Microstructure and microhardness of directionally solidified and heat-treated Nb-Ti-Si based ultrahigh temperature alloy

Directionally solidified （DS） specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at （1 500 ℃, 50 h） and （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, respectively. The results show that the microstructures become uniform, the long and big primary （Nb,X）sSi3 （X represents Ti and Hf elements） plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the （Nb,X）sSi3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution （Nbss） in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at （1 500 ℃, 50 h） ＋ （1 100 ℃, 50 h）, which is 72.8 % higher than that under DS condition.

Effect of high temperature treatments on microstructure of Nb-Ti-Cr-Si based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy,coupons were homogenized at 1 200-1 500 °C for 24 h,and then aged at 1 000 °C for 24 h.The results show that the heat-treated alloy is composed of Nb solid solution（Nbss）,（Nb,X）5Si3 and Cr2Nb phases.With the increase of heat-treatment temperature,previous Nbss dendrites transformed into equiaxed grains,and petal-like Nbss/（Nb,X）5Si3 eutectic colonies gradually changed into small（Nb,X）5Si3 particles distributed in Nbss matrix.A drastic change occurred in the morphology of the Laves phase after homogenizing treatment.Previously coarse Cr2Nb blocks dissolved during homogenizing at temperature above 1 300 °C,and then much finer and crowded Cr2Nb flakes precipitated in the Nbss matrix in cooling.Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti,Hf and Al in Nbss,（Nb,X）5Si3 and Cr2Nb phases reduced.

DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN ULTRA-HIGH TEMPERATURE Nb-Si-Ti-Hf-Cr-Al ALLOY

The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), eutectic colonies of Nba, plus (Nb, Ti)3 Si/(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss+ (Nb, Ti)3 Si/(Nb, Ti)5 Si3 eutectic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.

Friction of Alloys at High Temperature

A brief review is given about the friction and wear properties of high temperature alloys. Above a critical temperature, if the oxide becomes ductile, it will flow over the surface and prevent metal-to-metat contact. In order to study the tribology of oxide lubrication. a series of tests were carried out using Cu(ReO4)2 as a lubricant. The effects of time. Surface finish. substrates. load and temperature were investigated. A mechanism of lubrication is proposed in which the surface slip predominates along with mechanical attachment of oxide to the surface.

Experimental investigation of high temperature thermal contact resistance with interface material

Thermal contact resistance plays a very important role in heat transfer efficiency and thermomechanical coupling response between two materials,and a common method to reduce the thermal contact resistance is to fill a soft interface material between these two materials.A testing system of high temperature thermal contact resistance based on INSTRON 8874 is established in the present paper,which can achieve 600 C at the interface.Based on this system,the thermal contact resistance between superalloy GH600 material and three-dimensional braid C/C composite material is experimentally investigated,under different interface pressures,interface roughnesses and temperatures,respectively.At the same time,the mechanism of reducing the thermal contact resistance with carbon fiber sheet as interface material is experimentally investigated.Results show that the present testing system is feasible in the experimental research of high temperature thermal contact resistance.

Phase Transformation and Thermal Stability of Aged Ti-Ni-Hf High Temperature Shape Memory Alloys

The use of Ni-rich TiNiHf alloys as high temperature shape memory alloys （SMAs） through aging has been presented. For Ni-rich Ti80-xNixHf20 alloys, their phase transformation temperatures are averagely increased more than 100 K by aging at 823 K for 2 h. Especially for the alloys with Ni-content of 50.4 at. pct and 50.6 at. pct, their martensitic transformation start temperatures （Ms） are more than 473 K after aging. TEM observation confirmed that some fine particles precipitate from the matrix during aging. The aged Ni-rich TiNiHf SMAs show the better thermal stability of phase transformation temperatures than the solutiontreated TiNiHf alloys. The fine particles precipitated during aging should be responsible for the increase of phase transformation temperatures and its high stability.

Effect of yttrium on microstructure and mold filling capacity of a near-α high temperature titanium alloy

The addition of rare earth yttrium(Y) can improve the performances of high temperature titanium alloys,such as the tensile ductility,thermal stability and creep property,etc.However,few studies on the effect of Y on the castability of titanium alloys have been carried out,which is significant to fabrication of thin-walled complex titanium castings by investment casting.In this study,the microstructure and mold filling capacity of a Ti-1100 alloy with different Y additions(0,0.1wt.%,0.3wt.%,0.5wt.% and 1.0wt.%) were investigated systematically through investment casting experiments,and the casting experiments were carried out in a centrifugal titanium casting machine.The microstructures of the alloy were observed via the optical microscopy,scanning electron microscopy and transmission electron microscopy.The mold filling capacity was tested by using of a grid pattern and was evaluated by the number of segments completely filled by the cast alloy.The results indicate that the grain size is decreased and the mold filling capacity is improved significantly with increasing the addition of Y from 0 to 1.0wt.%.The average primary grain size of Ti-1100 alloy is reduced from 250 μm to 50 μm and the mold filling capacity is increased from 61.5% to 100%.Considering the potential harmful effect on tensile properties of titanium alloys due to high concentrations of Y,it is suggested that Y addition should be about 0.3wt.%.

