the fatigue limit in the highly β-stabilized titanium alloy due to a homogeneous dense distribution of acicular alpha precipitates. The success of the modified heat treatment depends very sensitively on the parameters applied for the solution and pre-aging treatment. In this study, duplex aging led to an increase of the fatigue limit to 700 MPa and 750 MPa under symmetrical push-pull and rotating bending loading, respectively. A change of the tensile fracture mode from intergranular to predominantly transgranular was observed accompanied by a slight gain of ductility. Fatigue cracks were shown to initiate in surface-near regions after aging Beta-C according to the duplex route.

Heat Treatment Optimization Studies on PIT ${\rm Nb}_{3}{\rm Sn}$ Strand for the NED Project

For the Next European Dipole (NED) program, a Powder-In-Tube (PIT) strand was successfully developed by SMI. This high-performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand presents a non-copper critical current density of ~ 2500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 T applied field and 4.2 K and a filament diameter around 50 mum. Extensive heat treatment optimization studies were performed in order to maximize both critical current and RRR, with a plateau temperature down to 625degC and duration up to 400 hours. It appears that a critical current enhancement of ~ 10% can be achieved for a reaction schedule of 320 hours at 625degC with non-copper critical current density respectively exceeding 2700 and 1500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 and 15 T (4.2 K). Thanks to this modified heat treatment, this strand completely fulfills the NED stringent specification.

Novel ultrahigh-strength nanolath martensitic steel by quenching–partitioning–tempering process

A modified heat treatment process designated quenching–partitioning–tempering (Q–P–T) process is developed based on the quenching and partitioning process proposed by J.G. Speer et al. [Acta Mater.51, 2611 (2003)] and D.K. Matlock et al. [Mater. Sci. Forum426–432, 1089 (2003)]. A Fe–0.485C–1.195Mn–1.185Si–0.98Ni–0.21Nb steel after Q–P–T process satisfies the designed requirement of tensile strength over 2000 MPa and elongation over 10%. The microstructure characterization indicates that this ultrahigh-strength steel consists of nanomicrostructures including lath martensite, filmlike retained austenite, and dispersive Nb-containing carbides. The effect of tempering time on the mechanical properties is analyzed based on microstructures.

Development of low thermal expansion nickel base superalloy for steam turbine applications

AbstractThe target operating temperature of ultrasupercritical power plants is increasing and is planned to reach 700°C. Austenitic superalloys are promising materials for these applications to replace ferritic heat resistant steels, because of their high strength at 650–700°C. In general, austenitic nickel base superalloys show higher creep rupture strength than ferritic heat resistant steels; however, they have higher coefficients of thermal expansion, lower creep rupture ductilities, and higher costs. The effect of the Mo and Co content, amount of γ' phase, and Al/Ti ratio in the γ' phase on the thermal expansion behaviour of a Mo containing superalloy has been investigated by use of the conventional Mo containing Alloy 252 as a reference. Tensile and creep rupture properties were also measured. Following a modified heat treatment, the Co free superalloy developed on the basis of these tests showed higher creep rupture ductility than Alloy 252, while retaining comparable low thermal expansion and high ...