“Burst-like” Characteristics of the δ/α′ Phase Transformation in Pu-Ga Alloys

2003 ◽  
Vol 802 ◽  
Author(s):  
Kerri J.M. Blobaum ◽  
Christopher R. Krenn ◽  
Jeffery J. Haslam ◽  
Mark A. Wall ◽  
Adam J. Schwartz
Keyword(s):  
Phase Transformation ◽  
Room Temperature ◽  
Large Temperature ◽  
Temperature Hysteresis ◽  
Scanning Calorimetry ◽  
Α Phase ◽  
Δ Phase ◽  
Annealing Cycle ◽  
Heat Of Transformation ◽  
Start Temperature

ABSTRACTThe δ to α′ phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc δ-phase transforms martensitically to the brittle monoclinic α′-phase at approximately −120°C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of α′ to δ involves a large temperature hysteresis and begins just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we are examining the δ/α' transformations in a Pu-0.6 wt% Ga alloy using differential scanning calorimetry (DSC) and resistometry. Both techniques indicate that the martensite start temperature is −120°C and the austenite start temperature is 35°C. The heat of transformation is approximately 3 kJ/mole. During the α ′ → δ reversion, “spikes” and “steps” are observed in DSC and resistometry scans, respectively. These spikes and steps are periodic, and their periodicity with respect to temperature does not vary with heating rate. With an appropriate annealing cycle, including a “rest” at room temperature, these spikes and steps can be reproduced through many thermal cycles of a single sample.

Restitution conditions of the parent cubic texture from the inherited hexagonal texture in the β–α phase transformation

1999 ◽  
Vol 32 (1) ◽  
pp. 21-26 ◽  
Author(s):  
Michel Humbert ◽  
Nathalie Gey
Keyword(s):  
Phase Transformation ◽  
Distribution Function ◽  
Correlation Function ◽  
Room Temperature ◽  
Hexagonal Phase ◽  
Parent Phase ◽  
Orientation Distribution ◽  
Variant Selection ◽  
Qualitative Information ◽  
Α Phase

Different factors calculated from theCcoefficients of the inherited orientation distribution function (ODF) allow us to check the importance of the variant selection. When no variant selection or a slight variant selection occurs by the cubic-to-hexagonal phase transformation, it is possible to calculate the ODF of the cubic parent phase present at high temperature from the ODF of the inherited hexagonal phase at room temperature. When a stronger variant selection occurs, qualitative information about the parent ODF can be obtained by using a specific correlation function, which we have named R(g).

Using a R-Phase SMA Methodology to Design an Energy Harvesting Unit With Tight Temperature Hysteresis

2013 ◽  
Author(s):  
Konstantin Lygin ◽  
Alexander Czechowicz ◽  
Horst Meier
Keyword(s):  
Phase Transformation ◽  
Energy Harvesting ◽  
Shape Memory ◽  
Thermal Energy ◽  
Smart Materials ◽  
Large Temperature ◽  
Temperature Sensitive ◽  
Temperature Hysteresis ◽  
Energy Potential ◽  
Sma Actuators

Shape memory alloys (SMA) are smart materials with the ability to remember a previous imprinted shape after plastic deformation. The shape memory actuator effect is induced by thermal energy. Its bases on crystalline transformation of NiTi-alloys from the austenitic B2 to the martensitic B19′ phase. Usually, SMA actuators show a large temperature hysteresis with high strokes and loads. A lot of technical applications are realizable with SMA-actuators, for example in automotive sector. Under special conditions an additional phase, the R-phase occurs in SMA. This transformation from the austenitic B2 phase to the R-phase is characterized by a very small temperature hysteresis. Compared to conventional transformation characteristic of B2 ⇒ B19′, the R-phase transformation has small stroke and load. Because of this the R-Phase transformation qualifies for temperature sensitive and autarkical applications in air-conditioning or heating systems. Today’s problem for engineers designing R-phase actuators is that a lot of rules and parameters need to be considered. This paper presents a methodology, which can be used as a guideline to support engineers designing R-phase actuators. The whole process from defining requirements to prototyping will be discussed for the development of an energy harvesting actuator, which uses a low thermal energy potential. This paper contribution is to fill this gap in product development methodologies for shape memory actuators.