Martensite Aging Effect and Thermal Cyclic Characteristics in Ti-Pd and Ti-Pd-Ni High Temperature Shape Memory Alloys

The effect of thermal cycling and aging in martensitic state in Ti-Pd-Ni alloys were investigated by DSC and TEM observations. It is shown that the thermal cycling causes the decreases in M, and Af temperatures in Ti50Pd50-xNix (x=10, 20, 30) alloys, but no obvious thermal cycling effect was observed in Ti50Pd50Pd40Ni10 alloys and the aging effect shows a curious feature, i.e., the Af temperature does not saturate even after relatively long time aging, which is considered to be due to the occurrence of recovery recrystallization during aging.

Thermodynamic Calculation of A_(r3) Temperature inLow Alloy Steels

On the basis of superelement model, Cahn's transformation kinetics theory and Scheil's additivity rule, actual γ/α transformation start temperature, A.3 in Fe-Σ Xi-C (Xi=Mn, Si, Ni, Mo etc.)multi-component low alloy Steels during continuous cooling process was calculated. Influences of chemical composition, hot deformation of γ and cooling rate on Ar3 temperature were analyzed. Calculated Ar3 temperatures are in reasonable agreement with measured ones.

Evaluation of Oxidation of Ti-Al and Ti-Al-Cr Coatings Arc-ion Plated on Ti-60 High-temperature Titanium Alloy

High-temperature titanium alloy for aeroengine compressor applications suffers from high-temperature oxidation and environmental corrosion, which prohibits long-term service of this kind alloy at temperatures above 600℃. In an attempt to tackle this problem, Ti-48Al （at. pct） and Ti-48Al-12Cr （at. pct） protective coatings were plated on the substrate of alloy Ti-60 by arc ion plating （ALP） method. Isothermal and cyclic oxidation tests were performed in static air at elevated temperatures. Phase composition, morphology of the coatings and distribution of elements were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray analysis （EDX）. The results showed that the Ti-48Al coating exhibited good isothermal oxidation resistance during exposure at 800℃, but poorer resistance against oxidation at 900℃. By contrast Ti-48Al-12Cr coating demonstrated excellent isothermal oxidation resistance at both temperatures. Cyclic oxidation tests performed at 800℃ indicated that resistance and no spallation of coatings was observed. But both coatings demonstrated good cyclic oxidation at 900℃ only Ti-48Al-12Cr coating demonstrated excellent cyclic oxidation resistance.

Effect of Training on Two-way Shape Memory Effect and Its Stability in a Ti-Ni-Hf High Temperature Shape Memory Alloy

The Effect of the thermal cycling training under constant strain on the two-way shape memory effect (TWSME) in a Ti36l\li49Hf15 high temperature shape memory alloy (SMA) has been investigated by bending tests. The results indicated that the training procedure is beneficial to get the better TWSME. The two-way shape memory strain increases with increasing the training strain. And it decreases with increasing the training temperature. The TWSME obtained in the present alloy shows poorer stability compared with that obtained in the TiNi alloys.

DEFORMATION BEHAVIOR OF FeAl INTERMETALLIC ALLOY AT ELEVATED TEMPERATURES

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Factors Affecting Transformation Temperatures in Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The effects of prestrain and annealing temperature on phase transformation temperatures in Fel4Mn5Si8Cr4Ni shape memory alloy have been studied. The results showed that when the annealing temperature was 673 K, both the At and the Ms temperatures increased appreciably as the prestrain increased, the As temperature increased slightly with increasing prestrain; the resistivity difference at 303 K between the heating and cooling curve also increased with increasing prestrain, which agreed with the recovery strain. The shape memory effect in Fe-Mn-Si-Cr-Ni shape memory alloy is caused by the stress-induced γ→ε martensite transformation and its reverse transformation. When the prestrain was 10%, the Ms temperature decreased remarkably as the annealing temperature increased.

