Boron and Hydrogen in NI3AL: Part II. Mechanical Testing of Bicrystals

1993 ◽  
Vol 319 ◽  
Author(s):  
Patricia E. Johnson ◽  
W. Gourdin ◽  
A. Gonis ◽  
N. Kioussis ◽  
M. Vaudin
Keyword(s):  
Water Vapor ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Tensile Tests ◽  
Stress And Strain ◽  
Boron Segregation ◽  
Environmental Embrittlement ◽  
Boron Doped ◽  
Low Symmetry ◽  
Strength And Ductility

AbstractTo provide a sensitive measurement of the effect of boron segregation on the strength and ductility of Ni3A1 grain boundaries, bicrystal tensile tests were performed on small specimens of boron doped Ni76A124 cut from extremely large-grained boules. Five specimens with the same “random” or low-symmetry grain boundary (disorientations measured by means of backscattered Kikuchi patterns) and two specimens with a second random grain boundary were tested in quenched and slow-cooled conditions. Duplicate tests performed in a low (7 ppm) water-vapor environment showed that the fracture mode and the stress and strain at fracture are altered by environmental embrittlement at individual, partially strengthened grain boundaries.

Local Electronic Structure and Cohesion of Grain Boundaries in Ni3Al

1994 ◽  
Vol 364 ◽  
Author(s):  
D. A. Muller ◽  
S. Subramanian ◽  
S. L. Sass ◽  
J. Silcox ◽  
P. E. Batson
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Cohesive Energy ◽  
Bulk Material ◽  
Large Angle ◽  
Spatially Resolved ◽  
Boron Segregation ◽  
Environmental Embrittlement ◽  
Boron Doped ◽  
Local Electronic Structure

AbstractOne of the fundamental questions concerning Ni3Al is why doping with boron improves the room temperature ductility of the polycrystalline material. Boron is thought to prevent environmental embrittlement and increase the cohesive strength of grain boundaries since it changes the fracture mode from intergranular to transgranular. This change in cohesive energy must be reflected in the bonding changes at the grain boundary which can be probed using spatially resolved electron energy loss spectroscopy (EELS). We have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24 using EELS, EDX and ADF imaging in a UHV STEM. Ni-enrichment is seen in a 0.5–1 nm wide region at large angle grain boundaries, both in the absence and presence of B. EELS shows that B segregation can vary along the interface. The Ni L2, 3 core edge fine structure which is sensitive to the filling of the Ni d-band, shows only the boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. These results demonstrate that boron segregation increases the cohesive energy and hence improves the fracture resistance of the grain boundary, by making the bonding at boundaries similar to that in the bulk. The measured changes in d band filling may also affect the local solubility of hydrogen.

Miniaturized Disk-Bend Testing of Ni3 Al: Effect of Stoichiometry and Boron Content

1990 ◽  
Vol 213 ◽  
Author(s):  
H. Li ◽  
A. J. Ardell
Keyword(s):  
Grain Boundaries ◽  
Boron Content ◽  
Maximum Load ◽  
Tensile Tests ◽  
Uniaxial Tensile ◽  
Al Alloy ◽  
Independent Evidence ◽  
Al Content ◽  
Boron Segregation ◽  
Boron Doped

ABSTRACTThe results of miniaturized disk-bend tests on samples of Ni 3Al of different stoichiometry and boron content are presented. The yield strengths and ductilities of alloys containing 24, 25 or 26 %Al. either boron-free or doped with 0.3 or 0.35 %B, were measured. Specimens 3 mm in diameter and approximately 200 μm thick were tested, some of these having been cut from the grip sections of previously tested tensile bars. The yield strengths were in excellent agreement with the results obtained from the uniaxial tensile tests. The load-displacement curves for the brittle alloys (all but the boron-doped 24 %Al alloy) exhibited a maximum load corresponding to crack initiation. The shapes of the deformed specimens confirmed the assumption that they deform as if they were clamped even though they are not. The fracture surfaces of the brittle alloys are consistent with intergranular failure. Nevertheless, the ductility of the alloys increases with decreasing Al content and decreasing grain size, even for the boron-free alloys which are all brittle. The fracture stress of the boron-doped 26 %Al alloy is about 30% greater than that of the boron-free alloy. It is argued that this is most likely a consequence of the depletion of aluminum at grain boundaries, coupled with boron segregation. Independent evidence suggests that this should increase the cohesive strength of grain boundaries in the boron-doped 26 %Al alloy.

