The Role of Interfacial Precipitates on Creep Behaviour of Power Metallurgy (PM) Ti-48Al-2Cr-2Nb+1W Alloy

2010 ◽  
Vol 654-656 ◽  
pp. 496-499 ◽  
Author(s):  
Dong Yi Seo ◽  
Scott Bulmer ◽  
H. Saari ◽  
Han Liang Zhu ◽  
Peter Au
Keyword(s):  
Creep Resistance ◽  
Creep Behaviour ◽  
Hot Isostatic Pressing ◽  
Lamellar Microstructure ◽  
Primary Creep ◽  
Nominal Composition ◽  
Controlled Cooling ◽  
Creep Tests ◽  
Fully Lamellar

Pre-alloyed powders with a nominal composition of Ti-48Al-2Cr-2Nb+1W were consolidated by hot isostatic pressing (HIP). After the HIP process, a step cooled heat treatment (SCHT) with a carefully controlled cooling rate was applied to homogenize the HIP’ed microstructure and produce a fully lamellar microstructure. Following the SCHT, various isothermal aging at 950 °C and step aging processes form interfacial precipitates at the lamellar interfaces. The morphology, size, and distribution of the precipitates are dependent on the aging condition. Creep tests were carried out in air at 760 °C and 276 MPa to investigate the effect of interfacial precipitates. Primary creep resistance and creep life of the 8 and 144 hr aged conditions are improved substantially compared to the unaged condition due to the existence of the interfacial precipitates. However, the step aging process improves the creep resistance only slightly, probably because of the size and distribution differences of the interfacial precipitates compared to the 144 hr aged condition. Microstructure control is important since it has a substantial influence on creep behavior, especially primary creep resistance.

Effect of Equal-Channel Angular Pressing (ECAP) on Creep in Aluminium Alloys

2007 ◽  
Vol 539-543 ◽  
pp. 2904-2909 ◽  
Author(s):  
Vàclav Sklenička ◽  
Jiří Dvořák ◽  
Marie Kvapilová ◽  
Milan Svoboda ◽  
Petr Král ◽  
...  
Keyword(s):  
Equal Channel Angular Pressing ◽  
Aluminium Alloys ◽  
Creep Resistance ◽  
Room Temperature ◽  
Creep Behaviour ◽  
Coarse Grained ◽  
Pure Aluminium ◽  
Creep Tests ◽  
Angular Pressing ◽  
Compression Creep

This paper examines the effect of equal-channel angular pressing (ECAP) on creep behaviour of pure aluminium, binary Al-0.2wt.%Sc alloy and ternary Al-3wt.%Mg-0.2wt.%Sc alloy. The ECAP was conducted at room temperature with a die that had a 90° angle between the channels and 8 repetitive ECAP passes followed route BC. Constant stress compression creep tests were performed at 473 K and stresses ranging between 16 to 80 MPa on ECAP materials and, for comparison purposes, on the initial coarse-grained materials. The results showed that the creep resistance of the ECAP processed Al-Sc and Al-Mg-Sc alloys was markedly deteriorated with respect to unpressed coarse-grained materials.

scholarly journals Creep Behaviour and Microstructural Characterization of VAT 36 and VAT 32 Superalloys

Metals ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 877 ◽  
Author(s):  
Vagner Gobbi ◽  
Silvio Gobbi ◽  
Danieli Reis ◽  
Jorge Ferreira ◽  
José Araújo ◽  
...  
Keyword(s):  
Failure Analysis ◽  
Creep Resistance ◽  
High Performance ◽  
Creep Behaviour ◽  
Ductile Failure ◽  
Microstructural Characterization ◽  
Dislocation Slip ◽  
Creep Tests ◽  
Experimental Conditions ◽  
Wide Range