Temperature effect on the corrosion and passivation characterization of Ni_(82.3)Cr_7Fe_3Si_(4.5)B_(3.2) alloy in acidic media

The effects of temperature on corrosion and the electrochemical behavior of Ni82.3Cr7Fe3Si4.5B3.2 glassy alloy in HC1,H2SO4,and H3PO4 acids were studied using AC and DC techniques.Impedance data reveal that the susceptibility to localized corrosion increases with increasing temperature.Potentiodynamic polarization curves reveal that the bulk glassy alloy is spontaneously passivated at all the investigated temperature in H2SO4 and H3PO4 solutions.A localized corrosion effect in HCl solution is clearly observed.The apparent activation energies in the regions of Tafel,active,and passive,as well as the enthalpies and entropies of the dissolution process were determined and discussed.The high apparent activation energy（Ea） value for H3PO4 solution in Tafel region is explained by the low aggressivity of PO4^3- ions.

A Calorimetric Study Assisted with First Principle Calculations of Specific Heat for Si-Ge Alloys within a Broad Temperature Range

Calorimetric measurements are performed to determine the specific heat of Si-xat.% Ge(where x = 0, 10, 30,50, 70, 90 and 100) alloys within a broad temperature range from 123 to 823 K. The measured specific heat increases dramatically at low temperatures, and the composition dependence of specific heat is evaluated from the experimental results. Meanwhile, the specific heat at constant volume, the thermal expansion, and the bulk modulus of Si and Ge are investigated by the first principle calculations combined with the quasiharmonic approximation. The negative thermal expansion is observed for both Si and Ge. Furthermore, the isobaric specific heat of Si and Ge is calculated correspondingly from OK to their melting points, which is verified by the measured results and accounts for the temperature dependence in a still boarder range.

Effects of Pouring Temperature on Interfacial Reaction between Ti-47-5Al-2-5V-1Cr Alloy and Mold during Centrifugal Casting

Pouring temperature and time are the most important influencing factors on interfacial reaction during the centrifugal casting. When cast at high temperatures, the crucible becomes brittle and prone to cracking, and shows a low stability. In this paper, we studied the centrifugal casting of Ti-47.5-Al-2.5V-1Cr alloy, and explored the effects of pouring temperature on the interfacial reaction. Castings at 1 600, 1 650, and 1 700 ℃ were obtained by controlling the other parameters constant in the experiments. The microstructure, elemental distribution, thickness of the reaction layer and phase composition of the castings at the interface were studied. The results show that the thickness at the interfacial reaction layer is increased by raising the pouring temperature. The elements in the mold and the matrix were double-diffused and reacted at the interface during the casting process, and formed solid solutions with the precipitation of many new phases such as AlOand TiO. The roughness of interface structure and layer thickness of reaction increase with the rise of temperature, and the interfacial reaction is more intense. There is a minimum layer thickness of the reaction layer that is 80 μm when the temperature is 1 600 ℃.

Temperature Dependent Work-hardening Behaviour in an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The work-hardening behaviour in an Fe-Mn-Si-Cr-Ni alloy has been investigated using tensile test at different temperatures and TEM observation. It was found that besides the intersection of εmartensite, the intersections of ε martensite with stacking fault and the cross-slip of dislocation which is difficult to occur in the alloy with low stacking fault energy are also important factors to the temperature dependent work-hardening behaviour.

Hydrogen Release and Room Temperature Creep for Vanadium Alloys Containing Hydrogen

Vanadium alloy has been taken as one of the candidate structural materials for fusion reactors because of its excellent high-temperature mecha nical performances, high thermal stress factor and low radioactivity. It is a kind of potential materials for hydrogen storage as well. Because operated in an environment conta!ning hydrogen and its isotopes or the neutron irradiation resulting transmutation product of H, the problem that H induced degradation of mechanical properties and hydrogen embrittlement has been being one of the key issues for the application for vanadium alloys.

Microstructure and properties of novel quinary multi-principal element alloys with refractory elements

Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.

Effects of Composition and Thermal Cycle on Transformation Behaviors,Thermal Stability and Mechanical Properties of CuAlAg Alloy

The phase transformation behavior, mechanical properties, and the thermal stability of CuAlAg alloy were studied and minor rare earth (0.1 wt pct La+Ce) was added to improve the mechanical property of the studied alloy. It was found that Ag addition in the CuAl binary alloy can improve the stability of martensitic transformation and high Al content leads to the disappearing of martensitic transformation. The tensile strength and strain of the Cu-10.6AI-5.8Ag (wt pct) alloy were measured to be 383.5 MPa and 0.86%, respectively. With rare earth addition, the tensile strain increased from 0.86% to 1.47%. The CuAlAg alloy did not exhibit martensitic transformation on the second heating process. Its poor thermal stability still needs to be improved.