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Atlas of Formability

Inconel 718

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"T July 31, 1992 Final, Apri 30, 1992-July 31, 19924. TITLE AND SUBTITLE S. FUNDING NUMBERS

Atlas of FormabilityInconel 718 C-NO0140-88-C-RC21

6. AUTHOR(S)

Prabir K. ChaudhuryDan Zhao

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National Center for Excellence in MetalworkingTechnology (NCEMT)1450 Scalp AvenueJohnstown, PA 15904

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Naval Industrial Resources Support ActivityBuilding 75-2, Naval BasePhiladelphia, PA 19112-5078

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13. ABSTRACT (Maximum 200 words)

In this investigation, flow behavior of Inconel 718 alloy was studiedby conducting compression tests over a wide range of temperatures andstrain rates. Constitutive relations were determined from the flow behavior,and a dynamic material modeling was conducted on this alloy. Thus, theoptimum processing condition in terms of temperature and strain rate wasidentified. Microstructural changes during high temperature deformation werealso characterized.

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ATLAS OF FORMABILITY

INCONEL 718

by

Prabir K. Chaudhury and Dan Zhao

National Center for Excellence in Metalworking Technology1450 Scalp Avenue

Johnstown, PA 15904

for

Naval Industrial Resource Support ActivityBuilding 75-2, Naval Base

Philadelphia, PA 19112-5078

July 31, 1992

The views, opinions, and/or findings contained in this report are those of the authors andshould not be construed as an official Department of the Navy position, policy, ordecision, unless so designated by other documentation

TABLE OF CONTENTS

Introduction ................................... I

Experimental Procedure ............................ 1

R esults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Sum m ary .................................... 31

Implementation of Data Provided by the Atlas of Formability ......... 31

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LIST OF TABLE

Table 1. List of figures, testing conditions and microstructural observationsfor Inconel 718 ......................................... 2

ii

Inconel 718

Introduction

Among the commercially available superalloys, Inconel 718 stands out as the mostdominant alloy in production. High performance requirements in the application of superalloys,such as aircraft gas turbines, has increased the need to understand the behavior of superalloys.At the same time modern metalworking processes require a better knowledge of mechanical andmicrostructural behavior during high temperature deformation. Flow behavior of Inconel 718was studied by conducting compression tests at various temperatures and strain rates todetermine the constitutive relation. From the constitutive relation a dynamic material modelingon Inconel 718 was carried out to optimize processing conditions in terms of temperature andstrain rate. In addition, microstructural changes were characterized to show the effect of thedeformation on the resulting microstructure.

Experimental Procedure

The material used in this investigation was commercially available Inconel 718 wroughtbars in heat treated and aged condition. The typical microstructure of the as-received materials isshown in Figure 1 showing equiaxed grains. The initial grain size is 23 gnm (7.5ASTM).Cylindrical compression test specimens with a diameter of 12.7 mm and a height of 15.9 mmwere machined, and compression tests were conducted isothermally on an MTS testing machine.The temperatures selected include those which are both above and under 8 (1038 C) solvus. Thetest matrix was as follows:

Temperature, C (F): 982 (1800), 1010 (1850), 1038 (1900), 1079 (1975) and 1149 (2100);Strain rate, s-1: 0.01,0.14, 1.84,5 and 25.

The tests were conducted in air except the ones at strain rate of 5 s-1, which wereconducted in inert gas atmosphere. Load and stroke data from the tests were acquired by acomputer and later converted to true stress-true strain curves. Immediately after the compressiontest, the specimens were quenched in order to retain the deformed microstructure. Longitudinaland transverse sections of the quenched specimens were examined using optical microscope.The photomicrographs presented were taken from the center of the longitudinal section of thespecimens.

Results

Table 1 is a list of the figures, test conditions and the observed microstructures. All thetrue stress-true strain flow curves with the corresponding deformed microstructure are shown inFigure 2 to Figure 26. True stress versus strain rate was plotted in log-log scale in Figure 27 at atrue strain of 0.3. The slope of the plot gives the strain rate sensitivity m, which is not a constantover the range of strain rate tested. Log stress vs. I/T at the same true strain is shown in Figure28. Processing map at this strain was developed for Inconel 718, Figure 29. The optimumprocessing conditions from the map can be obtained by selecting the temperature and strain ratecombination which provides the maximum efficiency in the stable region. This condition isapproximately 1070 C and 0.01 s-I for Inconel 718.

" ' ' i i I I i I I I i 1

Table 1. List of figures, testing conditions and microstructural observations for Inconel 718.

Figure Temperature Strain Rate Microstructure PageNo CF) S-i Optical Microscop2y No

1Heat treated and aged wrought bars. Equiaxed 3_ _rains of -23p4m (7.5ASTM).