An Explanation for the Environmental Sensitivity of Ni3Al

1998 ◽  
Vol 552 ◽  
Author(s):  
D. B. Lillig ◽  
D. Legzdina ◽  
I. M. Robertson ◽  
H. K. Birnbaum
Keyword(s):  
Mass Spectrometry ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Secondary Ion Mass Spectrometry ◽  
Environmental Sensitivity ◽  
Boron Segregation ◽  
Boron Doped ◽  
Distribution Of Elements ◽  
Free Material ◽  
Secondary Ion

ABSTRACTSecondary Ion Mass Spectrometry has been used to study the distribution of elements in and near grain boundaries in boron-free and boron-doped Ni76Al24 alloys with and without ∼220 wt. ppm of deuterium. In boron-free alloys, sulfur was distributed about the grain boundaries in both deuterium- free and deuterium-charged samples. The distribution of deuterium followed that of sulfur and was segregated to grain boundaries. In the boron-doped material, sulfur was not found at most grain boundaries in the uncharged material, but was in the charged material. No deuterium was found at the grain boundaries in the boron-doped material. It is proposed that in the boron-free material it is the synergistic effect of sulfur and hydrogen that is responsible for the environmental sensitivity of this alloy. In boron-doped material, boron segregation to the grain boundary prevents sulfur, and to some extent hydrogen, segregating to the grain boundary.

scholarly journals Effect of Crystallographic Orientation and Grain Boundaries on Martensitic Transformation and Superelastic Response of Oligocrystalline Fe–Mn–Al–Ni Shape Memory Alloys

2021 ◽  
Author(s):  
A. Bauer ◽  
M. Vollmer ◽  
T. Niendorf
Keyword(s):  
Martensitic Transformation ◽  
Grain Boundary ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Grain Boundaries ◽  
Tensile Tests ◽  
Image Correlation ◽  
Stress Concentrations ◽  
Grain Orientations ◽  
High Degree

AbstractIn situ tensile tests employing digital image correlation were conducted to study the martensitic transformation of oligocrystalline Fe–Mn–Al–Ni shape memory alloys in depth. The influence of different grain orientations, i.e., near-〈001〉 and near-〈101〉, as well as the influence of different grain boundary misorientations are in focus of the present work. The results reveal that the reversibility of the martensite strongly depends on the type of martensitic evolving, i.e., twinned or detwinned. Furthermore, it is shown that grain boundaries lead to stress concentrations and, thus, to formation of unfavored martensite variants. Moreover, some martensite plates seem to penetrate the grain boundaries resulting in a high degree of irreversibility in this area. However, after a stable microstructural configuration is established in direct vicinity of the grain boundary, the transformation begins inside the neighboring grains eventually leading to a sequential transformation of all grains involved.

Effect of Aluminum Concentration And Boron Dopant on Environmental Embriitlement In Feal Aluminides

1990 ◽  
Vol 213 ◽  
Author(s):  
C. T. Liu ◽  
E. P. George
Keyword(s):  
Aluminum Alloys ◽  
Grain Boundaries ◽  
Aluminum Concentration ◽  
Al Alloy ◽  
Test Environment ◽  
Environmental Embrittlement ◽  
Boron Doped ◽  
Weak Boundaries ◽  
Boron Dopant ◽  
Strong Segregation

