Improvement Of High-temperature Properties Research Articles

High-temperature creep mechanism of Ti-Ta-Nb-Mo-Zr refractory high-entropy alloys prepared by laser powder bed fusion technology

Creep resistance, which is one of the most important deformation modes, is rarely reported for refractory high entropy alloys (RHEAs). The experiment investigated the high-temperature creep mechanism of Ti-Ta-Nb-Mo-Zr RHEA prepared by laser powder bed fusion (LPBF) technology. The high cooling rate of LPBF suppresses most of the elemental segregation, but there are still over-solidified precipitates and a few continuous precipitates (CP). In the range of 923–1023 K, the stress exponent and activation energy were determined to be 3.2–3.4 and 261.5 ± 19.5 kJ/mol, respectively. Compared with other conventional alloys and HEAs, a large reduction of the minimum creep rate is found in the LPBF-built Ti1.5Ta0.5NbZrMo0.5 RHEA, indicating a significant improvement in high-temperature properties. The dislocation tangles at the interface is formed during the creep process and new Zr-rich CP phases are generated in the dislocation tangles region. The interfacial dislocation tangles is the result of the interaction between dislocations and two-phase mismatch stresses. The dislocation tangles prevents dislocations from further cutting the matrix phase, which is very favorable to the high-temperature creep performance. At the same time, the formation of this dislocation tangles greatly accelerates the nucleation process and growth rate of the new CP phase. The present work provides a pathway to design novel HEAs with improved high-temperature creep resistance.

High-temperature plastic flow of a precipitation-hardened FeCoNiCr high entropy alloy

In this work, we systematically investigated flow behavior of a high entropy alloy (HEA) strengthened by coherent γ′ precipitates in the temperature range of 1023–1173K. In contrast to the single-phase FeCoNiCrMn HEA, this precipitate-hardened alloy, i.e., (FeCoNiCr)94Ti2Al4, exhibited large reduction of the steady-state strain rate (by ~2 orders of magnitude) or drastic enhancement in flow stress, indicating significant improvement in high-temperature properties. Our results showed that the deformation could be divided into two regimes. At temperatures below 1123K, coherent γ′ precipitates effectively blocked the dislocation motion, thus resulted in a threshold stress effect. Above 1123K, however, γ′ particles dissolved and the deformation was controlled by the ordinary dislocation climb mechanism. In addition, we conducted transmission electron microscopy to characterize dislocation-precipitate interaction to provide microstructural evidences to support our conclusion of the specific deformation mechanisms in the two temperature regimes.

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

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.

Improvement in High-Temperature Properties of Al<sub>2</sub>O<sub>3</sub> Ceramics by Microstructure and Grain Boundary Control

Improvement in High-Temperature Properties of Al<sub>2</sub>O<sub>3</sub> Ceramics by Microstructure and Grain Boundary Control

934 高温損傷の自己修復と 304 ステンレス鋼の高温特性の向上

934 高温損傷の自己修復と 304 ステンレス鋼の高温特性の向上

Evaluation of Fatigue Reliability for Industrial Pt Alloy at Elevated Temperature(Strength & Fracture)

Platinum and its alloys are widely used in the glass-melting industry because of their high melting point, high resistance to oxidation and inertness with molten glass. Since they are usually used above 1273K, it is important to improve their mechanical properties at high temperatures above 1273K. For the further improvement of high temperature properties, it is essentially important to clarify the basic mechanical properties of platinum and its alloys at elevated temperatures. In this study, fatigue reliability at elevated temperature of platinum rhodium alloys were examined by the elucidation of fracture mechanism. Main results were follows ; (1) S-N characteristics of PtRh alloys were almost arranged in the straight line. (2) From the fractography using SEM, it was indicated that fatigue fracture mechanism was similar to the static destruction in the high stress side, on the other hand, that was effected by creep in the low stress side.

The Microstructure of Mechanically Alloyed MoSi<sub>2</sub> With Ni and Al Additions

The effect of Ni and Al additions on grain boundary silica in mechanically alloyed and hot isostatically pressed (HTPed) MoSi2 was investigated. Mechanical alloying Mo and Si in the absence of Al produced finely dispersed silica within a fine grained structure. Mechanically alloyed and HIPed Mo and Si with Ni and Al partially transformed the silica to crystalline oxide phases, including Al2O3. An improvement in high temperature properties is not expected due to the retention of a grain boundary silica film. Rapid grain growth resulted during HIPing, possibly due to the formation of a Ni/Fe/Al liquid phase.

Modification of polytetrafluoroethylene by radiation—1. Improvement in high temperature properties and radiation stability

Polytetrafluoroethylene (PTFE) has never been reported to form a network structure when subjected to high energy radiation. Results obtained in this work indicates that when irradiation is performed under 330–340°C in vacuo PTFE can be crosslinked through irradiation. Crosslinked PTFE was found to gain a great improvement in both high temperature mechanical properties and radiation stability.

Radiation crosslinking of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) is considered to be a typical radiation degradative polymer. In this work the polymer was crosslinked by means of radiation at 335 ± 5°C under vacuum. Crosslinked PTFE showed a great improvement in high temperature properties and radiation stability.

Improvement of hot workability of Ni 3AlCrZrB ordered alloy

The effect of minor additions of magnesium on high temperature plasticity in Ni 3AlCrZrB ordered alloy has been investigated. Results of hot compression tests in a Gleeble apparatus showed that the maximum allowable deformation of a polycrystalline alloy with an appropriate magnesium content was about 50% at 1100 °C. An applicable content of magnesium also notably increased the stress-rupture life and tensile ductility at high temperature. The improvement of high temperature properties by magnesium is caused by the magnesium mainly segregated at grain boundaries and increasing the grain boundary coherence.

Study on Improvement in High Temperature Properties of SAE 51431 Steel

Study on Improvement in High Temperature Properties of SAE 51431 Steel

Nimonic 100系合金の鍛造性と時効硬化性におよぼすAl, Tiの影響について

Nimonic type heat-resisting alloys have increased Co, Mo and Al+Ti content for the improvement of high temperature properties, at the expense of the forgeability. The principal cause for the loss of forgeability is due to the increase of Al+Ti content. Therefore, in the present paper, the effect of Al+Ti content on the forgeability of Nimonic-100 type alloys was investigated, and also next place, the age-hardening property, the oxidation resisting property and the metallographical structure of the alloys were examined. The results obtained were as follows: (1) Under the present experimental condition, the limit of possible hot-working of Nimonic 100 type alloys is on the line linking a point of about 4.5%Al and 0%Ti with a point of 0%Al and about 5%Ti, when the Al content is choosen as the coordinate and Ti as the abscissa. (2) The Vickers-hardness on the above boundary line is about 230∼260 in the case of solution-treated alloys. (3) The deformation resistance at 1100°C on the line is about 40∼45 kg/mm2. (4) The time required to reach maximum hardness in age-hardening at 900°C of the alloys is about 8∼30 hrs, and the amount of the softening is small in aging below 500 hrs. (5) From the result of age hardening test of the specimens containing a definite amount of Al+Ti, it was found that Ti contributed in age-hardening more than Al. (6) The constitutional diagram of the alloys solution-treated at 1150°C for 15 hrs was determined. (7) Al is effective for the improvement of oxidation resistance, but Ti has a detrimental effect on the contrary.