2 982(1800) 0.01 Deformed grains showing serrated grain 4boundaries (initiation of dynamicrec stallization), incipient necklacing.

3 982(1800) 0.14 Same as above, but the grains appeared to be 5more severely deformed.

4 982(1800) 1.84 Small and equiaxed dynamically recrystallized 6

grains (- a).5 982(1800) 5 Same as above, but the grains are smaller 7

(~5.6ptm), a very small proportion of deformedgrains still present, tested in an inertatmosphere.

6 982(1800) 25 n/a 87 1010(1850) 0.01 Deformed grains showing serrated grain 9

_boundaries , necklacing present.8 1010(1850) 0.14 Equiaxed dynamically recrystallized grains with 10

an averate size of 6.5 pm.9 1010(1850) 1.84 Equiaxed grains with an average size of 10.6pm, 11

_ twinning present10 1010(1850) 5 Equiaxed grains with a duplex structure; small' 12

grains 4-8g1m and large grains 20-30pn. Smallfraction of large deformed grains is also present,tested in an inert atmosphere.

11 010(1850) 25 Equiaxed grains with an average size of 7.6pn. 1312 1038(1900) 0.01 Equiaxed grains (-17gtm) developing twins. 14

Note that 1038 C is just bellow the 8 solvus.

13 1038(1900) 0.14 Equiaxed grains with an average size of lO•m. 1514 1038(1900) 1.84 Equiaxed grains (-12pAm) developing twins. 1615 1038(1900) 5 Equiaxed grains with a duplex structure; small 17

grains -12jin and large grains 25-301m. Smallfracuon of large defoimed grains is also present,tested in an inert atmosphre.

16 1038(1900) 25 Same as above, but the grains are slighy 18smaller, tested in an inert atmosphere.

17 1079(1975) 0.01 Large duplex equiaxed grains with a grain size 19in the range of 40-60 ptm. Tested in an inertatmosphere.1

18 1079(1975) 0.14 Equiaxed large grains with a duplex size 2019 1079(1975) 1.84 Equiaxed grains (22-301•n) developing twins. 2120 1079(1975) 5 Equiaxed grains (-29ptm), tested in an inert 22

atmosphere.21 1079(1975) 25 Equiaxed grains (22-25pm). 2322 1149(2100) 0.01 Large equiaxed grains (-61pm). 2423 1149(2100) 0.14 Same as above, but smaller grains 2524 1149(2100) 1.84 Large equiaxed grains. 2625 1149(2100) 5 Equiaxed grains (-56gim), tested in an inert 27

atmosphere.26 1149(2100) 25 Equiaxed grains (-41.im) 28

2

Figure 1. As-received microstructure of Inconel 718.

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Summary

Compression tests have been performed on Inconel 718 over a range of temperatures andstrain rates. The experimental conditions used in this work are representative of those used inmetalforming practices. From the stress-strain curves, the flow behavior was characterized and aprocessing map indicating the optimum processing condition was generated. This condition isapproximately 1070 C and 0.01 s-1.

The deformed microstructures were characterized from the quenched specimens byoptical microscopy and are presented for each testing conidition under the stress-strain curves.Dynamic recrystallization and grain growth occurred over the temperature and strain rate rangetested.

Implementation of Data Provided by the Atlas of Formability

The Atlas of Formability program provides ample data on flow behavior of variousimportant engineering materials in the temperature and strain rate regime commonly used inmetalworking processes. The data are valuable in design and problem solving in metalworkingprocesses of advanced materials. Microstructural changes with temperature and strain rates arealso provided in the Bulletin, which helps the design engineer to select processing parametersleading to the desired microstructure.

The data can also be used to construct processing map using dynamic material modelingapproach to determine stable and unstable regions in terms of temperature and strain rate. Thetemperature and strain rate combination at the highest efficiency in the stable region provides theoptimum processing condition. This has been demonstrated in this Bulletin. In somemetalworking processes such as forging, strain rate varies within the workpiece. An analysis ofthe process with finite element method (FEM) can ensure that the strain rates at the processingtemperature in the whole workpiece fall into the stable regions in the processing map.Furthermore, FEM analysis with the data from the Atlas of Formability can be coupled withfracture criteria to predict defect formation in metalworking processes.

Using the data provided by the Atlas of Formability, design of metalworking processes,dynamic material modeling, FEM analysis of metalworking processes, and defect prediction arecommon practice in Concurrent Technologies Corporation. Needs in solving problems related tometalworking processes can be directed to Dr. Prabir K. Chaudhury, Manager of the Atlas ofFormability project, by calling (814) 269-2594.

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