ABSTRACTThe room-temperature tensile properties of FeAl aluminides were determined as functionsof aluminum concentration (35 to 43 at. % Al), test environment, and surface (oil) coating. The two lower aluminum alloys containing 35 and 36.5% Al are prone to severe environmental embrittlement, while the two higher aluminum alloys with 40 and 43% Al are much less sensitive to change in test environment and surface coating. The reason for the different behavior is that the grain boundaries are intrinsically weak in the higher aluminum alloys, and these weak boundaries dominate the low ductility and brittle fracture behavior of the 40 and 43% Al alloys. When boron is added to the 40% Al alloy as a grain-boundary strengthener, the environmental effect becomes prominent. In this case, the tensile ductility of the boron-doped alloy, just like that of the lower aluminum alloys, can be dramatically improved by control of test environment (e.g. dry oxygen vs air). Strong segregation of boron to the grain boundaries, with a segregation factor of 43, was revealed by Auger analyses.

Fracture Modes in Ll2 Compounds

1988 ◽  
Vol 133 ◽  
Author(s):  
C. L. Briant ◽  
A. I. Taub
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Intermetallic Compounds ◽  
Boundary Segregation ◽  
Grain Boundary Segregation ◽  
Fracture Modes ◽  
Beneficial Effects ◽  
Boron Segregation ◽  
Total Composition ◽  
Carbon Additions

ABSTRACTThis paper reports a study of grain boundary segregation and fracture modes in Ll2 intermetallic compounds. Data obtained on Ni3A1, Ni3Si, Ni3Ga, Ni3Ge, and Pt3Ga will be presented. It will be shown that the amount of boron segregation and its ability to improve cohesion depends on the total composition of the compound. The beneficial effects of boron can be counteracted by the presence of borides on the grain boundaries. Carbon additions also produce some improvement in ductility in Ni3Si.

An atom probe field ion microscope analysis of grain boundary chemistry in boron and carbon doped NiAl

1992 ◽  
Vol 50 (2) ◽  
pp. 1220-1221
Author(s):  
Raman Jayaram ◽  
M.K Miller
Keyword(s):  
Grain Boundary ◽  
Enrichment Factor ◽  
Grain Boundaries ◽  
Atom Probe ◽  
Probe Field ◽  
Carbon Doped ◽  
Boron Segregation ◽  
Microanalytical Technique ◽  
Microalloying Elements ◽  
Boundary Chemistry

The low temperature brittleness of nickel aluminides has been a serious impediment to their technological applications. A commonly employed technique to ductilize these materials involves the addition of suitable microalloying elements and correlating grain boundary chemistry with fracture mode. In the well documented case of Ni3Al, boron segregation to grain boundaries is accompanied by suppression of intergranular fracture and a significant increase in ductility. The high resolution microanalytical technique of atom probe field ion microscopy (APFIM) has been used in this study to analyze grain boundaries in order to characterize similar attempts to ductilize NiAl. APFIM specimens were prepared from tensile specimens of stoichiometric NiAl doped with either 0.04 or 0.12 at. % boron or 0.1 at % carbon, respectively. A field ion image of a grain boundary in a B-doped NiAl specimen is shown in Fig. 1. The brightly-imaging spots decorating the boundary were determined by atom probe analysis to be boron atoms. The boron enrichment factor at the boundary depends on the assumed thickness of the segregation as shown in Fig. 2 with an enrichment factor of ∼850 times for a monolayer coverage (i.e. 0.2 nm).