Superalloys are used primarily for the aerospace, automotive, and petrochemical industries. These applications require materials with high creep resistance. In this work, evaluation of creep resistance and microstructural characterization were carried out at two new nickel intermediate content alloys for application in aerospace industry and in high performance valves for automotive applications (alloys VAT 32 and VAT 36). The alloys are based on a high nickel chromium austenitic matrix with dispersion of intermetallic L12 and phases containing different (Nb,Ti)C carbides. Creep tests were performed at constant load, in the temperature range of 675–750 °C and stress range of 500–600 MPa. Microstructural characterization and failure analysis of fractured surfaces of crept samples were carried out with optical and scanning electron microscopy with EDS. Phases were identified by Rietveld refinement. The results showed that the superalloy VAT 32 has higher creep resistance than the VAT 36. The superior creep resistance of the alloy VAT 32 is related to its higher fraction of carbides (Nb,Ti)C and intermetallic L12 provided by the amount of carbon, titanium, and niobium in its chemical composition and subsequent heat treatment. During creep deformation these precipitates produce anchoring effect of grain boundaries, hindering relative slide between grains and therefore inhibiting crack formation. These volume defects act also as obstacles to dislocation slip and climb, decreasing the creep rate. Failure analysis of surface fractures of crept samples showed intergranular failure mechanism at crack origin for both alloys VAT 36 and VAT 32. Intergranular fracture involves nucleation, growth, and subsequent binding of voids. The final fractured portion showed transgranular ductile failure, with dimples of different shapes, generated by the formation and coalescence of microcavities with dissimilar shape and sizes. The occurrence of a given creep mechanism depends on the test conditions. At creep tests of VAT 32 and VAT 36, for lower stresses and higher temperature, possible dislocation climb over carbides and precipitates would prevail. For higher stresses and intermediate temperatures shear mechanisms involving stacking faults presumably occur over a wide range of experimental conditions.

Creep Behavior and Microstructural Stability of Lamellar γ-T1AI (Cr, Mo, Si, B) with Extremely Fine Lamellar Spacing

2000 ◽  
Vol 646 ◽  
Author(s):  
Wolfram Schillinger ◽  
Dezhi Zhang ◽  
Gerhard Dehm ◽  
Arno Bartels ◽  
Helmut Clemens
Keyword(s):  
Creep Rate ◽  
Creep Behavior ◽  
Lamellar Microstructure ◽  
Lamellar Spacing ◽  
Primary Creep ◽  
Vacuum Arc ◽  
Internal Stresses ◽  
Microstructural Stability ◽  
Thermodynamical Equilibrium ◽  
Creep Tests

ABSTRACTγ-T1AI (Cr, Mo, Si, B) specimens with two different fine lamellar microstructures were produced by vacuum arc melting followed by a two-stage heat treatment. The average lamellar spacing was determined to be 200 nm and 25–50 nm, respectively. Creep tests at 700°C showed a very strong primary creep for both samples. After annealing for 24 hours at 1000 °C the primary creep for both materials is significantly decreased. The steady-state creep for the specimens with the wider lamellar spacing appears to be similar to the creep behavior prior to annealing while the creep rate of the material with the previously smaller lamellar spacing is significantly higher. Optical microscopy and TEM-studies show that the microstructure of the specimens with the wider lamellar specing is nearly unchanged, whereas the previously finer material was completely recrystallized to a globular microstructure with a low creep resistance. The dissolution of the fine lamellar microstructure was also observed during creep tests at 800 °C as manifested in an acceleration of the creep rate. It is concluded that extremely fine lamellar microstructures come along with a very high dislocation density and internal stresses which causes the observed high primary creep. The microstructure has a composition far away from the thermodynamical equilibrium which leads to a dissolution of the structure even at relatively low temperatures close to the intended operating temperature of γ-T1AI structural parts. As a consequence this limits the benefit of fine lamellar microstructures on the creep behavior.

The Effects of Heat Treatments and Tungsten Additions on Microstructures and Tensile Properties of Powder Metallurgy Ti-48Al-2Nb-2Cr

2010 ◽  
Vol 638-642 ◽  
pp. 1406-1411 ◽  
Author(s):  
Dong Yi Seo ◽  
S. Bulmer ◽  
H. Saari ◽  
Peter Au
Keyword(s):  
Tensile Properties ◽  
Hot Isostatic Pressing ◽  
Solution Treatment ◽  
Aging Treatment ◽  
Beta Phase ◽  
Mixed Mode Fracture ◽  
Controlled Cooling ◽  
Alloy Strength ◽  
Fully Lamellar ◽  
And Tungsten

The microstructures and tensile properties of a fully lamellar Ti-48Al-2Cr-2Nb, and two tungsten-modified versions, Ti-48Al-2Cr-2Nb-0.5W and Ti-48Al-2Cr-2Nb-1.0W (atomic percent) are investigated. Gas atomized powders are consolidated by hot isostatic pressing followed by solution treatment and aging. The microstructures are characterized by optical, scanning electron, and transmission electron microscopy and mechanical properties are characterized by room temperature tensile testing. The solution heat treatment, combined with controlled cooling, generates relatively fine, fully lamellar grains. Tungsten reduces the propensity for martensitic gamma formation during cooling, and slows down lamellar coarsening as well as the formation of equiaxed gamma phase during aging. The aging treatment stabilizes the microstructure and, in the tungsten-modified alloys, causes beta phase precipitation at lamellar interfaces and grain boundaries. Both aging and tungsten additions increase the alloy strength and reduce ductility. The fracture morphologies of the alloys are similar and exhibit mixed-mode fracture consisting of inter- and intra-granular cracking, as well as inter-lamellar cracking.