Effect of Aging Treatment on Phase Transformation of a Pseudoelastic NiTi Alloy

2014 ◽  
Vol 936 ◽  
pp. 1216-1223 ◽  
Author(s):  
Palloma Viera Muterlle ◽  
Eduard Benavides Villamarin ◽  
Paulo Herrera ◽  
Edson Paulo da Silva
Keyword(s):  
Phase Transformation ◽  
Shape Memory ◽  
Room Temperature ◽  
Niti Alloy ◽  
Aging Treatment ◽  
Single Step ◽  
Treatment Temperature ◽  
Scanning Calorimetry ◽  
Transformation Temperatures ◽  
X Ray

The pseudoelasticity (PE) and shape memory effect (SME) are the two main behaviors presented by the shape memory alloys (SMA's) and are associated respectively to mechanical and thermally induced martensitic transformations. The aim of this work is to investigate the effects of heat treatment temperature on the microstructure properties and phase transformation temperatures of a NiTi alloy with 57 w.t. % Ni. The X-ray diffraction (XRD) was carried out to obtain the phases present. The phase transformation temperatures were measured by differential scanning calorimetry (DSC). The alloy chemical composition and hardness were obtained by X-ray fluorescence (XRF), hardness (HRC) and microhardness (HV) tests, respectively. The analysis were performed in the state as received and after aging treatment at different temperatures between 350 °C and 600°C. The samples as received contained a fully austenitic microstructure at room temperature and the DSC analysis showed the presence of a phase transformation in multiple steps (B19'-R-B2). After aging at 350 °C the R phase was observed at room temperature with the austenite. With the aging treatment at 600 °C the R phase was solubilized and the alloy phase transformation occurred in a single step, ie, direct transformation from austenite to martensite and vice versa. The transformation temperatures Af, As, Ms and Mf changed with the aging treatment temperatures.

scholarly journals Phase Transitions and Molecular Reorientations in Bilayered n-Heptadecylammonium Chloride

1998 ◽  
Vol 51 (3) ◽  
pp. 557 ◽  
Author(s):  
E. C. Reynhardt
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Relaxation Rates ◽  
Scanning Calorimetry ◽  
Spin Lattice ◽  
Heating And Cooling ◽  
Α Phase ◽  
Monium Chloride

An investigation of phase transitions and molecular motions in polycrystalline n-heptadecylam- monium chloride (C17H35NH3Cl), employing differential scanning calorimetry, x-ray powder diffraction and nuclear magnetic resonance techniques, is reported. This compound can occur in two virgin polymorphs at room temperature, one interdigitated and one noninterdigitated. The temperature at which crystallisation occurs determines the polymorph that forms. If these polymorphs are heated transitions to noninterdigitated γ, β and α phases occur. Cooling to room temperature shows the same phase transitions, but the virgin phase is not formed. Instead, a noninterdigitated .epsi; phase is formed. Defect motions of chain-ends play a significant role in the spin-lattice relaxation rates in all the phases. In the α phase a degree of chain melting is present. The molecular dynamics of chains differs during heating and cooling cycles in the .epsi; phase. If the temperature is kept constant during a heating cycle in this phase, fourfold motions of chains are frozen over a period of several hours, but the sample remains noninterdigitated. It seems that the interdigitation process is hampered by the population of defect orientations of chain-ends. The methyl group executes classical threefold reorientations and the NH3 group jumps in an asymmetric threefold potential well.

scholarly journals Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

2020 ◽  
Vol 10 (21) ◽  
pp. 7628
Author(s):  
Sarah C. Hernandez ◽  
Franz J. Freibert
Keyword(s):  
First Principles ◽  
Room Temperature ◽  
Experimental Studies ◽  
Tetragonal Distortion ◽  
Ab Initio Study ◽  
Α Phase ◽  
Fcc Lattice ◽  
Δ Phase ◽  
A Site ◽  
Electronic Behavior

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.

scholarly journals Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

2005 ◽  
Vol 98 (2) ◽  
pp. 024303 ◽  
Author(s):  
M. Gich ◽  
E. A. Shafranovsky ◽  
A. Roig ◽  
A. Ślawska-Waniewska ◽  
K. Racka ◽  
...  
Keyword(s):  
Phase Transformation ◽  
Room Temperature ◽  
Temperature Stabilization ◽  
Σ Phase ◽  
Aerosol Nanoparticles ◽  
Α Phase

Effect of Al on Microstructures and Properties of Ti-45Al-8.5Nb-0.2B-0.2W Alloy

2013 ◽  
Vol 747-748 ◽  
pp. 44-49 ◽  
Author(s):  
Xiang Jun Xu ◽  
Jun Pin Lin ◽  
Dong Dong Han
Keyword(s):  
Phase Transformation ◽  
Room Temperature ◽  
Optimal Combination ◽  
Solidification Microstructure ◽  
Processing Route ◽  
Phase Transformation Temperature ◽  
Compressive Properties ◽  
Al Content ◽  
Α Phase ◽  
The Stability

