Use of multi-step loading small punch test to investigate the ductile-to-brittle transition behaviour of a thermally sprayed CoNiCrAlY coating

2017 ◽  
Vol 680 ◽  
pp. 203-209 ◽  
Author(s):  
H. Chen ◽  
T.H. Hyde
Keyword(s):  
Small Punch Test ◽  
Small Punch ◽  
Brittle Transition ◽  
Punch Test ◽  
Step Loading ◽  
Ductile To Brittle Transition ◽  
Conicraly Coating ◽  
Thermally Sprayed ◽  
Transition Behaviour

The Application of the Small Punch Tensile Test to Evaluate the Ductile to Brittle Transition of a Thermally Sprayed CoNiCrAlY Coating

2017 ◽  
Vol 734 ◽  
pp. 144-155 ◽  
Author(s):  
G.A. Jackson ◽  
Wei Sun ◽  
D. Graham McCartney
Keyword(s):  
Tensile Test ◽  
Room Temperature ◽  
Free Standing ◽  
Small Punch ◽  
Bond Coats ◽  
Brittle Transition ◽  
Barrier Coating ◽  
Ductile To Brittle Transition ◽  
Conicraly Coating ◽  
Thermally Sprayed

Thermally sprayed MCrAlY bond coats are important elements of thermal barrier coating (TBC) systems which are applied to the surface of gas turbine components to protect them in high temperature environments. Knowledge of their mechanical properties is essential in preventing TBC failure which can have catastrophic consequences. However, limited data on modulus, strength and ductility are available for such coatings. In this work, the ductile to brittle transition behaviour of a CoNiCrAlY coating has been investigated via the small punch tensile test (SPTT). Displacement controlled tests were carried out on free standing coatings at room temperature (RT) and between 400-750 °C at a rate of 1 μms-1. At low temperatures there was evidence of elastic-brittle behaviour and at high temperatures there was clear evidence of yielding and plastic deformation. The ductile to brittle transition temperature was found to be between 500-750 °C. The yield stress ranged from 1000-1500 MPa below 600 °C to less than 500 MPa above 650 °C. The elastic modulus was found to be approximately 200-230 GPa at 500 °C and 55 GPa above 700 °C. At room temperature the fracture surface showed flat, smooth features indicating brittle failure whereas at 700 °C there was evidence of ductile tearing.

Evaluation of the ductile-to-brittle transition temperature in a thermally sprayed CoNiCrAlY coating by small punch multi-step loading tests

2016 ◽  
Vol 231 (1-2) ◽  
pp. 6-13
Author(s):  
H Chen ◽  
J Yang ◽  
XL Xiao
Keyword(s):  
Transition Temperature ◽  
Elevated Temperatures ◽  
Free Standing ◽  
Small Punch ◽  
Brittle Transition ◽  
Loading Tests ◽  
Step Loading ◽  
Brittle Transition Temperature ◽  
Ductile To Brittle Transition ◽  
Conicraly Coating

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co–31.7%Ni–20.8%Cr–8.1%Al–0.5%Y, all in wt%) coatings of an approximate thickness of 0.5 mm. Small punch tests under multi-step loading conditions were performed between room temperature and 600 ℃ on these samples to evaluate the ductile-to-brittle transition temperature. The microstructure of the coatings was characterised using a scanning electron microscope with energy-dispersive X-ray analysis. A two-phase structure consisting of fcc γ-Ni and bcc β-NiAl was found to exist. The displacements obtained from small punch multi-step loading tests at each load increment were relatively small and similar at temperatures below 500 ℃ but a significant increase in displacement was noted at 600 ℃. Fractographic investigation showed that the main fracture mode was dominated by extensive γ matrix tearing at elevated temperatures. A distinct stress and strain behaviour was found at 600 ℃, indicating that the ductile-to-brittle transition temperature of this CoNiCrAlY coating occurred between 500 ℃ and 600 ℃.

Evaluation of Weibull Parameters by Different Small Punch Tests Samples Based on Beremin Model

2017 ◽  
Author(s):  
Kaishu Guan ◽  
Linling Guo ◽  
Mingxue Fu
Keyword(s):  
Transition Region ◽  
Test Specimen ◽  
Small Punch Test ◽  
Bulk Materials ◽  
Bulk Specimen ◽  
Weibull Parameters ◽  
Small Punch ◽  
Brittle Transition ◽  
Punch Test ◽  
Ductile To Brittle Transition

Small punch testing technology is used for quantifying the mechanical properties of bulk materials with greatly reduced volume. However, as the volume decrease, the ductile to brittle transition region of the specimen will move to lower temperature. That means a small punch specimen and a bulk specimen may fracture under different regime. For this case, it’s difficult to obtain fracture toughness of bulk materials from the small punch tests. In this paper, a new approach, which is based on Beremin local approach, to evaluate the Weibull parameters of bulk materials in the ductile to brittle transition region by small punch tests has been proposed. This paper focuses on the choice of specimen types of small punch test based on Beremin model. Four different types of specimens were tested at ductile to brittle transition region. i.e. ( a ) two kinds of linear notched small punch test specimen (LNSP), (b) two kinds of center holed small punch test specimen (CHSP). All notches of the specimens were machined by electric discharge method, and both sides of the specimens were grinded and polished. The results showed that 0.5mm CHSP (center holed small punch test specimen) is the best choice for small punch test based on Beremin model according to experimental curves, SEM (scanning electron microscope) analysis, and result of finite element simulation. Furthermore, the cumulative failure probability in ductile-to-brittle transition region by small punch tests was obtained and the method of calculating Weibull parameters (m,σu) is analyzed.

Validation of the Incremental Step Loading Technique Application to Small Punch Tests in Aggressive Environments in X80 Steel

2020 ◽  
Author(s):  
B. Arroyo ◽  
L. Andrea ◽  
P. González ◽  
J. A. Álvarez ◽  
S. Cicero ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Hydrogen Embrittlement ◽  
Threshold Stress ◽  
High Strength ◽  
Constant Load ◽  
Small Punch Test ◽  
Small Punch ◽  
Punch Test ◽  
Step Loading ◽  
Incremental Step

Abstract The Small punch test, which consists on punching a small plane specimen up to failure, is a technique to be taken into account for the estimation of mechanical properties when there is shortage of material. In recent works it has been applied to the estimation of mechanical properties steels in aggressive environments. In aggressive environments, tests under a constant load are usually employed for the threshold stress determination, but this a slow and sometimes inaccurate technique. The standard ASTM F1624 solves these issues; it consists on applying steps of constant loads subsequently increased up to the specimen’s failure. In a previous work, it was indicated how to implement this technique for Small Punch testing of steels in hydrogen embrittlement scenarios, adapting the steps duration. This proposal allows to obtain a threshold load by using at least 3 specimens in a total time of around a week. In the present work, the incremental step loading technique from ASTM F1624 standard is applied to the Small Punch test in order to estimate tensile threshold stress of a X80 high strength steel in hydrogen embrittlement environments by cathodic polarization in an acid electrolyte. Regular standard tests on cylindrical tensile specimens were carried out following the ASTM F1624 standard, in order to validate the methodology proposed.

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