The Role of Dislocation-solute Interactions on the Creep Behaviour of Binary Mg-RE Alloys

2020 ◽  
Author(s):  
Jing Li ◽  
Jialin Wu ◽  
Li Jin ◽  
Mert Celikin ◽  
Fenghua Wang ◽  
...  
Keyword(s):  
Solid Solubility ◽  
Creep Resistance ◽  
Creep Behaviour ◽  
Dislocation Motion ◽  
Dynamic Precipitation ◽  
Stem Analysis ◽  
Solute Interactions ◽  
Main Factor ◽  
Dislocation Patterns

Abstract The effect of dislocation-RE atoms interactions on the creep behaviour has been studied via creep testing and HAADF-STEM analysis of two extruded alloys; Mg-0.5Ce and Mg-2Gd (wt.%). Almost no Ce atoms are detected in the Mg matrix due to the low solid solubility in as-extruded condition. However, Gd solute segregations are observed along dislocations and hexagonal dislocation patterns. Such segregations can not only pin the dislocation motion and enhance the creep strengthening via dislocation patterns, but also lead to dynamic precipitation. Thus, combing with the stress exponent values, the transition of creep mechanism between Mg-0.5Ce alloys and Mg-2Gd alloys has been found and dislocation-Gd atoms interactions are determined to be the main factor for superior creep resistance of Mg-2Gd alloys.

Microstructures and Creep Properties of Powder Metallurgy Ti-48Al-2Cr-2Nb + W

2007 ◽  
Author(s):  
H. Saari ◽  
S. Bulmer ◽  
D. Y. Seo ◽  
P. Au
Keyword(s):  
Heat Treatment ◽  
Powder Metallurgy ◽  
Lamellar Microstructure ◽  
Controlled Cooling ◽  
Tial Alloys ◽  
Creep Properties ◽  
Rupture Ductility ◽  
Heat Treated ◽  
Fully Lamellar ◽  
Lamellar Interfaces

The microstructures and creep properties at 760 °C and 276 MPa of three powder metallurgy TiAl alloys (Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2Nb+0.5W, and Ti-48Al-2Cr-2Nb+1W (atomic percent)) are presented. The results indicate that the addition of W to the base composition, the use of a solution heat treatment combined with controlled cooling (to generate a fully lamellar microstructure), and the use of an aging heat treatment (to generate precipitate particles at the lamellar interfaces) improve creep properties dramatically. The solution heat treated and aged Ti-48Al-2Cr-2Nb+1W alloy has a time to 0.5% strain of 8.3 hours, a time to 1% strain of 46.4 hours, and a creep life of 412 hours with a rupture ductility of 16.9%.

Creep rates of spray ice

1990 ◽  
Vol 27 (2) ◽  
pp. 185-194 ◽  
Author(s):  
D. Shields ◽  
L. Domaschuk ◽  
E. Funegard
Keyword(s):  
Power Law ◽  
Constant Pressure ◽  
Creep Behaviour ◽  
Primary Creep ◽  
Control Device ◽  
Petroleum Exploration ◽  
Secondary Creep ◽  
Creep Tests ◽  
Pressure Tests ◽  
Polycrystalline Ice

Mars Island, a man-made spray ice island, was constructed in January and February 1986, and was used as a drill platform for petroleum exploration in the Alaskan Beaufort Sea. A series of pressuremeter creep tests was run in the spray ice of Mars Island in March 1986. Individual constant-pressure tests lasted up to 5 days.It is possible to compare the creep behaviour of the spray ice as interpreted from the pressuremeter tests with the creep behaviour interpreted from the island settlement records. These comparisons are made for both primary and secondary creep on the basis of conventional power law theory. The following points are of particular interest: (1) The primary creep data can be characterized using a simple power law. The exponent of time for spray ice is similar to that for solid polycrystalline ice. The exponent of stress is different for the two kinds of ice. (2) Pressuremeter tests gave secondary creep information that correlates well with the steady-rate settlement of the island. (3) Research into the possible range of primary creep parameters for spray ice is required, given that primary creep accounted for a large portion of the settlement of Mars Island. In particular, the effect of ice density on creep rates mast be resolved. (4) The pressuremeter is potentially an excellent design control device during the manufacture of future spray ice islands. The results of constant-pressure tests of 1–2 days duration could be used to check the design assumptions pertaining to the expected consolidation of the ice mass with time. Key words: spray ice, creep, artificial islands, pressuremeter, settlement.