In the present paper, the necessity of strictly controlling Al content is demonstrated for Ti-45Al-8.5Nb-0.2B-0.2W alloy by analyzing solidification microstructure, compressive properties at room temperature and α phase transformation point of alloys containing 43-47at.%Al. The results show that the alloy has an optimal combination of mechanical properties and microstructure. Increasing Al content leads to increase in lamellar colony size and decrease in B2 phase for the solidification microstructure. Yield strength decreases linearly with Al content, but total strain increases linearly with Al content. The phase transformation temperature of β + αα rises more than 20 °C when Al increases to 46at. %, and drops about 20 °C when Al decreases to 44at %. Therefore, Al deviation should be as small as possible in producing technical alloy to ensure the stability in terms of properties, microstructure and processing route.

scholarly journals Nanoindentation and Microstructure in the Shear Band in a Near Beta Titanium Alloy Ti-5Al-5Mo-5V-1Cr-1Fe

Materials ◽  
2019 ◽  
Vol 12 (24) ◽  
pp. 4065
Author(s):  
Bingfeng Wang ◽  
Xu Ding ◽  
Ying Mao ◽  
Lanyi Liu ◽  
Xiaoyong Zhang
Keyword(s):  
Electron Microscopy ◽  
Mechanical Properties ◽  
Titanium Alloy ◽  
Phase Transformation ◽  
Shear Band ◽  
Cryogenic Temperature ◽  
Room Temperature ◽  
Beta Titanium Alloy ◽  
Α Phase ◽  
Beta Titanium

Shear localization is the main deformation mode for the near beta titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe loaded at high strain rates at either room temperature or cryogenic temperature. Nanoindentation, transmission electron microscopy, and high-resolution electron microscopy technique are applied to character the microstructure features and mechanical properties in the shear band of near beta titanium alloy. A white and straight band is observed in the shear region. Both microhardness and nanoindentaion hardness in the shear region are inferior to those in matrix. The different microstructure in the edge and the center in the shear band contribute to different mechanical properties. The plasticity of the entire shear band is almost homogenous when specimens are deformed at the cryogenic temperature. Rotational dynamic recrystallization is responsible for the formation of the ultrafine grains in the shear band. The edge of the shear band is composed of elongated grains, while there are ultrafine equiaxed grains in the center of the shear band. Deformation temperature has significant influence on the process of the grain refinement and the phase transformation in the shear band (SB). The grain sizes of the shear band in the specimen deformed at room temperature are larger than those in the specimens deformed at cryogenic temperature. The shear band consists of α phase grains in the specimen deformed at room temperature, and the shear band consists of α phase and lath-like α′ phase grains in the specimen deformed at cryogenic temperature. Finally, the mechanisms for phase transformation in the shear band are illustrated.

Effect of Thermal Aging on the Transformation Temperatures and Specific Heat Characteristics of 9Cr-1Mo Ferritic Steel

2008 ◽  
Vol 279 ◽  
pp. 85-90 ◽  
Author(s):  
B. Jeya Ganesh ◽  
S. Raju ◽  
E. Mohandas ◽  
M. Vijayalakshmi
Keyword(s):  
Heat Capacity ◽  
Phase Transformation ◽  
Specific Heat ◽  
Room Temperature ◽  
Martensitic Steel ◽  
Thermal Ageing ◽  
Phase Transformation Temperature ◽  
Scanning Calorimetry ◽  
Temperature Range ◽  
Martensite Microstructure

The effect of thermal ageing on the heat capacity and transformation behaviour of behaviour of 9Cr-1Mo-0.1C (wt.%) ferritic / martensitic steel has been studied using differential scanning calorimetry (DSC) in the temperature range 473 to 1273 K. It is found that a-ferrite + carbide ® g-austenite phase transformation temperature is only mildly sensitive to microstructural details; but the enthalpy change associated with this phase transformation and especially, the change in specific heat around the transformation regime are found to be dependent on the starting microstructure generated by thermal ageing treatment. Prolonged ageing for about 500 to 5000 hours in the temperature range 823 to 923 K contributed to a decrease in heat capacity, as compared to the normalised and tempered sample. The martensite microstructure is found to possess the lowest room temperature CP among different microstructures.

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