Experimental Measurement of the Local Electronic Structure of Grain Boundaries in Ni3Al

1993 ◽  
Vol 319 ◽  
Author(s):  
D.A. Muller ◽  
P.E. Batson ◽  
S. Subramanian ◽  
S. L. Sass ◽  
J. Silcox
Keyword(s):  
Electronic Structure ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Bulk Material ◽  
Local Strain ◽  
Dark Field ◽  
Hole Density ◽  
Spatially Resolved ◽  
Boron Segregation ◽  
Annular Dark Field

AbstractWe have examined grain boundaries in both undoped and boron doped Ni0.76Al0.24 using electron energy loss spectroscopy (EELS), x-ray fluorescence (EDX) and annular dark field (ADF) imaging in a UHV STEM. A detailed study of a high angle grain boundary in nickel rich Ni3Al doped with 1000 ppm boron shows nickel enrichment occurring in a 5Å wide region. Boron segregation to the boundary is observed with EELS and is seen to vary along the boundary, coinciding with ADF contrast changes in the surrounding grains that may be due to local strain fields. Spatially resolved EELS of the Ni L2,3 core edge, which is sensitive to changes in the hole density in the nickel d band, shows boron rich regions of the grain boundary to have a bonding similar to that of the bulk material. Boundary regions without boron have an electronic structure similar to that of the undoped grain boundaries where the Fermi level lies deeper in the nickel d band. In addition to studying boron segregation, EELS provides a unique opportunity to examine the changes in bonding that control the local properties of the material.

The Effect of Grain Boundary Chemistry on the Slip Transmission Process Through Grain Boundaries in Ni3Al

1991 ◽  
Vol 238 ◽  
Author(s):  
I. M. Robertson ◽  
T. C. Lee ◽  
Raja Subramanian ◽  
H. K. Birnbaum
Keyword(s):  
Stress Concentration ◽  
Grain Boundary ◽  
Slip System ◽  
Grain Boundaries ◽  
Local Stress ◽  
Crack Advance ◽  
Boron Doped ◽  
Show Evidence ◽  
Fcc Alloy ◽  
Boundary Chemistry

ABSTRACTThe conditions established in disordered FCC systems for predicting the slip system that will be activated by a grain boundary to relieve a local stress concentration have been applied to the ordered FCC alloy Ni3Al. The slip transfer behavior in hypo-stoichiometric Ni3Al with (0.2 at. %B) and without boron was directly observed by performing the deformation experiments in situ in the transmission electron microscope. In the boron-free and boron-doped alloys, lattice dislocations were incorporated in the grain boundary, but did not show evidence of dissociation to grain boundary dislocations or of movement in the grain boundary plane. The stress concentration associated with the dislocation pileup at the grain boundary was relieved by the emission of dislocations from the grain boundary in the boron-doped alloy. The slip system initiated in the adjoining grain obeyed the conditions established for disordered FCC systems. In the boron-free alloy, the primary stress relief mechanism was grain-boundary cracking, although dislocation emission from the grain boundary also occurred and accompanied intergranular crack advance. Because of the importance of the grain boundary chemistry in the models for explaining the boron-induced ductility in hypo-stoichiometric Ni3Al, the chemistry of grain boundaries in well-annealed boron-doped and boron-free alloys was determined by using EDS. No Ni enrichment was found at the grain boundaries examined. These observations are discussed in terms of the different models proposed to explain the ductility improvement in the boron-doped, hypo-stoichiometric alloy.

Effect of grain shape on environmental embrittlement in Ni3Al tested at elevated temperatures

1989 ◽  
Vol 4 (2) ◽  
pp. 294-299 ◽  
Author(s):  
C. T. Liu ◽  
B. F. Oliver
Keyword(s):  
Grain Boundaries ◽  
Normal Stress ◽  
Elevated Temperatures ◽  
Grain Shape ◽  
Environmental Embrittlement ◽  
Boron Doped

This paper describes the effect of grain shape on environmental embrittlement in boron-doped Ni3Al (24 at. % Al). The alloy showed severe embrittlement when tested at 600 and 760 °C in air. The embrittlement can be alleviated by control of grain shape, and the material with a columnar-grained structure produced by directional levitation zone remelting shows good tensile ductilities when tested in oxidizing environments. The columnar-grained structure with vertical grain boundaries minimizes the normal stress and consequently suppresses nucleation and propagation of cracks along the boundaries.

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