Role of Equiaxed Primary Alpha (α׳) and Mechanical Properties of Ti-6Al-4V Alloy

2012 ◽  
Vol 510-511 ◽  
pp. 420-428
Author(s):  
A. Ahmad ◽  
A. Ali ◽  
G.H. Awan ◽  
K.M. Ghauri ◽  
R. Aslam
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Volume Fraction ◽  
Lamellar Microstructure ◽  
Phase Region ◽  
Bimodal Microstructure ◽  
Two Phase ◽  
Primary Alpha ◽  
Fully Lamellar

The paper presents the role of equiaxed α׳ in the bimodal microstructure to attain an optimal combination of ductility and strength. The study revealed that the production of bimodal microstructure and volume fraction of equiaxed α׳ were reliant on the forging temperature and subsequent heat treatment. The Ti-6Al-4V alloy was forged in the two phase region and different heat treatment cycles were employed to get the desired bimodal microstructure and thus the combination of strength and ductility. The mechanical properties of fully lamellar microstructure were compared with bimodal microstructure containing equiaxed α׳. The experimental results showed that the amount of equiaxed α׳ in the bimodal microstructure was critical for achieving a well-balanced profile of mechanical properties.

The Microstructure, Creep, and Hardness Properties of Powder Metallurgy Beta Gamma TiAl-4Nb-3Mn Alloy

2012 ◽  
Vol 706-709 ◽  
pp. 1100-1105
Author(s):  
Trevor Sawatzky ◽  
Dong Yi Seo ◽  
H. Saari ◽  
D. Laurin ◽  
Young Won Kim
Keyword(s):  
Heat Treatment ◽  
Titanium Aluminide ◽  
Hot Isostatic Pressing ◽  
Lamellar Microstructure ◽  
Beta Phase ◽  
Heat Treatment Condition ◽  
Tensile Creep ◽  
Nominal Composition ◽  
Gamma Titanium Aluminide ◽  
Heat Treated

Pre-alloyed beta gamma titanium aluminide powder with a nominal composition of TiAl-4Nb-3Mn is consolidated by hot isostatic pressing. After consolidation, a step cooled heat treatmentis performed to homogenize the material and produce a fully lamellar microstructure. Various agingheat treatments are then performed with the goal of forming interfacial beta phase precipitates alonglamellar interfaces. The step cooled heat treatment produces a relatively fine microstructure with alamellar spacing of 0.04 μm and an average lamellar colony size of 60 μm. The aging heat treatmentsgenerate beta phase precipitates along lamellar colony boundaries but not along lamellar interfaces,and result in lamellar degredation and grain growth. Constant load tensile creep and room temperaturehardness tests are performed on step cooled heat treated and step cooled heat treated and aged specimens.Creep resistance, generally, improves with aging time, even with no interfacial precipitation,and the lamellar degredation that occurs with aging. The microstructures of the as-tested specimensare characterized and related to the creep properties. The hardness values are also compared as afunction of selected heat treatment condition and microstructural features.

Effect of Element Re on Creep Behaviour of a Single Crystal Nickel-Based Superalloy

2011 ◽  
Vol 194-196 ◽  
pp. 1265-1269
Author(s):  
Hui Liu ◽  
Ming Gang Wang ◽  
Su Gui Tian
Keyword(s):  
Single Crystal ◽  
Creep Resistance ◽  
Creep Behaviour ◽  
Primary Creep ◽  
Steady State Creep ◽  
Nickel Based Superalloy ◽  
Microstructure Observation ◽  
Lower Strain ◽  
Dislocation Reaction ◽  
The Matrix

By means of measuring creep curves and microstructure observation, the influence of the element Re on creep behaviour of the single crystal superalloy has been investigated. Results shown that the creep resistance of nickel-based superalloy with element Re may be obviously improved and the 2% Re superalloy had lower strain rate and longer creep lifetimes compared with Re-free single crystal alloy. The activation energy and stress exponent of the 2% Re superalloy during steady state creep were measured to be Q =478.6 kJ/mol and n = 5.1 respectively. The deformed features of the 2% Re alloy during primary creep are dislocation slipping in the matrix channels, dislocation networks are formed by dislocation reaction, and creep resistance may be improved when networks on the γ/γ’ phases interfaces. In the tertiary creep stage, deformation mechanism is the <110> super-dislocation shearing into the rafted γ’ phase.

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